The role of the degree of weathering and groundwater fluctuation in landslide movement in a colluvium of weathered hornblende-gabbro

Shallow translational earth sliding, induced by heavy rainfall, in slopes made of colluvium of weathered homblende-gabbro was investigated in order to elucidate the relationship between weathering, soil strength and slope instability. The earth slide has a length of 100 m and a maximum width of 40 m and an average depth of about 6.4 m. Average rate of the initial sliding in early summer of 1976 was estimated to be about 50 mm/day and the sliding intermittently continued to the end of 1977. Sliding became active due to rising of groundwater table in response to rainfall. The average slope angle reduced from 13.9 degrees to 11.3 degrees as a result of the sliding. Mineralogical investigations indicated that (1) the sliding surface of this earth slide was located in the colluvium of weathered debris from the hornblende gabbro, and (2) hornblende in gabbro was altered due to weathering into swelling chlorite. Geotechnical properties of the colluvium were measured: shear tests gave values of c(p)' = 0.169 kgf/cm(2) and phi(p)' = 27.8 degrees for peak strength, and c(r)' = 0.122 kgf/cm(2) and phi(r)' = 10.6 degrees for residual strength. The infinite slope stability analyses using these values indicated that (1) the soil strength had reduced to the residual strength at the time of initial sliding, (2) the analyses using the residual strength well explain the results of the field measurements showing that the earth slide is active when the groundwater table rises to near the slope surface, and (3) the slope of 11.3 degrees, the post-sliding angle, is stable even under the most severe condition, even when the groundwater rises to the slope surface. These results suggest that the sliding surface had been already made before the 1976-1977 event. The existing literature shows that chlorite will alter to smectite minerals, having a smaller strength (phi(r)' = 4-5 degrees), as a result of further weathering. Using this value the ultimate angle of stable slope was estimated to be 8 degrees. Based on the discussion an idealized diagram illustrating the relationship between weathering, strength and slope instability of this hillslope was constructed.