Chapter 9 - Earthwork in Cold Regions

230

Embankment construction with frozen and unfrozen soil mate-rials is conducted throughout the year on many engineeringprojects where ambient air temperatures remain below 0 °C fora significant portion of the year. Construction for oil field sup-port facilities on the North Slope of Alaska required that opera-tions proceed over a 12-month period and not wait for theshort 3-month thaw season (Tart 1983). The operation of openpit mines requires effective and economic excavation and han-dling of frozen and thawed ground during the entire year. Diffi-cult problems can be experienced depending on the materialtype, temperature and water content, and time of year. Materialproperties of fill used on the project must be reviewed. Land-form analyses, borings, and geophysical techniques help assesspotential material sources. Borrow source characteristics deter-mine the techniques required for excavation (e.g., blasting, rip-ping, and scraping). Material type and volumes involved deter-mine the haul equipment that is most suitable for the project.In addition, placement and compaction techniques are depen-dent on embankment use and material type.

9.1 Site Considerations

Site considerations include drainage, thermal and frost actionfactors, subsurface conditions, and materials needed forembankment construction.

Drainage

Drainage is required on earthwork projects for snow and icemeltwater during the spring breakup period. Occasional sum-mer storms produce heavy runoff in some areas. Combiningthis with poor natural drainage—typical of relatively flat per-mafrost areas with impervious frozen ground at shallowdepths—can lead to ponding of water with accelerated thawunder and adjacent to embankments. Flowing water causes soilerosion, especially in thawing soils, and can quickly removelarge quantities of soil, leading to potential stability problems.

Adequate drainage facilities must be provided to handle bothsurface and subsurface water movement.

Ditching. In permafrost areas, excavation of ditches should beavoided wherever possible (Argue et al. 1981). Diversion orinterception of cross drainage by a highway embankmentshould be controlled by the use of culverts located at everydefinable water course. No significant volume of water shouldbe permitted to flow along the toe of the fill. Wherever pondingtends to develop, due to insufficient gradient, a berm may beconstructed along the embankment toe to guide flow to theculverts. Although generally avoided, Argue et al. (1981) statedthat interceptor ditches may be acceptable if they are con-structed at least 6 m from the embankment and have minimalgradients (Fig. 9-1). Special measures used to avoid erosion andkeep the ditch level and in line with culverts include use ofriprap on fill slopes, ditch blocks and checks, and lining theditch with granular materials, as is shown in Fig. 9-2.

Icings. The freezing of a sheetlike mass of ice on the groundsurface or at the outlet of culverts represents icing. The watersource may involve spring meltwater, artesian flow fromsprings, blockage of an inadequate drainage structure, orstreams where flow has been restricted by the formation of ice.An icing extending across a pavement is shown in Fig. 9-3. Thisicing occurred when the culvert became blocked and flow wasdiverted across the roadway. Brown and Kupsch (1974) statedthat the longitudinal dimension of the large icings mayapproach 10 km with a thickness of 1 m or more. Recognitionand prediction of potential icing locations are difficult butrequire careful attention in the planning and design of earth-work. They occur naturally at many locations and may be initi-ated in the area of new construction because of inadequatedrainage. Several techniques have been used to prevent icingconditions (Argue et al. 1981):

1. Raising grade. This approach is the simplest but is costly andassumes a knowledge of the limits of icing. Large storage

9Earthwork in Cold Regions

EARTHWORK IN COLD REGIONS 231

areas for the ice are required, and these are rarely available inthe steep terrain where icings commonly occur.

2. Ice fences or dams. Permanent earthworks may be constructedsome distance upslope to contain or limit the buildup of ice;sufficient storage space to contain the seasonal buildup isrequired; temporary fences of reinforced paper or burlap areeffective in areas where the seepage is slow and in thin layers.

3. Frost belts. Removal of the insulation cover of vegetation andsnow at some point above the road will allow rapid freezingof the active layer and start the icing at a point where it is

hoped it will not reach the road; this solution is temporaryat best but can be effective for one or two seasons.

4. Staggered culverts. Where ice collects behind a roadwayembankment and blocks the culvert, auxiliary culverts maybe placed above the anticipated ice level to carry spring run-off until the lower culverts are clear.

5. Application of heat. Steam lines and fuel oil heaters (firepots)are commonly used to open up culverts or thaw channels inicings in the spring and, in some cases, during the winter;electrical heating cables have also been used in Alaska.

FIGURE 9-1 Typical embankment cross section with interceptor ditch.Source: Adapted from Argue et al. 1981.

FIGURE 9-2 Typical ditch lining to prevent erosion.Source: Reproduced from Argue et al. 1981.

232 FROZEN GROUND ENGINEERING

6. Insulated subsurface drains. These can be used to interceptgroundwater and carry it away from the road; modern filterand insulation materials make such drains possible but insome cases expensive.

7. Road relocation. This is the most extreme solution. Icings ofsuch severity as to require relocation can probably be identi-fied before construction.

Carey (1973) summarized existing knowledge concerningthe occurrence, control, and prevention of icings that developfrom surface water and groundwater.

Thermal and Frost Action Factors

The thermal regime for an embankment, including the depthof freeze or thaw, determines to a large extent the embankmentperformance relative to settlement and stability. This thermalregime will usually differ radically from conditions existingprior to the construction of the embankment. Trees, brush, andsurface vegetation will have been removed, and different formsof snow cover will now form. In cold permafrost areas (meanground temperature close to –11 °C), a gravel embankmentthickness of about 1.5 m is usually adequate to prevent thawingof the underlying permafrost (Berg and Quinn 1977). Thinnerembankments may be used if they are surfaced with materials(pavements) of high albedo (high reflectivity to solar radiation)or if they are constructed with a layer of insulation placed in theembankment. Computational methods for frost depth andthawing of frozen ground are described in Section 3.2. Differ-ent albedos are accounted for by the n-factor used to convert airtemperatures to ground surface temperatures. Whenever possi-ble, surface boundary conditions obtained from field measure-ments made at typical locations should be used to improve thereliability of thermal calculations.

Selecting insulation thickness involves a thermal analysis ofa layered soil system consisting of gravel fill, insulation, and dif-

ferent subsoil layers. Change in moisture contents and variablesurface conditions usually require that computer methods beused to solve these problems. An exception to this approach isLachenbruch’s (1959) analytical solution of a three-layered sys-tem subjected to a surface temperature sine wave. Restrictionsto this solution are that there can be no phase change in any ofthe three layers and that the solution does not model the grad-ual change in unfrozen water content in the permafrost subsoil.Nixon (1979) stated that Lachenbruch’s solution has been usedwith reasonable success in cases where the gravel fill and insula-tion are assumed to be dry and the 0 °C isotherm remainsabove the permafrost. The solution illustrates which thermalvariables are involved, and it permits an inexpensive rough esti-mate to be made of the gravel and insulation thickness requiredfor protection of the permafrost.

Selecting reasonable thermal properties for gravel fill, poly-styrene insulation, and frozen ice-rich silt, Nixon (1979) haspresented a solution to the insulated fill problem, as is shown inFig. 9-4. The results are presented in terms of the dimensionlessratio between the mean annual pad surface temperature Tg (°Cbelow freezing) and the amplitude of the sine wave A o. Thick-ness of fill and insulation required to maintain the 0 °C iso-therm at or above the embankment base can be estimated interms of Tg and Ao using Fig. 9-4. Note that the same level ofthermal protection can be provided by different combinationsof gravel and insulation thicknesses. A choice is based on eco-nomics combined with the minimum thickness of gravel coveron the insulation required for structural reasons. Several possi-ble designs for insulated and uninsulated embankments areshown schematically in Fig. 9-5.

Subsurface Conditions

Soil and permafrost conditions on site—especially the type anddistribution of ground ice in different types of terrain—areimportant relative to the potential settlement and stability of theembankment. Determination of the type, distribution, proper-ties, and behavior of the materials is essential. Sampling of thematerials (soil or rock) for examination and testing in the fieldand/or laboratory is a part of the field investigation program(Chapter 10). If the subsurface materials consist of sound, ice-free rock or dense glacial till or of clean, non-frost-susceptible,well-drained sand or gravel, the frozen conditions can beneglected in most cases and conventional embankment founda-tion design is appropriate. If the subsurface materials are thawunstable, two approaches to design in permafrost areas are con-sidered. The passive method is the most desirable and involvesmaintaining the foundation materials in a frozen state for theservice life of the embankment. When permafrost degradationcannot be prevented, the active method must be considered. Twochoices are possible. The unfavorable materials are thawed andcompacted or are removed and replaced with more suitablematerials prior to embankment construction. The alternative isto design the embankment to accommodate the settlements thatwill occur as thawing progresses while any structure on theembankment is in use. A more detailed discussion of the passiveand active methods was given in Chapter 7.

FIGURE 9-3 An icing due to culvert blockage extending acrossGlenn Highway near Slana, Alaska, March 1967.Source: Courtesy of Kevin L. Carey, U.S. Army, Cold Regions Research andEngineering Laboratory.


Material sources

Local materials are normally used for embankment construc-tion after careful evaluation of their properties and potentialbehavior as fill. Excavation, hauling, and placement problems(both winter and summer) enter into the cost factor. Cleangravel and well-graded sand are the most desirable materials forearthwork construction, in general. For embankment dams andhazardous waste enclosures, fine-grained soils are often consid-ered more desirable. Sands and gravels perform well because oftheir strength and drainage characteristics, and they help over-come settlement and frost action problems. Sands and gravelscan be placed readily when ambient temperatures are below 0°C. In the Prudhoe Bay area, the cleanest and coarsest granularmaterials are generally found in the beds of large rivers (Tart1983). In areas where gravel is in short supply (e.g., northernCanada), low-ice-content glacial tills, quarried rock, and clay

shales have been used with considerable success (Argue et al.1981). Shales are subject to mechanical disintegration andrequire a cap or wearing surface of granular material whenexposed to heavy traffic. In discontinuous permafrost areas,cohesive soils are used for embankment construction whenthey can be obtained from borrow pits in unfrozen areas atsuitable moisture contents. When frozen (northerly areas),cohesive soils contain much ice, are difficult to excavate, andusually do not merit consideration.

Methods of assessing potential material sources includelandform analyses, borings, and geophysics. Landforms recog-nizable on aerial photographs are terrain features formed bynatural processes that have a definable composition and rangeof physical and visual characteristics occurring wherever thelandform is found. Landforms common to glaciated areas, andwhich consist primarily of sand and gravels, include eskers,kames, kame terraces, outwash, and beach ridges. More infor-

FIGURE 9-4 Lachenbruch analysis for thaw or frost protection.Source: Reproduced with permission from Nixon 1979. Copyright 1979 Canadian Geotechnical Journal.


mation on landforms can be obtained from texts on geomor-phology and engineering geology. Borings may involve a fieldand laboratory investigation, which is intended to disclose thephysical properties and arrangement of the underlying mate-rial. With preliminary information from borings available, amore extensive program may be indicated or only a geophysicalinvestigation may be needed to delineate boundaries betweendifferent elements of the soil deposit.

Projects may not have the time or funds to initiate theirown aerial survey to obtain current photographs of a site. Inthis case, inexpensive stereo photographic coverage of manysites in the United States can be purchased from the U.S.Department of the Interior, Geological Survey, Washington,DC 20240, or from the U.S. Department of Agriculture. Infor-mation concerning different types of maps available can beobtained from Topographic Maps, a publication available from

FIGURE 9-5 Possible designs for uninsulated and insulated embankments on permafrost when thefrozen condition is to be preserved.Source: Reproduced with permission of G. H. Johnston from Argue et al. 1981.


the Map Information Office, U.S. Geological Survey, Washing-ton, DC 20242.

9.2 Excavation and Transport

The technique used to excavate frozen soil depends on the timeof year and the soil moisture content. Surface thawing in thesummer may allow scraping and/or moderate ripping as ameans of removing gravel. Using heavy-duty rippers, frozensoils can be excavated at ambient air temperatures down to –40°C (Moore and Sayles 1980). Another technique common forice-saturated and frozen materials is blasting for both summerand winter excavation (Tart 1983). Thawing of frozen groundmay be used to simplify excavation or to permit densificationprior to other construction activities. Once the material hasbeen thawed or broken into chunks, large (15 to 23 m3 capac-ity) self-propelled scrapers or front-end loaders with bot-tom-dump or end-dump haulers are used for soil transport.

Mechanical Excavation

The conventional techniques used to excavate frozen soilsinclude ripping, shearing, and drag-bit cutting (Baker andJohnston 1981). Controlled explosive loading (blasting) is dis-cussed in a later section.

Machines and Cutting Tools. Excavation of natural soil mate-rials (frozen and unfrozen) involves technology that varies with

properties of the materials and with the scale of the operation.A distinction can be made based on the strength, cohesion, andductility of the material. In unfrozen soils with little cohesion,the forces and energy levels required for separation and disag-gregation are small compared with the forces and energy levelsrequired for acceleration and transport. In frozen soils withproperties similar to weak concrete, the forces and energy levelsrequired for cutting and breaking are high and the technicalemphasis is placed on the cutting and breaking process.

Mellor (1976) has directed a large research effort towarddeveloping a systematic analytical scheme that can be used tofacilitate work on the mechanics of cutting and boringmachines. This research has examined the kinematics, dynam-ics, and energetics for both the cutting tool and the completeexcavating machine. Kinematics deals with relationshipsdefined by the geometry and motion of the machine and itscutting tool. Dynamics deals with forces acting on the machineand its cutting tools. Energetics deals largely with specificenergy relationships that are determined from power consider-ations involving forces and velocities in various parts of the sys-tem. In his research, Mellor (1976) has applied mechanicalprinciples in accordance with the classification (Fig. 9-6) basedon the characteristic motion of major machine elements andthe actual cutting tools. Machines are classified as transverserotation, axial rotation, or continuous belt, and the cutting toolaction is divided into parallel motion and indentation.

Transverse rotation devices include bucket-wheel trenchersand excavators, continuous miners, and some dredge cutting

FIGURE 9-6 Classification of machines and cutting tools for excavation of ground materials.Source: Reproduced from Mellor 1976.


heads. Axial rotation devices include rotary drills, full-face tun-nel boring machines, and some face miners. Continuous-beltmachines represent a form of transverse rotation and include“digger chain” trenchers, ladder dredges, and similar devices.Operating ranges for various types of belt machines are givenin Fig. 9-7. Parallel motion tools (Fig. 9-6) involve a planingaction by the tool that is advanced more or less parallel to thesurface. Examples include teeth for dredging and ditchingbuckets, trencher blades, surface planers, and other abrasivecutters. Normal indentation involves a pitting or crateringeffect produced by a stroke normal to the surface. Tools work-ing in this way include reamers, disk cutters for tunnelingmachines, and percussive bits for drills and impact breakers.The indentation process may involve (1) brittle fracture withformation of loose fragments, (2) ductile yielding with dis-placement of material toward the free surface, and (3) compac-tion of a readily compressible material.

Equipment and Methods. Equipment used for excavationincludes all types and methods by which a frozen soil ismechanically fractured or broken. Compressed air jackham-mers are often used for small excavations and will break orloosen materials to a depth of about 0.3 m. The resilient behav-ior of frozen soil reduces their effectiveness. In warm perma-frost (–1 °C) with lower strengths, heavy power shovels havebeen used to make large excavations. Various types of excavat-ing equipment—including rippers, cutters, saws, impact orvibratory breakers, and bucket excavators—have been usedwith various degrees of success, as described in Table 9-1. Thistable, adapted by Baker and Johnston (1981) from Fos-ter-Miller Associates (1973), compares the rate of removal andenergy effectiveness of several mechanical methods of excavat-ing frozen ground. Excavation of frozen soils using rippers and

large self-propelled scrappers aredescribed in the next subsection.

Information on excavation ofslots and trenches in frozen groundusing saws, chain or bucket excava-tors, and drop hammers is includedin Table 9-1. Narrow slots may be cutin frozen ground for burial of cablesor to aid in excavating large blocks.Tooth and chain wear account forpoor performance of chain excava-tors in most frozen materials (Mellor1976). An Alkirk continuous miningmachine equipped with drag-bit cut-ters was used to excavate a tunnel infrozen silt (Tm = –1 °C) near Fair-banks, Alaska (Swinzow 1970). Othermethods, including coal cuttingmachinery and drilling and blasting,were used in the same tunnel with nodefinite conclusions made relative toadvantages of the different equip-ment and methods.

Frozen Soil Breakup by Ripping. Large areas of frozen mate-rial are loosened or fractured most economically by single- ordouble-toothed rippers employing the principle of shear ordrag-bit cutting. These rippers have shanks about 1.2 m long.Cross-ripping on a grid pattern with passes spaced at about 1.5m have provided the best results. Poor traction on the frozenground surface may permit only slight ripper penetration onthe first pass. On subsequent passes with better traction, deeperripper penetration is obtained. Scrapers utilized to load rippedmaterials are conventional self-propelled units of 15 to 23 m3

capacity (Fig. 9-8). Melting and drainage can be controlled byripping only small areas and excavating the material to gradebefore a cut is enlarged.

When an excavation extends well below the existing watertable, it is often practical to excavate these areas in the cold win-ter with ambient air temperatures as low as –40 °C (Moore and

FIGURE 9-7 Operating ranges of continuous belt machines.Source: Reproduced from Mellor 1976.

FIGURE 9-8 Self-propelled scraper unit loading frozen gravel.Source: Courtesy of Francis H. Sayles, U.S. Army, Cold Regions Research andEngineering Laboratory.


TABLE 9-1 Comparison of Various Methods of Excavating Frozen Ground

Rate of removal Effectiveness (× 10–9 m3/J)Method Details Material m3/min. m/min. Remarks Sourcea

D-8 Bulldozer, No. 8 ripper

Single-tooth rip-per, trench 0.2 m2 in cross-sec-tional area

Frozen clean beach gravel, w > satura-tion, air temp. –1 °C

13.7 — 1,295(short term)

Effectiveness based on rated power of bull-dozer; borrow pit at Barrow, Alaska

(1)

D-9 Bulldozer, No. 9 ripper

Single-tooth ripper Silty gravel (18% silt), w = 6%, air temp. –2 °C

— — 283(short term)

Borrow pit, Denali Highway, Alaska

(1)

D-8 Bulldozer, two-toothed ripper

Frozen clay 1.3 — 119(day long)

Stripping, Usibelli Mine, Healy, Alaska

(1)

Tractor-pulled ripper Tractor weight = 40,500 kg, developed 28,350-kg pull at ripper

1.5-m-thick layer sandy clay with cobbles, air temp. –21 °C

1.1 — 83(day long)

Area cut into 1- to 1.5-m square grid; cut pieces easily han-dled by shovel or bucket excavators

(1)

Disk saw, tractor mounted

Saw diameters var-ied from 0.8 to 2.5 m, with peripheral speed up to 15 m/s

Very dense clay, air temp. –13 °C

— — 62(highest)

Pebbles caused break-age of saw; stability of large disk saw was low

(1)

Combination disc cutter and shovel extractor

Narrow slits 60–80 cm apart, 0.8 m deep; 2.5-m-diameter cutter; peripheral velocity 3.5–20 m/s

— — — — High power consump-tion for the disk cutting process; high cutting speed causes rapid wear of cutting edges

(1)

Trench excavator Scoops on excava-tor chain, cuts trenches 1.8 m deep by 0.85 m wide

1.3-m layer of frozen material

0.1 0.1 52 Trench excavation made by cutting two narrow trenches and chop-ping in between

(1)

Arctic Ditcher Model 7-10, Banister Pipe-lines Ltd., Henuset Bros. Ltd., Ditcher, Barber-Greene Co.

Rotary-wheeled trenchers with buckets, conical carbide teeth

Sandy gravel, till, Churchill, Mani-toba

— — — Extensive teeth wear; additional weight necessary in the bucket wheel; greatly assisted by blasting

(3)

Chain saw 2.5-m-deep trench made in one pass

2-m layer of frozen material

— — — High wear, expensive materials for parts

(1)

Rotary excavator (FP-7A, FP-2M, FP-4) with special teeth

Trench cut 1.7 m deep by 1.2 m wide

2-m-deep layer of frozen material

0.2 0.2 — More efficient than chain swing

(1)

Rotary excavator Trench cut 1.1 m wide by 1.7 m deep, cutting speed 1 m/s

Frost line 50 cm deep 2.5 1.2 — Cutting speed of 1 m/s too high, causing rapid wear of surface

(1)

Frost line 120 cm deep

1.7 0.8 —

Rotary excavator Same size trench as above, cutting speed 0.5 m/s

Frost line 100 cm deep

1.7 0.8 391 Modifications made to bucket teeth design

(1)

continued on next page


aSources: (1) Foster-Miller Associates 1973, Tables V and VI; (2) U.S. Navy 1955, Table 3A3-1; (3) Joy 1973.

TABLE 9-1 Comparison of Various Methods of Excavating Frozen Ground (continued)

Rate of removal Effectiveness (× 10–9 m3/J)Method Details Material m3/min. m/min. Remarks Sourcea

Wedge-shaped hammer

50-cm-wide wedge splits 30–40-cm-wide pieces of frozen soil

— 0.3 — — — (1)

Wedge impact Wedge width 18 cm Frost line 1–1.2 m deep

0.2 — 18 Based on average value of 100 m/day of 1- to 1.2-m-deep fro-zen ground requir-ing 10 impacts/min. of 8,000 kg · mimpact energy

(1)

Vibrating hammer on excavator bucket

Hammer weight = 900 kg, width = 50 cm, fre-quency = 7–12 Hz, impact energy per cm of cutting edge = 30 J

— — — — Motor (22 kW), cost 50% of drilling and blasting

(1)

Vibrating hammer on excavator bucket

— — 1.43 — — — (1)

Alkirk cycle miner — Frozen Fairbanks silt, air temp. –2 °C

— — 801(short term)

Digging Fox tunnel (1)

79(day long)

Explosives Ammonium nitrate in diesel fuel

Frozen sand or gravel — — 1,006 Explosive energy only; does not include shot hole drilling

(1)

Explosive Cratering, single charge

Frozen till, Churchill, Manitoba

— — 81 — (1)

Thermal drill (Brown-ing)

Uses diesel fuel and compressed air

Frozen Fairbanks silt, air temp. –1 °C

Frozen gravel < 5 cm, air temp. 1 °C

— — 1 Slots cut at rates rang-ing from 3 m2/h in silt to 6.5 m2/h in gravel

(1)

Steam thawed, hand shoveled

Volume = 765 m3 Silt and gravel 0.13 × 10–3

— — Power plant excava-tion, Alaska

(2)

Blasting, hand shov-eled

Volume = 418 m3 Gravel and bedrock (mica schist)

0.13 × 10–3

— — Diversion dam and gates, Alaska

(2)

Jackhammer, hand shovel

Volume = 31 m3 Heavy gravel (partly frozen)

0.15 × 10–3

— — Water tower founda-tion, Alaska

(2)

Jackhammer, hand shovel

Volume = 176 m3 Heavy gravel with sand and silt (partly frozen)

0.14 × 10–3

— — Warehouse, Alaska (2)

Steam thawing, jack-hammer

Volume = 505 m3 Silty sand with gravel (frozen)

0.10 × 10–3

— — Steam lines, Alaska (2)

Hand shovel Volume = 689 m3 Silt, sand, and gravel (not frozen)

0.24 × 10–3

— — Sewer mains and later-als, Alaska

(2)

Hand shovel Volume = 230 m3 Sand (not frozen) 0.44 × 10–3

— — Garage and repair shop, Alaska

(2)


Sayles 1980). Frozen soft ground will support heavy equip-ment, and the risk of flooding an open excavation during con-struction will be minimized. The frozen soils are broken upusing heavy-duty single-tooth rippers mounted on large Cater-pillar tractors (Fig. 9-9). Large self-propelled scrapers (Fig. 9-8)are used to remove frozen materials from the excavation area.The exposure of unfrozen materials to cold air temperaturesallowed fog to form, as is shown in Fig. 9-8. This fog will usu-ally dissipate after the unfrozen soil and any surface water hasfrozen.

Questions as to whether various earth materials are rippablemay be based on their seismic velocities. Tart (1983) reportedthat frozen gravel deposits are usually rippable when seismicvelocities are under about 2,100 m/s. According to the Caterpil-lar Tractor Company performance handbook (1977), materialsare considered “rippable” by a D-9 tractor equipped with amulti- or single-shank ripper if the seismic velocity of thematerial is below 2,500 m/s and “marginal” if between 2,500and 3,000 m/s. Moore and Sayles (1980) reported that thesevelocities appear to be conservative on the basis of productionrecords for the Chena Project, Fairbanks, Alaska. Seismic veloc-ities for in situ frozen silts and frozen gravels for the Fairbanksarea (Mellor 1977) ranged from 1,900 to 2,900 m/s and 3,350 to3,700 m/s, respectively.

The strength of a frozen soil (Chapter 5) is highly dependenton the loading rate. For the practical speeds of trac-tor-mounted rippers (about 4 km/h), the loading rate is fastenough to produce a brittle failure in frozen soil. For thesespeeds, frozen soil strength is almost independent of the speedof ripping. Properties of frozen soils that do affect theirbreakup by ripping (Moore and Sayles 1980) include (1) soiltype (clay, silt, sand, or gravel), (2) density, (3) gradation, and(4) temperature and degree of saturation. In general,fine-grained soils rip more easily than coarse-grained soils.Within any type of homogeneous soil, experience has shownthat the denser soil is more difficult to rip. For the stratifiedsoils common to the Fairbanks area, it was noted that large

chunks of frozen soil would usually break apart along thin sandseams. Because water is normally drawn from coarse-grainedsoils to contiguous fine-grained strata, a reduced degree of sat-uration and lower strength of the sand would provide naturalbreaking planes for the mechanical rippers. All saturated andpartially saturated soils become stronger and harder to rip withlower temperatures. Moore and Sayles reported that extremedifficulty was experienced in ripping saturated gravels when theambient air temperature was –40 to –46 °C. At warmer temper-atures (April and May), when ground temperatures were in therange –1 to –4 °C, a single tractor could rip the same frozengravels easily. Frozen soils with very low moisture contents(drained sands and gravels) can readily be loosened at any tem-perature and in some cases with about the same effort as thatrequired in the unfrozen state.

Drilling and Blasting

Ice-saturated and frozen soil materials are very similar to rockformations. The excavation of these materials requires blastingin a manner similar to that used in quarries (Tart 1983). Bakerand Johnston (1981, p. 230) stated that “drilling and blasting isthe most economical method for dealing with the large quanti-ties of frozen material involved in a mining operation.”

The drilling operation and proper placement of explosivecharges determines the effectiveness of the blasting method.Foster-Miller Associates (1973) and Mellor and Sellmann(1975) discussed how a reduction in hole depth and diameter,or plugging of the hole (melting and slump within the hole),may not permit placement of the explosive charge at the correctdepth. A common problem in drilling is caused by meltwaterfreezing and forming an ice collar around the top of the hole.This ice collar and sloughing prevent proper stemming of thehole. Use of clean dry sand or fine rock chips to fill the holehelps achieve the required stemming. Particle size should notexceed 10% of the hole diameter. Swinzow (1963) achievedexcellent results in permafrost using a mixture of silt and claythat was tamped in the hole and allowed to freeze. Mellor(1989) provided more information on stemming of shotholes.

Explosive consumption in frozen materials is high. Tart(1983, p. 34) stated that “powder factors ranging from 1 to 2 lb/yd3 (0.6 to 1.2 kg/m3) are typically used.” A large proportion ofthe energy generated by the blast is absorbed by the ice for icecontents greater than 10% by dry mass. Because drilling andblasting are relatively expensive, most contractors prefer tokeep their hole spacings as large as practical. Tart gave a typicalspacing of 3 m between shot holes. This spacing, in overburdenwith high moisture contents, can give chunk sizes approaching3 m unless the powder factor is appropriately increased or thehole spacing is reduced. The larger chunk size requires usingend-dump haulers. Livingston (1956) and Bauer et al. (1965)reported on drilling and blasting trials used to determine opti-mum values for the type and amount of explosive and thedepth and spacing of holes.

Improper blasting of the face in deep excavations and openpit mines can lead to a series of undesirable events because ofunsatisfactory and extensive backbreak, as is shown in Fig. 9-10

FIGURE 9-9 Single-tooth ripper unit attached to a D-9G Cat-erpillar tractor.Source: Courtesy of Francis H. Sayles, U.S. Army, Cold Regions Research andEngineering Laboratory.


(Ives 1962; Lang 1976). The pit floor becomes higher as succes-sive blasts increase the backbreak, and removal of broken mate-rial by loaders or power shovels becomes impossible. Theunsuccessful primary blast requires reblasting of the toe andthe pit floor. Secondary drilling and blasting or mechanicalexcavation must be used on large blocks resulting from insuffi-cient fragmentation. Further drilling becomes difficult due tothe extensive backbreak. When time permits, the large frozenblocks can be left to thaw naturally.

Thawing Frozen Soil

Frozen ground thawing may be utilized to simplify excavationor to permit consolidation and densification prior to other

construction activities. Thawing can be accomplished in severalways, including solar thawing, steam injection, hot and coldwater injection, and electric thawing. They all are attractive forcertain soil conditions.

Solar Thawing. Warm summer surface temperatures lead tothawing of frozen surface soils to depths corresponding to theactive layer. The thaw rate and depth can be increased by sur-face modifications that increase the absorption of solar radia-tion. This can be accomplished by the removal of trees, brush,and moss cover, as is illustrated in Fig. 3-4. Additional surfacemodification is possible by placement of a reinforced polyethyl-ene sheet on the stripped ground surface (Esch 1985). Benefi-cial effects from the plastic sheet cover result from a decrease inevaporative and convective heat loses. Thaw depths reachedafter several years of surface modification can be estimatedusing Eq. (3.2-5). The major unknown is the n-factor. With aplastic cover, the initial n-factors may be lower than would beexpected from values listed in Table 3-5 due to changes in waterpercolation and surface evaporative losses. Calculated n-factorsfrom different surface modification plots are listed in Table 9-2.

Without the plastic cover, thawed material (70–150 mm)can be removed as soon as it has thawed so that a frozen surfaceis always exposed to the sun. This “thaw-scrape” operation canbe used to obtain borrow materials, make cuts, and prepare rel-atively large surface areas for construction. The method is suit-able only where time is not a major consideration and whererelatively large areas are involved. Stripping and covering with aclear polyethylene sheet helped thaw ice-rich soils to a depth of4 m during a 4-year period at a site used for a shopping centernear Anchorage (Esch 1985). The polyethylene sheets do dete-riorate and require periodic replacement to maximize thawingover several years. Preconstruction stripping (without the plas-tic cover) and thawing has been used successfully for roadwaysby the Alaska Department of Transportation (Esch). Solarthawing is very economical relative to other methods.

Steam Injection. Open system steam thawing involves inject-ing steam into the soil through a pipe and steam point during aperiod of time that is dependent on pipe spacing, the tempera-ture and pressure of the steam, and the permeability of thethawed soil. The method is relatively expensive, rapid, and canbe applied to all soil conditions at any time of the year. A typicalprobe consists of a conical nozzle tip fitted to a 19- to 25-mm-diameter pipe, usually of heavy or extraheavy-wall class. Wheresoils permit, probes are placed by their own weight or by light

FIGURE 9-10 Blasting problems in a deep excavation. (A) Ide-alized mining face: (a) and (b) are critical distances betweencenter of the “charge” (c) and the mining face; (a) is termedthe “burden” and (b) the “toe distance.” (B) Moderately suc-cessful blast on initial idealized face. (C) Successive blastleaving extended toe at (x); shattered ore in sector (y) is diffi-cult to remove because an electric shovel is unable toapproach over (x). (D) Actual mining face compared with ide-alized face, showing a situation that would result in a seriousproduction holdup; compare critical distances (a) and (b)with (a¢) and (b¢) and note that extensive secondary drillingand blasting of the extended toe (x) would be required.Source: Reproduced from Ives 1962.

TABLE 9-2 Calculated n-Factors for Different Surface Modifications

Source: After Esch 1984.

Thawing season

Stripped soil

Clear polyethylene

over stripped soil

Clear polyethylene over asphalt

on gravel

Gravel over

stripped soil

First 0.80 1.08 1.30 0.75

Second 0.93 1.16 1.58 1.05


hammering, with steam or hot pressurized water injected dur-ing driving. Where cobbles or boulders resist jetting or driving,drilling is used to develop pilot holes.

Unknowns in the design of a steam thawing system includeprobe spacing, depth, steam pressure, boiler capacity, and time.Steam probes are usually spaced at equal distances in a triangu-lar pattern. A hexagonal pattern (Fig. 9-11) is achieved by stag-gering the probes in alternate rows. Radial thaw from eachprobe during steaming results in the most uniform thawing.Heat transfer is most rapid when steam expands and flowsthroughout the soil column surrounding each probe hole.Return of steam alongside the probe may result in some uncer-tainty regarding thaw-rate calculations. Temperature measure-ment holes in the P region of Fig. 9-11 are used to determinethe completion of thawing.

Esch (1985) reported on a steam thawing project near Fair-banks that was initiated to provide for preconstruction thawand consolidation of ice-rich alluvial gravels. At the FairbanksPost Office site, steam probes consisting of 38-mm pipes, slot-ted at the lower ends, were installed in preaugered holes. Theholes were drilled to 2 m above the bottom of the frozen graveland were spaced at intervals of 2.4 m. Spacing between rowswas 3 m, with hole locations staggered in alternate rows. Steamwas supplied by three truck-mounted boilers rated at a total of65 boiler horsepower. Periodic steam injections to a depth of12.2 m during a 3½-month period were used to thaw an area of2,950 m2.

Water Injection. Thawing by water injection into the soilthrough a pipe and point with water ports should be limited torelatively free-draining granular soils. For lower-permeabilitysoils, the upward hydraulic gradients may create a “quicksand”condition. For cold water thawing, solar energy should be uti-

lized to increase water temperatures as much as possible beforecirculation through the system. Hot- and cold-water systemsare similar, with the exception of a heat exchanger or boilerstage required for hot-water thawing. Maximum efficiency willresult when the primary water flow path is along the interfacebetween frozen and thawed soil. Ideal conditions develop whenan open, porous soil structure forms along the thawing front asa cone of thawed soil settles away from the thawing face (Fig.9-12). Some soil fines may be removed by the upward waterflow, giving a small increase in soil permeability.

For summer conditions in the interior of Alaska (average airtemperature about 11 °C) and pipes located on 4.9- to 9.8-mcenters, water quantities required to thaw placer gravels rangedfrom 4 to 8 m3 per cubic meter of gravel thawed (Crawford andBoswell 1948). Time required for thawing was about 5 days permeter of depth for the 4.9-m spacing to 8 days per meter ofdepth for the 9.8-m hole spacing. Uncertainties as to water flowpaths and the conductive and convective heat transfers involvedin the problem make calculating thaw rates difficult for the var-ious hole spacings, depths, soil types, and water temperatures.

Electric Heating. Thawing of soil by electric heating involvesplacement of electrodes into boreholes, imposing an alternat-ing current between these electrodes, and using the soil’s resis-tance to generate a heating effect. The highest current flow den-sity will be along the shortest path between electrodes. Heatwill be generated along these current flow paths in proportionto the square of the current times the soil’s electrical resistance,resulting in more uniform heating than with other thawingmethods. The uniform heating and thawing permits consolida-tion to occur more uniformly. This method is considered mostsuitable for silt and clay soils and has the additional advantagethat it can be done in winter.

FIGURE 9-11 Plan view of steam pipes installed on an equi-lateral triangle pattern. Source: Reproduced from Esch 1985. Copyright 1985 American Society of CivilEngineers.

FIGURE 9-12 Sectional view of water flow during the coldwater thaw process.Source: Reproduced from Esch 1985. Copyright 1985 American Society of CivilEngineers.


Soil electrical resistance (resistivity) is expressed in units ofohm-meters. During thawing, this resistivity will decrease sig-nificantly, as is shown in Fig. 9-13. This will cause the electricalcurrent flow and heat input to rise after thawing of the mostdirect current path between electrodes. Lateral heat transfer byconduction now becomes a factor in the rate of thawing. Largevariations in resistivity values for different soil types and soilwith different salinities may cause erratic thaw patterns tooccur in nonuniform soils. These variations may increase thetime required for thawing in nonuniform soils.

Frozen and thawed soil resistivity and the volume of soil tobe thawed determine the size of the electrical power supply sys-tem. Electrical resistance (R) between two electrodes in ahomogeneous soil (Jumikis 1984) is given by

(9.2-1)

where R (W) is the soil resistance, r (W · m) the soil resistivity,Hel (m) the embedded electrode length, S (m) the electrode

RH

S

Re

=r

p2 el

ln

spacing, and Re (m) the electrode radius. The power (heat)input between two electrodes is

(9.2-2)

where P (watts) is the power available for heating the soil, V(volts) the applied voltage, and R (ohms) the soil resistance. Anestimate of the thaw time and energy requirements can be madebased on the energy required to raise the soil temperature to itsthaw point, to thaw the soil, and to raise the soil to the desiredfinal temperature. Some heat will be lost to the surface and tothe underlying soil. For a silty clay with a moisture content of30% and an initial ground temperature of –2 °C, Jumikis (1984)reported that the required thaw energy equals 100 kilowatt-hours (kWh) per cubic meter. This thaw energy indicates thatelectrical costs for thawing frozen ground appear to be reason-able, particularly if electrical transmission lines are accessible.

The voltage used for electrical thawing ranges from 110 to440. For safety, the site should be fenced off and a groundedelectrical system should be used. Esch (1985) described Russianpractice, in which electrodes of similar potential are arrangedin rows on 2- to 3-m centers with a row spacing of 2.5 to 4 m(Fig. 9-14a). Jumikis (1984) gave an alternative layout in termsof an equilateral triangular electrode array, which results in ahexagonal electrode arrangement or a square grid layout withground rods at the center of each square (Fig. 9-14b). Solidrods or waterpipes, used as electrodes, are installed in pre-drilled holes or are driven into the soil with vibrators or airhammers. When drilled holes are used, a calcium-chloridesolution should be poured around the electrodes to providegood electrical contact. The use of drilled holes and perforatedpipe electrodes provides a vertical drainage path for excesswater from thawing and soil consolidation. Drainage can beaccelerated by pumping from the electrode wells.

■ EXAMPLE 9.2-1: A frozen clay (PL = 14%, PI = 11%) masswith dimensions 8.56 by 8.56 m by 1.5 m deep is to be thawedelectrically during a 60-day period. The clay has a density of1,950 kg/m3, a saturation water content of 30%, and an unfro-zen water content of 7.7% for the initial average temperature of–6 °C. Steel electrodes (25.4-mm-diameter) are to be spaced at4.0 m with the hexagonal grid pattern shown in Fig. 9-15. Elec-trical resistivities for the clay include ru = 25 W · m and rf = 40W.m. The ground surface will be covered with thermal insula-tion so that heat loss will be limited to about 10% of the energyrequirements. Determine the actual thaw time, required gridpower, and required voltage. (After Jumikis 1984.)

Solution: Calculate the soil volume to be thawed equal to(8.56 m)2(1.5 m) = 110 m3. Assume that the final thawed soiltemperature will be close to +2 °C. Compute the soil dry den-sity: rdry = r/(1 + w) = 1950/ (1 + 0.30) = 1500 kg/m3. The soilvolumetric heat capacities are given by Eqs. (2.5-18) and(2.5-19) with wu = 7.7% for the frozen clay at –6 °C.

PV

R=

2

cvu = + = ◊∞1 500

1 0000 17 0 30 4 187 2 95

,

,( . . ) . . MJ/m C3

FIGURE 9-13 Resistivities for several soils and one rock typeas function of temperature.Source: Reproduced with permission from Hoekstra and McNeill 1973. Copy-right 1973 National Academies Press.


Compute the electrical resistances of the clay using Eq. (9.2-1)with an effective electrode embedment depth Hel = 1.00 m,electrode radius Re = ½(0.0254 m), and electrode spacing S =4.00 m:

cvf = + + -[ ]= ◊

1 500

1 0000 17 0 077 0 5 0 30 0 077 4 187

2 25

,

,. . . ( . . ) .

. MJ/m3 ∞∞C

Rthm

2 m

m

0.0127 m=

◊◊

=25

1 00

4 0022 9

WW

p .ln

..

Rfrm

2 m

m

0.0127 m=

◊◊

=40

1 00

4 0036 6

WW

p .ln

..

Compute the soil latent heat using (Eq. 2.5-23):

To both thaw and increase the soil temperature will require anenergy input Q = (Q l + Q 2 + Q 3 + Q l). The terms include Q 1 =energy needed to increase the soil temperature from –6 to 0 °C,Q 2 = soil latent heat, Q 3 = energy needed to increase the soiltemperature from 0 to 2 °C, and the estimated heat loss Q l ª0.1(Q l + Q 2 + Q 3). Compute

Recall that 1 J = 2.778 × 10–7 kWh. Conversion gives

Q = E = 15,853.74 MJ (2.778 × 10–7 kWh/J) = 4,404 kWh

For the 60-day period, compute the required grid power PT :

(9.2-3)

The electric grid shown in Fig. 9-15 includes six radial and sixcircumferential paths; thus the total power PT for the entiregrid (Jumikis 1984) is

(9.2-4)

Solve for the voltage Ureq required for this grid:

L Q= = -2 1 500 333 7, ( .kg/m kJ/kg)(0.30 0.077)

=111.6 MJ/m

3

3

Q = ◊∞ ∞ ++ ◊∞1 1 110 2 25 111 6

2 95

. ( )[( . .

( .

m MJ/m C)(6 C) MJ/m

MJ/m C)

3 3 3

3 ((2 C)]

=15,853.74 MJ

∞

PE

tT = = =(

(

,.

kWh)

h)

kWh

days(24 h/day)kW

4 404

603 06

PU

R

U

R

U

RT ( ( )kW) V /2= + ÊËÁ

ˆ¯̃

=6 6 4 302 2 2

W

FIGURE 9-14 Installation for ground thawing electrodes withsingle phase alternating current: (a) suggested pattern; (b)electric thawing circuit for a square grid. Source: Reproduced from Esch 1985. Copyright 1985 American Society of CivilEngineers.

FIGURE 9-15 Electrode grid for a single-phase source of elec-tric power. G, grounded central-tap electrode; u, voltage.Source: Reproduced with permission from Jumikis 1984. Copyright 1984National Academies Press.


for the line-to-ground center tap, or 2Ureq ª 124 V line to line.Using Eq. (9.2-4), with U = 62 V, compute the power dissipa-tion in the frozen and thawed soils:

The time required for heating the soil from –6 to +2 °Cincludes tth for warming to 0 °C, tm for thawing, and tu forwarming to +2 °C. Compute

In summary, the total approximate time for electrical thaw-ing of the frozen clay: ttotal = tth + tm + tu = 46.9 days < 60 days.For 62 V, the required grid power is 3.50 kW for the frozen soiland 5.57 kW for the unfrozen soil.

Hydraulic Dredging

Hydraulic dredges are utilized in the construction of embank-ments, causeways, and offshore artificial islands. Two types ofdredges that have been used in the Alaskan Beaufort sea (Schle-gel and Mahmood 1985) are: (1) Cutterhead suction dredgesand (2) trailer suction Hopper dredges. The Cutterhead dredgeloosens the seafloor soils with a rotating cone-shaped head. TheHopper dredge uses a drag or “draghead” pulled behind thedredge to excavate the seafloor soils. Both types utilize a vacuumto mechanically bring disturbed sediments to the surface. Onceon the surface, the sediments are either placed on barges or inpipelines and transported to the construction site, or directlydeposited from the barge. Schlegel and Mahmood reported thatwarm permafrost should not cause problems in excavation forthe Cutterhead dredge. Tightly bonded silts and clays and well-bonded granular materials with high frozen moisture contentsdo cause some difficulty for the “draghead” dredge.

LaVielle, Burke, and Pita (1985) described a project at Wain-wright, Alaska, where the sand and gravel materials involved acomplex structure of interlayered wedges of frozen and unfro-zen materials. When ice-cemented, highly abrasive materials

UR

PTreqfr kW V 6= Ê

ËÁˆ¯̃ = Ê

ËÁˆ¯̃

ÈÎÍ

˘˚̇

= ª30

36 6

303 060 61 1

0 5 0 5. ..

. . 22 V

PU

Rfrfr

2V)kW= = =

2

30

62

30 36 63 50

(

( . ).

W

PU

Rthth

2V)kW= = =

2

30

62

30 235 57

(

( ).

W

t th

3 3kWh/J) m MJ/m C)6 C]

3.5 kW=

¥ ◊∞ ∞

=

-( . ( )[( .

.

2 778 10 110 2 25

117 9

7

hh days= 4 9.

tm

3 3kWh/J) m MJ/m C)

3.50 kW

h

=¥ ◊∞

= =

-( . ( )( .

.

2 778 10 110 111 6

974 4 40

7

..5 days

tu

3 3kWh/J) m MJ/m C)2 C]

5.57 kW

h

=¥ ◊∞ ∞

=

-( . ( )[( .

.

2 778 10 110 2 95

32 4

7

== 1 4. dayswere encountered, the dredge operators were able to selectivelyguide the Cutterhead to undercut the frozen wedges and layersand remove unfrozen sand and gravel. The technique is illus-trated in Fig. 9-16. Reported production rates when mining thecombined thawing and frozen wedges averaged about 1,150 to1,530 m3 per day. Production increased to 2,300 m3 per day forthawed materials.

9.3 Field Placement

Placement of frozen soils in an embankment requires accept-able compaction and maintenance of the frozen state for the lifeof the structure. Thawing is the main concern, for it will causeexcessive settlement and loss of stability due to insufficientcompaction and low densities. Baker and Johnston (1981)stated that frozen low-ice-content soils can be placed in bermsand embankment sideslopes if on thawing these soils do notdetrimentally affect the main structure (Fig. 9-17). The mainportion of the embankment should be placed according to nor-mal compaction specifications. Use of dry, cohesionless,non-frost-susceptible materials is most important for backfill-ing foundation excavations and placement of materials behindwalls and abutments.

Compaction

The preferred method for handling frozen soils, to be used inembankments, is to allow the material to thaw and drain. This

FIGURE 9-16 Dredge undercutting a layer of frozen sands andgravels.Source: Reproduced from LaVielle, Burke, and Pita 1985. Copyright 1985American Society of Civil Engineers.

FIGURE 9-17 Placement of soils with low ice content so as tominimize detrimental effects to the embankment.Source: Reproduced with permission from Baker and Johnston 1981. Copyright1981 John Wiley & Sons.


material is then placed in the thawed state with compaction todesired standards (Tart 1983). The effect of temperature onoptimum moisture content for an unfrozen clay is shown inFig. 9-18. With decreasing temperatures, the optimum watercontent will increase and the maximum dry density willdecrease. Ctori (1989) explained this behavior in terms of anincrease in water viscosity at particle contacts with lower tem-peratures. Thawed materials cannot be used on many projectswhen fills must be constructed in winter with frozen soils. Thefrozen soils cannot be compacted satisfactorily, and those witha high ice content, upon thawing, may contain large voids,exhibit loss of strength, and flow or slump. The placement ofunfrozen soils in embankments and fills during freezingweather is limited or prohibited by most agencies due to thedifficulty of obtaining specified densities.

Sands and gravels may be compacted satisfactorily at lowtemperatures for low moisture contents. The required frozendensity will equal the maximum unfrozen density only whenthe moisture content is less than 1% (Bernell 1965). Gooddrainage of the borrow materials is required to achieve a lowermoisture content. Laboratory studies on the compactionbehavior of several soils at freezing temperatures (Heiner 1972)showed that the energy used to compact frozen soils has a largeeffect on relative compaction, and the effect increases with ahigher water content. This effect is shown in Fig. 9-19 for agranular soil where relative compaction (as a percentage of themaximum modified AASHO value) has been plotted againstwater content for several temperatures.

When air temperatures were below freezing (about –4 °C),Low and Lyell (1967) reported that placement and compactionof unfrozen core material was carried out for the PortageMountain Dam, provided the delivery temperature of thematerial was between 2 and 5 °C. The temperature of theunfrozen soil decreases slowly enough so that the material canbe spread and compacted in relatively small areas before freez-ing. Additives, such as calcium chloride, mixed with the soilprior to placement will lower the freezing temperature and

maintain a relatively plastic condition for a longer time period(Alkire, Haas, and Kaderabek 1975). If freezing of the soil afterplacement is a problem (dams and dikes), straw can be spreadover the exposed surface or tarpaulins can be laid down and hotair forced underneath to retard or prevent freezing.

A laboratory study on the compaction of frozen lumps ofsilty sand at –7 °C gives a bilinear relationship (Fig. 9-20)between dry density and water content (Alkire, Haas, and Kad-erabek 1975). The intercept (at a water content of 3%) corre-sponds to a coating of ice on all soil particles. The addition ofcalcium chloride to the soil before freezing gave an increase inthe compacted dry density, as is shown in Fig. 9-20. A lowershear strength of the frozen lumps corresponds to a lowersoil-water freezing temperature related to the addition of cal-cium chloride.

Placement in Water

When air temperatures are below –10 °C, cohesive soil place-ment in fills and embankments is normally not permitted.Placing frozen soils in temporary ponds has enabled construc-tion to proceed on earth dams and dikes during winter periods.

FIGURE 9-18 Temperature effect on dry density for clay atoptimum moisture contents.Source: Reproduced with permission from Ctori 1989. Copyright 1989 GroundEngineering.

FIGURE 9-19 Compaction curves for a granular materialplaced at freezing temperatures .Source: Reproduced with permission from Heiner 1972. Copyright 1972 SwedishResearch Council for Environment, Agricultural Sciences, and Spatial Planning.


Various petroleum companies have constructed more than 15temporary earthfill islands for use as exploratory drilling plat-forms in water with depths of 1.5 to 12 m. These operations arediscussed in the following sections.

Fills and Embankments. The placement of frozen soil in watercan be useful in the construction of cofferdams, temporarydikes, causeways for light traffic, and other earth structureswhere some settlement and distortion can be tolerated or cor-rected. Soviet engineers have successfully placed frozen cohesivesoils, such as loess and clay, in water during summer and winterconstruction of several earth dams (Batenchuk et al. 1968). Atthe Irkutskaya generating station, with air temperatures as lowas –25 °C, the water was heated electrically to 15 °C duringplacement of frozen soil in water. Salt and an air bubbler systemwere used to prevent formation of an ice cover on the water.

A number of successive operations are involved in the winterplacement of frozen soil in artificial ponds (Evdokimov 1970).Containment dikes must be constructed in the dry season priorto winter. Environmental restrictions mustbe satisfied when the soil placementinvolves lakes or rivers. The settling pondmust have sufficient area for the project,which means that the soil placed in perime-ter dikes can be a large portion of the totalrequired fill. Depending on the pond depthand surface area, heat may be required tokeep the water from freezing. If an ice coverforms, holes or trenches are cut in the iceand kept open during soil placement. Bakerand Johnston (1981) stated that soils withan ice content greatly in excess of the opti-mum moisture content should not be used.A small-scale test is appropriate at the start

of the operation to determine the characteristics of the thawedsubmerged soil.

Artificial Islands. Temporary earthfill islands have been con-structed for use as exploratory drilling platforms (working sur-face 100 m in diameter) in water with depths up to 20 m (Wangand Peters 1985). Cross sections of several island types con-structed in both the Canadian and Alaskan Beaufort seas areillustrated in Fig. 9-21. Design and construction considerationsinclude the availability of suitable borrow material, methodsfor transport and placement of the fill, sea bottom soil and per-mafrost conditions, and stability of the island relative to settle-ment or heave and to ice and wave action. Design and con-struction procedures have been described in several papers(Garratt and Kry 1978; A. D. Brown and Barrie 1975; Hayleyand Sangster 1974; Wang and Peters 1985). These islands nor-mally have a design service life of less than 1 year.

Summer construction involves the excavation of suitableunfrozen sea bottom materials by dredge and transport bybarge or floating pipeline to the site. For this method, the bor-row material should be an unfrozen coarse sand or fine gravel.At least 50% of the particles should be retained on a No. 100sieve to ensure that sedimentation occurs at a reasonable rate inthe barges. Long, flat underwater slopes (1:15 to 1:25) associ-ated with sand and fine gravels require that coarse materials beavailable immediately adjacent to the site; or, as in recent prac-tice, a sandbag dike may be constructed around the perimeterof the proposed fill to retain the material (Fig. 9-21). To protectthe fill against erosion at and above the water level, an armoredslope may be formed by placing sandbags on a synthetic filterblanket (1:3 to 1:5 slope) tied to the island with wire mesh. Thisarmored slope acts like a breakwater.

In areas where the water is too shallow for barges, winterconstruction is necessary. From the area to be filled, sea ice isremoved in stages, either by ripping when frozen to the bottomor by cutting and removing blocks with a mechanical ditchingmachine and loader. Clean, well-graded gravel can be placed byend dumping into the water until the fill is at least 0.3 m aboveice level. The desired height of fill above high-water level is thenachieved by placement and compaction of the gravel in lifts.Hydraulic fill placement is normally not feasible for armoredslope islands because of the flat slope that results.

FIGURE 9-20 Low temperature compaction of a silty sand.Source: Reproduced with permission from Alkire, Haas, and Kaderabek 1975.Copyright 1975 Canadian Geotechnical Journal.

FIGURE 9-21 Several types of artificial islands constructed in the Beaufort sea.Source: Reproduced from Wang and Peters 1985. Copyright 1985 American Society of Civil Engineers.


An island retained by caissons has three primary purposes:(1) to reduce fill quantity, thus reducing on-site constructiontime; (2) to provide adequate shore protection against severewave action; and (3) to reduce island cost by the reuse of cais-sons at other sites. If the island is located in deep water wherewave conditions are severe, caissons may be the only alternativefor slope protection.

9.4 Water-Retaining Embankments on Permafrost

Embankment dams constructed and maintained on permafrostare designed using a combination of soil mechanics principlesfor unfrozen soils and available information on permafrostbehavior. Sayles (1984, 1987) has reported that at least five siz-able hydroelectric water supply embankment dams on perma-frost have performed well in the former USSR. There has been amix of successes and failures with smaller dams. Problemsinvolved seepage, settlement, stability, slope protection, andconstruction methods. Adequate facilities for overflow and dis-charge must be provided to avoid overtopping of the embank-ment and subsequent erosion. The thermal regime for theembankment and foundation requires special attention, in thatthawing can precipitate uncontrolled seepage along with unsta-ble slope conditions. Design must also allow for wave and iceaction on upstream slopes and erosion on downstream slopes.

Water-retaining embankments on permafrost involve twotypes, frozen and unfrozen. Frozen embankments and theirfoundations are maintained frozen during the life of the struc-ture (Fig. 9-22a). Thawed embankments are designed with theassumption that the permafrost foundation will thaw duringconstruction and/or operation of the structure. They are lim-ited to sites with sound bedrock or thaw-stable foundationmaterials. In some cases where water is retained for short peri-ods of time, thawed embankment design has been used withthe assumption that the underlying permafrost will remain fro-zen. The embankment design for a particular site must includethe usual factors appropriate for soils in a temperate climatealong with factors common to cold regions. These factors weresummarized by Sayles (1984, 1987):

1. Service type: retain water continuously or intermittently.2. Water retained by embankment: width, depth, temperature,

and chemical composition.3. Site climatic conditions: regional and local, especially tem-

perature.4. Existing permafrost: temperatures, extent in area and depth.5. Material sources: availability of local materials and logistics

involving fabricated construction materials.6. Safety: consequences to life and property in the event of

embankment failure.7. Environment: effects of construction and operation of the

water-retaining structure.8. Solar radiation: orientation of the downstream embankment face.9. Frost action: problems on the embankment crest and down

stream slope.10.Cost: economics relative to a particular design.

Unfrozen Embankments

Unfrozen embankments are more suitable for sites where thefoundation materials are thaw-stable (i.e., where the thawedsoil strengths provide an adequate factor of safety against slopefailure and thaw settlements will not endanger the embank-ment integrity). These foundation materials include ice-poorpermafrost or reasonably sound bedrock. When a portion ofthe foundation contains ice-rich materials at shallow depths,this ice-rich soil will usually be thawed, excavated, and replacedwith suitable material to a predetermined depth. MacPherson,Watson, and Koropatnick (1970) selected this excavation depthsuch that thaw settlement would be limited to a specifiedamount during the embankment service life.

When the permafrost is not removed and the foundation isexpected to thaw during the service life, embankment designwill be similar to procedures followed in a temperate climate.With larger thaw settlements anticipated and potential crackingof the impermeable core, self-healing soils (Gupta, Marshall,and Badke 1973) are specified for this impervious zone. Soilsthat become stiff and brittle when compacted are to be avoided.Anticipated thaw settlements can be accommodated by over-building the embankment height and/or periodically adding tothose embankments that settle below a tolerable limit(Johnston 1969). Temperature change over several years andthe corresponding thaw settlements for a dike at the KelseyGenerating Station (Johnston 1969) are illustrated in Fig. 9-23.

FIGURE 9-22 Typical cross section of a water retainingembankment of permafrost: (a) impervious dam with frozensoil for stability and water tightness; (b) pervious dam. Notes: 1, zone of permanently thawed soil; 2, zone of permanently frozen soil; 3,zone of alternating thawing and freezing; 4, zone in which it is necessary tofreeze naturally unfrozen ground; and 5, zone in which it is desirable to thawpreviously frozen ground.

Source: Reproduced with permission from Bogoslovskiy et al. 1963. Copyright1963 National Academies Press.


The concept of using sand drains in a thawing foundation toincrease consolidation rates and more quickly improveembankment stability has been demonstrated (Johnston 1965,1969; MacPherson, Watson, and Koropatnick 1970). Methodsfor estimating rates of thaw and settlement of soils on perma-frost are outlined in Section 3.2 and Chapter 4. Heat fromwater seepage was accounted for by Brown and Johnston(1970) in estimating thaw and settlement rates of dikes on per-mafrost. At sites where foundation thawing was anticipated andseepage was expected to be a problem, foundation areas haveusually been thawed and grouted before embankment con-struction (Gluskin et al. 1974).

For thawed embankments, filters and drainage systems areessential to control seepage through and beneath these earthstructures. The design of these systems is similar to proceduresfollowed for earth dams in nonpermafrost areas (Cedergren1989). Special provisions are required to avoid plugging thedrainage system with ice during periods when seepage controlmay be needed most.

Placement of a thawed embankment on a permafrost foun-dation is normally limited to regions of cold permafrost wherewater will be retained for a relatively short period of time eachyear. Artificial cooling of the foundation may be required dur-ing construction and operation (Rice and Simoni 1963; Kitzeand Simoni 1972). As a further provision for keeping the foun-dation frozen, it is essential that a positive seepage cutoff beprovided (Borisov and Shamshura 1959; Trupak 1970). Such acutoff may take the form of sheet piling, a plastic membrane(Belikov, Panasenko, and Antsiferov 1968), or other waterproofmaterials that are sealed to the frozen foundation and extendup to the embankment crest. The most economical and effec-tive seepage cutoff often is a zone of frozen soil. This zone can

be created from the surface of the embankment slopes duringthe winter season by natural freezing when water is notretained. Effective temperature and water seepage monitoringsystems are necessary in operating this type of water-retainingembankment to detect possible thawing that would initiateseepage through the embankment or in the foundation beneaththe structure.

Frozen Embankments

Frozen embankments are more suitable for sites where perma-frost is continuous and the foundation would become unstableif thawed. Frozen embankments operating with permanent res-ervoirs normally are located in regions having a mean annualtemperature of –8 °C or colder (Sayles 1984). At these tempera-tures, embankments 10 m or more in height usually requiresupplemental artificial freezing for part of each year to ensurethat the foundation and embankment remain frozen. Withtime, a deep reservoir will develop a talik similar to thatobserved beneath wide lakes. As the talik becomes larger, thereis a risk of lateral thawing beneath the embankment leading toseepage and/or stability problems.

Johnston and MacPherson (1981) noted that almost anytype of earthen material can be used to construct frozenembankments, provided that the soil pores are filled with ice.Tsytovich (1975) suggested that ice could be used for theimpervious core. Although these suggestions have merit, theywill also cause problems if a portion of the embankment foun-dation thaws from heat of the reservoir. Thaw-stable materialsshould be used to maintain stability in the upstream slope. Pro-vision must be made to accommodate any differential settle-ment between the core and the upstream shell.

FIGURE 9-23 Observations at Kelsey Generating Station, Manitoba, Canada: (a) ground temperatures; (b) settlements.Source: Reproduced with permission from Johnston 1969. Copyright 1969 Canadian Geotechnical Journal.


Water outlet control structures require special considerationwith respect to their location and preservation of the frozenembankment. During water flow through these structures, heatis released from the water, which may result in thaw and settle-ment of the adjacent frozen soils. Extensive thawing can resultin breaching of the seepage barrier. Sayles (1984) reported thatmost problems are related to thawing followed by seepagearound and beneath outlet structures. To avoid these problems,spillways and outlet structures are normally located at a remotesite, preferably in competent bedrock away from any ice-richpermafrost. Refrigeration is used to preserve the soil in a frozenstate. A chute spillway elevated above the embankment surface(Bogoslovskiy et al. 1963) is another solution. For small dams,siphons and pumps have been used to control reservoir levels(Gluskin and Ziskovich 1973). Sayles (1984) gave emphasis tothe importance of a positive seepage cutoff so as to prevent for-mation and enlargement of a talik beneath the outlet structureand the adjacent embankment.

Maintaining the Frozen State

Both natural and artificial techniques are used to maintainembankments in the frozen state. Natural winter cooling is lim-ited to a depth of about 10 m for a snow-free surface and a verycold climate (Tsytovich 1975). Several techniques used to facili-tate natural cooling include (1) snow removal from the down-stream slope, (2) placement of a shelter to provide shade and tokeep rain and snow off the surface, (3) using berms to protectthe downstream slope from tailwater warming effects, and (4)keeping the foundation frozen during construction. Zarling,Braley, and Esch (1988) reported that the use of snow shedsalong a roadway embankment in Alaska lowered the meanannual soil surface temperature from 3.9 °C for exposedground to –2.3 °C beneath the sheds. Snow removal from theembankment slopes during the winter months was less effectivein decreasing ground surface temperatures.

Frozen earth dams with heights of up to about 25 m havebeen constructed using artificial refrigeration (Sayles 1984).Refrigeration involved circulation of natural chilled air or arti-ficially chilled liquid brine in vertical pipes installed along theaxis of the embankment, and in two-phase thermosiphons(Appendix C, Section C.5). Air is allowed to flow through thesystem only when its temperature is lower than a specifiedvalue, usually about –15 °C. Trupak (1970) indicated that cool-ing by air can be used effectively at locations where the meanannual air temperature does not exceed –5 °C. In regions ofhigh humidity, rust and ice can form inside the freeze pipe,resulting in reduced heat removal efficiency and potentialblockage of the pipes (Gluskin and Ziskovich 1973; Biyanovand Makarov 1978). Refrigeration cooling systems are of thetype described in Chapter 6.

Thermal and Stability Considerations

The importance of the thermal regime relative to embankmentand foundation behavior has been emphasized in the precedingsections. Reasonable estimates of the initial temperature distri-

bution for undisturbed sites can be made when accurate mete-orological data are available (Sayles 1984). Heat balance equa-tions are used to account for solar radiation, air temperatures,wind velocity, evapotranspiration, geothermal gradient, andflowing groundwater and surface water. Heat transfer coeffi-cients at the surface depend on the type of material: gravel,stone, snow, vegetation, or other. The soil temperature distri-bution can be determined by in situ measurements. For a non-steady state, within a homogeneous, pervious, and imperviousembankment, Bogoslovskiy et al. (1963) recommended thefinite-difference method for temperature prediction.

The long-term-equilibrium (steady-state) temperature con-dition will ultimately be reached after completion of theembankment and many years of operation. Transient condi-tions will exist at various times during the life of the structure.These thermal problems are complex, usually requiring two-and three-dimensional analyses. They are complicated by phasechanges (water/ice) at the moving boundary between thawedand frozen zones. Heat transfer caused by seepage water (con-vection) and by conduction will occur in the pervious materi-als. The unknown distribution of ice and unfrozen zones in theembankment cannot be known precisely. The accuracy of tem-perature predictions is significantly affected by the variousparameters used in the calculations. These parameters includetotal moisture content (ice and unfrozen water); thermal prop-erties of the soils; and temperatures of the soil, water, and air.

Temperatures in an embankment and foundation about 75years after the storage reservoir was filled are illustrated in Fig.9-24. The embankment consists of three zones: a central imper-vious ice core, an upstream rockfill with ice-filled cavities, anda downstream rockfill. The data in Fig. 9-24 show that thawingof the lower part of the ice core should be expected during theinitial period of operation. Considering the core thawing areaand the frozen part of the rockfill, the refrigeration plant capac-ity required to maintain a frozen core could then be deter-mined. These data would greatly facilitate design of the coolingsystem. Calculation methods can be checked using computerand physical models of the embankment and foundation tem-perature field. The discrepancy between a nonsteady tempera-ture field (75 years) and a steady-state field was no more than1.5 to 2.0%.

The function of each zone within the embankment must beconsidered when evaluating stability. Embankments designedto remain frozen during their service life consist of an upstreamthawed zone and a downstream frozen zone. The upstreamzone provides thermal insulation for protection of the down-stream frozen zone from heat of the reservoir water and as pro-tection from moving water or ice. The frozen downstream zoneprovides an impervious water barrier and resistance to hori-zontal forces exerted on the embankment. Thawing within theupstream zone and its foundation may cause failure if the shearstrength of the soil decreases as a result of excess pore pressures.Excess pore pressures develop when the thaw front advancesmore quickly than meltwater can drain. A rapid drawdown ofthe reservoir will correspond to the most critical case.

The stability of unfrozen embankments on permafrost isevaluated using methods appropriate for unfrozen soils in


temperate climates. Special consideration must be given toexcess pore pressures and low shear strengths that may exist atthe thaw interface. Trial failure surfaces will normally includethe thaw zone. Similarly, shear strength at the thaw interfacewill be a major consideration in assessing the resistance to hor-izontal forces. Vertical sand drains terminating in an intercept-ing horizontal drainage system have been used to acceleratethe dissipation of excess pore pressures and increase shearstrength at the thaw front (Johnston 1969; Gupta, Marshall,and Badke 1973).

9.5 Embankment Performance

The construction of an embankment over natural terrainchanges the surface thermal characteristics, thereby alteringboth the mean temperature and amplitude at the ground sur-face and at various depths. This change will increase the depthof frost penetration and with certain soil conditions will causefrost heaving. In permafrost areas, construction of the embank-ment will lead to an adjustment of the permafrost depth with adeeper (thicker) active layer. For thin embankments, frozenice-rich, fine-grained soils may subside to or below the level ofthe surrounding terrain. The changes in surface heat balance

caused by the construction of an embankment are associatedwith the problems of frost heave, settlement, and thaw stability.

Frost Heave

Embankments and underlying soils heave when three condi-tions are satisfied: (1) a frost-susceptible soil, (2) the presenceof water, and (3) freezing temperatures. The heave is caused byformation of ice lenses parallel to the freezing plane, with thedirection of heave perpendicular to this plane. These lensesincrease in thickness when water is available and will usuallyresult in differential surface movement due to nonuniform soilconditions. Longitudinal splitting as well as transverse crackingmay be observed in highway embankments. The effect on apavement surface is illustrated in Fig. 1-18. A significant loss ofsubgrade soil strength will occur on thawing of the ice lenses.Repeated wheel loads will cause a breakup of the pavementstructure.

Frost-heave prevention requires that one of the three neces-sary conditions for heaving be controlled. Several options areavailable. The frost-susceptible material can be excavated andreplaced with non-frost-susceptible soils to the depth of frostpenetration. Non-frost-susceptible soils have been defined inSection 2.4. When the depth is sufficient to contain the zero-

FIGURE 9-24 Temperature variation in a water-retaining embankment and foundation 75 years after filling the reservoir.Source: Reproduced with permission from Bogoslovskiy et al. 1963. Copyright 1963 National Academies Press.


degree isotherm or to provide enough overburden pressure toprevent or reduce frost heave to tolerable limits, the excavationdepth can be reduced. Nixon’s (1987) empirical model for pre-dicting heave using a segregation potential parameter (Section2.4) takes into account the effect of overburden pressure.Rooney and Johnson (1988) reported that observations by theAlaska Department of Transportation and Public Facilities haveshown that 1.8 m of embankment over frost-susceptible non-plastic silts reduced differential heave to tolerable levels forhigh-speed roads in areas where the maximum frost depth wasabout 3.6 m.

Another option is to use insulation (see Sections 3.3 and11.3) to prevent the frost penetration from reaching thefrost-susceptible material. Insulation materials may includepeat, bark, wood chips, foamglass, sulfur foam, and polyure-thane or polystyrene foam products. Closed-cell extruded poly-styrene board has been shown to hold its R value on manyprojects and is commercially available. Taking into account theallowable dead and live loads on the insulation, the thermallyoptimum location of the insulation is as near the surface aspossible. Methods for estimating the required thickness ofinsulation and placement details are discussed in Section 11.3.

Limiting the supply of water available to the freezing planewithin frost-susceptible soil can reduce the amount of heave totolerable limits. Lowering of the groundwater table may notprevent frost heave because of the high suction pressures gener-ated in fine-grained soils. Where a high water table exists,drainage is usually designed in conjunction with one of theforegoing techniques. Placement of a capillary cutoff, either ageotextile placed at the interface of the natural ground and theembankment (Bell, Allen, and Vinson 1983) or a granular layeras illustrated in Fig. 1-16, will reduce heave within the embank-ment. The procedures outlined above are normally addressedin the design of most highway embankments.

Settlement

Thaw related settlement at the toe of highwayembankment slopes is responsible for longitu-dinal cracking of the roadway surface (Fig.9-25). Thawing of frozen ground results fromchanges in the ground thermal regime causedby the embankment construction. Themechanics of thaw settlement have beendescribed in Chapter 4. Preliminary designwill normally identify anticipated levels ofthaw strain, time-dependent thaw penetrationdepths, and acceptable limits for total and dif-ferential settlement. The design criteria mustattempt to establish a balance between accept-able embankment deformation and continuedmaintenance required to relevel the embank-ment surface.

Most of the embankment settlement onpermafrost foundations results from thawingof ice inclusions and the associated drainage ofwater. Recognizing that this initial thaw settle-

ment is the predominant characteristic, Johnston andMacPherson (1981) prepared Fig. 9-26, which predicts the per-cent thaw settlement based on initial soil water content and theanticipated final void ratio. For natural soil deposits, higher icecontents are usually associated with soils that are also com-pressible in the unfrozen state. Variation of both soil types andice content within the embankment foundation is oftenobserved. When thaw progresses laterally at unequal ratesunder the embankment, significant differential settlements canbe anticipated, as is illustrated in Fig. 9-25.

Tart (1983) described the performance of fills constructed offrozen granular soils during the winter period. The mostimportant concern with these fills is the settlement that occursduring seasonal thaw. For settlement prediction, Tart firstdetermined the minimum dry density of thawed material froma potential material source. Assuming that the thawed granularsoils will attain at least this density and that the in situ densitycan be measured or estimated, Tart estimated the minimumpotential thaw strain (DH/H) as

FIGURE 9-25 Embankment instability as a result of thaw con-solidation at the toe.Source: Reproduced with permission from Esch 1983. Copyright 1983 NationalAcademies Press.

FIGURE 9-26 Thaw settlement based on initial water contents and final voidratios.Source: Reproduced with permission from Johnston and MacPherson 1981. Copyright 1981 JohnWiley & Sons.


(9.5-1)

where DH is the change in fill height, H the final fill height, andgl and g2 the initial and final dry densities of the fill material.Following Tart’s procedure and using a void ratio ef of 0.5 forthe frozen fill and a borrow water content of 10%, Fig. 9-27gives an initial dry density close to 1,430 kg/m3. If one assumesthat laboratory tests on the thawed fill material give a mini-mum dry density of 1,920 kg/m3, the anticipated minimumthaw strain equals [(1,920 – 1,430)/1,920]100 = 26%. Thisstrain will be reduced by compaction of the frozen fill materialduring construction. For example, a reduction of ef to 0.3 willreduce the thaw strain to about 13%.

Stability

When an embankment is placed over permafrost, the surfacethermal balance is always changed. With proper design andconstruction, the permafrost table beneath the central portionof the embankment will remain at its original level. Placementof insulation or an adequate embankment depth will cause thetop of the permafrost to rise into the non-frost-susceptibleembankment material (Fig. 9-25). The embankment sideslopesusually do not have enough thermal resistance to protect thepermafrost from increased heat input caused by their bare sur-face. During the winter, additional snow cover on the side-slopes produces a thicker insulating cover, preventing cold win-ter air from cooling and maintaining the permafrost below thesideslopes. The result involves thawing of frozen soil below thesideslopes, as is shown in Fig. 9-25.

For ice-rich permafrost, the sideslopes will slowly subsideinto the thaw channel that forms below the toe of the slope.This lack of foundation support causes a downward and out-

DH

H=

-g gg

2 1

2

ward movement of a portion of the embankment. Longitudinalcracks (Fig. 9-25) will develop at the embankment surface,causing damage to a pavement or any structures located nearthe cracks. To overcome this problem, a combination of ther-mosiphons (Appendix C, Section C.5) and/or insulation layerswere installed in 1990 for long-term studies by the AlaskaDepartment of Transportation. Another approach to loweringsideslope temperatures involves the use of a platform-likestructure constructed on the embankment sideslope that allowsair movement beneath it. This structure keeps both snow andsun off the slope. Zarling and Braley (1986) reported that thesesnow sheds lowered the average sideslope temperatures by 3 to6 °C and appeared to stabilize the sideslopes better than eitherthermosiphons or cooling ducts.

Artificial Islands

Performance prediction of an artificial island requires informa-tion on water depths, consistency and density of unfrozen seabottom sediments, the presence of permafrost within criticaldepths, and its potential for thaw settlement and stability prob-lems when the island is constructed. The presence of salinefoundation soils with different properties and behavior shouldbe considered. Extra fill can be incorporated into the design toaccount for settlements, but the height of fill (surcharge load)will be limited by allowable seabed bearing pressures. Conven-tional slope stability methods are used to determine acceptableslope angles when unfrozen fill and seabed materials are consid-ered. The extent of frozen and thawed zones in the interior andperimeter zones of the island are assessed by one- and two-dimensional thermal calculations. The ice cover, which formsduring winter, will attain thicknesses of 2 m or more. It eventu-ally becomes landfast in shallow water near shore and is subjectto only limited motion. Croasdale and Marcellus (1978) havediscussed the ice forces that might act on these artificial islands.

The potential failure modes for artificial gravel and caisson-retained islands are illustrated in Fig. 9-28. Slope failure is gov-erned by the slope angle, fill and foundation soil properties,and the height of the island. When seabed sediments consist ofweak, fine-grained materials, shear strength will usuallyincrease with depth. In some areas, shear strength will decreasewith depth, with the minimum strength occurring near thepermafrost table. Wang and Peters (1985) reported that the fac-tor of safety calculated using a two-dimensional slope stabilityanalysis was about 30% lower as compared with a 3-dimen-sional analysis considering the circular island shape. Slope sta-bility was most critical at the end of construction and improvedwith time due to consolidation.

Edge failure involves passive resistance of the island slope tothe ice load (Fig. 9-28). Stability increases with frozen fill thick-ness and the strength of the frozen fill. According to Wang andPeters (1985), the geometry of a gravel island is usually notdesigned to prevent an edge failure; therefore, the designershould ascertain that an edge failure will not cause damage tothe top side facilities. This type of failure has occurred on exist-ing gravel islands, pushing sandbags upslope (Wang and Peters1985).

FIGURE 9-27 Chart for estimating as-placed density of frozengranular soils.Source: Reproduced from Tart 1983. Copyright 1983 American Society of CivilEngineers.


Truncation failure (Fig. 9-28) involves displacement of theisland mass above the horizontal plane across the island at thelevel of the ice sheet bottom. Stability increases with island free-board height and the angle of internal friction of the island fill.It also increases with island diameter for both thearmored-slope and caisson-retained island designs. Bottom

sliding failure is complicated by the island geometry andinvolves effects of ice loading on a portion of the island mass.Wang and Peters (1985) considered a statically admissible stressfield which gives a lower bound estimate of the island stability.For islands constructed during the summer, consolidation offine-grained foundation soils will increase strength consider-ably by the time winter ice loads are applied.

The bearing capacity of caisson-retained islands should bechecked for caisson loads. On sandy seabed soils, bearingcapacity will usually not be a problem (Wang and Peters 1985).Island fill excavated and placed in the winter may be in anexceptionally loose state, especially the underwater portion.Strains of up to 10% have been observed in the underwater filldue to thaw settlement. Post construction settlement can beallowed for by adequate freeboard.

An additional concern in gravel island design involves waveovertopping caused by wave run-up. The island freeboarddetermination is directly affected. Wang and Peters (1985) out-lined the factors involved. Gadd, Machemehl, and Manikian(1985) have reported on a comparison of wave overtoppingpredictions to measurements from large-scale model tests. Thestudy showed that freeboard can be reduced when informationfrom model studies is used for final design. The topic is omittedhere due to lack of space and the complexity of wave-structureinteraction.

FIGURE 9-28 Potential failure modes for the artificial islands.Notes: 1, slope failure; 2, edge failure; 3, truncation failure; 4, bottom slidingfailure; and 5, bearing capacity failure.

Source: Reproduced from Wang and Peters 1985. Copyright 1985 AmericanSociety of Civil Engineers.

Chapter 9 - Earthwork in Cold Regions

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