Mineralogical Study of "Masa" with Special Reference to the Effect of Compaction

Weathered granitic rocks called "Masa" are commonly developed in the Chugoku district, southwest Japan, and for the construction of a dam, Masa has been used as the core materials of rock fill type dams. This paper deals with quantitative examination of the degree of weathering, compaction and pre-crushing effects and permeability of natural state of Masa at five locations in the district. Special attentions were paid on mineralogical studies to elucidate the effects of pre-crushing and pre-compaction as well as on the interrelations of soil constants. Among the engineering soil constants of Masa, natural moisture content and content percentage of fine grains less than 74 µm (P-74) have been confirmed as effective barometers for the degree of weathering, i.e., at each investigated area, the value of P-74 varies characteristicaly according to the weathering degree of Masa. The compaction properties of natural state of Masa depend also on weathering degree and are proved to have intimate relation with maximum dry density (ρdmax) and optimum moisture content (Wopt), i.e., the former becomes smaller and the latter greater as weathering proceed. On the other hand, fine grained Masa subjected pre-crushing have higher ρdmax and lower Wopt compared with those of natural state. This means that physical and/or mineralogical properties of soil grains of Masa have changed to those with resistant against high compaction energy.
Existence of maximum value of dry density (ρdmax) was confirmed with increasing pre-crushing times. That is, the compaction density conversely lowers when the pre-crushing energy are excessive. This is probably caused by the fact that the specific surface of fractured quartz and K-feldspar increase due to crushing resulting relative decrease of matrix with which the interstices of soil grains are usually filled. Furthermore, arrangement of voids in the compacted Masa exhibits no particular orientations. This property surely increases the safety of dam concerming permeability, since the compacted Masa shows no weak direction. Finally, practical usefulness of Masa as core materials were discussed based on the results obtained.