Surface deformation as a guide to kinematics and three-dimensional shape of slow-moving, clay-rich landslides, Honolulu, Hawaii

Two slow-moving landslides in Honolulu, Hawaii, were the subject of photogr ammetric measurements, field mapping, and subsurface investigation to learn whether surface observations can yield useful information consistent with results of subsurface investigation. Mapping focused on structural damage a nd on surface features such as scarps, shears, and toes. The x-y-z position s of photo-identifiable points were obtained from aerial photographs taken at three different times. The measurements were intended to learn if the sh ape of the landslide failure surface can be determined from systematic surf ace observations and whether surface observations about deformation are con sistent with photogrammetrically-obtained displacement gradients, Field and aerial photographic measurements were evaluated to identify the boundaries of the landslides, distinguish areas of incipient landslide enlargement, a nd identify zones of active and passive failure in the landslides. Data rep orted here apply mainly to the Alani-Paty landslide, a translational, earth -block landslide that damaged property in a 3.4-ha residential area, It beg an moving in the 1970s and displacement through 1991 totaled 4 m, Thickness , determined from borehole data, ranges from about 7 to 10 m; and the slope of the ground surface averages about 9 degrees. Field evidence of deformat ion indicated areas of potential landslide enlargement outside the well-for med landslide boundaries, Displacement gradients obtained photogrammetrical ly and deformation mapping both identified similar zones of active failure (longitudinal stretching) and passive failure (longitudinal shortening) wit hin the body of the landslide. Surface displacement on the landslide is app roximately parallel to the broadly concave slip surface.