Satoru Shibuya

Application of suction stress for estimating unsaturated shear strength of soils using direct shear testing under low confining pressure

The primary objective of this study is to evaluate effects of suction on shear strength of unsaturated soils under low confining pressure and to examine the relationships between suction, shear strength behavior, and volumetric deforma- tion using newly developed direct shear testing equipment for compacted weathered granite soils. The soil-water retention curves (SWRCs) of unsaturated soils were obtained under various overburden pressures. To analyze test results from the direct shear test under unsaturated conditions, a new method, suction stress-SWRC method (SSM), is proposed to deter- mine the suction value for each overburden pressure and the suction stresses. As a result, it has been found that the stress states at the peak shear strength point are on the same failure line for the saturated state when the suction stress is treated as a component of confining pressure. The relationship between stress ratio and dilatancy for the saturated state can be ex- tended to the unsaturated state. It is also noted that the estimated unsaturated shear strengths using the SSM agree well with the measured values from laboratory testing.

A non-linear stress-stiffness model for geomaterials at small to intermediate strains

A double exponential fitting model (DEFM) capable of expressing the non-linear stress-stiffness relationship of geomaterials has been proposed by Shibuya et al. (1997). The model comprises two material constants; the elastic stiffness at very small strains and the strength, together with other free parameters to determine the complete stress-stiffness relationship. In this paper, the capability of the original function used for DEFM in simulating the tangent stiffness-stress relationship of geomaterials is first discussed. Second, the methods for determining the free model parameters, as well as its conversion to obtain a stress-strain relationship are proposed. The applicability of DEFM to predicting non-linear stress-stiffness relationship is examined in detail in a total of forty-nine fitting cases of compression test data on sedimentary rock, artificial soft rock and soft clay. It is found that the DEFM is effective in expressing the non-linear stress-stiffness relationship of various kinds of geomaterials at small to intermediate strains, say less than 0.5%. The superiority of this model compared to other fitting models currently in use is also demonstrated in some of the fitting cases.

L-Shaped Geodrain in Embankment Model Test and Numerical Simulation

An embankment is usually made of unsaturated soil. Unsaturated soil when properly compacted is stiff and strong, since it sustains the matrix suction. The soil suction disappears quickly in the process of wetting, which in turn brings about large deformation or even failure of the embankment. In order to prevent seepage flow into the embankment, and also accumulation of water behind it. Design for an L-shaped geodrain in the embankment is proposed in this paper. A numerical simulation was carried out to examine the efficiency of the L-shaped geodrain system. The analysis was based on the results of a seepage flow test performed using a decomposed granite soil.

IN-SOIL HYDRAULIC TRANSMISSIVITY OF GEOSYNTHETIC DRAINS IN THE LABORATORY

When geosynthetics such as plastic board drains are employed for drainage material in embankment, the drainage ability could be deteriorated owing to the sustained earth pressure, implying that the cross sectional area of the geosynthetics could be decreased by the compressive overburden stress. In engineering practice, it is thus of great importance to evaluate the in-soil performance of geosynthetic drains. In so doing, the in-soil hydraulic transmissivity of geosynthetic drains should be properly measured in the laboratory. In this paper, the hydraulic transmissivity of two types of plastic board drains was carefully examined in the laboratory by using fine-grained clay, poorly graded fine sand and well-graded granular material.

Short-Term Stability Evaluation of an Embankment Constructed on Marine Clay Foundation at Kobe Airport

This article introduces the application of a stability analysis method that accounts for strength anisotropy on the embankment constructed at the Kobe airport. Shear strength anisotropy is induced by the in-situ state of stress along a potential slip surface of the embankment. In addition, the effects of the shearing rate on the undrained shear strength of the marine clay are examined in detail based on the results of a series of constant-volume direct shear box (DSB), triaxial compression (TC), and triaxial extension (TE) tests varying shearing rates. The most critical slip surface was selected from the deterministic slope stability analysis using unconfined compression tests of undisturbed clay samples from the field. For the most critical slip surface, the potential factor of safety of the embankment is estimated to be within the range of 1.21 to 1.28, as indicated by the results of the stability analyses using the strengths obtained from the DSB, TC, and TE tests and varying shearing rates.

Effects of Grain Shape on Mechanical Behaviors of Granular Material under Plane Strain Condition in 3D DEM Analyses

In this paper, we carried out the true triaxial test simulation under plane strain condition with three dimensional DEM analyses (3D DEM) to study behavior and development of shear band. The maximum mobilized internal friction angle corresponds to the shape factor, FU, on a linear relationship, and the inclination of the linear relationship seems to become larger with the microscopic angularity of the grain. The thickness of the shear band increased with the grain size. Also the grain shape affects on the relationship of residual stress ratio to relative shear displacement along the shear band.

Mechanical Behavior of Structured Clay and Its Simulation

The mechanical behavior of natural sedimentary clay and cement-mixed clay is different from the comparable behavior of the reconstituted sample as the consequence of structuration induced over the geological period due to natural conditions or rather quickly due to the addition of chemical bonding in fabrics. In this paper, the mechanical behavior of such structured clays as highlighted by reduced compressibility, higher stiffness and higher strength was carefully examined in the laboratory. In comparing the elastic shear modulus at very small strains, G between each structured clay and the reconstituted sample, the clay structure was evaluated quantitatively by metastability index MI(G) proposed by Shibuya. The stress-strain and strength behavior when subjected to undrained shear in compression was also simulated by the so-called super/subloading yield surface (SYS) Cam clay model proposed by Asaoka et al.

Influence of Various Experimental Conditions on Shear Behavior of Compacted Sandy Soil under Unsaturated Condition

The objective of this paper is to examine the influences of moisture content, loading speed, and degree of compaction on the shear behavior of compacted sandy soil under unsaturated conditions. A series of triaxial compression tests with various experimental conditions were carried out by using triaxial apparatus for unsaturated soil. The test results show that the shear strength of unsaturated soil decreases with the increase in moisture content and increases due to increment of loading speed. The effect of degree of compaction is also discussed. The results reveal that moisture content, loading speed, and degree of compaction have strong influences on the shear behavior of unsaturated sandy soil.

EFFECTS OF MOULDING WATER CONTENT AND COMPACTION METHOD ON DEFORMATON AND STRENGHT CHARACTERISTICS OF COMPACTED SOIL

締固めた細粒分質礫質砂の変形・強度特性に及ぼす締固め時の含水比および締固め方法の影響を圧密非排水三軸圧縮試験とベンダーエレメント試験を実施して検討した．最適含水比よりやや乾燥側で締固めた供試体では，圧縮性が小さく，非排水せん断時の偏差応力が大きい結果となった．締固め方法の影響は，圧縮性が静的よりも動的に締固めた供試体の方が小さく，偏差応力は静的供試体の方が大きくなった．これらの圧密非排水三軸圧縮試験の結果は，ベンダーエレメント試験より得られた土構造の配向性を反映する弾性係数の異方性と強い相関があった．このことから，締固め時の密度や含水比，締固め方法によって生じる変形・強度特性の違いは，締固めに必要とされるエネルギーや締固めによって生じる土構造の変化と密接な関係にあることが示唆された．

Advanced Laboratory Stress-Strain and Strength Testing of Geomaterials in Geotechnical Engineering Practice

Advanced laboratory stress-strain and strength testing of geomaterials is a need for geotechnical engineering practice. In this keynote paper, recent developments in advanced stress-strain and strength testing of geomaterials, together with engineering application of the test results are reviewed by showing two case histories with reference to excavation work in urban area, and the failure of reinforced wall due to heavy rainfalls.