Bentonite-based engineered barrier materials transfer from unsaturated state as emplaced to saturated state after closure of deep geological disposal facilities for radioactive waste, while decrease in saturation could occur due to heat and gas generation from the wastes. Modelling for mechanical characteristics and water retention properties of unsaturated “bentonite-based engineered barrier materials” are indispensable to predict such important transient behavior. In this study the following characteristics are obtained based on controlled-suction oedometer or triaxial compression tests : i) swelling and shrinking caused by respectively wetting and drying, ii) consolidation under constant matric suctions, iii) shear strength depending on matric suction, iv) hysteresis of water retention. In addition, applicability of existing constitutive models such as Barcelona basic model, Komine’s model, vG model, and suction stress concept are discussed based on these results.
It has been recognized that gas hydrate-bearing sediments clearly exhibit time-dependent behavior such as long-term consolidation, strain rate dependency, and creep deformation due to its viscoplasticity of gas hydrates. The main objective of the present study is to understand the time-dependent behavior of gas-hydrate-bearing sediments by a series of experiment and to establish a constitutive equation that can describe the time-dependent behavior of gas-hydrate-bearing soils. First, a series of undrained triaxial compression tests with step-changed strain rate is conducted on synthetic CO2-hydrate-bearing sand specimens in order to investigate the strain rate dependency of the hydrate-bearing soils. Second, the elasto-viscoplastic constitutive equation has been developed based on the obtained experimental results. The proposed model is then applied to the experimental results to find that the model can reproduce the mechanical behavior of the CO2-hydrate-bearing specimens. The main findings obtained from the present study are as follows: First, the unique stress-strain-strain rate relations, namely, Isotaches, is observed for both the CO2-hydrate-bearing sand specimens and the sand sample without hydrates. In addition, the CO2-hydrate-bearing specimens clearly show the strain rate dependency, and it increases with increase in the hydrate saturation. Second, from the experimental results, it is found that dependency of hydrate saturation on both the strength and the stiffness does not appear in the case where the strain rate is quite slow. This is because the strength of the CDH-bearing sediments may be determined by the balance of the hardening effect and viscoplasticity of the hydrates. In the case of the slow strain rate, the viscoplasticity is more dominant on the mechanical behavior of the CDH-bearing sand than the hardening effect. The proposed constitutive model can well express those mechanical responses of the CO2-hydrate-bearing specimens by introducing the effect of the hydrate saturation into the viscoplastic parameters.
Ground anchors (hereinafter anchors) are structures that apply a tensioning force to a tendon to improve the stability of a slanting surface or slope. The change in the tensioning force depends on the amount of stretching of the free length of the tendon. One way of measuring tensile load of anchor is using a load cell. The measurement results in this case can be used to detect ground movement, similar to those of the displacement observation techniques commonly used for slopes or other surfaces. However, the relationship between displacement observation and load change in a load cell has not yet been sufficiently evaluated.
In this study, we evaluated the relationships between various displacement observations and load cell changes on four slopes where ground movement has caused an increase in the anchor load. We found a robust linear relationship between the changes in displacement observation and the amount of tendon elongation due to the increased load. Load cell installed on ground anchor has been confirmed possibility which can be used as displacement sensor.
In the vacuum consolidation method, the pressure under the air-tight sheet increases according to ground surface settlement. An air-water separation system for the vacuum consolidation method (VCM) was developed to improve this issue and enhance the vacuum effect. Although it has been demonstrated up to today that the system can obtain a higher vacuum pressure, the suppression effect of the new type VCM against deformation has never been discussed. In this study, therefore, mainly to reveal the suppression effect of the new method against deformation, several numerical simulations were conducted by using a soil-water coupled analysis code, GEOASIA, which mounts the macro-element method with discharge and drainage functions of vertical drains. The main findings of this study are as follows: 1) The air-water separation system has a high efficiency against deformation. Especially, the method remarkably restricts settlement that occurs after stopping vacuum pumps because it has a high preloading effect. 2) The conventional VCM also restricts deformations including a lateral deformation even if large settlement occurs. This is because the vacuum loading effect is maintained even though the pressure under the airtight sheet increases according to ground surface settlement. 3) Short-term constructions that have never been attempted can be performed in both the type VCMs. The above discussions demonstrated that the numerical simulation method used in this study allows the selection of effective countermeasures and economical construction conditions.
It is required to reinforce leaning type retaining wall so as to avoid catastrophic failure of the wall because huge numbers of leaning type retaining wall were constructed as railway retaining structure. In this study, series of shaking table model test and associated analyses were carried out so as to develop an effective aseismic countermeasure and its design methodology. Preliminary literature surveys clarified that overturning failure of the wall could be extracted as a catastrophic failure mode of the leaning type retaining wall. Series of shaking table model tests revealed that soil reinforcement at the top of the wall could effectively restrict overturning of the wall. Based on observation on the location of failure plane developed in the backfill and the amplitude of the dynamic earth pressure, a procedure to evaluate the effect of reinforcement was also proposed. It was also found out from the case study on the damaged leaning type retaining wall in the 1995 Hyogoken Nanbu earthquake that installation of the reinforcement at the top of the wall could be effective tools to reinforce existing leaning type retaining wall.
Seismic response analysis of rock slopes for deformation estimation has been studied intensively in recent years, and Discontinuous Deformation Analysis (DDA) is one of the feasible methods in this field. However, conventional DDA has difficulty in setting appropriate analysis parameters which can predict sliding between the blocks precisely. In this study, to enhance the accuracy and the easiness of parameter setting of DDA, the integration method of friction law is focused. The implicit updating scheme of friction force (return mapping algorithm) and Newton-Raphson iteration is newly introduced to DDA, and the effectiveness of the proposed method is confirmed through fundamental numerical examples.
Previous researchers have proposed a simplified method for the design of seismic underground structures (such as cut-and-cover tunnels and utility ducts), which is capable of calculating the seismic sectional force with a lower computational load. However, it is not known if this method can be applied to box culverts for roads that have structural properties. Therefore, we evaluate the applicability of this method to the box culvert, and try to enhance the accuracy of the calculation by making an improvement. This simplified method consists of three calculation processes. The scope of our study contains two processes: First, we estimate the correlation of shear strain between the ground and the structure. Second, we determine the cross-sectional force in the box culvert elements using the shear strain obtained from the previous calculation process. Comparing with the results of dynamic analysis (which can accurately estimate the seismic response), we evaluate the accuracy and applicable range of the simplified method that we propose in this research.