The proposed site for an offshore structure is located in a region of gradually varying bottom slope off the California coast. Due to the seismicity of this region the owner has asked for your preliminary evaluation of the stability of the slope during an earthquake.
The significant water depth contours (in fathoms) are shown on the plan view of the site in Fig. 1. Previous exploration in the vicinity of the site indicates that the soil profile is relatively uniform over a large area, and for this and economic reasons only one soil boring was taken for the current investigation. The location of this boring (Boring A) is indicated in Fig. 1 and the soil profile data are summarized in Fig. 2. The shear strength values shown in Fig. 2 were measured in unconsolidated undrained triaxial compression tests.
As a first approximation for the stability during an earthquake compute the static factor of safety of the slope using an infinite slope analysis and a cross-section along the line A-A'. For many clay slopes a factor of safety of 2.0 or greater with respect to static stability is adequate to ensure that stability will be maintained during an earthquake. Write a letter to the owner explaining the results of your calculations and the recommendations you would make regarding the static and seismic stability of the slope.
TIPS:
1) Note that the depth contour intervals are not unitform. Convert from fathoms to feet and plot the profile. Then estimate the average slope angle of the middle steeper section.
2) Make a table in excel and calculate the FS at each depth. Be sure to use the correct FS equation.
3) At each depth, use the undrained strength at that depth, but the average unit wt above that depth.
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