Seismic response of the George Massey Tunnel

• Main Author: Puar, Surinder S.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4646

The George Massey Tunnel, in Richmond, British Columbia, is a 630-meter long submerged concrete tunnel, with 550-meter and 335-meter long approaches on the north and south ends, respectively. The tunnel crosses the Fraser River and is founded on a deep deposit of unconsolidated sediments consisting mainly of sands and silts that are susceptible to liquefaction during earthquake loading. This thesis represents a comprehensive analytical investigation to evaluate the liquefaction potential of the foundation soils and the performance of the tunnel during a major earthquake. The evaluation procedures and post-liquefaction stability and deformation results are presented. Liquefaction potential analyses based on the total stress approach were conducted. Liquefaction was predicted by comparing the earthquake-induced stresses to soil resistance. Dynamic ground response analyses were performed to assess the magnitude of the cyclic stresses; the cyclic resistance of the soil was computed using various methods, depending on the soil type. Estimated acceleration levels could potentially trigger liquefaction in substantial zones of the tunnel's foundation. The residual (peak post-liquefaction) shear strength of liquefied soils was estimated to be adequate to maintain post-earthquake stability of the tunnel at all of the locations analyzed. The main problem to be addressed, therefore, was the displacements due to triggering of liquefaction i n directions transverse to- and parallel to the tunnel alignment, as a result of the 475-year seismic event. Post-liquefaction deformations of the tunnel were computed using both empirical and numerical methods. The numerical methods incorporate post-liquefaction stressstrain relationships and account for the effects of both gravity and inertia forces. Analyses suggested that liquefaction would occur at four of the five locations. Liquefaction was not predicted at the south shore. The liquefaction resistance at the south shore location was on the borderline in terms of the triggering criteria. The south shore location stratum is very similar to that of the north shore (where significant liquefaction is predicted). The displacement analyses at the two locations were compared and contrasted, revealing what movements could be expected at either end if liquefaction were to occur or not (i.e., depending on assessment of different earthquake magnitudes). Since the computed liquefaction-induced displacements were, often, beyond tolerances, potential remedial options were analyzed at the offshore location detennined to be the most susceptible to liquefaction. Those analyses showed that the use of certain remedial schemes will decrease the displacements significantly. Because it is very difficult to access the stratum directly beneath the tunnel, densification of zones adjacent to the tunnel is the most effective and economically feasible solution to limit displacements.