The shear strength and stress-strain behaviour of residual soil are known to be affected significantly not only by the initial porosity and stress history, but also by the bonds between particles. Although residual soil is commonly encountered during constructions in the tropical region, studies on its engineering properties are far from adequate. There is a lack of in-depth study to characterize the strength and deformation behaviour of intact residual soils in Singapore, especially under more representative testing conditions such as plane-strain conditions. In relation to a tunnelling construction project in Singapore, the engineering properties of an intact residual soil were characterised using laboratory tests. Large block undisturbed soil samples taken from a construction site were used. K₀ consolidated undrained triaxial compression (CK₀UC), extension (CK₀UE), direct simple shear (CK₀UDSS), and K₀ consolidated undrained plane-strain (CK₀UPS) tests were conducted. The undrained shear strength ratio c_u/σ′₁₀ and overconsolidation ratio (OCR) relationships were established. The test results indicate that the undrained shear strength (c_u) of the intact residual soil is highly anisotropic in term of c_u/σ′₁₀ ratio. It is also noted from the experimental results that the secant friction angles were highly dependent on consolidation stresses, as well as the testing methods. These anisotropic properties will affect considerably the design methods and the selection of parameters for analyses.

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Torrential rainfall in mid-July 2009 triggered numerous geodisasters such as slope failure and debris flow in Chugoku and Northern Kyushu areas of Japan. A number of slope failures and debris flows occurred in Yamaguchi and Fukuoka prefectures resulting in extensive damage to human life and infrastructure. One of the most serious geodisasters included a slope failure followed by debris flow at Sasaguri-machi and Fukuchi-machi, Fukuoka prefecture, Japan. This paper summarizes the results of geotechnical investigations on the geodisaster sites in Fukuoka prefecture. The geotechnical investigation included determining a series of grain size distributions, consistency limits and conducting direct box shear tests for collapsed soils collected at six disaster sites. The generation mechanisms of slope failure followed by debris flow were also investigated by analyzing the precipitation, topography, geology, and strength properties of the collapsed soils. Moreover, slope deformation and stability analyses were coupled with an unsaturated-saturated seepage analysis to investigate the slope failure mechanism. The main findings from the study are summarized as: The physical properties, such as the grain size distribution, the plastic limit and liquid limit of collapsed soils, are summarized and compared with the results of other failure slopes in the literature. The collapsed soil was characterized as being a well grained soil (the uniformity coefficient >50) and highly weathered (the ignition loss >5%), however, with regard to the liquid limit and plastic index, there were no remarkable findings. The original shear strength for collapsed soils with natural water content is relatively large and slope failure doesn't occur because the cohesion in the shear strength is induced by a suction force between the soil particles under unsaturated condition. However, water seepage into the soil induces a drastic decrease in the shear strength, which is mainly caused by a decrease in cohesion (losing suction) resulting from soil saturation. In addition, the drained/undrained condition in the shear process is also sensitive to shear strength. For example, both water seepage and the shear process with constant volume cause an approximate 30% reduction in shear strength for Fukuchi-machi and Sasaguri-machi soil samples. Therefore, the reduction of cohesive strength due to water seepage and the low permeability of the slope are the parameters which trigger geodisaster. Based on the results of slope deformation and a stability analyses which took the change in water pressure and cohesive strength into account, the geodisaster at Fukuchi-machi was simulated, it is reasonable to assume that the shallow failure near the top of slope occurred due to torrential precipitation of about 100 mm per hour which triggered a debris flow.

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Recent constitutive models for sands that incorporate the effects of particle breakage have emphasised the change of location of the critical state line in the void ratio: logarithm of the mean effective stress plane as the grading changes. This approach differs from earlier experimental work in which a unique and static critical state line was assumed: the basic difference between the two approaches being the question of whether the soil “knows” about the breakage that it has undergone. A series of triaxial tests was therefore conducted to investigate the effect of particle breakage on the current location of the critical state line. Two different shearing stages were used: the first to produce particle breakage and the second one to see if the material “remembers” the original state if sheared again. It was found that the critical state line does move with particle breakage, so indicating that the soil does “know” about the breakage that has occurred. However, large amounts of breakage were required to create a significant shift. The results show, furthermore, that the effect of the change of grading is not only a verticle movement in the the critical state line but also a rotation. Comparisons with the behaviour of reconstituted samples with the same grading as the pre-sheared samples demonstrated that while the soil does have some “knowledge” that it has undergone breakage, the initial grading remains more important than the current grading in determining its behaviour. An analysis of thin sections showed that this is probably because when particles break, the broken fragments remain in close proximity to each other and are not distributed uniformly throughout the soils.

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Fibre reinforced soils have been investigated for several decades and different models have been suggested to estimate their improved shear strength. The shear strength of such composite materials is affected by the micro and macro mechanical characteristics of both the fibres and the soils (e.g., relative sizes of fibres and soil grains, fibres aspect ratio, stress state, mechanical properties of the fibres), yet no model is available to explicitly take all of them into account. The aim of this work is to establish a new expression for the shear strength of the reinforced material, able to consider the main characteristics of the soil and the fibres as well as the effect of fibre to grains relative dimensions. Data from triaxial tests carried out on fibre reinforced soils with distinct grain size distributions (from clayey sands to sandy gravels) and from previous experimental works were considered and have been analysed successfully within the proposed framework.

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Cement Treat Granulate Soil (CTGS), a new artificial granular material, has been developed recently by mixing the dredged marine clay with appropriate amounts of cement and polymer. The CTGS particles are crushable and deformable, thus forming a compressible material. Besides being a lightweight material, CTGS is a granular material, and is therefore expected to be applied in reclamations or as a back fill or subsoil materials. This study investigates the deformation and crushing of the CTGS particles and their effects on the stress-strain behaviors. The comprehensive investigation of the principle of treatment, the micro-structure of particles, triaxial stress-strain behaviors, induced particle crushing and particle deformation are first presented via the experimental work done on two types CTGS produced from a lean-mixture design of cement and polymer. Subsequently, the results of X-ray Computer Topography (CT) scanners along with triaxial CD tests on CTGS and conventional gravel having rigid particles are presented. The test results reveal local failure mechanisms between the individual particles of the CTGS and gravel, from which the failure models of the granular materials formed by deformable and crushable grains and non-crushable grains are interpreted.

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Mixing a loose clean sand with random discrete flexible fibres has been found beneficial in decreasing the susceptibility to the phenomenon of liquefaction under monotonic loading. The addition of fibres can convert the strain softening response, typical of a loose unreinforced sand, into a strain hardening response by affecting the pore pressure generation and the effective stress path response. A new constitutive model based on the rule of mixtures has been used to simulate the undrained response of fibre reinforced sands. The model superimposes the individual contributions of the sand and the fibres according to their volumetric fraction. An apparent densification of the sand matrix induced by the presence of the fibres is accounted for in the model by assigning some of the void space to the fibres. This apparent densification is considered responsible for the observed strain hardening behaviour of reinforced sands. The proposed model is able to accommodate any distribution of fibre orientation: the orientation of fibres plays a key role in explaining the experimentally observed effective stress paths.

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The elastic behaviour of compacted soils subjected to very small strains (smaller than 10⁻⁵) is essential, because the serviceability of most geotechnical structures depends on soil elastic properties. Small-strain stiffness of soils was studied in the past years using different experimental devices (mainly resonant columns). However, the results have been relatively inconsistent. The bender elements technique, now extensively used in soil mechanics, offers an efficient non destructive alternative, since it is based on the propagation of shear waves. To enrich the common bender elements testing results providing only shear modulus values, an evolution of the bender elements technique, named bender-extender element's is used in this paper. This device allows the simultaneous measurement of the shear and compression wave velocities. As a result, the two independent elastic constants are measured for the same sample in order to avoid indirect estimations. Also, for unconfined sample's the GrindoSonic test is performed: this test is based on the transient vibrational response of the sample to a slight shock. The present study provides new results about the elastic properties of intermediate unsaturated soils made of mixtures of sand and clay. The results for the elastic properties are presented as a function of the suction level. Finally, a model relating the elastic behaviour of these unsaturated soils is proposed.

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Cement mixing is one of the popular ground improvement technologies in geotechnical engineering practice. In order to effectively and confidently design cement-mixed soil structures for specific purposes, its stress-strain behavior needs to be well understood. Though there have been many studies on cement-mixed soils using different types of soils, their behaviors have not been generalized yet. As is the case with concrete materials, the hydration of cement in cement-mixed soil continues with time, thereby improving the strength and deformation characteristics of cement-mixed soil over time. In the field, the cementation bonds are formed under stress in case of in-situ soil. However, in the usual testing techniques, cementation bonds under stress has not been a point of consideration in most of the previous studies. This has led to an underestimation of the stress-strain behavior of cement-mixed soil. On the other hand, soils are subjected to confining stress during loading which has also some effect on the strength and deformation characteristics of soil which has not been considered yet in the case of cement-mixed sand. This study investigates the effect of curing stress and period on the strength and deformation characteristics of cement-mixed sand. The effect of confining stress in the triaxial test is also investigated in another series of specimens. A series of consolidated drained (CD) triaxial compression (TC) tests were done along with the small strain cyclic loading and bender element tests during monotonic loading to determine the small strain Young's modulus (E_v) and shear modulus (G_vh) respectively. The effect of the curing period is significant in the peak strength, stiffness, E_v, G_vh and also in the post peak regime. The curing stress also has a significant effect on the peak strength, E_v and G_vh. The confining stress has an effect on the peak strength, stiffness and in the post peak regime. However, the effect is small compared to clean sand.

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Changes in air pressure during monotonic and cyclic loading are in some cases important for the behavior of unsaturated soil. For example, in order to investigate the stability of embankments and slope failure during earthquakes, it is necessary to consider the effect of the pore air or the pore gas pressure as well as the pore water pressure and the interaction between the soil and the pore fluids. In the present study, we carried out a series of monotonic and cyclic loading tests on sandy soil used for the improvement of river embankments. The effects of the initial suction, the confining pressure, and the degree of compaction under fully undrained conditions, namely, constant water and constant air shearing tests, as well as under drained conditions for both air and water, were studied. For the stress variables of the unsaturated soil, the skeleton stress was used to describe the experimental results and was defined as the difference between the total stress tensor and the average pore pressure of water and gas (Oka et al., 2010). From the monotonic and cyclic test results, we found that the stress-strain behavior of unsaturated sandy soil strongly depends on the initial suction, especially under fully undrained conditions, due to the difference in pore pressures. In the cyclic loading tests under fully undrained conditions, the mean skeleton stress decreased due to the increase in air pressure and led to the failure of the specimen in the case of a lower level of initial suction. In addition, the test results exhibited the strain rate effect on the stress-strain behavior during cyclic loading under fully undrained conditions.

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For the assessment of the quality of laboratory samples, a number of methods are available, though not universally applicable to any soils. This paper examines the issue of sampling quality and its assessment using comparisons between shear wave velocity measurements in situ and in recovered samples as the base, which is very useful in naturally structured granular soils, like residual soils. For this purpose, cross-hole and down-hole tests were performed in the field and bender elements measurements were made on triaxial samples collected from two experimental sites on residual soil from Porto granite. Various sampling techniques and tools were used, including block sampling and different tube samplers. The analysis of the results has led to a new classification of sampling quality and sample condition based on the comparison of normalised shear wave velocities in the field and in the laboratory.

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The influences of inherent cross-anisotropy on soil strength on the homogeneous deformation and bifurcation characteristic of the stress-strain relationship are studied. By neglecting the cohesion term and employing an elliptical shape function in Mohr-Coulomb failure criterion, a 3D anisotropic failure criterion is proposed to describe the strength of cross-anisotropic sands under true triaxial conditions. Based on the proposed failure criterion, the influence of the anisotropic parameter on the failure curve on the deviatoric plane and the relationship between the peak friction angle and the intermediate principal stress ratio are obtained. The suitability of the criterion is justified by comparing with a series of true triaxial tests on sands without strain localization. The proposed failure criterion is adopted to build a three dimensional anisotropic elasto-plastic model, which allows the bifurcation analysis to be incorporated with the non-coaxial flow rule for the purpose of studying the onset of strain localization. Compared with the true triaxial tests under several intermediate principal stress ratios conditions in the literature, the proposed model and bifurcation analysis is shown to be capable of predicting the influence of inherent cross-anisotropy on the onset of strain localization.

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Numerical analyses using the PFC^2D are conducted to study the relative changes of particle crushing and the shear behavior of granular materials according to the quantified particle shapes. A total of seven particle shapes are standardized and quantified. Three different particle models, including a circular particle model, a non-crushing particle model, and a crushing particle model, are developed and analyzed. The results show that shear strength is mobilized in size order: 3 ball>6 ball_T>2 ball>6 ball_R>4 ball>9 ball>1 ball model, corresponding to triangle>rectangle>square>circle shape in both the non-crushing particle model and the crushing particle model. Within the same shape but with a different number of sub-particles, it is found that an increase in the number of sub-particles within a particle coincides with smaller shear strength per model. The non-crushing particle model shows the increase of porosity not only in the shear band, but also in other layers. However, in the case of the crushing particle model, the increase of porosity is mainly focused within the shear band. It is found that with a larger circularity and convexity, that is, as a particle becomes more circular in shape, the shear strength decreases, regardless of particle crushing. It can be concluded that the standardized particle shape model suggested in this research has broader applications for future studies.

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The negative pore-water pressure in unsaturated soils increases the inter-particle force and small-strain stiffness, though this concept is only valid in wettable soils. The non-wetting nature of soils originating from the organic contamination of geoenvironments and natural hazards causes unexpected geo-events such as impermeation of water and hillslope runoff due to the changes in soil wettability. This study presents an experimental and numerical investigation to understand the evolution of capillary force and pressure for unsaturated soils whose surface wettability is wettable (hydrophilic) and water-repellent (hydrophobic). Hydrophobic granular materials are synthesized by the silanization technique with 0.5 mm diameter glass beads. The small-strain shear stiffness and corresponding degree of saturation are continuously monitored during evaporation for both specimens. The peak value of maximum shear stiffness is captured at a degree of saturation S~5.5% for hydrophilic specimen, while the hydrophobic specimen shows a quasi-constant small-strain stiffness during evaporation. The minimization of free energy for the liquid bridge between the two-particle system allows the attractive and repulsive capillary force and pressure produced between particles to be numerically estimated. The regime of zero-capillary pressure is identified depending on the contact angle and volume of liquid bridge. The measurement of small-strain stiffness combined with the numerical simulation of both hydrophilic and hydrophobic specimens clarifies the governing factors to determine capillarity in the granular materials and provides insight into the phenomenological observation of capillary pressure for unsaturated soils.

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Some slope failures along oblique faults experience arch to the extent that the stable scarp can be observed after collapse. The load resisting the scarp is transferred to the side supports by the circumferential forces across the exposed face, separating it from the force acting normal to the slope and the frictional resistance which runs along it. Therefore, the failure mechanism involves a passive condition where the major principal stresses dominate the force supporting the arches. This study presents the groundwork carried out to confirm the assumption of a passive condition appearing in the scarp of the inclined slope by measuring the pressure changes and the surface movement of a physical model. Moist sand was uniformly layered inside a rigid acrylic frame fixed on a slope and base made of rigid acrylic plates. The movement distributions of the slopes were recorded and analyzed using particle image velocimetry (PIV) and image processing software. As the toe of the slope was sliced and cut in sequence, starting from the middle part, the removal of the propped portion resulted in a slip along the surface. Since the moist sand fails inward rather than outward, it is reasonable to assume that the lateral compression is greater than the inclined compression, to the extent where it is sufficient to initiate arch action in a passive condition.

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This paper describes the initial stiffness of reconstituted kaolinite clay in both vertical and horizontal planes under three different stress histories. The initial shear stiffness was obtained from bi-directional bender element tests during isotropic and K₀ stress loading and unloading. An empirical correlation was established based on the initial stiffness of normally consolidated soils. Unlike the unique relationship of the initial vertical stiffness of normally consolidated clays, the initial stiffness in the horizontal plane is dependent on the stress ratio and previous stress history; thus, three different relationships of the initial horizontal stiffness were obtained for the three loading programs. The effect of the stress history on the initial horizontal stiffness can be considered properly by defining the degree of overconsolidation in terms of the horizontal effective stress. The change in the initial stiffness has a directional dependency on the stress history in the direction of the particle motion and wave propagation in the bender element tests.

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Coaxiality between the principal directions of the stress tensor and the principal directions of the plastic strain increment tensor is assumed in conventional plasticity models. In order to investigate coaxiality, or non-coaxiality, between these two principal directions, a series of drained tests on dry Hostun sand was carried out using a precision Hollow Cylinder Apparatus (HCA). The applied stress path includes large Principal Stress Axes Rotation (PSAR). Two of the three principal stresses are kept constant. Therefore, among the three principal stresses, only the intermediate principal stress, which is confining pressure (same pressure outside the hollow cylinder for internal and external lateral surfaces), changes during loading. During these tests, at different stress levels, elastic (or quasi-elastic) properties are also investigated, using small amplitude quasi-static cycles. These small cycles are performed in two different directions by successively changing only the axial stress σ_zz or the shear stress σ_θz. Elastic experimental properties are well simulated using the DI Bendetto-Geoffroy-Sauzeat (DBGS) hypo-elastic model, which takes into account PSAR. For each test, the elastic part of deformation is calculated using the DBGS model and removed from global strain so that it is possible to to focus only on the irreversible part (plastic part). Then, the principal directions of stress and plastic strain increment are compared. Experimental results show that there is no coaxiality between these directions. This observation attests to the existence of a non-coaxial plasticity. In addition, the coupling between tge coaxial and non-coaxial part is clearly shown. Experimental results reveal that the plastic strain part is very important for the first large amplitude cycles and remains greater than the elastic part even after 20 cycles.

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Small strain shear modulus (G_max) has been a parameter of choice used to assess the strength and deformation behavior of cemented and other sensitive soils. The influence of density, effective confining stress, stress anisotropy, and cement content on shear wave velocity (v_s)/shear modulus has been studied extensively and published. There are, however, very few studies on the effects of cement/strength degradation during shear on the shear wave velocity/shear modulus, which may be important for reliable and accurate prediction of mechanical behavior of cemented sands. The objective of this study is to evaluate the effect of cement degradation on shear wave velocity/shear modulus by measuring continuously the shear wave velocity during shear. A laboratory testing program was performed using samples of silty sand artificially cemented with Ordinary Portland Cement (OPC). Shear wave velocity was measured continuously within the triaxial cell during the shear phase using torsional ring transducers. G_max was calculated using the shear wave velocity and the corresponding density during shear. Results from this study suggest that G_max reaches a peak value before σ′₁ reaches a failure stress and this behavior is believed to be an indicator of bond breakage or destructuring. G_max calculated at various stages during shear showed that the cement and modulus degradation can be represented by a simple index using G_max. The results of this study suggest that there may be a unique relationship between small strain shear modulus and effective stresses at failure for dilative soils implying that in situ shear wave velocity measurements may be used to estimate effective stress strength parameters or as a precursor to failure in weakly cemented soils.

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SOILS AND FOUNDATIONS Vol. 51, No. 2, April 2011, pp. 227-238
The following correction needs to be noted: in the line 5 in the right column on page 227 of the original paper, 24 m should read 2.4 m.

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