Landslides commonly occur in the tertiary-mudstone zone found throughout Hokkaido's southern Sorachi district. The authors carried out measurements to determine levels of construction safety and the validity of landslide countermeasures. Measurements were performed in actual landslide conditions in the direction of soil movement using an automatic system with groundwater level gauges and pipe strain meters installed in two to four bore holes. Investigation of four landslides was carried out at measurement intervals of six hours.

The results showed that displacement in the initial stages of landslides caused by elevated groundwater levels often began at the lower part of sliding surfaces with a flat or slight inclination in most landslides. The displacement of the sliding surface then propagated to the head, and the entire landslide body moved between 12 and 24 hours the initial shift. Meanwhile, when a big earthquake occurred during measurement period, the whole landslide slid almost simultaneously and displacement subsided within a relatively short time.

These start areas of sliding displacement in slide surface were compared with the stress conditions of landslide bodies as calculated the slicing method which has been often adopted in structural design. The results indicated that displacement does not start in the domain near the heads of landslide bodies with a low partial safety factor but the start area tends to coincide with areas featuring high increment of sliding force calculated in significantly relative terms at the time of a groundwater level rise.

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