The geotechnical investigation was carried out at 4 valleys, which are designated as landslide risky valleys in heavy rainfall time by Hiroshima Prefectural Government. Using the models of valleys, the rainfall seepage analysis, the calculation of ground water level and the slope stability analysis were carried out, making the risk assessments of valleys in heavy rainfall time. The practical geotechnical investigation method using lightweight dynamic cone penetration test was also proposed, with which the geotechnical investigation of one valley can be completed by 2 personals in a day. By the laboratory rainfall seepage test and the one-dimensional analysis, it is found that the volumetric water content and degree of saturation and the descent velocity of high water content band (HWCB) is determined by the rainfall intensity. The method for calculating the groundwater level after HWCB reaches the base layer is newly obtained. Using the findings, the risk assessments of 4 valleys were made by comparing the safety factors calculated for continuous rainfalls with several intensities.
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