Suction Stress Research Articles

Analytical solution to estimate the point of application of resultant passive earth thrust against unsaturated retaining structures

ABSTRACT This paper describes a simplified analytical solution for computing the magnitude and the point of application of the resultant passive earth thrust, as well as the passive earth pressure distribution against inclined walls retaining unsaturated soils under different steady flow conditions. The present analysis was performed based on the assumptions consistent with the Coulomb approach within the framework of the limit equilibrium method. Possible modifications of induced negative pore pressure or matric suction within the unsaturated zone above the water table have been considered depending upon the steady flow conditions, soil types, and groundwater table positions. Results of the parametric study showed considerable alterations in the passive earth pressure of unsaturated soils due to different parameters and highlight the importance of incorporating suction stress changes directly in the stability analysis of earth-retaining structures.

Bearing Capacity Analysis of Pile-Stabilized Slopes under Steady Unsaturated Flow Conditions

AbstractWhen analyzing slopes stabilized by piles, the soil has been commonly considered dry or saturated, whereas soils are usually unsaturated in practice. Suction stress is caused by positive an...

Failure potential of a circular tunnel face under steady-state unsaturated flow condition

In natural geotechnical structures, especially for those in the vicinity of water bodies or subjected to rainfalls, the soil properties are more likely governed by the unsaturated soil constitutive model rather than the fully saturated or dry one. In this contribution, the theory of unsaturated soil mechanics is introduced to tackle the stability problem of a circular tunnel face under steady-state unsaturated flow condition. By making reference to the existing findings, a generalized analytical solution for the vertical suction stress profile is discussed which permits to deal with unsaturated soil problems with the classical limit analysis method. An extended Mohr-Coulomb yield criterion is presented to characterize the tunnel face failure in unsaturated soil where the effect of suction stress on tunnel face stability is interpreted as the apparent cohesion. The increase in shear strength of unsaturated soil due to the existing of apparent cohesion is considered as a combined effect of n, α, ks and q, which respectively correspond to the pore size parameter, the inverse of air entry value (AEV), the saturated hydraulic conductivity and the flow rate. Within the framework of limit analysis, an advanced three-dimensional (3D) failure mechanism is used to include the unsaturated shear strength parameters into the work rate equation which eventually yields the critical retaining pressure for tunnel face stability. Parametric analysis is performed to show the influence of different unsaturated soil parameters on tunnel face stability. Four representative soil types are investigated to give some suggestions for tunnel excavations and designs in unsaturated soil.

Face stability analysis of tunnels under steady unsaturated seepage conditions

Tunnel face stability analyses are generally conducted assuming completely dry or saturated soils. However, a vast majority of practical conditions involve unsaturated soils. This paper presents an analytical framework to estimate the safety factor of a tunnel face with zero supporting pressure under steady unsaturated seepage conditions. The framework is implemented using the kinematic limit analysis method combined with the strength reduction technique. A closed-form solution for the unsaturated steady seepage problem is incorporated into the suction stress-based formulation of effective stress to obtain the profiles of suction stress and apparent cohesion. Based on a three-dimensional log-spiral mechanism of collapse, the safety factor is obtained using an iteration method for finding a zero supporting pressure. Several charts are provided to analyze the effects of unsaturated parameters and tunnel geometry. The results indicate that accounting for the role of unsaturated seepage increases the safety factor. The effect size of unsaturated seepage depends on the soil type, tunnel geometry parameters, and seepage conditions. The main contribution of this work is development of a useful approach for exploring the impact of steady unsaturated seepage on the face stability of tunnels.

Relative density effects on the bearing capacity of unsaturated sand

This study focuses on the impact of relative density on the bearing capacity of unsaturated sand using both theoretical predictions and measurements from physical modeling tests. The theoretical predictions incorporate the effective stress, quantified using the suction stress concept and friction angles obtained from direct shear tests on unsaturated sand specimens at different relative densities and degrees of saturation, into conventional bearing capacity equations. The suction stress values inferred from the failure envelopes were found to match well with values predicted from the soil-water retention curves for sands with different relative densities. Moreover, the bearing capacity values measured in physical modeling experiments involving loading of a circular footing atop unsaturated silty sand layers having different initial degrees of saturation matched well with the predicted bearing capacity values from an effective-stress based model. As expected, the bearing capacity was greater for soils with increasing relative density, but an interesting observation is that a transition from general to local shear failure occurred at a certain combination of relative density and degree of saturation. For the silty sand tested, this transition occurred at a relative density of 0% for degrees of saturation between 4 and 16% and at a relative density of 40% for degrees of saturation between 30 and 90%. General shear failure was always observed at relative densities of 70 and 90%.

Suction Stress of Clay Over a Wide Range of Saturation

Quantifying the relationship between water retention and internal stress state of unsaturated soils has long been a challenge. Current effective stress formulations can effectively capture the relationship for coarse-grained soils, where capillary mechanisms dominate internal stress state over a wide range of saturation, but fail for fine-grained soils, particularly at low to intermediate saturation, where surface adsorption mechanisms dominate water retention and corresponding stress state. Internal stress state of compacted kaolinite specimens spanning saturation (S) from 0 to 0.73 is measured through Brazilian tensile strength tests. Changes observed in strength, strain, and stiffness are analyzed to calculate evolution of internal stress with saturation. A model is proposed to quantify the relationship between water retention and internal stress for fine-grained and coarse-grained soils at saturations over the full range (0 ≤ S ≤ 1). Performance of the model is evaluated by comparison with experimental results from the literature.

Earth pressure profiles in unsaturated soils under transient flow

Assessing the effect of heavy precipitations on earthen structures warrants studying earth pressure profiles in unsaturated soils under transient flow. This paper presents an analytical framework to determine the changes in at-rest, active, and passive earth pressures of unsaturated soils due to transient infiltration. A closed-from solution for one-dimensional transient unsaturated flow is incorporated into a suction stress-based representation of effective stress to obtain the tempo-spatial changes in matric suction, suction stress, and effective stress. The profiles are used to extend Hooke's law and Rankine's earth pressure theory, leading to the determination of unsaturated at-rest, active, and passive earth pressures at different depths and times. The analytical framework is used in a set of parametric study for three hypothetical soils of clay, silt, and fine sand. The results reveal the nonlinear characteristics of earth pressure profiles during transient infiltration for all three soils, whereas a linear trend is generally seen under steady-state flow conditions. Further, the depth of tension crack under transient flow is different than the one resulted from steady-state analysis. A transition from tension to compression stress is seen near the soil surface. This observation can be particularly important for fine-grained soils, implying that the depth of tension zone is affected by suction stress changes. Findings of this study can contribute toward a better understanding of service state behavior and forensic studies of earthen structures under heavy precipitations.

Uncertainty Quantification in Water Retention Characteristic Curve of Fly Ash Using Copulas

Abstract Water retention characteristic curve (WRCC) is a primary input for modeling the unsaturated behavior of geomaterials. The inadequacy of measurement methodologies in capturing the measurable range of the suction of geomaterials along with material variability induce uncertainties in the quantification of WRCC. The goal of this study is to demonstrate the usefulness of copulas for quantifying the uncertainties in WRCC attributed to the aforementioned reasons. Four instruments measuring different ranges of suction of four fly ashes were used to generate 54 combinations of measured data that would represent possible cases of data-induced uncertainties. van Genuchten WRCC equation parameters (a and n) were determined for these combinations. Statistical analysis of the parameters determined from the 54 combinations indicates a negative correlation with each other; therefore, they represent a bivariate random vector. Four commonly used copulas, Gaussian, Frank, Plackett, and No. 16, were utilized to construct the bivariate density function of the parameters, among which the Gaussian copula was found to be the best fit. The impact of the copula-based model developed in this study on two important unsaturated functions, the suction stress characteristic curve (SSCC) and hydraulic conductivity function (HCF) of fly ash, was investigated and found to be significant. The dispersion in SSCC ranges from monotonically increasing to sharp post peak reduction in the suction range of 20–50 kPa. The limiting state of suction required to reach the theoretical minimum permeability was found to vary widely in the range of 300–9,000 kPa. The study shows that a Gaussian copula with lognormal marginals can adequately capture the significant range of uncertainties associated with WRCC.

Effect of suction stress on strength characteristics of a compacted silty soil under low confining pressure

Effect of suction stress on strength characteristics of a compacted silty soil under low confining pressure

Stability of Unsaturated Soil Slopes with Cracks under Steady-Infiltration Conditions

Most classical approaches for evaluating the stability of soil slopes with tension cracks are performed under dry and/or saturated conditions. The suction-induced effect of unsaturated soils is generally neglected in assessing slope stability. This article presents an analytical framework for assessing the stability of unsaturated soil slopes with tension cracks under one-dimensional (1D) steady-infiltration conditions using the limit-analysis method. The boundary-value problem was formulated based on the log-spiral failure mechanism by taking the effects of suction stress, effective unit weight of soil, and tension cracks into account. A simplified method was developed to calculate the external work rate of unsaturated soil weight. Detailed investigations of the effects of suction, Poisson’s ratio, steady infiltration, and water in tension cracks on the slope stability are presented.

Numerical analysis of landslides caused by rainfall in a reduced physical slope model

To assess slopes’ movements as heavy rainfall consequences, the strategy to replace the field monitoring by the combination between the reduced physical model and numerical simulations can be envisaged. This strategy reduces the cost and makes the monitoring well efficiency by reducing its duration and considering variably saturated conditions. In this article, the macro gravity physical model is used as an alternative of the 1 g model. So, the hydraulic gradient method is proposed to follow the slope movements under different rainfall conditions. This method which is based on the increase of the mass force would avoid the use of centrifuge technique. It is studied numerically to demonstrate the optimum dimensions of the physical model and prioritizing the influence of the main parameters against the increase of mass forces. The before-failure behaviour was modelled by establishing an elastoplastic model based on the effective stress concept and non-associative Mohr–Coulomb yield function. The coupling between the strain and the suction was introduced using both the proposal effective stress–suction relation and the balance mass equations of solid and fluid. The validation of the proposal model was done using experimental results previously published. A good agreement between the numerical and experimental results was highlighted.

Laboratory and field evaluation of modulus-suction-moisture relationship for a silty sand subgrade

Abstract A change in soil modulus in response to suction-moisture variation after compaction plays a vital role on modulus-based compaction control during earthwork construction. This study investigated the small-strain modulus-suction-moisture relationship for a silty sand subgrade using the laboratory free-free resonant frequency (FFR) and the in-situ spectral analysis of surface waves (SASW) in a trial section of highway construction project in Thailand. The post-compaction soil water retention curves (SWRCs) at different compaction states were investigated over the entire range of suction. Relationships between the SWRC parameters, i.e., air-entry suction, water-entry suction, residual suction etc., and as-compacted dry density and water content were obtained using the multiple-linear regression analysis. Such relationships are useful for the prediction of post-compaction suction in the field. The variation of SASW and FFR shear moduli with the suction stress, inferred from SWRCs, was found to be linear. A modified model was proposed, considering the combined effect of void ratio and the suction stress on the shear modulus, represented as Parameter A which was found to decrease as suction increased. The inclusion of suction stress and void ratio within the model variables yields better accuracy of prediction as compared to those models that considered either only suction or water content.

Long-term landslide monitoring using soil-water relationships and electrical data to estimate suction stress

Soil-moisture fluctuation in the unsaturated zone of shallow colluvial landslides is influenced by many geologic, geomorphic, and seasonal conditions. Long-term field monitoring of variables such as water content, water potential, and electrical conductivity that can establish relationships used for geotechnical and landslide hazard investigations is deficient, particularly in regard to the shallow unsaturated zone. In addition, using electrical-conductivity data as a means to calculate related parameters such as shear strength and suction stress is even less common. The hydrologic conditions in the soil, rainfall, and movement were monitored within an active landslide in southeastern Kentucky to (1) assess soil moisture fluctuations within the landslide and (2) establish soil-water relationships across the slope using constitutive models and a new equation for predicting suction stress. In-situ measurements of volumetric water content and water potential were used in a framework that incorporates electrical conductivity to estimate unsaturated soil properties (soil-water characteristic curves) and suction stress. The framework proves that the relative constitutive equations are valid for long-term soil hydrologic monitoring and that electrical data can be used as a predictor of suction stress. The practical application of developing this framework is to further the understanding of the dynamic movement of water through shallow colluvial soils, which can support landslide hazard and geotechnical investigations.

Stability study of slope reinforced with piles under steady unsaturated flow conditions

In this study, based on the log-spiral upper bound limit analysis method that integrated with the generalized effective stress, a new formula for calculating the lateral stabilizing force provided by a row of piles required to increase the slope safety factor to a desired value under vertical steady unsaturated flow was developed, and the most critical solutions were obtained by the optimization method. Moreover, the proposed method can be simplified to the case without piles for computing the stability number for a slope in the limit state of failure. By comparing the results of the proposed method to others for some examples, its feasibility was verified. Numerical study for the parametric effects was presented, indicating that limit lateral stabilizing force depends strongly on the soil types, the slope geometry, the seepage conditions and that in routine design without consideration of suction stress can lead to a less conservative result in clay and sand, but can obtain a relatively safe result in silt and loess. This paper gives some useful suggestions for engineering application and provides a useful method for further study.

Experimental approach to estimate strength for compacted geomaterials at low confining pressure

It is important to estimate the shear strength of shallow compacted soils as a construction material. A series of constant water content triaxial compression (CWCC) tests under low confining state in this study were performed on compacted geomaterials. For establishing a relationship of the shear strengths between saturated and unsaturated states on compacted geomaterials, the suction stresses were derived by two methods: the conventional suction-measured method and the Suction stress-SWRC Method (SSM). Considering the suction stress as an equivalent confining stress component in the (sigma_net, tau) plane, it was found that the peak deviator stress states agree well with the failure line of the saturated state from the triaxial compression test when the SSM is applied to obtain the suction stress. On the other hand, the cavitation phenomenon on the measurement of suction affected the results of the conventional suction-measured method. These results mean that the SSM is distinctly favorable for obtaining the suction value in the CWCC test because the SSM is not restricted by the cavitation phenomenon. It is expected that the application of the SSM would reduce the time required, and the projected cost with the additional equipment such as a pore water measuring device in the CWCC test.

A Comparison between Simple Shear and Triaxial Tests for Evaluating the Variations of G and D with Matric Suction

Shear modulus (G) and damping ratio (D) are both well known as principal soil dynamic parameters. In the present study, cyclic triaxial and simple shear tests are performed on Firuzkuh silica sand at various shear strain amplitudes using the developed testing devices and peripherals. It is well-understood that degrading curvature of G with shear strain appears in both triaxial and simple shear results. Nevertheless, mean confining stress has dissimilar effects in each of the two tests that does not provide comparable empirical correlations. It is noticed that the variations of G and D with suction stress in triaxial differs from those in simple shear. On the basis of cyclic simple shear results, the increase in suction pressure from zero to the end of transition zone in SWCC leads to increase in G values. In triaxial method, on the other hand, similar increase occurs only up to the inflection point in SWCC, starts reducing afterwards down to a limit value at residual water content. The damping ratio variations with shear strain are generally ascending despite local drops at the strain order of 0.1%, which has appeared in both triaxial and simple shear results.

Determination of Consistency Limits Based on Suction Stress Characteristic Curve

Consistency limits, especially the liquid and plastic limits, are prominent parameters to be determined in geotechnical investigations for classification and identification of finegrained soil. An investigation of the soil suction stress characteristic curve (SSCC) for determination of consistency limits is conducted in this work. The typical SSCC can be divided into five zones according to the state of pore water retention and the mechanisms of solid-liquid interactions. The correspondences between consistency limits of fine-grained soils and characteristic points of every zone in the SSCC are investigated. A detailed discussion of the intrinsic relationships between zones in the SSCC and soil consistency reveals the physical nature of each zone and the critical moisture content. Using experimental data from soil-water characteristic tests of undisturbed loess and disturbed loess, and van Genuchten's soil water characteristic curve (SWCC) model, consistency limits are determined for SSCCs with different void ratios. The feasibility of the presented approach for determining consistency limits of fine-grained soils based on SSCC is demonstrated.

MECHANICAL CHARACTERISTICS AND WATER RETENTION PROPERTIES OF UNSATURATED “BENTONITE-BASED ENGINEERED BARRIER MATERIALS” BASED ON CONTROLLED-SUCTION TESTS

放射性廃棄物の地層処分においては，処分場閉鎖後に地下水の浸入に伴い人工バリアシステムは不飽和状態から飽和状態に向かって遷移するが，廃棄体からの発熱やガス発生により飽和度が一時的に低下する現象も起こる．しかし，ベントナイト系人工バリア材の不飽和力学特性や保水特性に関しては十分なデータが取得されておらず，上記のような過渡的な挙動を予測するための適切な数値モデルを構築できないのが現状である．本研究では，ベントナイト系人工バリア材を対象にマトリックサクションを制御した各種要素試験を行い，吸水・脱水による膨潤・収縮変形特性，不飽和圧密変形特性，不飽和せん断強度特性および保水特性をマトリックサクションとの関係で整理，考察するとともに，それぞれの特性に対する既往の構成モデルの適用性について議論した．

Active earth pressure for soils with tension cracks under steady unsaturated flow conditions

The calculation of active earth pressure has commonly been performed under dry or saturated conditions. However, in practice soils are usually unsaturated. With the aim of estimating active earth pressure under steady unsaturated flow conditions, this paper describes a new method within the framework of the kinematic approach of limit analysis. A closed-form formula is adopted to estimate the suction stress of unsaturated soils subjected to vertical steady flow. A rotational failure mechanism composed of a log-spiral curve and a vertical crack is presented for calculations of the external work rate and internal energy dissipation rate. Pre-existing cracks as well as cracks that form as part of the failure mechanism are considered in the present work. The maximum depth of cracks is determined by requiring the vertical crack boundary to be stable. Explicit expressions about the thrust of active earth pressure for different crack types are derived from the work–energy balance equation, and the most critical solutions are obtained by optimizing the variables from the failure mechanism. Numerical results for different parameters are calculated and given in the form of graphs for practical use. A parametric study is performed to investigate the impact of different parameters on the active earth pressure.

Generalized stress framework for unsaturated soil: demonstration and discussion

The long-lived debate on the principle of effective stress is rooted in the obscure physical significance of stresses. For the sakes of clarifying stress concepts and establishing a reasonable principle of effective stress, unsaturated soil is divided into six phases and the bearing structure of it, named generalized soil structure, is defined based on considering soil as a special structure. Then the essence of effective stress equation, named stress relation equation, is derived according to analysis of interphase interactions and independent-phase equilibrium. The stress relation equation indicates the corresponding relation between two series of stress variables used in mixed and multiphase continuum models, respectively. Furthermore, a reasonable concept of suction stress is redefined to describe interparticle connection properties. Then, a generalized stress framework is constructed by associating stress relation equation with suction stress. After demonstrating the concept of neutral stress, a generalized principle of effective stress is established and the total soil skeleton stress is searched out, which is the predominant stress controlling the strength and deformation of soil. Finally, the collapse phenomenon is analyzed and the time- and spatial-dependent stress frameworks are developed.