Mamoru Kikumoto

An Experimental Method to Determine the Elastic Properties of Transversely Isotropic Rocks by a Single Triaxial Test

A novel method is proposed for determining the deformation anisotropy of rocks by a single triaxial test using a single specimen sampled from an arbitrary direction. Transversely isotropic elasticity is assumed for the purpose of application of the test method to sedimentary and metamorphic rocks, and the non-axial symmetric stress–strain relationships of anisotropic rocks are determined by triaxial testing by means of a simple improvement to the cap in the triaxial testing apparatus. Both the elastic parameters and the directions of the transversely isotropic elasticity can be obtained by measuring the shear deformations that occur under triaxial stress conditions. An overview of the method for determining transversely isotropic elasticity is presented in this paper, along with the results of a sensitivity analysis performed assuming simulated strains with random measurement errors. The results show that the directions of anisotropy can be determined precisely using the directions of the principal strains measured during isotropic compression and that the elastic parameters can be determined uniquely from the stress–strain relationships observed during both the isotropic and axial compression processes.

Interaction Effect of Retaining Wall and Existing Foundations in Braced Excavation

Two-dimensional model tests are conducted to investigate the deformation mechanism of the ground and the earth pressure of retaining wall. Numerical simulations with finite element method using FEMtij-2D are also carried out for the same scale of the model tests. Subloading tij model is used in the analyses to model the ground material. Several patterns of the model tests are performed varying the length of the retaining wall and changing the distance between the foundation and wall. It is revealed in this research that maximum surface settlement does not always occur just behind the wall, but mostly at the position of the existing building. The rotation of the foundations depends on the distance between the foundation and wall. The numerical analyses can well simulate the observed earth pressures, surface settlements and deformation mechanism of the ground.

Simple modeling of stress-strain relation for unsaturated soil.

Though stress-strain characteristics of unsaturated soils are complicated and rather different from those of saturated soils, it should be described properly by a constitutive model for soils because soil usually stays under an unsaturated condition in actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one applicable to unsaturated soils. The proposed model is formulated using the Bishop’s effective stress and the residual strength is, therefore, assumed to be constant. In the proposed model, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric and distortional behaviors of unsaturated soils are properly described. In addition, a simple method to extend classical water retention curves such as van Genuchten’s equation to be able to incorporate the influences of suction histories and density is proposed and applied to the proposed model. In the present paper, the outline of the proposed model is explained and applicability of the model is discussed through typical results of simulations.

Particle Crushing in Granular Assemblies

Idealised assemblies of equally sized spherical particles are subjected to a range of macroscopic compressive principal stresses and contact forces are determined. Stress fields within individual particles are studied and a failure criterion for brittle materials imposed indicating that crushing occurs when the maximum contact force reaches a threshold particle strength value, irrespective of the presence and magnitude of other lesser contact forces acting on the particle. Failure surfaces are drawn in the three dimensional principal stress space.

New concept to describe three-phase capillary pressure–degree of saturation relationship in porous media

The Leverett concept is used conventionally to model the relationship between the capillary pressures and the degrees of saturation in the water-nonaqueous phase liquid (NAPL)-air three-phase system in porous media. In this paper, the limitation of the Leverett concept that the concept is not applicable in the case of nonspreading NAPLs is discussed through microscopic consideration. A new concept that can be applied in the case of nonspreading NAPLs as well as spreading NAPLs is then proposed. The validity of the proposed concept is confirmed by comparing with past experimental data and simulation results obtained using the conventional model based on the Leverett concept. It is confirmed that the proposed concept can correctly predict the observed distributions of NAPLs, including those of nonspreading ones.

Field performance of hand-mixed and machine-mixed asphalt in labour-based asphalt maintenance works

Abstract Asphalt premix for labour-based maintenance works can be produced manually (hand-mixed) or by mechanised asphalt plants – the conventional method. Hand-mixed asphalt (HDMA), common especially in most developing countries, is claimed to equal machine-mixed asphalt (MMA) in terms of durability. Recently, however, there have been concerns about reduced durability of labour-based asphalt (LBA) works. The current study compared durability of HDMA and MMA with a view to establish suitable scope and appropriate traffic conditions for LBA maintenance works. The study was based on laboratory tests – on binder, aggregate and site premix – and field experiments which involved asphalt production and placement of small and big patches of HDMA and MMA on a heavily trafficked Likuni Road and a lightly trafficked Tsiranana Avenue in Lilongwe, Malawi. The study monitored the patch durability for two months after the maintenance work. Effects of the study variables – production method (HDMA and MMA), scope of works (small and big patches) and road category (heavily trafficked and lightly trafficked roads) – on patch durability were statistically analysed. The study has shown that production method affects durability of LBA the most. HDMA had 67% less durability than MMA. Scope of works and road class affected LBA durability by 27% and 23%, respectively. At a level of significance (p-value) of 0.05, the effect of production method was significant, while that of patch size and work scope was not statistically significant. It is imperative of asphalt producers to adhere to proportions of premix components if quality of HDMA is to improve. Apart from improving the HDMA production process, the quality of LBA works could be improved by increasing contractors’ access to MMA. The study recommends contractors’ cooperation and adoption of production techniques (cost-cutting techniques) such as use of recycled asphalt pavement and warm mixture asphalts in order to improve contractors’ access to MMA.

An Elastoplastic Model for Soils Exhibiting Particle Breakage

An elastoplastic constitutive model exhibiting particle crushing is developed based on the framework of continuum mechanics. The proposed model is formulated based on the extension of the critical state soil model [1, 2] to firstly incorporate the effect of packing density on the stress-strain characteristics by employing the concept of subloading surface [3]. Then, the effect of particle crushing is further implemented by incorporating the evolution of particle size distribution curve due to crushing stress and its effect on the constitutive behavior. Finally, the performance of the proposed model is validated with experimental test. The simulation results show that in spite of a simple formulation of the constitutive model, the behavior of crushable soils is well predicted.

Application of coupled elasto-plastic NMM-DDA procedure for the stability analysis of Prasat Suor Prat N1 Tower, Angkor, Cambodia

There are many historic remains in the world and among them many masonry structures are in danger of collapse. To select suitable methods for restoration and preservation, it is important to evaluate the stability of masonry structures considering the interaction between foundation ground and masonry building, and accurate calculation of stress distribution inside the masonry structure is required. In the previous studies, NMM–DDA (coupled numerical manifold method and discontinuous deformation analysis) one of the discontinuum-based numerical methods was developed to satisfy these requirements and used for the stability analysis of Prasat Suor Prat N1 Tower in the Angkor monuments, Cambodia. However, the original NMM–DDA code treat only elastic bodies (for both DDA blocks and NMM elements) and cannot treat the failure of soil. Hence, in this study, the elasto-perfectly plastic constitutive law (Drucker–Prager model) was newly introduced to the original NMM–DDA code, and the model was verified by performing biaxial test numerically and comparing with analytical solution. The newly developed elasto-plastic NMM–DDA was applied to the stability analysis of Prasat Suor Prat N1 Tower. The simulation results were compared with the on-site observation/investigation and the applicability of elasto-plastic NMM–DDA was also discussed.

A Simple Method to Consider Density and Bonding Effects in Modeling of Geomaterials

A simple method to describe stress-strain behavior of structured soils under normally and over-consolidated states in one-dimensional stress condition is first presented by introducing a state variable to represent the influence of density. To describe the one-dimensional stress-strain behavior of structured soils, attention is focused on the density and the bonding as the main factors that affect a structured soil, because it can be considered that the soil skeleton structure in a state which is looser than that of a normally consolidated soil is formed by bonding effects. The extension from one-dimensional model to three-dimensional model can be done only by defining the yield function using the invariants of modified stress ‘tij′ instead of one-dimensional stress σ and assuming the flow rule in modified stress space tij.

Mechanical Behavior of Geosynthetic-Reinforced Soil Retaining Wall

Mechanical behavior of geosynthetic-reinforced soil retaining wall is investigated and the reinforcing mechanism by geosynthetics is discussed through two-dimensional laboratory model tests and their corresponding finite element analyses. A series of mode tests and simulations are carried out varying arrangement, length and number of the reinforcements. It is revealed that the effectiveness of the geosynthetics mainly depends on the increase of internal stability, which is local stability within the reinforced ground, and external stability, which is overall stability of the surrounding backfill and subsoil. It is also indicated that the results of the numerical analyses show good agreement with the results of the model tests.