ABSTRACT
The uncontaminated mud disposal area at East of Ninepins is an open seafloor disposal facility in Hong Kong waters. The dredged mud disposed of at the facility is typically sandy clayey silt, which stands at an average slope of about 1 in 100.
As part of the routine monitoring, bathymetric survey of the disposal area in July 1997 indicated that the average level of the disposal mud surface was about −25 mPD (Principal Datum), with isolated mounds of up to −21 mPD, as compared with the original seabed level of −31 mPD. Subsequent survey in October 1997 showed movement of about 0.42 Mm3 of the disposed mud over a distance of about 1 km. One of the major mounds was lowered by an average of about 2 m and the slope angle was reduced from 1 in 50 to 1 in 150. Side scan sonar data also showed well-defined flow features in association with the displacement.
Based on the disposal records, geophysical data and weather information, it is concluded that the submarine landslide was probably triggered by cyclic wave loading during the passage of a typhoon in August 1997, which resulted in sudden increases in pore water pressure in the disposed material. Results of the back-analysis suggested that the landslide was not a single large event, but a series of small scale failures progressing from the edge of the mound.
