This paper presents the physical properties of volcanic soil sampled at Takanodai area, Minamiaso village, Kumamoto Prefecture in JAPAN, where long-runout slope failures took place by the main shock from the 2016 Kumamoto earthquake. A series of box shear tests under various conditions such as constant stress, constant volume and cyclic shear has been carried out for undisturbed samples of pumice fall deposits to evaluate strength properties and seismic behaviors. Based on the results of Newmark displacement method, the seismic displacement of pumice fall deposits layer shows rapid increase because of the strength reduction induced by the crushability of pumice fall deposit while pumice fall deposit layer has enough stability under normal condition, which is considered to be one of the factors to cause long-runout slope failures at the above site.

The authors hypothesized that displacement on a model slope would not occur unless the initial quasi-saturated volumetric water content was exceeded. This prediction was based on a past experiment using a small model slope for the purpose of predicting slope failure beforehand. However, in previous experiments, the soil-layer thickness of the slope was as thin as 100 mm, so the behaviors of sensors installed at the upper, middle, and lower slopes were nearly the same. Also, the size of the model slope was small, and the characteristics of each point could not be grasped. Therefore, in this study, we verified whether the hypothesis obtained from past experimental results was valid even when the topsoil-layer thickness to 300mm and the model size are increased. The following findings were obtained. Displacement was confirmed to occur in the vicinity of the toe of the slope after the volumetric water content exceed the initial quasi-saturated volumetric water content. On the other hand, in the middle part of the slope, while still below the initial quasi-saturated volumetric water content, displacement was confirmed. Thus, depending on the installation position, the displacement may occur before the initial quasi-saturated volumetric water content is exceeded.

Capillary barrier (CB) is used as a rainwater infiltration control technique. And the CB effect results mainly from differences in water-holding capacity and permeability between the sand layer and the gravel layer. Water-holding capacity or CB diversion length has been evaluated by the laboratory soil experiments. But, there is no case where the diversion length that important parameter for the design of CB was evaluated by numerical simulation. In this paper, in order to study the above problem, we investigated whether the penetration phenomenon of slope CB ground can be reproduced by two-dimensional numerical simulation(HYDRUS-2D) for the observation result of the diversion length obtained by laboratory soil experiment. And, evaluated focusing on the diversion length and the bottom flow rate distribution. As a result, compared with the experimental data of the large soil tank, it is possible to show moisture dynamics considerably accurately and clarified the possibility of being applicable to the design of CB.

A series of stress-controlled undrained cyclic loading tests using a torsional simple shear apparatus are carried out on sand specimens by applying harmonic and irregular shear stresses to evaluate cumulative dissipated energies during the liquefaction process and to examine the effect of stress irregularity on the dissipated energy versus induced strain and pore-pressure buildup.　Tests with harmonic stresses show that the energy for the same soil, which is roughly proportional to effective confining stress, is almost uniquely correlated with pore-pressure buildup and induced strains during cyclic loading irrespective of the stress amplitudes and the number of loading cycles.　Irregular loading tests also yield almost unique correlations between them despite widely varied stress amplitude and the number of zero-cross N_zc.　In the irregular loading, however, the dissipated energy corresponding to larger post-liquefaction strain tends to fluctuate and also decrease with increasing N_zc due to wave irregularity, necessitating some allowance of variation for design.

In this research, in order to examine the mechanical properties of sands improved by chemical grout using acidic silica, a series of unconfined compression tests for the improved sand specimens was carried out. As a result, it was confirmed that the unconfined compression strength of the chemical grout sand depends on 50% particle size of the sand, uniformity coefficient and void ratio. Through discussing these factors on the strength of improved sand, a strength estimation equation for chemical grout soil was proposed using a soil pore size estimation method. It was revealed, however, that the strengths of improved sands estimated by the proposal method overestimated the test results for some of sands taken from fields. As it was found that fine particle content and calcium carbonate composition in sand made the strength decrease, the proposed equation was modified by taking those factors into account.

On July 21, 2009, the torrential rain resulted in debris flow at more than 480 mountain streams in Hofu City, Yamaguchi Prefecture. In the present study, the layered samples were collected using a Geo-slicer at the location of the alluvial flow generation area in Ishihara, which was one of the districts where a remarkable debris flow disasters occurred. It is located in the flooding area of the Saba river, which flows south within the central part of Hofu City. The soil layers consist primarily of river/lake deposits, and the thin layers of debris flow deposits are sandwiched. In addition, we estimated the formation age of carbide in the old and new debris flow deposit by radiocarbon dating (accelerator mass spectrometry, AMS). Debris flows that have reached the flooding deposit area have occurred at least four times after A.D.1230, including the debris flow in A.D.2009 in the Ishihara area.