| Research Abstract |
To clarify the generation mechanism of shallow landslides, I studied the weathering profiles, their formative mechanisms, and the relationships between the profiles and landslide generation for granitic rocks, ignimbrite, and sedimentary rocks. The major results are as followings :
1) As major weathering processes of granitic rocks, micro-sheeting and spheroidal weathering were recognized. Granitic rocks, which are micro-sheeted by unloading, are loosened intensively to a depth of a few meters with a well defined front. This loosened zone increases its thickness and finally slides. Spheroical weathering proceeds by micro-cracking and chemical leaching. Moderately weathered granite, which are made through geological time, is loosened rapidly to a depth of a few meters when it is exposed by artificial cutting or natural denudation ; rainwater infiltrates through this loosened zone preferentially. The saturation, generation of pore pressure, and the decrease of cohesion by the loss of suct
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ion of this loosened material become the basic factors for the generation of landslide.
2) Ignimbrite varies from unwelded to strongly welded tuff. Unwelded ignimbrite is weathered to have a well-defined front, which acts as a capillary barrier between the fresh permeable tuff and the overlying less permeable weathered tuff. Infiltrating rainwater increases the weight of the weathered tuff and decreases its cohesion because of suction disappearance. These are the basic factors of the landslide of unwelded ignimbrite.
3) Vapor-phase crystallized ignimbrite has a well defined weathering front between the fresh or hydrated impermeable tuff and the overlying strongly weathered, permeable tuff. The weathered tuff on the front is exfoliated parallel to slope surface by the intensive chmical leaching. The saturation, generation of pore pressure, and the decrease of cohesion by the loss of suction of this weathered tuff become the basic factors for the generation of landslide.
4) Sedimentary rocks generally have oxidation and dissolution fronts, which are made by the sequential reaction between the rocks and the infiltrating water. This weathering process and the resultant change of mechanical properties are the major causes of the common type of landslide of sedimentary rocks, which have slip surfaces along or below the oxidation front. Less
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