Soft rocks, especially smectite bearing tuffs and mud stones, sometimes cause the serious troubles such as ground pressure, landslide and crumbling of cut slope.
To study preventive methods of the surface deterioration for smectite bearing soft rocks, experiments in laboratory were carried out. In these experiments, four kinds of preventive methods such as a soak method, a wet curing by covering with mat, an ion exchange method and a seal method have been attemped to six samples of soft rocks. In result, it has become clear to be closely related with exchangeable cation composition of smectite and effect of preventive method. All of the methods are effective to Ca-type smectite bearing soft rocks. On the other hand, only the seal method is applicable to Na-type one.
Field test by the seal method was carried out in tunnel consisted of Na-type smectite bearing tuff. The preventive affect using polymer cement or high molecular resin has been ascertained to extend from three to six months. This result indicates that the prevention of deterioration by the seal method is applicable in the case of short term to foundation rock at dam and cut slope etc.

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The area in the Tohoku district where Neogene mudstone is distributed is well known as a typical place with high frequency of rock-slides.
The study of the overfolded structure and the anomalous sedimentary structure of the mudstone in the westernmost part of the Ou mountains has revealed that they have generated a gravity folded structure.
This means that the rise of the sediment on the sea bottom and volcanic activities owing to the crustal uplift during the Miocene and the early Quarternary brought about the Ou mountains, and it can be considered to be a geological factor in the recent landslides. In the area in question rock-slides subsequently took place due to the climate, topography, and volcanic activities during the Pleistocene and the early Alluvium.
These facts show that the recent landslides have a close connection with the ancient landslides during the Miocene.

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Mass movement process of a valley-head slope is studied at a site where Kanto-loam partly covers basal granite, by means of the investigations of microtopography, geologicalmicro-structure, vegitation, and tilt-fluctuation.
It is found that loam layer of the valley-head slope creeps with every rainfalls and increases the topographical instability at the lower end of valley-head slope. Consequently, slope failure on this slope is thought to occur at the lower end of valley-head slope periodically.
The above mass movement process is different from the past investigated case at other valley-head slope where soil-pipe has developed at the lower end of slope. The difference of the process is probably due to the difference of the position of ground water table in slope, namely creep dominant slope has a lower ground water table and soil-pipe dominant slope has a higher one.

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Recently, in Japan, not a few dam sites are composed of soft and weak rocks or sand and gravel deposits. Since these dam foundations are low in strength, it is required to evaluate seepage failure potential in the designing stage.
There are two types of seepage failure tests (SFT), that is, laboratory SFT for undisturbed specimens and in-situ SFT. Although in-situ test is more expensive than laboratory one, heterogeneity and existence of very weak part of foundation can be taken into account with in-situ test. Therefore, in-situ test can give us more realistic results than laboratory one. Conventional in-situ SFT is the test by use of the water injection from a single borehole (single-hole injection type in-situ SFT). However, because the water flow from a borehole is not onedimensionalbut radial, large potential loss takes place around a borehole. This seepage is much different from the under seepage in dam foundation, and the evaluation of critical hydraulic gradient is very difficult.
In this paper, we have explained this problem about single-hole injection type in-situ SFT quantiatively by using a mathematical model. In addition, we have developed new in-situ SFT by use of the water injection from plural boreholes drilled on the line parallel to the open side plane (multi-hole injection type in-situ SFT) in order to establish approximate one-dimensional flow condition. We have shown the advantage of multi-hole injection type in-situ SFT by use of a mathematical model, and reported results of case study of this test.

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