Ground-water flow analysis in the slope above Shum Wan Road, Hong Kong

On August 13, 1995, a slope above Shum Wan Road failed due to high rainfall and caused a 30-m section of Nam Long Shan Road to collapse. The slope con sists of weathered tuffs with a clay layer on the surface of the failure. A hydrogeological study was carried out by saturated finite difference grid model, MODFLOW, for the slope at the Shum Wan Road area. From the ground-wa ter model, it was found that the ground-water level reached three meters be low the ground surface during failure. The model is sensitive to recharge a nd specific yield. The presence of the clay layer helped to maintain a high ground-water level. Stability analyses were performed using SLOPE/W. The r esult of stability analyses showed that the factor of safety, F, decreased due to the rising initial water table. On the 31st of July, the factor of s afety was 1.41, and dropped down to 1.01 on the 3rd of August. The factor o f safety again rose back to 1.31 on the 8th of August and it finally droppe d down to 0.99 on the morning of the 13th of August. The present study show ed that the antecedent rainfall had some influence on stability of the slop e. The amount of water in the form of seepage, which drained out from the s eepage surface from the lower part of the slope, is quantified and found to be 790 m(3). Preventive measures can be taken by inserting horizontal pipe s in the slope to drain out the ground water in the form of seepage or by c overing the slope with shortcrete or chunam.