June 2022 G-01483.84-017-R-Rev0 Because the 25 Table 9: Stability analysis results for a static short-term case - Section A-A’. Loading Condition Soil Parameters Slip Surface Groundwater Condition Scenario 1 Scenario 2 FoS Figure No. FoS Figure No. Short Term Undrained Circular 0 1.14 A1 1.34 A5 0.1 1.01 A2 1.23 A6 Noncircular 0 1.03 A3 1.15 A7 0.1 0.97 A4 1.08 A8 Table 10: Stability analysis results for a static short-term case - Section B-B’. Loading Condition Soil Parameters Slip Surface Groundwater Condition Scenario 1 Scenario 2 FoS Figure No. FoS Figure No. Short Term Undrained Circular 0 1.32 B1 1.45 B5 0.1 1.21 B2 1.36 B6 Non-circular 0 0.84 B3 1.11* B7 0.1 0.78 B4 1.15 B8 *The failure surface has changed location with the improved ground conditions. 6.8.2 Seismic Stability For the short term design life of the WRS, analysis of seismic stability is not considered necessary. The likelihood of significant earthquake shaking occurring during the design life of the structure is considered low. If significant earthquake shaking does occur during the design life of the structure, the effects are anticipated to be minor, including limited deformation and cracking. This is because the tunnel spoil material has inherently high internal strength and that the foundation material is not susceptible to liquefaction or significant strength loss once the stiff clay is removed from the toe area. The effects of earthquake shaking could include limited damage to the roads and storm water channels on the surface of the WRS. This damage would require minor regrading as repair, which could be readily achieved with the equipment available on site. 6.9 Conclusions and Implications for WRS Design ▪ Stability analysis results show that prior to the firm clay being removed beneath the toe of the WRS the FoS is less than 1.3 and that some foundation preparation will be required prior to placement of the tunnel spoil. With the weak surficial soils (firm clay) removed from beneath the toe, the FoS increases to above 1.3. The change in FoS shows the impact removing the softer surficial soils has on the FoS and short-term stability of the WRS.
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