Abstract
It is often troublesome to tunnel through the areas occupied by serpentinite which is usually dealt with as fragile earth for civil engineering. This paper presents the physioal and chemical properties of serpentinite in Himekawa, Niigata Prefecture from the standpoint of engineering geology. Many tunnels penetrating serpentinite masses in other parts of Japan are also referred.
Serpentinite in Himekawa is divided into two bodies, the Asaoyama and Hiraiwa. The latter shows petrologically more marginal facies than the former and both include chlorite minerals. The degree of alteration of serpentinite is fairly consistent with an index (Fc′) which is designated by the average ratio of hardportion within every ten meters interval of drill core. Chemical analyses have shown that aluminum oxide (Al2O3) and calcium oxide (CaO) tend to increase as the alteration of serpentinite proceeds. Talc and carbonate minerals also tend to increase in that course. Determined physical properties are as follows; ranging from 2.6 to 2.8 for apparent density, less than 3% for water absorption rate, and less than 5cm3 for swelling degree. The primary ultrasonic wave velocity (Vpc) for drill core has the following relationship with the secondary one (Vsc) :
Vpc=2.075Vsc-0.259
Serpentinite itself does not have any expanding nature but when it undergoes serpentinization along fractures, talc is readily formed followed by foliation. Eventually upon clay mineralization, serpentinite mass has a tendency to swell and slide due to the earth pressure and the leaching of ground water.
