Measured Lateral Earth Pressure Changes During Deep Excavation Construction

G. Creer

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: SOIL AND TERRAIN CHARACTERIZATION - VII Instrumentation & Monitoring

ABSTRACT: Design of underground structures, whether temporary or permanent, requires an understanding of the lateral earthpressures acting on the structure.A section of tunnel was being constructed using cut and cover methodsinstiff to verystiff glacial siltyclay/clayey silt till, with groundwater at a depth of about 2m below grade.A monitoring array, including anumber ofspade cell pressure plates, was installedin the soil behindthetemporary shoring wall to measure the changesin total lateral pressurethrough the course of construction.The in situ horizontal pressures were calculated andestimated using in situ and laboratory testresults.However, it was difficult to determine the in situ lateral pressuredirectly from the spade cells due to over stressing during installation, which is common in over consolidated soils.Overthe course of construction, the horizontal pressures •relaxed† and at the end of construction the spade cells indicated a15% to75% reduction in the lateral stress, depending on spade cell location and depth.Longterm monitoring of thespade cell array is planned to assess the changes in lateral stresswithtime.R‡SUM‡La conception d…ouvrages souterrains, qu…il soit temporaire ou permanent, exige une compr—hension despressions desterres lat—rales agissant sur l…ouvrage. Un tron–on du tunnel —tait entrain d…ƒtre construit en utilisant des m—thodes ⁄ cielouvert dans le till glaciaire argileux limoneux / limoneux argileux raide ⁄ tr‹s raide, avec la nappe phr—atique situ—e ⁄ 2mde profondeur d'environ au-dessous du niveau du sol. Un r—seau de surveillance de sentier, y compris un certain nombrede plaques de pression cellulaire spade, —tait install— dans le sol derri‹re le mur de sout‹nement temporaire pourmesurer les variations de la pression lat—rale totale durant la construction. Les pressions horizontales in situ ont —t—calcul—es et estim—es ⁄ l'aide des r—sultats des essais de laboratoire et in situ. Toutefois, il —tait difficile de d—terminer lapression lat—ralein situ directement ⁄ partir des cellules spade due ⁄ une surcharge lors de l'installation, qui estcommune dans des sols consolid—s. Durant la construction, les pressions horizontales se sont dissip—es et ⁄ la fin de laconstruction des cellules spades ont indiqu— une r—duction de 15% ⁄ 75% de la contrainte lat—rale, selon l'emplacementet la profondeur de la cellule spade. La surveillance ⁄ long terme du r—seau de cellules spades est pr—vue pour —valuerles changements dans la contrainte lat—rale avec letemps.1INTRODUCTIONDesign of underground structures, whether temporary orpermanent, requires an understanding of the lateral earthpressures acting on the structure. In deep excavations,such as those for the construction ofsubwaycut andcover tunnels,stationsand otherinfrastructure, temporaryshoring is designed to resist lateral earth pressures duringconstruction and the permanent structure is designed toresist thelong-termlateral loads.Designers generallypresume thatthe temporary shoring is not present forthelifetime of the structure and the structure itself must resistthe lateral loads.The purposeofthisinvestigation was to measure thechanges in total stress during construction of asection ofcut and coversubway boxstructure.This paper providesa general overview of the site conditions, theinstrumentation that was installedto monitor the stresschanges in the soils, an overview of the results obtainedand a brief discussion of the results.2THE SITEAND PROJECTWilson Yard is the main storage,maintenance andoverhaulfacility forTTC…s new Toronto Rocket Subwayfleet. As part of theToronto-York Spadina SubwayExtension, Wilson Yardhas undergone an expansion toaccommodate the newsubwayfleet andincludes theconstruction of a cut and cover tunnelallowing foroperational improvements.The general configuration ofthenewtunnel section is shown in Figure 1.Thenew tunnel consistsof a single track box structureapproximately6.5m high,7mwide, foundedat a depth ofapproximately 9 to 10m below grade(elev. of about185.8m)and will have approximately4 to 5mof soil coveron completion.The construction of the tunnel sectionusedtemporaryshoring consisting ofconventionalsoldierpiles andlaggingwith internal struts and braces. A section of theshoring adjacent to TTC…s active main subway line wasconstructed usingastiffer secant caisson wall.Construction work commenced inmid to late January2015 with the installation of the shoring walls, followedshortlythereafterby the excavation work. The box tunnelsections were constructed through the spring, summerand early fall of 2015. The box tunnel was backfilled inearly tomid-September2015.

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