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 consists of weathered tuffs with a clay layer on the surface ofthe 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-water model, it was found that the ground-water level reached three meters below the ground surface during failure. The model is sensitive to recharge and specific yield. The presence of the clay layer helped to maintain a high ground-water level. Stability analyses were performed using SLOPE/W. The result 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 safety was 1.41, and dropped down to 1.01 on the 3rd of August. The factor of safety again rose back to 1.31 on the 8th of August and it finally dropped down to 0.99 on the morning of the 13th of August. The present study showed that the antecedent rainfall had some influence on stability of the slope. The amount of water in the form of seepage, which drained out from the seepage surface from the lower part of the slope, is quantified and found to be 790 m• Preventive measures can be taken by inserting horizontal pipes in the slope to drain out the ground water in the form of seepage or by covering the slope with shortcrete or chunam.