Two case studies of using diaphragm wall as earth retaining structure for deep excavation are reported. Case A concerns excavation down to a depth of 14.7 min alternate layers of soft silty clay and silty sand. Diaphragm walls of 70 m thick and 22 m deep were installed into a dense silty sand stratum. In Case B, diaphragm walls of 60 m thick were constructed in very soft clayey soil without penetration into any firm stratum. In both cases, instrumentations including piezometers, inclinometers, earth pressure cells, reinforcing bar transducers, heave stakes, settlement points and strut strain gauges were installed and monitored throughout the construction. Comparison of predictions based on simplified theory and empirical relationships were made with the actual performance behavior of the diaphragm wall and the subsoils. It was found that elastic mode of soil-wall system can reasonably predict the behavior of diaphragm wall. In sandy soils, arching effect has significant effect on the magnitude and distribution of earth pressure on the wall. With the assistance of instrumentation monitoring during excavation work, factor of safety against base failure as low as 1.05 was used in the design
