柿岡盆地北部,東山地すべりにおける斜面勾配とその力学的安定について

抄録

The angle of earth-slide slope of Higashiyama Hill is investigated from the dynamic viewpoint: the earth-slide actually occurred in June, 1976. The slide surface (plane) of this earth-slide is located inside the weathered debris of gabbro (Fig. 1). The slope section indicates that this earth-slide belongs to the “slab-slide”. The average rate of the initial sliding is estimated at about 50 mm/day. Field observations performed in July to October, 1977, approximately one year after the event indicate that groundwater table continues to rise in approximately parallel with the slide surface following to the rainfall, hence the rate of slide-movement increases (Fig. 2).
Application of the Infinite Slope analysis would be reasonable for the present case. The equation given by Skempton and DeL.ory (1957, p. 309) was modified:
F_s=c'+[(γ₁Z₁+γ₂Z₂+γ₃Z₃)-mγ_wZ]cos²βtanφ'/(γ₁Z₁+γ₂Z₂+γ₃Z₃)sinβcosβ
where γ and Z denote respectively the saturated unit weight of the soil and the thickness of the soil, with suffixes 1, 2, and 3 indicating respectively volcanic ash soil, pumice, and earth-slide soil, F_S is the factor of safety, γ_w is the unit weight of the water, β is the slope angle, c' and φ' are the cohesion and the angle of shearing resistance of earth-slide soils, respectively, and m is the fraction of Z such that mZ is the vertical height of the ground-water table above the slide surface.
Drained direct shear tests were performed on the intact earth-slide soils using a 6-cm diameter shear box adapted for reversals. The tests were conducted under a strain control of 0.03_??_0.045 mm/min. Figure 3 shows the one example of stress-strain relations. The strength envelops in Fig. 4 give the following data: c'_p=0.169 kgf/cm², φ'_p=27.8° for peak strength, and c'_r=0.122 kgf/cm², φ'_r=10.6° for residual strength.
Substituting 1) these strength parameters and 2) Z₁=2.8 m, Z₂=0.5 m, Z₃=3.1 m, Z=6.4 m, γ₁=1.74 gf/cm³, γ₂=1.20 gf/cm³, and γ³=1.79 gf/cm³ into equation (1), the relationships between F_S and β were obtained under various m-values (m=0, full drained; m=1.0, water table to the slope surface) (Fig. 5). At the actual slopes, as shown in Fig. 2, the groundwater table fluctuated between the half depth of slide mass (m=0.5) and the slope surface (m=1.0).
The average angle of pre-slide slope is estimated to be about 13.9°. The factor of safety calculated using this slope angle and the peak strength and assuming m=1.0 is 1.55 (Fig. 5). Thus, the pre-slide slope might be stable under this condition. On the other hand, at the critical stable under a condition that the m-values fluctuate between 0.5 and 1.0, the calculated residual factor ranges from 0.72 to 1.01. This suggests that the soil strength had reduced nearly to the residual strength when the initial failure occurred.
The slope angle of one year after the initial slide is estimated at about 11.3°. The factor of safety calculated using this slope angle and the residual strength and assuming m=0.5 to 1.0 lies in a range of 1.24_??_0.97 (Fig. 5).