Current models of the elasto-plastic behaviour of unsaturated soils contain important underlying assumptions that have not been tested due to a lack of adequate experimental data. To address this issue, the objective of this paper is to provide a comprehensive set of experimental data. An extensive experimental program has been performed on a remoulded unsaturated silt. To characterise its elasto-plastic behaviour, samples were taken through various stress paths, including wetting, drying and compression. Experimental results were analysed to provide (1) evidence of suction-induced preconsolidation; (2) dependence of cohesion and shear strength at failure on suction; (3) stiffness in relation to suction and (4) uniqueness of the critical state line.

View full abstract

The catastrophic failure of the 71-m high Gouhou concrete-faced rockfill dam in Qinghai Province, China, has drawn many studies. This paper aims at providing new understanding on the modes and process of this failure, which could be applicable to similar dam failures. The geometrical and hydraulic criteria for internal erosion in rockfill materials are assessed. Unsaturated-saturated seepage theory is used to analyze the Gouhou dam since the rockfill materials of the dam were unsaturated before reservoir water infiltration. The water infiltration is simulated by several cases of transient seepage analyses. According to the study, a perched-water table formed in the dam when the reservoir water infiltrated into the rockfill from the top of the concrete face. The perched water spread nearly horizontally along stratifications in the rockfill and exited from the downstream slope at a high elevation as observed before the breach of the dam. The hydraulic gradient of the perched water table was the highest near the wetting front, which might provide necessary hydraulic conditions to trigger internal erosion and piping failure. Based on the geometrical conditions of the rockfill materials and the hydraulic conditions in the dam, the susceptibility of internal erosion is evaluated and the possible modes and process of seepage failure of the dam are described.

View full abstract

The nature of the particles of seven alluvial rockfill materials and three quarried rockfill materials are expressed in terms of uncompacted void content and unconfined compressive strength of each of the rockfill materials. Drained triaxial tests are conducted on modeled rockfill materials. The behaviour of the rockfill materials is related to the uncompacted void content and unconfined compressive strength of the rockfill materials. A prediction procedure is proposed to determine the angle of shearing resistance of the rockfill materials using uncompacted void content and unconfined compressive strength of these materials.

View full abstract

In this paper we present an experimental study performed as an attempt to exhibit non-localized, diffuse failure deformation modes in sand specimens submitted to test conditions leading to unstable failure. In particular, we are interested in diffuse modes which may appear in common triaxial compression tests on undrained loose sand specimens. A special series of triaxial compression tests are reported. These tests resemble the common triaxial compression tests but are modified in such a way that after the peak in the stress-strain curve, test control is changed from strain-control to stress-control. More precisely, a special kind of load-control is used, based on an original device which allows to recover displacement-control after a dynamic step in the post-peak regime. It is shown that the control used allows for the observation of the failure modes occurring in the unstable branch of the test.

View full abstract

It is important to estimate the seepage properties of unsaturated soils when performing an unsaturated-saturated seepage analysis, which is used to clarify the slope failure mechanism and predict the occurrence time of slope failure due to rainfall. Kitamura et al. (1998) proposed a numerical model to quantitatively estimating the seepage properties of unsaturated soils. And they have compared numerical results with laboratory soil tests in order to examine the validity of the numerical model. Consequently, it has been found that the numerical model must be improved moreover. In this paper a practical numerical model for seepage behavior of unsaturated soil is proposed to improve the previous model proposed by Kitamura et al. (1998). Firstly, the basic theories of the previous model are explained. Calculation results are compared with those obtained from the laboratory soil tests in order to examine the validity of the previous model. Then, we perform some improvements of the previous model based on hydro-mechanical properties, and propose a new parameter, named “parallel translation index (I_pt)”. From the results we showed that the relationship between the new parameter and the uniformity coefficient (U_c), expressed by logarithm is linear relation. Therefore, the reasonable seepage properties of soils can be computed from the improved numerical model using only the popular soil parameters.

View full abstract

The paper pertains to the development of a simple semi-empirical method of analysis for predicting the uplift capacity of pile groups embedded in sand assuming an inverted truncated rectangular pyramidal failure surface. Various pile and soil parameters such as length, diameter of the pile, group configuration, spacing of the piles and the soil properties such as density, angle of internal friction and the pile-soil interface friction angle having direct influence on the uplift capacity of the pile group are incorporated in the analysis. The predicted values of uplift capacity of pile groups with different configuration and length to embedment ratio are then compared with model test results carried out as a part of the present investigation and also with the values reported in literature. The predictions are found to be in good agreement with the measured values validating the developed method of analysis.

View full abstract

Engineering problems concerned either directly or indirectly with unsaturated soils are more and more common, ranging from aspects related to seepage to those related to shear strength and volume change. A few constitutive models have been conceived to describe and quantify these problems all of them with some advantages and drawbacks. Regarding mechanical behavior, the so-called Barcelona Basic Model (BBM) has achieved wide acceptance in geotechnical engineering, basically due to its simple mathematical formulation and good description of phenomena associated with unsaturated soils, especially collapse. Another concept that has emerged as a powerful framework to tackle three-dimensional states in granular materials is that of a modified stress tensor, such as t_ij. In this paper both BBM isotropic structure and t_ij concepts are combined to propose a new elastoplastic model for unsaturated soils, called t_ij-unsat. Modifications and generalizations for unsaturated states are introduced to accommodate those frameworks and overcome some inherited drawbacks. At the end of this work, experimental data gathered from specialized geotechnical literature is used to support the theoretical framework. The satisfactory agreement between simulations and tests results emphasizes the applicability of the proposed formulation to real boundary value geotechnical problems.

View full abstract

In this study the compression behavior of high water content cement-treated soft Bangkok clay is further investigated by conducting a constant stress ratio (CSR) test at various stress ratios (η). The test utilized cement-treated clay specimens with cement contents (A_w) of 10% and 15%, each of which was in combination with 100% and 130% total clay water contents. The test results confirmed that the ratio of after-curing void ratio to cement content (e_ot/A_w) can effectively characterize the compression behavior of cement-treated clay. The specimens with higher values of e_ot/A_w yielded higher volumetric and shear strains at the same stress ratio. While those with lower values of e_ot/A_w resulted in lower shear strains, with consequent higher values of strain increment ratios (dε_v/dε_s) both before and after transitional yield points. Significantly, the e_ot/A_w ratio has described the relationship of the compression yield loci of cement-treated clay at various stress ratios and mixing conditions.

View full abstract

This paper discusses the influence of fabric and particle shape on K₀-values of cohesionless materials using micromechanical analysis and the results from a series of tests. Following a simple analysis using Harr's probabilistic theory that illustrates the dependency of K₀ on particle shape, a rigorous micromechanical analysis is provided to take into account the effect of interparticle friction, particle shape and particle arrangement. The fabric effect is introduced via a joint density function of branch vectors. Both micromechanical analyses and experimental results show that the K₀-values are affected by fabric related to both the direction and the length of branch vectors. The effect of particle shape may be undermined if one only considers the directional variation of the density of branch vectors (or contact normals). The K₀-values are shown to be also affected by the direction in which deformation is restrained.

View full abstract

A multi-phase physicochemical method for simulating the durability of cementitious composites is used to predict the long-term degradation of underground concrete and cemented soil by calcium leaching. The objective is to develop a unified approach that can be used for both cemented soils and concrete. This paper describes a computational model based on physicochemical thermodynamics that can calculate the voids of micron-to-millimeter scale in soil foundations as well as the gel and capillary pores of nanometer-to-micron scale in cement paste. The proposed model shows that the soil-structure interaction of calcium-leaching transport and underground water advection are very important when assessing long-term durability of cementitious composites.

View full abstract

The viscous property of sand under triaxial compression (TC), triaxial extension (TE) and cyclic triaxial loading conditions was evaluated by using reconstituted loose samples of three types of relatively fine uniform and relatively angular sand (Silica No. 8, Toyoura and Hostun). The viscous property was quantified mainly by changing stepwise the axial strain rate many times and partially by performing drained sustained loading, both during otherwise monotonic loading (ML) at a constant strain rate. Such a peculiar feature of viscous property as that the viscous stress increment decays with an increase in the irreversible strain during ML at a constant strain rate (i.e., the so-called TESRA viscosity), which has been observed with Toyoura and Hostun sands in previous studies, was observed also with Silica No. 8 sand. The magnitude of viscous property is represented by the rate-sensitivity coefficient, β, defined as the slope of the ΔR/R−log {(γ^ir)_after/(γ^ir)_before} relation, where ΔR is a sudden change in the principal stress ratio, R=σ₁′/σ₃′, caused by a step change in the irreversible shear strain rate from (γ^ir)_before to (γ^ir)_after at a given R value during otherwise ML. The following was found. The same definition for β is relevant to drained TC and TE tests. In drained cyclic triaxial loading, the β value is obtained from the above-shown equation after re-defining the sign of ΔR and proportionally scaling the value of R. With the respective type of sand, the β values under these different loading conditions are very similar. The effect of overconsolidation on the β value is insignificant. The β values of these three types of sand are similar to each other and also to those of other ordinary types of sand and gravel having largely different particle sizes. The decay rate of viscous stress increment with an increase in the irreversible shear strain, which is another factor of the viscous property, is rather similar among the three types of sand, while the decay rate is slightly lower in ML TC than in ML TE. The effect of overconsolidation on the decay rate is insignificant.

View full abstract

Following days of heavy rainfall, a large-scale landslide occurred in Southern Leyte Province, Philippines, burying almost the entire village of Guinsaugon and causing the death of more than 1000 people. The landslide, which occurred along the steep slope of Mt. Can-abag in the middle of the province, mobilized large amount of rocks and debris with estimated volume of about 10-15 million m³. This paper discusses the results of the damage investigation conducted in the area after the disaster, with emphasis on the general features of the landslide and on the hydro-geological aspects of the disaster. Based on the observations made from the field investigation as well as additional information obtained from various sources, the main causes of the landslide are identified, and several sources of potential hazards are pointed out.

View full abstract

It has been recognized that the soil resistance to liquefaction increases significantly as the degree of saturation decreases. However, the effect of the degree of saturation reported in the literature varies widely between researchers. In this study, influential factors of the liquefaction resistance of partially saturated sand are derived from theoretical consideration and effects of the factors are examined through a series of triaxial tests. It was confirmed that the degree of saturation has a significant effect on the liquefaction resistance. It also appeared that the liquefaction resistance depends on the initial confining pressure and the initial pore pressure; the higher the confining pressure and the lower the initial pore pressure, the higher the liquefaction resistance of partially saturated sand. A unique relationship between liquefaction resistance ratios and the potential volumetric strain was found, which enable to estimate the liquefaction resistance of partially saturated sand with the effects of the three factors taken into account.

View full abstract