Many slope failures occur in the granitic rock regions during heavy rainfalls and their causes are explained from the atmospheric, geological and topographical points of view. Piping phenomenon is considered as one of the causes and many studies as to slope failure have been reported.
Several studies related to the piping phenomenon have already done. Terzaghi (1951) has developed the concept of the critical hydraulic gradient for sandy soil at which the seepage force and the submerged weight is balanced. Moreover, Zasraysky & Kassiff (1965) have paid attention to the balance mentioned above for the case of cohesive soil. Recently, Nobari, Lee and Duncan (1978) have reported the causes of the failure of Teton dam and postulated that one of the causes is hydraulic fracturing.
However, in spite of majority of studies, the mechanism of piping phenomenon has not yet been made clear. In paticular, the study of piping on the undisturbed sample of weathered granite has never been done, because of difficulty of preparation of the specimens.
The authors designed an apparatus especially for finding out the piping mechanism of the undisturbed specimen and for examining it in detail.
The samples used are the weathered granite in the region of the Ryoke granitic rocks, in Kawachinagano city, Osaka Pref. and Tuna district Hyogo Pref.. Costituent minerals identified by X-ray diffraction are quartz, oligoclase, microcline, biotite, hornblend, vermiculite and illite. According to the degrees of weathering of the weathered granite, the profile in the field can be classified into three zones, which are named U-zone, M-zone and L-zone respectively on the basis of the properties of solid phase (Nishida & Aoyama 1979). These physical properties are given in Table 1 and the characteristics of each zone are as follows; U-zone slightly keeps the original structure of the parent rock, but the soil particles are aggregated togethere because of re-compression and mixing with regolith. M-zone is a transition zone from rock to soil in structure, and is the most characteristic portion in the profile of weathered granite.
L-zone consists of weathered rock, corresponding to the grade CL by Tanaka's rock classification, and joint system becomes clear below this zone. It is difficult for the weathered granite to be obtained as the undisturbed specimen. To solve this problem, the authors devised a sampler as a piping test apparatus.
(Table in full paper)
As shown in Fig. 1, the piping test apparatus should have, at least, a free surface of specimen. The side wall of the apparatus is composed of 18 nails about 15 cm length, and of epoxy cement whose tensile strength, compressive strength and adhesive strength are 9.8 MPa, 26.5 MPa and 2.45 MPa respectively.
In the piping test, the seepage force causes the failure of soil specimen as a result of increasing water pressure. In each stage of water pressure, the excess pore water pressure, the total amount of water flowed through the specimen, deformation and the degree of muddy water were measured.
