Geology, hydrology, and mechanics of a slow-moving, clay-rich landslide, Honolulu, Hawaii

The Alani-Paty landslide has damaged streets, utilities, and homes built on a debris apron in Honolulu, Oahu, Hawaii. Failure of weathered, crudely stratified, highly plastic, debris-apron deposits has created several similar landslides in southeastern Oahu. The Alani-Paty landslide affects about 60 residential lots. It is about 300 m long, 160 m wide, 7-10 m thick, and consists of two main kinematic elements that are separated by a right-lateral shear zone. One element has moved about 4 m, mainly by translation, down a slope of about 12°, and the adjacent element has moved about 3 m down a slope of 9°. Longitudinal stretching in the upslope third and shortening in the downslope two-thirds characterize deformation in each element; landslides in Ohio, Utah, and Colorado have deformed similarly. Smectite-rich clay layers within the deposits are medium to stiff, and measured angles of residual friction range from 6° to 11° with cohesion intercepts less than 12.5 kPa. Saturated hydraulic conductivity within the landslide decreases with depth; below the slip surface, the hydraulic conductivity increases. Rainfall infiltrates at the ground surface, percolates downward and perches on the zone of low hydraulic conductivity near the slip surface, keeping the slide mostly saturated year round. The main body of the landslide moves during rainy periods, when the ten-day average rainfall exceeds 25 mm/day and the pore-water pressures in the upslope quarter of the landslide increase 10-30 kPa. Pore pressure increases within the landslide occur 1-2 days following the onset of rainfall and result from infiltration of rainfall and runoff; after materials above the perennial water table become saturated, downward propagating pressure waves triggered by bursts of intense rainfall produce further, short-lived increases in pore pressure. This elevated pore pressure at the slip surface triggers movement. The ground-water response in the upslope quarter of the landslide is relatively rapid compared to responses in other landslides described in the literature.