Research results

Deep-rapid landslide Mechanism

Figure 1. Occurrence Mechanism of Deep-rapid Landslides Caused by Rainfall

Figure 2. Numbers of Deep-rapid Landslides Caused by Torrential Rain or Snow Melt in Japan since 1868. (after Uchida et. al (2007))

Figure 3. Relationship of the rainfall amount which falls from the beginning of rainfall until a deep-rapid landslide occurs with the rainfall intensity at the time of the deep-rapid landslide occurrence
(after Tamura et al. (2009))

In Web Magazine No. 14, we introduced a method of extracting small catchments where deep-rapid landslides are likely. Here we introduce basic issues about deep-rapid landslides.

What is a deep-rapid landslide?
As introduced in Web Magazine No. 14, landslide phenomena (slope failure) are roughly divided into two types: shallow landslide, which is landsliding of the surface soil layer alone, and deep (-rapid) landslide, which is simultaneous landsliding of the surface soil layer and its underlying weathered bedrock. Generally shallow landslides are between 0.5m to 2.0m in depth, but deep landslides are large scale slides and the depths of deep landslides are several tens of meters.

Why does rainfall cause deep-rapid landslides?
Deep-rapid landslides are occasionally triggered by torrential rainfall. If torrential rain falls on a slope consisting of weathered/fractured bedrock, the rainwater percolates deeply into the ground through the cracks in the bedrock. Then, groundwater is concentrated in the bedrock, reducing the strength which supports the bedrock against force acting to cause it to slide. Consequently, a deep-rapid landslide occurs. It can be presumed that cracks in bedrock form over long periods of time, as the bedrock is deformed by the impacts of gravity or groundwater (this is usually referred to as, “mass-rock creep”).

Are deep-rapid landslides increasing?
Deep-rapid landslides are definitely not a new phenomenon. It was reported that in 1889 for example, in Totsukawa Village in Nara Prefecture, more than 20 deep-rapid landslides with collapsed sediment volume exceeding 1 million m3 (approximately equal to the volume of the Tokyo Dome) occurred.
Recently, many experts have pointed out “Deep-rapid landslides have become conspicuous in recent years”. It is certainly true that during the past 10 years, deep-rapid landslides have been triggered by torrential rain or snow melt almost every year (Fig. 2). And assuming that the probability of deep-rapid landslides increases with an increase of rainfall magnitude, it can be presumed that the number of deep rapid landslides occurring will increase with the increase of rainfall magnitude as a result of climate change. However, further researches must be performed to answer the following questions: “Are deep-rapid landslides tending to increase in the long term (ca. 100yr.)?” and “To what degree is the increase in the rainfall magnitude increasing deep-rapid landslides?”

What degree of rainfall causes deep-rapid landslides?
Research to discover what degree of rainfall causes deep-rapid landslides is now in progress. Figure 3 shows the relationship of the rainfall amount which falls from the beginning of rainfall until a deep-rapid landslide occurs with the rainfall intensity at the time of the deep-rapid landslide occurrence. We mainly compiled cases of deep-rapid landslides which have occurred in recent years. Since cases are still few and most cases occurred in Kyushu and Shikoku, we cannot provide clear conclusions, but the figure shows that these deep-rapid landslides occurred after a total of 400mm of rain had fallen since the beginning of the rainfall. This figure also indicates that some of deep-rapid landslides occurred at the time when the rainfall intensity was nearly zero, revealing that there are also deep-rapid landslides which have occurred after the rain had almost stopped.

(Contact: Volcano and Debris Flow Research Team)

Durability Improvement of Concrete Structures
- Technologies to improve the durability of concrete by using admixtures -

Bridge with scaling deterioration

Scaling control effect (left: early-strength cement, right: high early-strength cement + ground granulated fine blast furnace slag)

Evaluation of chloride osmotic resistance

Test construction of a product (dish-type side ditch)

In concrete structures in cold, snowy regions, deterioration particular to such regions (frost damage), such as surface scaling and cracking, may occur in winter due to repeated freezing and thawing of water in concrete. In regions near the coast and where surface deicers are spread, frost deterioration is further accelerated by the influence of salt. Therefore, the service lives of concrete structures tend to be shorter than those in warm areas, and technologies to improve durability of concrete in such severe environments are especially required.
To improve the durability of concrete, it is important to use high-quality materials (water, cement, fine and coarse aggregate), determine an appropriate mix ratio of these materials and secure an appropriate content of entrained air. In particular, cement is the most important material that determines the quality of concrete, and it is possible to accelerate strength development or reduce the heat generated at the time of setting (heat of hydration) by changing the type of cement. It is also possible to improve long-term strength and durability by replacing part of the cement with blast furnace slag (impurities in minerals generated when refining iron by reducing/melting with coke), fly ash (a part of coal ash generated in boilers of thermal power plants) or other materials. These are known as mineral admixtures and have chemical components similar to those of cement. They generate hydrates when mixed with cement and densify the pore structure of the concrete.
Against the above backdrop, our team has been studying measures to improve durability and other properties of concrete by combining a variety of types and proportions of cement and admixtures. Past studies have revealed that scaling deterioration and the penetration rate of salt (effective diffusion coefficient of chloride ion) that causes corrosion of reinforcement in concrete can be controlled by combining admixtures appropriately, compared with conventional concrete. Further studies for practical use are currently under way with the production of concrete products using such cement, and test construction in actual environments.

(Contact: Material Research Team, CERI)