Quantitative risk assessment of collapses of deep excavations in Hong Kong

A study has been carried out to quantify the risk to life from deep excavations in Hong Kong. The study was based on the past records of collapses and excessive displacements as catalogued by the Geotechnical Engineering Office (GEO) and supplemented by other data. This paper summarises the study that included a review of previous incidents in Hong Kong involving private developments between 1981 and 1995. Hazard identification studies, frequency assessment, consequence assessment and risk estimation were carried out, It was found that the contribution to the overall risk is significantly higher for sheet pile walls than for other types of walls, The high failure rate for sheet pile walls is mainly due to inadequate penetration or strutting. The public are most at risk from buildings on pad foundations adjacent to excavations. The risk from this case is compared with the accepted annual risk criteria for hazardous installations and is seen to be reasonably high, It should be noted however that the risk from an excavation is transient whereas for a hazardous installation the risk is present over many years, The most effective ways of reducing the risk arc improved site control and routine monitoring to maximise the warning of a large collapse.