Sierra Slope Evaluation

Project: Sierra Slope Evaluation
Location: Vancouver, British Columbia, Canada
Client: Tensar International Corporation

In 2009, MEG Consulting Limited (M+EG) was contracted by Tensar International Corporation to perform a series of non-linear dynamic FLAC analyses to evaluate the seismic performance of a reinforced soil slope that was to be built at the West Abutment of the new Port Mann Bridge located to the east of Vancouver, in the Lower Mainland of BC.  The Sierra SlopeTM is a structural alternative to a retaining wall whereby synthetic geogrid is employed as soil reinforcement.  The analyzed sections of the Sierra SlopeTM are part of the Port Mann Highway 1 Design-Build project, which is being constructed by the Kiewit-Flatiron (K-F) partnership.

 

At the west abutment of the new Port Mann Bridge, a vertical reinforced soil wall is to be constructed over stone-column improved ground.  Due to space limitations at the abutment due to the existing bridge foundations (bridge still in operation), the abutment was designed to consist of a combined MSE and EPS wall.  The thickness of the EPS is a maximum at the front of the abutment, with the height of granular fill increasing towards the approach structure.  The initial fill is retained by a vertical MSE wall, which transitions into the Sierra SlopeTM a short distance behind the abutment.  The  Sierra SlopeTM is about 9 m high at the abutment, reducing to about 2 m to the west, and is replaced by a natural soil slope at the east side of the B5913 underpass.

 

Project Highlights:

  • 2D non-linear dynamic analyses have been performed using the program FLAC, which can simulate a full range of static and dynamic conditions with coupled fluid flow and soil-structural interaction.  Constitutive model - UBCSand was used to simulate the dynamic pore pressure generation under earthquake loading.  The geogrid reinforcement was modeled in FLAC as built-in strip elements under plane strain conditions.
  • The results of the FLAC analyses including the expected levels of ground deformation at the Sierra SlopeTM and the maximum tensile forces generated in the slope reinforcement were assessed in order to compare/evaluate standard design approaches with more rigorous non-linear time-history numerical analyses.
  • Based on the results of the analyses, it would appear that the seismic response of the geogrid wall is greatly enhanced by the presence of the flexible reinforcement