Shear Strength Of Unsaturated Soils Research Articles

Undrained Shear Strength of Unsaturated Soils under Zero or Low Confining Pressures in the Vadose Zone

Core Ideas We present a model to estimate the variation of undrained shear strength of unsaturated soils. Environmental factors influenced the undrained shear strength of unsaturated soils. The proposed model can be implemented into a numerical analysis. In geotechnical engineering practice, many projects such as stability analysis of slopes or trenches and pavement design involve soils in vadose zones at shallow depths, where soils are under unsaturated conditions. For these types of projects, undrained shear strength test results obtained under zero or low confining pressure can be effectively used to analyze the behaviors of unsaturated soils. Experimental procedures to determine the undrained shear strength for different soil suction values, however, is time consuming even under low confining pressure. For this reason, we attempted to develop a semi‐empirical model to predict the undrained shear strength of unsaturated soils at a shallow depth assuming zero confining pressure as a function of soil suction. In addition, existing empirical or semi‐empirical models are also presented along with the characteristic behaviors of unsaturated soils under axial forces at low or zero confining pressures. Finally, the advantages and disadvantages of each model are discussed based on the comparison between the measured undrained shear strengths under zero or low confining pressures with those predicted using the existing and proposed models.

Contribution of osmotic suction to shear strength of unsaturated high plasticity silty soil

The engineering behaviour of unsaturated soils is affected by suction. It is widely accepted that soil suction consists of two major components: matric and osmotic suctions. Matric suction is associated with the capillary effects whereas osmotic suction is associated with the salt content of the pore water. Soil suction has been incorporated as “effective” stress or suction stress when describing the engineering behaviour of unsaturated soils. Usually, the suction component that is incorporated is matric suction while the contribution of osmotic suction is seldom investigated. In this paper, the contribution of osmotic suction to shear strength of unsaturated soils is investigated through unconfined compression tests on three series of compacted soil prepared using distilled water and sodium chloride solution. Specimens were prepared at three water contents corresponding to dry densities at dry of optimum, optimum and wet of optimum on the standard Proctor compaction curve. Series one and two consist of specimens compacted using distilled water and sodium chloride solution, respectively. In series three, one half of the specimen was compacted using distilled water and the other half compacted using sodium chloride solution. Unconfined compression tests were conducted on the soil specimens at various elapsed times after compaction to allow for ionic equilibrium due to the osmotic gradient in the third series of soil specimens. The test results show soil specimens have similar strength at each compaction water content regardless of elapsed time and type of pore water. No effect of osmotic suction or osmotic suction gradient on shear strength of the compacted soil specimens was found. It was concluded that the salt content of the pore water affects the soil structure of clayey soils and does not act like matric suction. Hence, matric suction should be measured in order to quantify the engineering behaviour of unsaturated soils.

Including degree of capillary saturation into constitutive modelling of unsaturated soils

The degree of saturation (S) of soil can be separated into two components: the degree of capillary saturation (S′) that is based on the capillary water and the degree of adsorptive saturation (S″) that is based on the adsorbed water. This paper discusses the role of the degree of capillary saturation (S′) in modelling the coupled hydro-mechanical behaviour of unsaturated soils and proposes a new constitutive model for unsaturated soils by using the degree of capillary saturation (S′) and the effective inter-particle stress (σij′). An enhanced hydraulic model is introduced to describe the hydraulic hysteresis and hydro-mechanical interaction in terms of the degree of capillary saturation (S′). In the proposed constitutive model, the shear strength, yield stress and deformation behaviour of unsaturated soils are governed directly by the above two constitutive variables, namely σij′ and S′. To be in line with the existing finite element frameworks for unsaturated soils, the proposed model is eventually generalised to constitutive functions consisting of only primary variables such as the net stress (σij), suction (s) and degree of saturation (S). The typical performance of the model for simulating the characteristic trends of unsaturated soil behaviour is discussed in several different scenarios. The model is then validated against a variety of experimental data in the literature, and the results show that a reasonable agreement can be obtained using this new constitutive model.

Using adaptive neuro-fuzzy inference system for modeling unsaturated soils shear strength

Determining shear strength of unsaturated soils is generally more complex, time-consuming, and costly than that of saturated soils. In spite of numerous studies conducted on unsaturated test methods, there is still a critical need for the existing studies to be applied while extending the researches on the subject matter of shear strength determination in unsaturated soils. This is primarily in order to save time and money spent on laboratory tests. For this purpose, several empirical approaches to unsaturated soils shear strength testing have been proposed by different researchers. Being based on shear strength and soil matric suction parameters, these empirical methods can be divided into linear and nonlinear variants. Nonlinear empirical methods are usually based on saturated soil shear strength parameters and soil–water characteristic curve (SWCC). However, it is still a time-consuming and costly practice to determine SWCC parameters in the laboratory. As such, several correlations are proposed to estimate SWCC parameters; some of these correlations are, however, based on fitting parameters, and this is again a relatively difficult task to undertake. Recently, artificial intelligence has led to applications in solving nonlinear problems and cases where an accurate understanding of the problem is required. In this research, aiming at proposing adaptive neuro-fuzzy inference system (ANFIS) models to predict unsaturated soils shear strength, two clustering approaches are followed, namely fuzzy c-mean clustering and subtractive clustering. ANFIS input parameters were taken to be the same parameters commonly chosen in most empirical models utilized for determining shear strength; these include net normal stress ( $$\sigma _{\mathrm{n}}-u_{\mathrm{a}})$$ , matric suction ( $$\sigma _{\mathrm{a}}-u_{\mathrm{w}})$$ , effective cohesion ( $$c^\prime $$ ), and angle of frictional resistance ( $$\varphi '^\prime $$ ), and also other especial parameters which were taken in empirical models such as Lamborn (A micromechanical approach to modeling partly saturated soils, Texas A&M University, College Station, 1986), Vanapalli et al. (Can Geotech J 33:379–392, 1996), Oberg and Sallfors (ASTM Geotech Test J 20:40–48, 1997), Bao et al. (Keynote lecture, proceedings of the 2nd international conference on unsaturated soils (UNSAT 98), Beijing, China, pp 71–98, 1998) and Khalili and Khabbaz (Geotechnique 48:681–687, 1998) models collected from SoilVision database. Continuing with the research, a couple of comparisons were made on five empirical models along with the two ANFIS models. The results indicated that both ANFIS models were of high capability in terms of shear strength prediction using the chosen inputs so that more satisfying and adequate predictions be provided compared to empirical models results.

The relationship between the swelling pressure and shear strength of unsaturated soil: the Yanji Basin as a case study

The swelling pressure is a unique and important parameter for reflecting the engineering characteristics of expansive soil, providing a method to study the shear strength of expansive soil. The results of previous studies indicate that the shear strength of unsaturated soil is composed of three parts, and the swelling pressure contributes to the matric suction. Unsaturated direct shear tests and swelling tests of undisturbed expansive soils in the Yanji Basin were performed in this study, and the relationship between the swelling pressure and water content, as well as between the total cohesion and initial water content, were analyzed. According to a data regression analysis, a variety of fitting functions were used for comparisons and a linear relationship between the swelling pressure and the matric suction was established. The study indicate that the exponential fitting function, power function, and linear function should be used and compared with one another to find the best fit. This research is significant for the study of the relationship between shear strength and the swelling pressure of unsaturated soil.

Centrifuge Model Test on Unsaturated Expansive Soil Slopes with Cyclic Wetting–Drying and Inundation at the Slope Toe

The influence of dry unit weight and matric suction on the shear strength and deformation characteristics of unsaturated expansive soil was studied by triaxial tests. Artificial rainfall system and lighting–winding system were, respectively, set up for the unsaturated expansive soil slope centrifuge models to simulate wetting–drying cycles. Three kinds of centrifuge model tests concerning different sequences between centrifuge running and wetting–drying cycles on slope surface and inundation at the slope toe were conducted on the unsaturated expansive soil slope models with two different dry unit weight, water contents and slope ratios. The slope settlement and horizontal displacement, failure mechanism, and cumulated cracks on unsaturated expansive soil slope centrifuge models were investigated and analyzed. It was found that the crack accumulation induced by wetting–drying cycles or softening behavior induced by inundation at the slope toe was crucial to understanding the rainfall-induced progressive failure, which occurred from the slope toe and developed upwards in unsaturated expansive soil slopes. Therefore, cover layer should be built for unsaturated expansive soil slopes from rainfall infiltration into the cracks induced by wetting–drying cycles, and at least enough drainage measures should be taken to reduce water infiltration into the unsaturated soil slope and inundation at the slope toe.

A Revised Formula to Compute Shear Strength of Unsaturated Soils

In the western part of Sichuan province of China, moraines are widely distributed along the Valley of Zagunao River. The moraines at high altitude have deformed constantly under the influence of precipitation. It is considered as a severe threat to the local community. By on-site geological investigation and lab experiments, the sensitivities of moraines under different saturation rates are observed. The cohesion and friction angle both have negative correlations with the saturation rates of the morainic materials. Traditional mechanical strength models for unsaturated soil analysis failed to explain the mechanical characteristics explicitly. A revised Mohr-Coulomb criterion formula with an added cohesive compensation is proposed in this research to compute the unsaturated strength. The revised criterion accurately predicts the strength and is valuable for future research of special types of breccia.

Corrigendum: Capillary water retention curve and shear strength of unsaturated soils

<i>Corrigendum:</i> Capillary water retention curve and shear strength of unsaturated soils

The use of soil water retention curve models in analyzing slope stability in differently structured soils

This study analyzes whether and at what rate the parameterization of the Soil Water Retention Curve (SWRC) affects the analysis of shallow slope stability for differently structured unsaturated soils. Advanced empirical or physically-based equations of SWRCs have been proposed in literature to describe soil systems characterized by the so-called bimodal porous domain. In unsaturated soils, SWRC affects the stability assessment in two ways. It influences the resistance properties in terms of shear strengths, which depend on the soil water suction; and it affects the hydrological process modeling (e.g. infiltration) directly influencing soil moisture patterns and indirectly influencing slope stability. Most of the formulations proposed to predict the shear strength of unsaturated soils require the definition of an χ parameter that tunes the contribution of the suction effect to a rate proportional to the saturation conditions.In this study, a set of experiments was carried out in order to analyze both the mechanical and hydrological effects of SWRC on slope stability. First, three SWRC models were calibrated on different soil textures. Then, slope stability analyses were carried out on a synthetic hillslope supposed to be characterized alternatively and homogenously by the different soils. The factor of safety (FS) of the slope was computed first, at given states of hydrological conditions (i.e., fixed soil moisture), and then at dynamic hydrological conditions simulated by solving the 1D Richards's equation. Two different formulations of the χ parameter were also used. Finally, a sensitivity analysis of the SWRC models and the χ formulations for slope stability were evaluated for different slope angles and mechanical properties.The results indicated that for clayey (and bimodal) soils, changes in FS obtained with different SWRC models can be significant, especially at soil moisture values close to the residual zone. In sandy (and unimodal) soils, the choice of χ formulations can be more important. The variation of FS decreases as the slope angle increases or the friction angle decreases.

Discussion of “Capillary-based effective stress formulation for predicting shear strength of unsaturated soils”

Discussion of “Capillary-based effective stress formulation for predicting shear strength of unsaturated soils”

Reply to the discussion by Mesri and Kane on “Capillary-based effective stress formulation for predicting shear strength of unsaturated soils”

Reply to the discussion by Mesri and Kane on “Capillary-based effective stress formulation for predicting shear strength of unsaturated soils”

Incorporating Temperature Effects in Soil-Water Characteristic Curves

The study of water movement in unsaturated soils under non isothermal conditions requires understanding the soil water characteristic curves (SWCC) as a function of temperature. Physical processes such as water permeability, and shear strength of unsaturated soils are a function of the SWCC. These curves are generally assumed to be a unique relationship for a soil for all practical purposes. However, factors such as temperature and volume change can affect this uniqueness. The aim of this paper is to present a rational approach to describe the effect of temperature on the SWCC under isothermal conditions. A simple equation based on the relationship between temperature and the capillary rise is applied to find the air entry value at different temperatures when its value is known at a reference temperature. The same equation can be applied to the residual water content term of the SWCC. An experimental procedure for two different coarse-grained soils in the low suction range (i.e., 0–30 kPa) at three different temperatures (4, 20, and \(49\,^{\circ }\hbox {C}\)) were conducted to validate the proposed approach. The comparison between experimental results, published SWCC data from literature and the proposed equation indicate that the proposed method can be used to successfully predict the measured SWCC with respect to temperature.

Stiffness and shear strength of unsaturated soils in relation to soil-water characteristic curve

This paper introduces a methodology to predict the non-linear stiffness–suction and shear strength–suction relationships for unsaturated soils within the lower suction range from the non-linearity of the soil-water characteristic curve (SWCC), using a normalised function formulated with ‘suction times exponential degree of saturation’. The information required in this methodology includes (a) measurements of the shear strength or stiffness properties at saturation condition and one unsaturated condition and (b) the SWCC. Published experimental data on the stiffness and shear strength properties and the SWCC obtained from 25 different soils varying from coarse-grained sands to expansive clays are used to validate the proposed normalised function and to calibrate the exponent value. It is found that the normalised function, using exponent values of 1·0 and 2·0, respectively, for cohesionless and cohesive soils, provides reasonable predictions of the stiffness–suction and shear strength–suction relationships for all the soils used in this study, taking account of various influencing factors including external stress, soil structure, anisotropy, hydraulic hysteresis and testing technique.

Measurement of unsaturated shear strength parameters of silty sand and its correlation with unconfined compressive strength

Abstract Compressive strength and direct shear testing parameters on unsaturated soil are often used in analysis and design of several geotechnical infrastructures. Rarely, any attempt has been made to quantify shear strength of unsaturated soil under different net normal stress conditions. The objectives of this study are to quantify shear strength parameters of unsaturated silty sand and to establish a simple correlation of it with unconfined compressive strength. Consolidated drained direct shear tests and unconfined compressive strength tests were performed to determine shear strength of unsaturated completely decomposed granite (CDG) soil. To interpret this, a series of typical soil water characteristic curves (SWCC) of CDG soil under different net normal stresses were also measured. Experimental results show that the unsaturated shear strength significantly increases with increase in net normal stress and matric suction. A new model for prediction of unconfined compression strength of unsaturated soil from shear strength and initial suction was proposed. The predictions from newly proposed model were found to be consistent with the measured unconfined compressive strength. This correlation can be useful in economical and rapid design/analysis of geotechnical infrastructure.

Capillary water retention curve and shear strength of unsaturated soils

This paper proposes a new water retention model for unsaturated soils, which takes into account capillary condensation of adsorbed water. In this model, the degree of saturation of a soil is separated into that based on capillary water and that based on adsorbed water. Through analysis of a partially saturated two-cylinder system, a new shear strength criterion for unsaturated soils is proposed, in which only the degree of saturation based on capillary water contributes to the variation of shear strength with suction. The proposed shear strength criterion is justified against thermodynamic principles and is compared against existing criteria in the literature, which shows that it provides a much improved prediction of the experimental data for a wide range of suction values.

Shear strength of unsaturated granular soils: three-dimensional discrete element analyses

To investigate the shear strength behavior of unsaturated granular soils, the three-dimensional discrete element method has been used to model soils in triaxial compression tests. A simple but effective contact model with an attractive capillary force was implemented. The effects of matric suction and packing density on shear strength of unsaturated granular assemblies were examined. The Mohr–Coulomb strength parameters (apparent cohesion and friction angle) were fitted for each matric suction examined. The results show that matric suction can increase the strength and modulus of granular soils and lead to increased dilation. The peak friction angle depends on the packing density but seems independent of matric suction. The apparent cohesion increases with matric suction non-linearly at a decreasing rate. Similar values of cohesion were observed for both dense and loose assemblies, which can be explained by the anisotropic distribution of capillary force network. Based on the microscopic observations, the stress-induced anisotropy of contact distributions leads to an anisotropic distribution of capillary water and, as a consequence, the capillary stress is anisotropic, imposing a shear effect on an assembly in parallel with that imposed by the externally applied loading. Consequently, the strength of unsaturated granular soil is controlled by the combined effects of packing density, and the magnitude and degree of anisotropy of the capillary stress. A new shear strength function for unsaturated soils, considering the anisotropic effects of matric suction, is proposed and validated using experimental data in the literature.

New Progress in the Study of Intergranular Suction and Shear Strength of Unsaturated Soil

&lt;p&gt;The suction between soil particles is the basis and core problem in the study of unsaturated soil. However, is the suction between soil particles just the matrix suction, which has been widely used since the discipline of unsaturated soil mechanics was established. In fact, the concept of matrix suction is from soil science and reflects the water- absorbing capacity of the soil. Matrix suction characterizes the interaction between soil particles and pore water rather than the interactions between soil particles, which are not in conformity with the principle of the effective stress of soils. The suction of unsaturated soil, in essence, is the intergranular suction composed of absorbed suction and structural suction. In this paper, first, the basic concepts of absorbed suction and structural suction were briefly introduced. Then, with soil mechanics, powder science, crystal chemistry, granular material mechanics and other related disciplines of knowledge for reference, the quantitative calculation formulas were theoretically deduced for the absorbed suction for equal-sized and unequal-sized unsaturated soil particles with arbitrary packing and the variable structural suction for equal-sized unsaturated soil particles with arbitrary packing and unequal-sized unsaturated soil particles with close tetrahedral packing. The factors that influence these equations were discussed. Then, the shear strength theory of unsaturated soil was established based on the theory of intergranular suction through the analysis of the effective stress principle of unsaturated soil. This study demonstrates that the shear strength of unsaturated soil consists of three parts: the effective, cohesive force, the additional strength caused by external loads and the strength caused by intergranular suction. The contribution of the three parts to the shear strength of unsaturated soil depends on the following influence factors: soil type, confining pressure, water content and density. Therefore, these factors must be comprehensively considered when determining the strength of unsaturated soil.&lt;/p&gt;

Shear strength of unsaturated soils from vicinity of Gorlice

Shear strength of unsaturated soils from vicinity of Gorlice

Relating shear strength of unsaturated soils with capillary water retention curve

This paper proposes a new water retention model for unsaturated soils, which takes into account capillary condensation of adsorbed water. In the proposed water retention model, the degree of saturation of a soil is separated into that based on capillary water and that based on adsorbed water. Through the analysis of a partially saturated two-cylinder system, a new shear strength criterion for unsaturated soils is proposed, in which only the degree of saturation based on capillary water contributes to the variation of shear strength with suction. The proposed shear strength criterion is justified against thermodynamic principles. The proposed strength criterion is compared against existing criteria in the literature, which shows that it provides a much improved prediction of the experimental data, for a wide range of suction values.

Determination of the relationship among capillary pressure, saturation and interfacial area: a pore unit assembly approach

Three state variables namely, degree of saturation (S w ), capillary pressure (P c ) and specific air-water interfacial area (a aw ) are indispensable for modelling coupled processes relevant to unsaturated soils mechanics, agriculture, and contaminant hydrology. They play a key role in simulating various phenomena and the determination of various parameters and physical characteristics such as the unsaturated soil shear strength, field capacity, wilting point, air and water diffusivity and the rate of dissolution of contaminants. The determination of soil water retention curve (S w -P c ) as well as the specific interfacial area (a aw ) using available experimental techniques is a challenging and time consuming task. Therefore, a numerical technique that employs basic soil properties to obtain these variables is of much value and high practical and theoretical importance. In the current study, the porous network extracted from a discrete element model (the so-called pore unit-assembly) has been used to directly model the drying and wetting processes inside a granular soil packing and to obtain the values of P c , S w and a aw . The results of the simulations are in good agreement with the experimental data, which points to the efficacy and adequacy of the introduced algorithms and involved assumptions for this purpose.