Load Tests On Rammed Aggregate Piers For Ground Improvement In Problematic Soils

M. Smith

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: GROUND IMPROVEMENT - III

ABSTRACT: Rammed Aggregate Pier• (RAP) Elementsare increasingly being used in British Columbiatoimprove the bearing capacityofproblematic soils for the support of new buildings. The stiffness ofRAPsis commonly measured by an axial compression(or modulus) test, which measures the load-settlement response of the pier. This paper presents the results of three loadtestsRAPs in BC. The first test was performed on a single pier in the Kelowna area. The second test was performed ona single pier in theRichmond area, and athird test was performedona group of four piers beneath a footing loadon thesame site.The results of the modulus tests performed on individual piers indicate high-stiffness foundation pierswhen installed insandy and silty soils commonly found in British Columbia. The results of the research demonstrate the increase in limitequilibrium bearing capacity that is afforded by the construction process. This paper is of particular significance astraditional analysis proceduresof granular columns in BCdo not take into account the high-stiffness of aRAPnor theincrease in lateral stress in the matrix soil.R†SUM†Piliers en Agr‡gat Dam‡ (Rammed Aggregate Piers,RAP) sont de plus en plus utilis‡es en Colombie Britannique afin derenforcer et d…am‡liorer la capacit‡ portante de sols probl‡matiques, pour permettre le soutien de nouveaux b—timents. Eng‡n‡ral, la rigidit‡ desRAPest mesur‡e gr—ce –un test de compression axiale, qui mesure le tassement d…une colonnepar rapport – unechargeimpos‡e– celle-ci. Cet article pr‡sente les r‡sultats de trois tests de chargement sur desRAPen ColombieBritannique. Le premiertesta ‡t‡ effectu‡ sur une colonne dans la r‡gionde Kelowna. Le deuxiƒmetesta‡t‡ r‡alis‡sur une colonne,r‡gion Richmond, et le troisiƒmetests…est fait sur un groupe de quatre colonnes soumisessous une charge portante ‡quivalente a celle d…une semelle, au m⁄me site.Les r‡sultats des tests effectu‡s sur lesRAPindividuelles indiquent une haute rigidit‡ lorsqu…elles sont install‡es dans dessols sablonneux et silteux, couramment trouv‡sen ColombieBritannique. Les r‡sultats de la recherche d‡montrent qu…ily a une augmentation de la limite de lacapacit‡ portantegr—ce auprocessus de construction. Ce document est d…uneimportance consid‡rable, compte tenu que les proc‡dures d…analyses traditionnelles desRAPen Colombie Britanniquene prennent pas en compte la rigidit‡ desRAP, ni l…augmentationde contraintes lat‡rales induites – la matrice du sol.1INTRODUCTIONMany sites in British Columbia are underlain by loose,liquefiable and/or soft, compressive soils. Larger buildingswere traditionally supported by deep pilefoundationsextending through problematic soils but in recent years,have been supported on large raft foundations afterpreloading, soil densification, or removal/replace methods.More recently,granular columns calledRammedAggregate Pier• (RAP) Elementshave been used toreinforce and improve the bearing capacity of theseproblematic soils for the supportoffootingor matfoundations fornew buildings.The results of research performed byHughes andWithers (1974) andHughes et al. (1975)on thebehavior ofgranular columns in soft cohesive soils is still consideredfor design today. Hughes et al. (1975)notethat the bearingcapacity of granular columns is dependent upon thebulging of the column;the ultimate loadisdependentonthe friction angle of the gravel used to form the column andthelateralrestraint of thesoilaround the column.Unlikethe stone columns constructed by the vibro-replacementmethod studied byHughes and Withers (1974) andHugheset al. (1975),RAPsare constructed byramming stone intothe groundwith vertical compactive energy usinga beveledtamper head. It has been demonstrated that this methodof construction (1) results in friction angles of 48 to 52degrees of theconstructedgravel column, and (2)increasesthelateral stress in the soil surrounding thecolumn(White et al. 2000; Handy 2001; Wissmann et al.2001).The as-built strength and stiffness of theRAPsaremeasured by fieldloadtests, or modulus tests.This paperpresents the results of three load tests on RAPs in BC. Thefirst test was performed on a single pier in the Kelownaarea. The second test was performed on a single pier inthe Richmond area, and a third test was performed on agroup of four piers beneath a footing load on the same sitein the Richmond area.

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