P 0201- Caisson Foundations for Cellular Telephone Monopoles

8/12/2019 P 0201- Caisson Foundations for Cellular Telephone Monopoles

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Proceedings: Third International Conference on Case Histor ies in Geotechnical Engineering St. Louis MissouriJune 1-4 1993 Paper No. 1.44-

aisson Foundations for ellular Telephone MonopolesDonald A. BenviePrincipal Tectonic Engineering Consultants P.C., Highland MillsNew York

Jeffrey B. KirbyChief Engineer Tectonic Engineering Consultants P.C., HighlandMills New York

SYNOPSIS: Design and construction procedures tha t were implemented for the instal la t ion ofcaisson foundations to support cel lu lar telephone monopoles a t fifteen s i tes throughoutmetropolitan New York are presented herein. Subsurface investigation procedures, developmentof geotechnical design cr i ter ia methods of structural analysis and caisson design, andactual construction ins ta l la t ion of the caissons have been provided along with a comparisonof so i l conditions and their impact on design and construction of the caissons.INTRODU TIONAs the cel lu lar telephone industry expands i t scoverage areas and upgrades services to exist ingusers, the construction of new ce l l s i test ransceiver s tat ions) has prol ifera ted acrossthe country. In the Northeast, and part icular lywithin the metropolitan New York area, the demandfor cel lu lar telephone services has prompted onemajor ce l lu la r telephone provider to undertake athree year construction program to developapproximately 200 new s i t es with a completedvalue in excess of 150 million.Typically, a ce l l s i te i s comprised of a shel terto house communications equipment and some typeof elevated s t ructure for mounting of antennas.In densely populated areas, exis t ing s t ructuressuch as multistory buildings or water tanks arecommonly used for placement of antennas. In l essdeveloped areas where s t ructures of adequateheight do not exis t or the locations of suchstructures do not coincide with the optimumtransmission locat ions, erect ion of a newantenna-supporting s t ructure i s necessary. onetype of structure commonly used by the cel lu larindustry for antenna mounting a t such cel l s i tesi s a s ingle cantilevered pole, known as amonopole.The weight of the monopole s t ructure includingthe antenna mounting platform s nominal,resul t ing in small compressive loads applied a tthe foundation level . However, these structuresare subjected to substantial overturning momentscaused by wind and ice loading that must beaccommodated by the foundation. To support theseloading conditions, accommodate a wide variety ofsoi l types, and provide an expedient andeconomical ins ta l la t ion method, the use of asingle large diameter caisson or dr i l led pier hasproven to be the preferred type of monopolefoundation.

The metropolitan New York area, encompassing NewYork ci ty Long Island, Westchester County, andNorthern New Jersey, as shown in Figure 1,exhibits a broad range of subsurface conditions.These include sands of varying density, sof ta l luvia l deposits of clay and s i l t glacial t i l ldecomposed rock, and man made f i l l s . Although thesuperstructure type, loading conditions a.ndperformance c r i t e r i a are essen t i a l lystandardized, the wide variation of subsurfaceconditions has precluded a standard approachwhereby one type of caisson design and method ofconstruction could be implemented uniformly.Instead, a wide variety of caisson foundationtypes were necessary to accommodate s i te specificfoundation conditions.

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These caissons are constructed with a variety ofmethods tha t varied from dry uncased excavationsto conventional augured casing insta l la t ions tobentonite s lurry dr i l l ing with tremie concreteplacement methods. In addit ion, special designconsiderations and construction procedures werenecessary to minimize the impact of caissonsins ta l led direct ly adjacent to exist ing buildingfoundations.Procedures used to evaluate subsurfaceconditions, develop foundation design cr i ter iaperform s t ructural design of caisson foundations,and the actual construction proceduresimplemented to ins ta l l the caissons a t 15 s i t e sthroughout the metropolitan New York area arepresented herein. Additionally, a comparison ofso i l conditions encountered a t various s i t es andt he i r impact on design and construction of thecaissons wil l also be discussed.


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MANY

CTPA

FIGURE 1 - Metropolitan New York AreaMONOPOLE CONFIGURATION AND LOADING CONDITIONSThe monopoles ut i l ized in th is program range from35 to 150 feet in height, with a majority being100 feet t a l l . Each was f i t ted with a t r iangular-shaped structural steel platform a t the top tofac i l i ta te mounting of the antennas.The pole shaft s uniformly tapered, with adiameter of one to two feet a t the top,increasing to between three and f ive feet a t thebase, depending on the total height. The cross-section i s either 12 or 16 sided, and i sfabricated from s teel . Except for the platform,the structure resembles in many ways the polesused for mounting of l ighting fixtures or powerl ines in various parts of the country. A typicalins ta l la t ion i s shown in Figure 2.The most significant loading condition to whichthe pole i s subjected i s wind load acting on theshaf t platform, antennas and otherappurtenances, and ice which may accumulate onthe structure . The magnitudes of wind and iceloads are based on design cr i ter ia recommended byEIA(1991), which has been established as anAmerican National Standard. The load defini t ioni s consistent with the familiar approachpresented in ASCE 1990), which was formerlypublished as ANSI A58. 1 and incorporated intonumerous building codes.

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ANTENNA

MONOPOLE

__; IMONOPOlE FOUNDATION_. .........-_L 1\ L

FIGURE 2 - Typical Monopole Ins ta l la t ionWithin the metropolitan New York area, theminimum wind speed required for design (based ora 50 year mean recurrence interval) varies freD70 to 90 mph. However , t was decided a t thes ta r t of the program that a single conservativeloading cr i ter ion should be used for monopoledesign. This was to fac i l i ta te use of a stockpole at. any location, thereby reducing lead timeand the overal l construction duration.The pole manufacturer normally performs thenecessary analysis to determine service loadreactions a t the foundation level , includingshear, overturning moment, and compression.Foundation reactions are presented in Table 1 foxa range of monopole heights. I t i s noted t ha t amajor portion of these reactions i s due to thepresence of the platform atop the pole.


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TABLE 1MONOPOLE BASE REACTIONSMONOPOLE AXIAL SHEAR MOMENTHEIGHT ft} k} k} k f t )

75 13 14 760100 20 19 1320125 32 31 2530

SUBSURFACE INVESTIGATIONA geotechnical invest igat ion was performed a teach ce l l s i t e within t he l imi t s of the proposedmonopole s t ruc ture to i den t i fy types anddis t r ibu t ion of subsurface mater ia ls , delineateengineering charac ter i s t i cs of the subsurfacemater ia ls and es tab l i sh caisson foundat ion designc r i t e r i a . Generally, each s i t e i nves t iga t ionconsistedof one boring dr i l l ed to a depth whichvar ied from 25 to 42 fee t . Borings were advancedthrough overburden so i l s using e i ther a hollowstem power auger or a ro l l e r b i t with cas ing andwater . Standard penetra t ion t es t ing SPT} ands p i i t spoon sampling were general ly performed a tf ive . foot in tervals in accordance with ASTM01586. Where rock was encountered, an Nx s izediamond core barrel was used to advance t heborehole a minimum of 10 f ee t into sound rock.An abbreviated labora tory t es t ing program wasusual ly performed for each s i t e to ver i fy f i e ldc lass i f i ca t ion of samples and es tab l i sh indexpropert ies o f t he foundat ion mater ials .Gradat ion analysis , moisture content , andAtterberg l imi t s t es t ing were rout inelyperformed. Unconfined compressive s t reng tht es t ing of so i l samples and uniaxial compressiont es t ing of rock cores were only occasional lyperformed to develop addi t ional shear s t reng thcharac ter i s t ics of the foundation mater ia ls onmarginal s i t es .REGIONAL SUBSURFACE CONDITIONSSubsurface condit ions vary considerablythroughout the metropol i tan New York area . Thegeologic condit ions can be character ized,however, by severa l di s t inc t regions.Long Is land, including Br?oklyn and Queensiden t i f ied as Region A Figure 1) , i sgeneral ly comprised of granular coas ta l pla insedimentary deposi ts . These sediments arecomprised l a rge ly of s t r a t i f i e d sand and siltlayers with varying amounts of gravel . Thedis t r ibu t ion and extent of these sedimentdeposi ts i s fa i r ly consis tent , al though moderatevar ia t ion in silt content was found a t thevar ious s i t es invest igated. Soil dens i t iesgenera l ly var ied from compact to medium dense asdetermined during Standard Penetra t ion Test ingSPT). Groundwater l ev e l s varied from 10 to 35f ee t below grade and were considered to berepresenta t ive of s t a t i c condit ions based on thepervious nature of the so i l deposi ts .

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Embayment deposi ts of organic clays and s i l t sover lying the granular sediments were encounter7dalong the coasta l areas of Queens. ~ h e s e o r g a n ~ cdepos i t s are generally 5 to 10 f ee t ~ n t h ~ c k n e s sIn many of these t ida l areas , f i l l s o f between 5and 10 f ee t th ick have been placed over theembayment deposi ts to recla im these areas fo rdevelopment.Westchester County and the boroughs of Manhattanand the Bronx Region B in Figure } arecharacter ized by predominantly granular depos i t soverlying glac ia l till Direct ly under lying t ~ sg l ac i a l till is bedrock. The granular d e p o s ~ t sare primari ly tan sands and gravel with little tomoderate amounts of s i l t . The g l ac i a l tillcons is t s of a random matr ix of grey and grey-brown sand, gravel, s i l t and clay t ha t ex i s t s i nvarious proportions throughout the region. Thetill i s qeneral ly qui te dense, with SPT valuescons is ten t ly greater than 50 blows per foot , andi s underla in by bedrock. The depth to bedrock i swidely varied throughout the region. Bedrockformations encountered dur ing the s i t einvest igat ions included g ran i t i c and granodiar i tegneiss in the upper and cen t r a l areas o fWestchester and mica sch i s t in the more souther lyareas . Depth to groundwater var ied substant ial ly ,and when encountered, was usual ly due to perchedcondi t ions.Subsurface condit ions n Northern New Jerseyindicated as Region C in Figure 1) are s imi larto Westchester, although so i l co lor and bedrocktype di f f e r markedly. Addi t ional ly , the granulardepos i t s overlying the glac ia l till are genera l lydeeper in extent than those encountered a t theWestchester s i tes . The glac ia l tills are aheterogeneous mixture of reddish brown sands,gravel , silt and clay, w i th sand predominatingt h e mix proportion. The under lying bedrock wasmost commonly found to be pa r t o f the Brunswickshale formation, which s genera l ly weathered a tt h e contact surface and becomes more massive withdepth. Discont inui t ies n the shale weregeneral ly a t low angles to the hor izon ta l .Groundwater condit ions were qui te var iab le , withdepths ranging from three fee t to grea ter than 3 5fee t .FOUNDATION DESIGN CRITERIASelect ion and development of geotechnical designc r i t e r i a for the caisson foundat ions i s afunct ion of so i l type, loading condi t ions ands t ru c t u ra l design methodology. The so i l typedetermines which of the genera l shea r s t r eng thparameters, f r ic t ion angle andjor cohesion, arenecessary to character ize t h e s t r eng th andbehavior of the foundation mater ia l s . Asmentioned previously, the loading condi t ionsconsidered most c r i t i c a l are overturning due tosubs tan t ia l l a t e ra l loading of t h e monopole and,to a l es ser extent , compressive loads imposed bythe weight of the s t ruc tu re .Since l a te ra l capaci ty of t h e caisson toaccommodate overturning moments was considered tobe the most c r i t i c a l parameter , development o ffoundation design c r i t e r i a had to consider themethod of analysis fo r caisson design.I n i t i a l l y , hand solut ions developed by Brems wereused to s i ze caissons and determine concretere in forc ing requirements. Subsequently, the


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For cohesive so i l s , ul t imate l a t e r a l so i lre s i s t ance i s uniform with depth and determinedby:(4)

Th7 f ~ r e m e n t i o ~ e d l t ~ r l so i l re s i s t ance designcrJ.terJ.a were J.nput J.nto Broms' equations todevelop maximum re s i s t ance of the ca issons asdiscussed fu r the r on.The use of .sof tware to ass i s t in designing thecaissons necess i t a t ed development of a l t e rna tefoundat ion design c r i t e r i a for use as input .Brie f ly , s o i l def lec t ion under l a t e r a l loading i smodeled with p-y ( load vs . def lect ion) curves inthe LPILE program. These p-y curves can ei therbe input manually o r generated from preprogrammedp-y curves fo r severa l di f feren t so i l condi t ionsfrom the LPILE da ta base. These preprogrammedfamil ies of p-y curves were developed by theLPILE authors based on extensive fu l l scale t i e ldload t e s t resu l t s . The basic types of so i lcondit ions are :

Dry and Submerged Sands Saturated Sof t Clays

sa tura ted S t i f f Clays Dry S t i f f Clays

To f ac i l i t a t e analysis and minimize design costs ,computer generated p-y curves were used fordesign and ana lys i s of the ca issons a t thevar ious s i t e s .Geotechnical design parameters tha t are s imi la rfo r both Broms' and LPILE methods of analys isinclude shear st rength, f r ic t ion angle andjorcohesion) and ef fec t ive u n i t weight of the so i l s .For t he LPILE analysis using the computergenerated p-y curves, addi t iona l geotechnica li nput included the modulus of ho r izonta l subgrader eac t ion (kh) These values were determined basedon es tab l i shed empir ica l re l a t ionsh ips with SPTr e s u l t s .For c lays , values of s t r a i n correspondingto one h a l f the maximum principal s t r essdi f fe rence determined from unconfined or t r i ax i a lcompression t e s t s a re also needed. Typical valueso f were obtained from es tabl ishedr e la t ionsh ips cor re la t ing shear s t r ength (cu) tos t r a i n .STRUCTURAL DESIGN PROCEDUREMagnitudes of the l a t e r a l loads imposed on themonopole and t r ansfe r red to i t s foundat ion aredeveloped in accordance with parameterses tab l i shed by t he EIA as discussed previously.Using t he foundat ion loading data andgeotechnica l design c r i t e r i a , t he followingprocedure was used to design the ca isson . Theprocedure descr ibed herein di f fe rs from tha tpresented in ACI(1985) by incorpora t ing thebehavior o f layered so i l s , and focuses on the useo f computer sof tware to perform the ana lys i s .

2 5

1 .

2.

3.

4 .

5.

6.

7.

Establ ish minimum diameter of thefoundation based on t he s i ze of thebase pla te and c learances for anchorbolts and r e inforc ing . Minimumver t ica l reinforcement required byACI(l989) i s ca lcu la ted as:5)

Model the so i l as a layered medium,based on material types, ground water ,and s t i f fness parameters. Theseparameters are expressed as a loaddeformation re la t ionship (p-y curve)indexed to the so i l type and factoredby the hor izonta l subgrade modulus.Depending on the so i l s t i f fness , themaximum moment wil l occur a t a pointapproximately 1. 5 diameters belowgrade. Estimate th i s maximum momentand calcula te a t r i a l value for theeffect ive s t i f fness of t he caisson andrebars as:

6)where I ef f i s the t ransformed momentof inere1.a of the cracked sectionunder the given moment and axial load.In pract ice , th i s i s ca lcu la ted by theSTIFFl program, which can alsoincorporate the s t i f fness contr ibut ionof a permanent s tee l cas ing.Run the LPILE analys is to determinedef lect ion of the foundat ion a t gradeand maximum bending moment under thegiven service loads. The def lect ioni s compared to an allowable value of3/8 , and the moment i s compared tothe est imated value used in thes t i f fness computation. Determine i faddi t iona l caisson s t i f fness or acorrec t ion to the estimated s t i f fnessvalue i s needed.Adjust the caisson length, diameter,or ver t i ca l reinforcement to obta inthe required s t i f fness . Repeat theanalysis u n t i l the def lect ionc r i t e r ion i s sa t i s f i ed and convergencei s obtained.Modify the so i l parameters to assessthe sens i t iv i ty of t he design andconfirm tha t predicted behavior wil lbe consis ten t throughout a reasonablerange of varia t ion.Verify t ha t the s t ruc tura l capacity isadequate in accordance with ACI(1989),and tha t so i l pressures are withinacceptable l imi t s .

A typical caisson foundation for a monopole i sshown in Figure 3.


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TOP OF CONC

GRADE\ l_ I / BOT IOM OF i ~ . ~ - - B ASE PLATE~ ' - ~ I ~ - ' W, / , / / , / , /

I ;:)0

I 0::CIANCHOR BOLT1 I~ CONCRETE CAISSOtr- ~ ~ I)wC> Fz~ I -1- TEMPLATE zz 0N

:::E 120 I 000 I :X::::Ew -VERTICAL BARSIBOTIOM OF A I tAC A I S S O N - ~ I

L CAISSON

SECTION A-AFIGURE 3 - caisson FoundationAt th is point, the economy of the design i sreviewed to determine i f appreciable savingscould be achieved by a l aring the soi lparameters. This can be accomplished in someinstances by excavating the natural soi l aroundthe top of the caisson to a reasonable depththree to four feet) and replacing t withcompacted backfi l l , to increase the horizontalsubgrade modulus of the surficial soi ls . Thel a te ra l extent of excavation and replacement i sshown in Figure 4. This soil replacement methodi s also effect ive n reducing the requiredcaisson length by as much as five to ten feet .

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EXCAVATE EXISTINGMATERIAL REPLACEW/ COMPACTED B C ~ L L

w

CAISSON

I

ItFIGURE 4 - Soil Replacement

1'-0MIN

Table 3 contains physical character ist ics of them ~ n o p o l e st ructures and caisson foundations for

f ~ f t e e n . representat ive cel l s i tes in them e ~ r o p o l ~ t n New York area, including monopoleh e ~ g h t and resultant caisson dimensions.I t i s not uncommon for these monopoles to belocated close to exist ing buildings and otherst ructures . iue B-2 required a unique design toaccommodate i t s proximity to an exist ingbuilding. The centerl ine of t h i s monopole is only6 feet from the exist ing basement wall whichextends approximately 7 feet below grade.Preliminary analysis indicated tha t the minimumdiameter of the caisson would be 5.5 feet , whichwould place the edge of the caisson within 3 feetof the building. Due to the age and condition ofthe building foundation Mall, t was necessary toavoid t ransferring the substant ial la tera l loadfrom the monopole to the adjacent wall .Therefore, a caisson with a double s teel casingseparated by a layer of compressible material asshown in Figure 5 was designed to absorb la tera lloads induced by the monopole. The 5. 5 footdiameter inner casing extended fu l l depth to thetop of sound rock to enclose the caissonconcrete. The larger outer casing extended tojus t below the building footing. The four inchannular space between the two casings was f i l ledwith polystyrene granules which can compress andabsorb the la tera l deflection and minimize loadt ransfer from the monopole. A neoprene rubbergasket was installed a t the top of the annularspace to protect the compressible material andminimize intrusion of water.


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TABLE 3CAISSON DESIGN DATASITE MONOPOLE DIAMETER LENGTH As/AcHEIGHT ft) (ft)O (f t )A-1 100 5.0 25A-2 100 6.0 30A-3 125 7.0 35A-4 132 6.5 35A-5 100 5.0 30A-6 100 5.5 25A-7 60 5.0 30B-1 150 5.5 16B-2 150 5.5 20B-3 125 ** **C-1 75 5.5 25C-2 100 6.0 30C-3 100 6.0 30C-4 100 6.0 25C-5 35 4.0 18

*Plus permanent s tee l casing**Not yet determined

bIb'

( )ziii=


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concern for protection of the exis t ingstructures, the contractor elected to leave thecasings in place.The use of bentonite slurry to advance caissonexcavations was used primarily in granular soi lswith high groundwater levels tha t were typical lyencountered on Long Island and in Queens. Inmost circumstances, sidewall s tabi l i ty abovegroundwater was maintained with temporary casing.Once groundwater was encountered, a bentoniteslurry mix was placed into the excavation. Theslurry level was generally maintained severalfeet above the groundwater level to maintain aposi t ive head and a l levia te the possibil i ty ofbase disturbance due to l iquefaction. Once theexcavation was advanced to the appropriate depth,the re inforcing cage and anchor bol t assembly wasinserted into the excavation and the concrete wasplaced using tremie methods. The displacedbentonite s lur ry was usually disposed of onsi te.CONCLUSIONSThe growth of the cel lu lar telephone industry hasled to the development of numerous antenna s i t e sin. the metropolitan New York area. Singlecaisson foundations have been found to be themost desirable method of supporting monopolesused as antenna supporting st ructures .As par t of th is construction program, thestructural configuration and loading conditionswere standardized, enabling the design to focuson s i te-specif ic subsurface conditions. Threedis t inct regions were identified, and foundationdesign cr i ter ia were developed based ongeotechnical investigations.Although simplified f ield investigation andlaboratory test ing procedures were used todevelop the foundation design cr i ter ia for thesest ructures , the quali ty of the data obtained hasturned out to be more than adequate.

_A structural design procedure based on computeranalysis methods allowed rapid evaluation of thesens i t iv i ty of the overall caisson design to thevarious soi l parameters. A major benefi t derivedfrom t h i s capabi l i ty was obtained in severalinstances where surf ic ia l so i l excavation andreplacement was used to decrease predictedcaisson deflections. This was accomplished a t asignif icant reduction n cost compared todeepening or enlarging the caisson.The various construction procedures employed havealso been discussed. By implementing carefulplanning and supervision a t each s i te prior toand during construction, sat isfactory caissonins ta l la t ions resulted a t a l l locations. Thiswas accomplished despite the acceleratedconstruction schedules and the inevitableobstacles tha t were encountered during=onstruction within the urbanjsuburbananvironment.l'he performance of each of the completedn.onopoles has been sat isfactory in the two yearsince the s ta r t of the program, which includedaveral severe wind storms, and is expected to>ntinue for the useful l i f e of the st ructures .

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SYMBOLS

NcNsptpqaqultzyYdEsop

gross concrete areaarea of reinforcing s teelcohesionundrained shear st rengthcaisson diametermodulus of elast ic i ty for concreteeffect ive moment of iner t ia of transformecracked sectioneffect ive caisson f lexural stiffnessmodulus of horizontal subgrade reactionRankine coefficient of passive ear thpressureembedment lengthbearing capacity factoraverage uncorrected blow countult imate la tera l resis tanceallowable bearing capaci tyult imate bearing capacitydepth along caissoneffect ive unit weight of so i ldry uni t weight of soi ls t ra inangle of internal f r ic t ion

REFERENCESACI(1985), Suggested Design and ConstructionProcedures for Pier Foundations, ACI336.3R- 72 (Revised) , American ConcreteInst i tu te Detroit , MI.ACI(1989), Building Code Requirements forReinforced Concrete, ACI 318-89 ,American concrete Inst i tu te Detroit , MI.ASCE(1990), Minimum Design Loads for Buildingsand Other Structures, ASCE 7-88 , AmericanSociety of Civil Engineers, New York, NYEIA(1991), Structural Standards for s teel

Antenna Towers and Antenna SupportingStructures , ANSI/EIA/TIA-222-E-1991Electronic Industries Association 'Washington, D.c. 'Reese, L.C. and M W O'Neill(1988), Dril ledShafts: Construction Procedures and DesignMethods , ASDC: The Internat ionalAssociation of Foundation Dril l ing Dallas,TX 'Reese, L.C. and s.-T.Wang(l989)Documentation of Computer ~ o g r a m LPILE ,Ensoft, Austin, TXWang, s . - T. and L.C.Reese(1987),Documentation of the Computer Program

STIFF1 , Ensoft, Austin, TX

P 0201- Caisson Foundations for Cellular Telephone Monopoles

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