Abstract Identifying the slip surface in a landslide area is one of the most important tasks for evaluating the scale of countermeasures. Conventionally, a slip surface is identified by a combination of visual observations of borehole cores and dynamic observations using pipe strain gauges in the borehole. However, during the limited investigation period, landslides can stop sliding either temporarily or for a long period after a sudden change, such as earthquake-induced landslides, for which identification of the slip surface is currently based only on visual observations of the cores. In this study, the slip surface was identified by conducting various laboratory tests on the “Tanomoki landslide,” where no apparent landslide movement was observed, using pipe strain gauges. Evidence of the slip surface was obtained by using data from an X-ray diffraction analysis, chemical analysis, physical tests, and an improved swelling pressure test based on an oven-drying method. The data properties obtained allowed us to identify an objective and a topographically consistent slip surface, thus providing an example of slip surface determination.

Since the Edo Era, debris flows have been analogously called “mountain tsunamis”, implying that strategies historically used to mitigate tsunamis in coastal regions in Japan could provide insightful points of reference for debris flow management. The recent use of secondary product blocks and terracing for sediment control has seen nationwide acceptance, including their impending implementation in sabo dams and diversion levees for volcanic sediment control. Volcanic sediment control includes measures that manage pyroclastic flows during eruptions and general methods that counter ashfall mudslides triggered by rain. Therefore, deploying these block-centric measures within volcanic regions requires their stability against external disturbances such as debris flows, and their trapping functionality must be maintained on mutable ground. In this study, we introduce novel sabo dams that combine conventional block sabo dams with breakwater structures that are typically deployed in coastal zones. We examined their stability during debris flow interception in basic experiments, and explored their flexural capacity and extent of deformation on mutable ground in analogous rudimentary experiments. We propose a sabo dam configuration that incorporates breakwater structures for maintaining stability against disturbances such as debris flows. Our results suggest the potential efficacy of the interlocking permeable block design for augmenting the flexural resilience of a block sabo dam on mutable ground. A novel phenomenon, the interlocking-beam effect, could potentially bolster displacement resistance.