Engineering geological investigation of the landslides in Boloc.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
This study concerns the landslides in Boloc, Philippines, an area underlain by sedimentary sequences of the Iloilo Sedimentary Basin with active tectonism, frequent and large contemporary earthquakes, and set in a humid tropical and monsoonal climate. The aim of the project is to provide engineering geological input to the landslide management in Boloc with emphasis on maintaining road access. Based on the failure model which shows dominantly pore-pressure-driven and periodic activity with 4m per event movement rate, it is recommended that the 4-hectare landslide (Landslide A) affecting the 100m segment of the municipal road in Boloc to be (1) partially mitigated by installing subsurface drains in its pore-pressure-build up prone landslide head, (2) monitored for changes in movement and activity to determine the effectiveness of subsurface drains, and (3) use a gravel fill road for accommodation of landslide movement and for simple inexpensive maintenance. Landslide A forms the larger Boloc Landslide Complex which includes Landslide B, a landslide which also has its toe delimited by the Central Stream in Boloc. Landslide A experiences significant changes in groundwater level (>10m) during wet season and dry season as observed from boreholes and face logs, has a ground water compartment at its head inferred from ponding and distribution of springs., and has a local reservoir at its middle part as inferred from low groundwater table and lack of springs Landslide B is interpreted to be also driven by pore pressure, as suggested by occurrence of a groundwater compartment at its head, as inferred from springs and ponding. The increase in pore pressure sufficient to drive movement is associated with occurrence of heavy and prolonged rainfall as suggested by the delayed start and end of landslide activity in Landslide A with respect to the onset and end of heavy rainfall periods as shown by analysis of data from borehole inclinometers, rain gauge, surface monitoring, and anecdotes from local residents. Accumulation of talus and colluvium at the head also contributes to destabilisation but to a lesser extent than by pore pressure based its relatively small proportion affecting the landslide. The materials of Landslide A and Landslide B are both predominantly consisting of large (>3m) blocks of intact, in-situ-derived sandstone sequence and/or interbedded sandstone-siltstone-mudstone, as suggested by the proximity of their source bedrocks which were also mapped in this study. The block materials of these landslides are associated with their rigid and predominantly failure-surface accommodated movement. This is suggested by the sideways-rotational movement of Landslide B interpreted from satellite imagery and orientation of main scarp, and by the partitioning of Landslide A into zones defined by sub-failure surfaces inferred to comprise graben-forming surface, thrust surface, and tensional surface which were inferred from surface morphology. The significant influence of pore pressure and the predominantly block materials of Landslides A and B observed and interpreted from detailed surface and subsurface data are consistent with the anticipated conditions inferred from the general ground model for the area of Boloc. Local build-up of pore pressures in the areas of Landslides A and B were anticipated from the lithology-controlled vertical confinement of sandstone units and the occurrence of a concave fold limb. Disaggregation of bedrock into large blocks via weakened lithological contacts was anticipated from the contrasting stiffness (inferred from unconfined compressive strengths) of sandstone sequence units versus interbedded sandstone-siltstone-mudstone units.