Equations for estimating the vertical bearing capacity of logs in sandy and clayey soft ground was examined using the results of 29 vertical loading tests at 6 sites. The experiments were conducted using logs with a bottom diameter of 0.110–0.190 m and length of 2.0–12.0 m. In addition, screw weight sounding tests, standard penetration tests, unconfined compression tests, and electric cone penetration tests were conducted and the vertical bearing capacity of the logs was estimated using data from these soil investigation methods. The logs were pushed into the ground so that their tops were 0.5–1.5 m below the ground surface to avoid wood decay and insect damage. As a result, it was found that vertical bearing capacity of logs can be estimated by multiplying a previously proposed equation by ξ depending on the soil investigation method, and based on this, estimating equations for the design was proposed using the lower limit values of the variation of ξ.

In-situ rock shear tests using large rock blocks are commonly used to investigate the strength characteristics of rock mass. However, the initial stress and boundary conditions are unknown; therefore, several difficulties in the interpretation of test results are encountered, among other problems. In recent years, with the advancements in aseismic design in the evaluation of the foundation rocks and surrounding slopes of nuclear power plants, dynamic nonlinear analysis methods that account for the destruction of rocks are being developed. However, there are few achievements for the natural rock mass, and there is a lack of data for verification. Therefore, in this study, we developed a laboratory cyclic box shear test method using large rock blocks. This method can be utilized to acquire verification data for dynamic nonlinear analysis and resolve the problems with the in-situ rock shear test. In addition, the applicability of the test method developed herein was verified by comparing the results obtained through the method with the results of laboratory element tests using the same rock blocks.

An 8-m-long core was taken from the seafloor from shallow gas hydrate province in the off San’in Region. Based on the observation of the cores, the muds were divided into three categories - carbonate nodules, laminated muds, and massive muds. The core samples were subjected to physical property, cone penetration, vane shear, and undrained triaxial compression tests. The void ratio of the massive mud decreased with an increase in depth, whereas that of the other lithofacies did not. Only muds containing carbonate nodules contained coarse fraction, and the activity was large in all lithologies. Cone penetration resistance and vane shear strength increased with depth only the massive muds. The difference in cohesion due to lithology was small, c’=3.1 kN/m² or less, c_cu=5.0 kN/m² or less.

The suffusion phenomenon that occurs in embankment structures, such as river levees and reservoir dams, may reduce the stability of the entire structures and can be a factor of aging. However, the transfer mechanism of soil particles through the gaps is unclear, and how suffusion progresses has not been sufficiently clarified. In this study, a one-dimensional water-passing experiment was performed, using a column device that enables the outflow of particles, to examine the time change of the particle size composition of discharged soil particles during the process of suffusion. The relationship between the amount and the size of the discharged soil particles was also examined. As a result, by inferring the particle size composition of the discharged soil particles from the relationship between the concentration and the turbidity of the drainage, it was clarified that the particle size composition of the discharged soil particles changed during the process of suffusion. Moreover, it was confirmed that, as more soil particles were discharged under highly saturated conditions, larger particles were discharged.

Cation exchange capacity (CEC) and amounts of leached cations (LC) of bentonite were used to predict the water permeability and swelling property in researches related to bentonite buffer for radioactive waste disposal project. The purpose of this study is to examine various methods of measuring CEC and LC of bentonites and to propose proper method depending on bentonite types. Three methods using respectively ammonium acetate, barium chloride and ben-zyltrimethylammonium chloride (BTM) to obtain CEC or LC or both were examined. The following conclusions are obtained: i) CEC obtained by the improved method using barium chloride is generally in agreement with by the method using ammonium acetate; ii) Summation of LC measured by the method using BTM generally agrees with CEC meas-ured by the method using ammonium acetate; iii) Method using BTM can reduce the effect of calcium carbonate dis-solution, which is often a problem for other two methods; iv) Summation of LC measured by the method using BTM may be regarded as CEC.

Sedimentary soft rocks such as mudstone weather over time when exposed to the surface by cut excavation. And the cut slopes that have been excavated for a long time often caused surface collapse. However, there have been no cases of observing the weathering process of sedimentary soft rocks for decades, and prediction of weathering, stability of cut slopes, and rational measures are major issues. In this study, we conducted field surveys such as elastic wave exploration, soil tests, and X-ray diffraction analysis for about 50 years after excavation of significantly weathered mudstone cut slopes. Then, the weathering mechanism of the slope and its effect on stability were examined. As a result, it was confirmed that the strength of the slope surface layer decreased due to the effects of stress release and repeated dry and wet conditions due to unloading immediately after excavation. On the other hand, chemical weathering such as oxidation and dissolution of clay minerals that compose mudstone is still in progress, and it is expected that weathering will continue in the future.

In the cement stabilization method, laboratory tests are conducted to determine the type and adding amount of stabilizer. In this study, unconfined compression tests and thermo-gravimetric analysis were performed on cement-treated clay cured at various temperatures in order to develop a testing method that can estimate the strength of cement-treated soil of target age in a short term. As a result, it was observed that the accelerating effects on the strength development of cement-treated clay can be evaluated accurately by using the equivalent age based on the Arrhenius law. In addition, the progress of the hydration reaction was similar when the equivalent age was the same, regardless of the curing temperature. When the curing temperature was higher than 55°C, the acceleration effects of strength development and hydration reaction became smaller. This indicates that there is an upper limit to the temperature at which the acceleration effects can be maintained according to the Arrhenius law.

Several types of true triaxial test apparatus have been used for real materials with a membrane boundary (flexible boundary). However, the friction at the end face of the specimen have influence on the true triaxial stress state. In this study, a true triaxial test with a membrane boundary was reproduced without end-face friction using the distinct element method, and the true triaxial tests with constant Lode angle stress paths were simulated. The obtained stress-strain relationship of specimens is similar to that obtained under the wall-element boundary (rigid boundary) condition. This indicates that the difference in boundary conditions does not significantly affect the macroscopic behavior. On the other hand, the micromechanical properties for the inside of the specimen indicate that the membrane boundary contributes to the uniformity of the boundary force distribution, the void ratio distribution near the boundary, and the uniformity of the void ratio inside the specimen. These results suggest that the present analytical conditions reproduce a more ideal true triaxial test. Furthermore, from the examination of three different failure criteria, it was found that the Matsuoka-Nakai’s criterion could be the most suitable for the critical state under an intermediate magnitude of confining pressure used in this study.

The relationship between the initial water content of clay and the mixing quality of cement treated marine clay was investigated by a series of laboratory experiments. In the range where the initial water content w₀ of clay is lower than the liquid limit w_L of the clay, the strength of cement treated clay showed the peak strength at w₀ = 0.925w_L. At w₀=0.85 w_L, the strength was smaller than that at w₀=0.925 w_L, while at w₀=0.775 w_L, it was unstable with respect to the strength of 0.925w_L, depending on the conditions, such as large or small. Using the strength estimation formula obtained from the strengths of the initial water content ratios of 1.0 w_L and 1.2 w_L, the strength of the cement treated clay decreased significantly as w₀ became smaller than wL. It is probable that the strength was not sufficiently developed in the sample with a small initial water content because the mixing quality was low. The reason for the deterioration of the mixing quality may be that, along with the decreased fluidity, a part of the sample and the cement slurry were mixed at a high concentration and strongly adhered to the blade to prevent uniform mixing.

In the previous paper (Kokusho, 2019), nonlinear numerical analyses were conducted wherein the Newmark-type slope model was shaken underneath by propagating SH wave, where wave energy as the difference between upward and downward SH wave was confirmed to contribute to slope sliding together with other associated energies. Residual slope displacements δr were uniquely evaluated from the wave energy despite the difference in earthquake waves, indicating that δr can be readily obtained with no need of acceleration time-histories. In this research note, in order to minimize the numerical errors involved, the slope model has been improved so that the numerical analysis can be conducted with high accuracy up to large slope displacements. The analytical results have been summarized in a normalized unique function for widely varying pertinent parameters for slope failures. A design chart previously developed to calculate earthquake-induced slope displacements has been partially revised accordingly though mostly unchanged.