Pos Selim landslide stability assessment

Abstract

The main task of the study is to conduct several sets of limit equilibrium analyses (LEA). The target is to estimate the maximum friction angle mobilized at the basal rupture surface (BRS) below the main body and head graben at failure in 2003, and to assess the self-stabilizing effect of landslide movements 2003-10 and the destabilizing 2004 excavation at the Northern Spur. Also, aerial photo interpretation is conducted and shaded relief models (SRM) are examined to review site geology. The limit equilibrium analyses used basal rupture surfaces constructed by Cruden’s method (Cruden 1986) using surface displacement dip values (Lee 2019). The piezometric lines are straight lines inclined at various angles through a common point at or near the toe of the basal rupture surface. The Set One analyses comprise a set of runs for each of four longitudinal sections to determine the inclination angle (β) of the piezometric line for the factor of safety (FoS)= 1.00 condition for a given friction angle value in the range 30-35 degrees (for the BRS under the main body and head graben). The friction angles for a reasonable maximum β value of 17°, are 33°-34°, which is significantly lower than 38°, the minimum value of peak friction angle at the BRS for first time failure assigned base on judgement supported by the results of preliminary rock testing (AML 2007, section 5.4.5). This difference might be due to the occurrence of progressive failure: this is plausible as there is photographic evidence of pre-failure movement at main scarp and toe of the basal rupture surface. In Set Two analyses, FoS are derived separately for the same BRS with 2003 and 2010 profile for two sets of phi/beta values determined from Set One result to assess the self-stabilizing effect of landslide movement. The FoS lying in between 5% to 11%, positive for each set of phi/beta value increases and progressively increase in magnitude to the north, are supportive evidence indicating from 2003 to 2010, the landslide stabilizes and experiences greater displacements in the north. In Set Three analyses, two sets of beta/phi values (at phi 30° and 31°) are selected from the modeling result FoS=1.00 for Seepage case (western end piezometric line at toe BRS) with 2003 CNS section, and calculate FoS with 2005 ground profiles to assess the effect of ground excavation in 2004 at the northern toe of the landslide. The small decrease in FoS magnitude within 0.007 from 2003 to 2005 (for both sets of beta/phi) suggests the 2004 excavation theoretically destabilizes the landslide and significantly impacts the velocity, triggering it up to 0.06m per day (Lee 2015 Figure 4.2) Aerial photos taken in 2005 by Dr Andy Hansen, and 2005 and 2010 shaded relief models constructed by him reveal geological features within the landslide footprint (conjugate and minor faults) not mapped by on-site mapping. A tectonic evolution model is built using the new observations and information from previous studies.