Back analysis of an earthquake-triggered submarine landslide near the SW of Xiaoliuqiu

Occurred in the offshore of SW Taiwan on 26 December 2006 with a magnitude of 7, the Pingtung earthquake had triggered numbers of submarine landslides. This event provides an excellent opportunity to incorporate the back analysis approach to evaluate the in situ shear strength parameters. According to the chirp sonar images of the seabed near the SW Xiaoliuqiu obtained before and after the earthquake were adopted to establish the slope profile and identified the location of a circular sliding surface. Consequently, the in situ, effective strength parameters under the critical condition can be calculated by back slope stability analysis. Submarine sediment sampler was obtained via gravity sampling method and the laboratory tests were performed to determine the index properties and strength parameters. Test results indicate the cored sediment has the characteristics of normally consolidated (NC) clay. The effective friction angle (phi) is 15.3 degrees with cohesion (c') of 19.4 kPa. The effective and total stress methods were used to perform the back analysis. The strength parameters derived from back analysis of effective and total stress methods all indicate values approach the CIU triaxial tests results. Consequently, the representativeness of the marine sediment characteristics obtained from laboratory tests is identified. The total stress approach yields an undrained strength ratio c(u)/sigma' v0 of 0.26 which well fit the ratio used in geotechnical practice for estimating NC clay. According to the analytical approach, the landslide was applied seismic forces (seismic coefficient k(h) = 0.14) and generated excess pore pressure of 31 kPa at the sliding surface.