Abstract
In general, it is difficult to explain the occurrence of a landslide with stability analysis based on routine tests on an overconsolidated clay like the Shimajiri mudstone, and a problem always arises in selecting the kind of strength parameters to be used.
This report deals with the relationship between the occurrence of landslide at Maaji Housing Area and the characteristics of residual and peak strength, slaking, geology and so on, and also considers the determination of average shear strength parameters required to compute the stabilization of unstable slopes and design of cuttings both in the Shimajiri mudstone.
Results obtained are as follows:
1. The landslide mentioned above is a compound slide where the slip surface passes through the foundation rock and the fill.
2. Geologically, the foundation rock consists of a mudstone in the Shimajiri group which belongs to the Neocene period, and the rock is widely distributed in the central and southern part of Okinawa-jima. The Shimajiri group has the tectonic discontinuities such as minor fault, bedding plane, joint and hair crack exist, and their main strikes are almost in accord with a direction of landslide movement. Such geological conditions mentioned above are closely related to the occurrence of this landslide.
3. Due to the physicochemical and strength characteristics of the mudstone, the cutting plane and natural slope are weathered and softened as time goes, and many fissures are developed and become weak planes in the mudstone.
4. The parameters of residual strengths obtained by ring shear test are: cdr=0 and φdr=10.7° in the mudstone sampled close to the slip surface, cdr=0 and φdr=15.1° in the mudstone from scarp, and cdr=0.075kg/cm2 and φdr=19.2° in the slurried mudstone.
5. The parameters of peak strengths obtained by triaxial compression test are: cf′=2.779kg/cm2 and φf′=43.2° in the mudstone sampled close to the slip surface.
6. The brittleness index of the mudstone IB is about 90% for a normal effective stress of 2.0kg/cm2 and it is larger than that of the Blue London Clay.
7. The parameters of average shear strength acting along the slip surface at the failure, c′ and φ′, can be determined by introducing the residual factor R and the failure criteria of Mohr-Coulomb to the c′-φ′ relationship obtained by back analysis. The parameters in case of the present landslide are: c′=2.17t/m2 and φ′=13.9°.
8. R is larger, being above 0.90. This fact indicates that the average shear strength in the landslide is close to the residual shear strength as in other landslides occurred in the Shimajiri mudstone.
