Design of Piles in Liquefiable and Laterally Spreading Soil Conditions, Low Level Road Project, North Vancouver, BC

The Low Level Road (LLR) project includes the design and construction of the Neptune/Cargill Overpass in North Vancouver, BC. This two-span bridge structure is 78 m long and 12 m wide with flared roadway geometrics. The bridge structure is located directly south of steeply sloping terrain with soil conditions in the area comprising of primarily granular fill over native granular deposits. The presence of liquefiable granular soil was predicted in the event of a design level earthquake having a return period of 1 in 975 years. The thickness of the liquefiable soil varied and was estimated to be negligible at the north abutment and up to 15 m at the south abutment. Furthermore, the bridge structure site was located approximately 300 m northeast of Burrard Inlet and it was estimated that lateral soil spreading in the order of 300 mm towards Burrard Inlet could occur due to seismic loading conditions. Pile foundations were determined to be the preferred foundation option for both abutments and pier. The requirement of continued operation of numerous rail tracks between the pier and abutments reduced the possibility of implementing ground improvement to mitigate the soil liquefaction and lateral soil spreading hazards. Thus, the pile design included consideration of seismically induced soil liquefaction and laterally spreading soil conditions. These effects were incorporated into the analytical software programs L-Pile and Group by using p-y curves representative of liquefiable soil and including the free-field lateral soil spreading movement. Furthermore, the piles at the south and north abutments were installed behind and relatively close to the facing of 9 m high MSE walls, the effect of which was incorporated into the lateral pile design. Subsequently, soil-structure-interaction (SSI) analyses were completed to evaluate the effect of seismic loading conditions on the entire bridge structure. The bridge superstructure and substructure was modeled using CSiBridge software (SAP2000) with the behaviour of the pile foundations represented by linear soil springs. These soil springs with six degrees of freedom at each pile head were determined using the Group software program. An evaluation was performed of the structure based on site-specific response spectra developed for the 975-year design earthquake. Several iterations were completed of the SSI analyses to ensure convergence of the results from the CSiBridge and Group analyses.