Mechanisms of shallow translational landsliding during summer rainstorms: North Island, New Zealand

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Abstract
Measurements of the physical properties of strongly weathered soils from greywacke, in the South Auckland region of New Zealand, indicate that the dominant clay mineral is kaolinite, that the hydraulic conductivities of subsoils are greater than those of surface horizons and that the soils are cohesive. Strength changes from natural moisture content to saturation are 22 percent with c' 17 to 14 kPa and ø' 44° to 36° for the clay material immediately above the failure planes of translational landslides, and 38 percent for the silty clay below the failure plane. Infinite slope analysis supports the conclusion, drawn from field observations, that intense summer rainstorms trigger landslides by causing high hydrostatic pressures in tension cracks. The high pore water pressures are then transmitted through the permeable subsoil. The only effective soil conservation measure is reafforestattion which adds to the apparent cohesion of the soils.

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