It is widely accepted that the majority of post-wildfire debris flows are triggered by surface runoff and sediment bulking in response to short duration, high intensity rainfall events. Infiltration-triggered debris flows that result from discrete bodies of soil dislodging from the slope are less frequent, and tend to appear after longer periods of time or with the vegetation re-growth. Understanding the processes that control the transition, from runoff to infiltration-initiated, will dictate the approach for hazard assessment: rainfall intensity-duration relations for runoff-initiated and slope stability for the infiltration-initiated. Here, through a series of physical experiments we document the role of soil particle wettability in this transition. We found that the two extremes, fully wettable and water repellent gave opposite responses, slumps for infiltration-initiated in wettable sand and erosion by surface runoff in water repellent sand. From the tests, a continuous capping effect generated by strong water repellency was a necessary condition for erosion and sand bulking to occur i.e. the generation of runoff-initiated debris flows.

View full abstract

The immense destructive impact induced by debris flow in mountainous region endangers human lives and infrastructure facilities. The previous studies have indicated that the fatalities in a debris-flow event depend on the volume of debris flow, which may be conspicuously amplified by entraining bed-sediment along the trajectory as it descending the slope. Several lines of evidence by the previous studies have also highlighted this viewpoint that debris flow can grow dramatically in magnitude as accompanying by the entrainment. In this paper, we present an elementary model to estimate the dynamic entrainment rate of bed-sediment when overridden by debris flow. Following the preliminary studies done by Iverson (2012) and Medina et al. (2008), a temporal entrainment rate can be computed basing on the momentum conservation of the two-layer system. To represent the influence of the pore water pressure on the entrainment rate, we employ two parameters λ1 and λ2 into the model, which denote the saturate degree of debris flow and bed-sediment, respectively. Our approach demonstrates that if flow layer and bed layer shares the same friction angle, the condition λ2>λ1 should be satisfied to provoke the entrainment. The performance of the approach is prior tested on a simple scenario with the parameters of typical value, and then a debris-flow event that occurred in 2010 at the Yohutagawa torrent, Japan. The computed entrainment rate and accumulated depth show a good agreement with the in-situ surveys. Another advantage of our approach rests on the fact that entrainment rate is written in a differential form, thus it can be easily incorporated to the mass constitutive equation of the numerical model using a shallow water approximation.

View full abstract

In spring 1994 a landslide occurred on a natural slope of about 25 m height just below a ski-lift station in the Austrian Alps near Lech. The slid soil mass blocked the road at the toe of slope. The upper part of the sliding surface reached the foundation of the lift station on top of slope destroying the earth platform in front of the station and endangering the entire building. A quick solution for the slope reconstruction and stabilization had to be developed and executed before the beginning of the next ski season in September. The solution had to meet a wide range of requirements, some of them caused by the specific position in the high mountains. Finally, a geogrid-reinforced full-height slope was designed and constructed reusing the local soils and reconstructing approximately the former natural slope shape before the failure. Note, that at that time the experience with such structures was still modest, some decisions were a bit risky due to the lack of knowledge and were based on engineering judgement. The system was successfully built in less than two months and is still stable after twenty years of service, say twenty season cycles winter-summer in the Alps. The problems, boundary conditions, philosophy and design from 1994 are described together with the unknown factors and specific solutions, and the construction technology and experience as well. The current state is shortly described and commented.

View full abstract

In the past, horizontal semi-infinite soil grounds were utilized to analyze the response of the stabilizing piles due to lateral soil movement or lateral force loading. In this study, a limit equilibrium method analyzing the lateral force on stabilizing piles that embedded in semi-infinite slopes is presented. In addition, the soil arching effects between two neighboring stabilizing piles are analyzed, and the lateral active stress in the rear of the piles is obtained. Furthermore, the squeezing effect between the two piles proposed by Ito and Matsui is combined with the lateral active stress in the slope to evaluate the distribution of the lateral force of the stabilizing piles in sandy slopes. A numerical simulation using FLAC3D is utilized to validate the proposed approach. The simulation shows that the proposed model could reasonably predict the lateral force acting on the stabilizing piles embedded in the slope.

View full abstract

A landslide-dam hazard mapping method is proposed so as to provide the necessary information to countermeasures against earthquake induced landslide disaster chain. The procedures of the method are introduced together with newly developed slope identification technique and new slope stability analysis GIS module. A practical application is presented to show the practical usefulness of the hazard mapping system.

View full abstract

Expressway embankments have collapsed in many locations in Japan recently because of torrential rains and earthquakes striking the areas. Because the soil from collapsed embankments contains much water, we have assumed that penetrating rain and groundwater affected the embankments, in some way, and led to the collapse. To reinforce the slopes and prevent such disasters, we have dug slits where are excavated at the foot of the slope and filled with crushed stones to reduce the water content ratio of the slope. To verify the effectiveness of this measure, we have conducted the following two tests. First, we conducted laboratory tests, changing the water content ratio, to examine the changes in the strength characteristics of the embankment soil. This test confirmed that soil strength weakens as the water content increases, even if compaction of the soil remains the same. For the second test, we made a 1/50-scale model embankment and carried out large-scale dynamic centrifugal force loading tests to verify the reinforcement effects of this measure. The test confirmed that embankments reinforced with the drainage works held down deformation to half of that of embankments which were not reinforced.

View full abstract