Field and laboratory investigations as well as slope hydrodynamic simulations were carried out to observe the mechanism of landslide at Puncak Area in West Java, Indonesia. Monitoring on water level fluctuations in piezometers were conducted to calibrate the simulation results. It was found that the inducement of landslide was started from the lower part of middle slope. The seepage from upper slope as well as back-up migration of saturation front in response to rainfall played an important role in triggering the slope failure. Indeed, the largest increment of pore water pressure occurred in blue clay under the lower part of middle slope in which the failure surface developed.
Landslide occurred at Puncak area in West Java, Indonesia (Figure 1) in February 1984 after the heavy rainfall and destroyed a highway section as well as several houses. Such landslide was located at the lower part of Mount Megamendung (Figure 2) and resulted in the translational movement of colluvial deposit (clay - silt layer), and sand layer (Figure Ib). These deposits were underlain by blue clay which was investigated as the failure surface (Sugalang, 1989). Such clay was montmorillonite and interpreted as the lake deposit, originated from tuffaceous sediments (Karnawati 1999).
According to field observation as well as the evaluation on geological data and rainfall record, the interaction of rain precipitation, morphology, geology, hydrogeology, and existing landuse conditions were considered as factors encouraging landslide. The landslide occurred on a gentle slope which was only about 15 0 (Figure 5), when the monthly precipitation was high (exceeding 770 mm as illustrated in Figure 3). This gentle slope should be stable, however it was located on the foot of mountains which was surrounded by higher mountainous and hilly morphology (Figure 2).
