花崗岩体表層部に発達する面状破壊構造に関する研究

Studies on the Planar Fracturing Structures Developed in the Surficial Part of Granite Mass

HASHIKAWA Kunitake

廣島大學地學研究報告

Geological report of the Hiroshima University

In this paper are dealt the planar fracturing structures in the surficial part of granites at the several localities in the Hiroshima Pref. with special reference to the significance of the sheeting joint (nearly horizontal joint) in the processes of weathering. The sheeting joint seems not only to affect the weathering process but also to give rise to the anisotropy in physical properties such as P-wave velocity and elastic moduli of the weathered granites.
Important result of this study are summarized as follows:
1. Horizontal or nearly horizontal fracture surfaces are characteristic in the weathered granites examined here, irrespective of the grade of weathering. They have about the same orientation as that of the sheeting joint in comparatively unweathered rocks which are subparallel to the land surface. This suggests that they in question are mostly derived from the fracture structure in the rocks prior to the weathering. The sheeting joints may have been formed as shear and/or extension fractures as a result of release in lithologic burden in the vertical direction by removal of rocks by erosion. That at least some of the sheeting joint represent shear fracture stems from the fact that the so-called rib-structure, wedge splitting, or conjugate set of joints are observed relating to some of the sheeting joint.
2. Another type of horizontal fracture, here termed the "lamination" in the purely descriptive sense, is frequently observed in weathered granites. Characteristic features of the lamination are that (a) it develops at an interval of the millimeter order, (b) it is nearly parallel to the sheeting joint, (c) it develops on a regional scale, (d) it is most conspicuous just prior to complete disintegration of constituent mineral grains, (e) it cuts through mineral grains such as quartz and feldspars, (f) it formes the surface most readily split off in the weathered granites. The lamination may have emerged during the weathering from the sheeting joint and the latent fracture, e.g., horizontal microfractures or incompletely developed fracture.
3. Laboratory and in situ measurements of the P-wave velocity in weathered granites of various degrees of weathering reveal an anisotropy of the granites as high as up to 50 percents with respect to the P-wave velocity and the elastic moduli. The anisotropy is charactelized by the vertical axial symmetry ; that is, the P-wave velocity in the horizontal direction is 0.4 to 4.0 km/sec, and the velocity in the vertical direction is always smaller than this by up to 50 percents. Likewise the elastic moduli in the vertical direction is 0.2 to 0.5 times lower than those in the horizontal direction.
4. The internal structures of the weathered granites can be classified into three types, each of which corresponds to the topography of the region; i.e., (a) the erosional surface with low relief, (b) the piedmont gentle slope, or (c) the slope of the V-shaped valley. The degree of weathering decreases in this order. The weathered granite under the piedmont gentle slope consists of two nearly horizontal layers with different rates of increase in the P-wave velocity with depth. The structure of the weathered granite under the erosional surface with low relief is similar to that of the above type, but the lower weathered zone is thinner in this type owing to the progressive weathering from the upper zone into the lower in the above type. The weathered granite under the erosional slope of the V-shaped vally is composed of a single mirage layer.
From the fact described above, the author has concluded that (a) differences of the weathering structure of granites reflect the history of topographic development and the development of the horizontal fractures (b) the fracture-like sheeting joint and lamination were formed primarily during various stages of offloading pressure release resulted from denudation, and have been emerged finally in conformable situation to the present topography, and (c) the character of dynamical anisotropy in the weathered granites is important on the stand point of fundmental design of civil engineering and stability of slopes.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676