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Rainfall-induced slope failures often occur within a short period of time during or immediately after heavy rainfall. To analyze the mechanism and conditions leading to such failures, constant shear stress drained triaxial tests, which mimic the field stress path, were conducted on three sets of sandy samples obtained from sites of previous disastrous landslides. The effects of various parameters, such as initial relative density and principal stress ratio, on the temporal development of deformation were examined. Behavior after initiation of failure was significantly affected by the mode of deformation, i.e., whether compressive or dilative. Compressive soil failed rapidly due to increased pore-water pressure associated with the shearing process, while dilative soil exhibited slow failure due to loss in pore-water pressure during dilation that increased the shearing resistance of soil. Moreover, conventional triaxial drained and undrained tests were also performed to illustrate the difference in soil response under different stress paths. The results demonstrated that in analyzing the stability of slopes under rainwater infiltration, the use of strength parameters associated with failure initiation in constant shear stress drained tests may be more appropriate than those obtained from conventional triaxial tests.

In order to understand the mechanism and conditions leading to failure of sandy slopes due to the infiltration of rainwater, constant shear stress drained triaxial tests were conducted on three sets of soil samples-normal sand, gravelly sand and silty sand-taken from natural slopes where large-scale landslides have occurred in the past. Water was infiltrated from the bottom of an initially unsaturated soil specimen under constant shear stress drained condition until failure occurred. Such a loading pattern simulated the stress path followed by a soil element on a potential failure plane on a slope subjected to rainwater infiltration. The effects of various parameters reflecting the initial condition, such as relative density, principal stress ratio, degree of saturation and infiltration rate, on the development of deformation during the infiltration process were investigated. The test results confirmed that the development of pore water pressure within the soil is the main reason for the failure of slopes during heavy rainfall. The results obtained can serve as guidelines in developing warning systems against impending rainfall-induced slope failures.

An earthquake of magnitude 7.6 occurred in the south-east coast of El Salvador on January 13, 2001, causing widespread damage to buildings and several kinds of civil engineering structures due to ground shaking and earthquake-induced ground failures, including several large-scale landslides. This paper discusses the results of the damage investigation conducted in the area after the earthquake, with emphasis on the general features of the earthquake and its effects on the ground damage, specifically the landsliding which occurred in natural slopes and the liquefaction of soil deposits. The occurrence of liquefaction in the alluvial plain exhibited several characteristics in common with those observed in other areas of similar condition. On the other hand, the main cause of slope failures in volcanic deposits was not clearly identified, and several possible sources of slope instability were pointed out.

Recent studies have revealed that damage to gas pipelines buried underground is mainly caused by liquefaction of the surrounding soil mass. Therefore, if the occurrence of liquefaction in underground soil where gas pipelines are buried can be detected immediately after an earthquake, a more efficient and quick measure can be implemented to prevent the expansion of damage. With this in mind, a “liquefaction sensor” which is based on the measurement of water level within a hollow pipe inserted underground is developed, instead of the conventional pore water pressure meter which is less durable. Several experiments, ranging from laboratory boiling tests and shaking table experiments to field flow and vibration tests, were carried out to examine the adaptability of the sensor. From these tests, it was observed that the water level inside the pipe increases as the excess pore water pressure in the adjacent ground is increased, and the two variables have a nearly proportional relation to each other. Therefore, the occurrence of liquefaction can be detected by measuring the water level within the pipe of the liquefaction sensor.