Several Uses of Airphoto Interpretationto the Soils Engineer*

WILLIAM W. HOLMAN, Soils Engineer,Greer Engineering Associates, Inc., Montclair, N.J.

ABSTRACT: The soils engineer can make use of airphoto interpretationin many of the various types of problems with which he becomes con­cerned. Three of these uses or applications are: highway location anddesign; reconnaissance and soil evaluation of a proposed industrialarea; and in the foundations investigation for a group of heavy structures.Each of these uses is discussed, as aPPlied to specific engineering projects.

AIRPH~TO interpretation IS rapidly be­

comll1g one of the more useful toolsof the soils engineer. Formal courses insoils engineering now include at least abrief description of the methods and pro­cedures of the photoanalyst. The more re­cent texts on soils usually point out, theadvantages to be gained by using airphotointerpretation. In addition, there havebeen numerous recent bulletins and papersdescribing specific engineering projects andhow the results of airphoto interpretationhave been applied in each instance. Aerialphotographs will not displace drilli ngequipment nor will they replace the soilslaboratory. However, they will serve as avaluable supplement in certain soils in­vestigations by uncovering additionalfacts that can be considered in the evalua­tion of a soils problem.

The soils engineer is concerned with allof the problems associated with the ma­terials upon which a foundation is con­structed. In addition, the geologic historyof the area and its numerous boundaryconditions are of importance in his workand usually have a direct influence uponthe recommendations he makes. Problemsof area reconnaissance, site investigation,foundation design, and structural main­tenance are within the scope of soilsengineering. It is understandable, there­fore, why airphoto interpretation is usedin so many different ways.

The following are three of these uses asapplied to specific engineering projects;namely that of highway location and de­sign, reconnaissance and soil evaluation of

WILLIAM W. HOLMAN

a proposed industrial area, and in thefoundations investigation for a group ofheavy structures.

1. HIGHWAY LOCATION AND DESIGN

In 19S2 plans were being formulated forthe location and construction of anothersection of the Garden State Parkway inNew Jersey. This 16S-mile long, four-lanedivided toll road now runs the length of theState, close to the Atlantic shore line. Thethen proposed 34-mile section, in Middle­sex and Monmouth Counties, was to havea SOD-foot right-of-way with an allowablespeed of 60 miles per hour.

Monmouth and Middlesex Counties aremostly in the Inner Coastal Plain of New

* Presented at the 22nd Annual Meeting of the Society, Hotel Shoreham, Washington, D. C.,March 21-23, 1956. This paper is a part of the Panel on Photogeology.

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PAN~L-INTERPRETATION FOR SOILS ENGINEER 843

Jersey. The natural materials present inthis area consist of a series of unconsoli­dated beds of marine sediment each bedconsisting of various proportions of sand,silt, and clay. Some of the more than 15beds or formations are of dense uniformmedium to coarse quartz sand, whereasother formations are of soft to firm micace­ous clay. Overlying this wide assortmentof materials are more recent formations ofintermixed gravel and sand. These morerecent or Quaternary deposits are presentas scattered ou tcrops; however, their val uefor use as base, subbase, and quality fillmakes even the smaller deposits soughtafter.

Because of the narrow, meandering pat­tern of the formation exposures, and alsothe spotty and random occurrence of themore granular materials, the soil conditionscould justly be considered as complex withregard to highway engineering. A detailedsoils investigation is of paramount im­portance in such areas; however, time wasdefinitely not available to make an ex­tensive soil survey by drilling, soil sam­pling, and testing. As a result, the NewJersey Highway Authority requested ofthe Joint Highway Research Project ofRutgers University the preparation of asoils strip of this 34-mile section of park­way, using the airphoto interpretationmethod.

Previous investigation by the Projectstaff in this physiographic province of NewJersey was only of Ii mited extent. A numberof reconnaissance trips had been made inthe area in addition to a series of handauger borings for soil classification pur­poses. Soil testing consisted of grain sizeanalyses, consistency limits, and optimummoisture-standard density relationships.No airphoto interpretation of the area hadbeen performed.

In approximately three weeks an Engi­neering Soils Strip Map to a scale of oneinch equals one-half mile was prepared,together with a 50 page report describingthe various materials present along theproposed right-of-way. The strip map,which delineated the various soil types andtheir drainage characteristics, was of anarea three miles wide, and borrow materi­als u'ere shown of a strip six miles wide.This soils strip map was compared toexisting conditions in the field following therough grading operations. The accuracy ofthe map was most encouraging.

Some of the more important items con­sidered in preparing this soils map werespeed, accuracy, and clarity. A team of fiveengineers and geologists worked togetherto prepare the necessary data. Referenceliterature was investigated; the logs ofprevious borings together with test resultswere correlated with the geologic forma­tions in the area; a rapid examination wasmade of the airphotos to note major soilpatterns and to determine those areaswhich would require field investigation;several reconnaissance trips were made toclarify confusing airphoto patterns; andfinally the soil types were assigned mapunits and the final airphoto interpretationwas performed. In several areas the ou t­crop pattern of the soil types was so inter­mingled that only general mapping infor­mation could be presented in the timeallotted. Clarity was assured by usingmapping designations which were not com­plicated and by omitting highly technicalterms and long discussions in the writtenreport.

The more important uses of this Engi­neering Soils Strip Map were as follows:

A. Area' Familiarization. For initial re­connaissance, such a map provides generalsoil information concerning the entire areasurrounding the right-of-way. The loca­tions and the in-place relationships be­tween the various geologic deposits and soiltypes are determined for the soils engineer.This is important in planning futurephases of soil study, and such informationwould normally not be available until thecompletion of an extensive field surveybased on borings, soil sampling, and labo­ratory soil testing.

B. Alignment and Profile Selection. Suf­ficient soils information is presented for theselection of both preliminary alignmentand a general profile. vVhere several align­ments are under consideration, the soil andfoundation problems of each can be evalu­ated and a final selection can be made usingthe soil strip map as a guide.

C. Establishing Additional Field Work.In areas where more involved foundationproblems occur, the soil map and reporthelp in organizing a more worthwhileboring program. Borings are not necessaryat definite intervals along an alignment todetermine the soil types that may be pres­ent. This general information is shown onthe strip map. Subsequent detailed boringsin critical area can be obtained at a

844 PHOTOGRAMMETRIC ENGINEERING

mll1lmUm cost and, of equal importance,such information will be available for usein the early stages of highway design.

D. Locating Borrow Material. The loca­tions of the various types of borrow ma­terial for use in highway construction areshown on the soil map. A field party doesnot have to "scout around" for such ma­terials, but can quickly check on deposi tsas indicated on the map.

E. Pavement Design. By correlating pastperformance of several types and designsof highway pavements with various soilareas, it is possible to arrive at a pavementtype and design that best fits the soil con­ditions in a certain area. For this sectionof the Garden State Parkway, a definiteflexible pavement thickness (surface, base,and subbase) was established for themajor soil areas according to the soil stripmap. In addition, variable thicknesses ofselected borrow material were establishedfor the critical soil areas. Modifications inthis design were made as the subsequentfield work progressed.

For an economical, rapid, and accuratereconnaissance soil survey in highwayengineering, an engineering soil 'strip mapprepared by airphoto interpretation isideal.

2. RECONNAISSANCE AND SOIL EVALUATION

FOR A PROPOSED INDUSTRIAL AREA

In Cuba, this coming year, there will beconsiderable construction of oil refineryfacilities. Oil products are in demand inCuba, and also there is much activity in'the fields of prospecting and drilling fornatural petroleum deposits. Soil andfoundation studies were made for at leasttwo proposed refinery sites in 1955 andaerial photographs were instrumental inboth investigations.

The geologic deposits at the proposed re­finery sites are mostly of sedimentaryorigin. These materials consist of calcare­ous sandstones, shales, and conglomerates;limestones, chalks, and tripolite. As a re­sult of subsequent folding, uplift, anderosion, the dips of the various interbeddedstrata are nearly vertical. To add to thisrelatively complex bedrock condition,masses of serpen ti ne were intruded upthrough the sedimentary country rockwhich further obscured the continuity ofthe structure. In addition to these con­solidated formations, there are soft alluvial

deposits in former gully systems whichwere developed in the area, and organictidal marsh deposits.

The soil developed in the area is a later­itic material and is much different fromthe majority of the soils present in theU.S.A. The process of laterization is typi­cal of tropical regions where the heat andthe moisture appear to carry the ordinaryprocess of rock disintegration and decaysomewhat farther than in more temperateclimates.

The present soil is mostly a silty claywhich is usually dried out or desiccated.Surface cracks to three inches wide arepresent in many places and vertical fis­sures, seams, and fracture planes extenddeep into the profile. The depth of the soilis extremely variable as a result of thesteep bedrock dip and of the differentialrock weathering. In many places thesteeply dipping beds of calcareous bedrockare interbedded with clay or silty clay soilswhich were developed from the weakersediments. Much of the bedrock is so softand weathered that it can be broken intonumerous small pieces by finger-tip pres­sure.

The primary objectives, when initiatinga foundations study in a comparativelynew area, are to become familiar with itsgeology, the normal process of soil de­velopment in that particular climatic zone,and the over-all conditions at the site. Allof these items could be termed areafamiliarization. Prior to the performanceof actual field work, such familiarizationwas facilitated greatly by the interpreta­tion of aerial photographs. For the sites inquestion a materials map was preparedwhich delineated: the soils of greaterstrength; areas in which fhe more resistantlimy bedrock was close to the ground sur­face; former gully systems since filled withsoft soils; man-made fill areas; organictidal marsh deposits; and outcrops of theresistant serpentine bedrock.

To a soils and foundations engineer, eachof these items is of utmost importance. Thesoils of greater strength can readily with­stand the loads imposed by oil storagetanks, process units, and other refinerystructures. Foundations are relatively in­expensive and construction can proceedrapidly. In areas where bedrock crops outor is close to the ground surface, particu­larly where the hard more resistant rockis interbedded with weathered strata or

PANEL-INTERPRETATION FOR SOILS ENGINEER 845

soil, foundation settlement will not be uni­form. Such differential settlement canover-stress a flexible structure and the re­sultant strain could throw the structuralframe out of line and possibly even leadto complete failure. This settlement can beavoided, but special foundation designs arenecessary. Soft, poorly drained sedimentand strata of organic materials dictate theuse of either costly construction proceduresor of expensive foundation types. Theseconstruction procedures could be pre­loading the area and thus pre-consolidatingthe soft soils, or completely removing theobjectionable material and replacing itwith suitable well-compacted fill. The ex­pensive foundation types could be deepfootings, piles, and drilled piers.

The materials map prepared by airphotointerpretation was constantly referred toas the actual field work progressed. Insome places corrections were necessary,whereas in other areas additions were war­ranted. There were places where the mapindicated conditions that would have goneunnoticed by only a cursory field inspec­tion. In one particular area, through a pro­posed tank farm, a former inland extensionof marine tidal marsh was obscured byrecently placed fill material. From thegroundsurface, it was impossible to detectthe boundary of the former tidal area withsufficient accuracy. However, by airphotoanalysis, the lateral extent of the organicsoils was closely defined as well as the dis­covery of several areas where sandier soilswere present rimming the tidal marsh.Borings and soil sampling to explore all soiltypes could then be directed and plannedwith a greater degree of efficiency.

Following the field work, the soils engi­neer gathers all of the facts together andanalyses each set of conditions to arrive atthe most suitable foundation recommenda­tion for each structure. The airphotosagain were useful for they· helped theengineer visualize the site better than anyother available reference. Each area wasreviewed, misinterpretations were cor­rected where necessary, and a final ma­terials map was then prepared. Such a mapwas useful in the present foundationsstudy, and its cost will be repaid many

times over for it can be referred to for allsubsequent work in the area.

For area reconnaissance of a proposedextensive industrial site, airphoto analysisis practically a necessity. Not many itemscan remain over looked once the airphotoshave been studied.

3. FOUNDATIONS INVESTIGATION FOR A

GROUP OF HEAVY STRUCTURES

The final use of airphoto analysis to bereferred to in this paper concerns the found­ations investigation for a group of heavilyloaded structures. A large corporationwished to expand its production facilitiesby building a new plant area consisting ofshops, furnaces, warehouses, and otherexpensive units. The site was chosen, pur­chased, and plans were completed for theplant layout. A few foundation boringswere made which indicated dense granularsoil to a depth below the practical zone ofinduced soil stress. From all indications,comparatively inexpensive foundationtypes could be utilized, even consideringthe proposed high unit loads. As a routinesafety measure, airphotos of the area werereferred to and a startling fact was re­vealed.

About 12 feet beneath the level groundsurface, and running diagonally through theproposed plant area, was a former narrowstream. This drainage path had developedsome 5 to 10 feet of soft organic soilswhich were subsequently filled over withmore recen t gran ular deposi ts.

Additional borings were made and theairphoto analysis was veri fied. Part of theproposed industrial plant was movedaway from the former stream, whereasother parts were redesigned so as to over­come the presence of the weak and com­pressible subsurface stratum. The endresult in this case history was success;however, it could very well have beenfailure.

Not all of the uses of airphoto inter­pretation can be brought to light as vividlyas the preceding ones. Nevertheless, eachengineering use is just as important to thesoils engineer: a better job can usually beaccomplished in less time when airphotoscan be put to use.