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Slope stabilization procedures for seven landslides in the Niigata Prefecture of Japan had provided an excellent and rare opportunity to examine the composition of ground water, and residual shear strength of the sliding surface soil, as well as unsheared mudstone. Ring shear tests were conducted on the soil samples collected from sliding surface soil, revealed during the installation of 3 m diameter drainage galleries. Shear strength and index properties of the soil were measured by mixing the soil with distilled water and sea water, and after leaching the NaCl from the pore water. The residual shear strength of the soil sample with distilled water was 3–5° lower than that with the sea water. The residual shear strength of the intact rock powder after leaching the dissolved salt was close to that of the soil from sliding surface, and the residual shear strength of the soil from sliding surface after mixing with sea water was close to that of the intact rock powder. The increase in residual shear strength...

Slope stabilization procedures for six landslides in the Niigata prefecture in Japan afforded an excellent opportunity to examine the residual drained shear strength of weathered mudstones. Installation of 3 m diameter drainage galleries allowed undisturbed block samples to be collected which contained the slickenside surface of the failure plane. Ring shear tests were conducted on undisturbed samples and remolded samples of the same material. In addition to the laboratory testing, back analysis was performed to calculate the residual shear strength parameters of the slopes. Good agreement was found between the residual friction angles determined from back analysis, tests on undisturbed samples, and tests on remolded specimens. Many of the landslide materials exhibited a small cohesion at low values of effective normal stress, but the secant friction angle achieved a constant value at higher normal stresses.

Mineralogical composition, measured with x-ray diffraction for soil specimens collected from more than 80 natural disaster areas, show smectite, kaolinite, mica, feldspar, and quartz as major constituent minerals. Based on the results of more than 35 mixtures of smectite, kaolinite, and quartz in different proportions, a new correlation method was proposed for estimation of the residual friction angle ( ϕr′ ) with the proportion of the minerals. The mineralogical compositions of the natural samples were used to estimate the ϕr′ based on the proposed triangular model. The estimation method resulted in values as close as 90% to the measured ϕr′ , and as close as 80% to the case of 53 samples tested by other researchers. The paper incorporates the correlation of ϕr′ with the liquid limit, the plasticity index, the clay fraction, the specific surface area, and the proportion of smectite. Various ranges are proposed, according to the influence of the constituent clay minerals. In addition, the applicable range...