Constant Matric Suction Research Articles

Analytical and experimental evaluation of two-layered unsaturated sand bearing capacity

Pavement design methods based on principles of unsaturated soil mechanics take into account high soil shear strength due to matric suction resulting in more economical design especially in long roads. In this study, the bearing capacity of two-layer unsaturated sand was investigated using both analytical and experimental methods. At first, using the limit equilibrium method an analytical formula was proposed to determine the bearing capacity of two-layer unsaturated sand in which linear suction profile was considered in soil layers. It should be considered that the constant matric suction distribution assumed by the previous researchers does not show the real profile of matric suction within the soil, sometimes resulting in miscalculated unsaturated bearing capacity. Also, the bearing capacity of two-layer unsaturated poorly graded sand was investigated experimentally in different suctions by a special unsaturated chamber apparatus (UCA) designed for this purpose. The results show more than double increase of unsaturated soil bearing capacity with Sr = 25% compared to saturated soil. The formation of failure wedges in all experiments was investigated by image processing. An acceptable agreement was obtained between the theoretical and experimental bearing capacity results.

Laboratory Investigation of the Unsaturated Shear Strength Characteristics of Sand–Clay Mixtures

Sandy‐clay mixtures are commonly used in civil engineering projects, such as transportation and water resources, due to their unique construction properties. While the macromechanical characteristics of these mixtures under saturated conditions are well understood, their behavior in unsaturated states is still not fully elucidated. This study aims to investigate the mechanical behavior of sand–clay mixtures under unsaturated conditions by examining the influence of varying sand content and matric suction on their shear strength. The research includes unsaturated shear strength tests and nuclear magnetic resonance (NMR) experiments to analyze the pore characteristics within the mixtures at different sand content levels. The correlation between changes in pore structure and shear strength is systematically analyzed. The unsaturated shear strength of the mixtures is predicted using the Fredlund shear strength formula and compared with the measured values. The results indicate that the unsaturated shear strength of the mixture decreases as the sand content increases but stabilizes once the sand content reaches 20%. Below 20% sand content, the changes in micropore distribution are not significant with increasing sand content. However, beyond 20% sand content, there is a noticeable increase in the ratio of large pores and a decrease in the proportion of small pores. The sand content of 20% represents a critical threshold for the strength and porosity variations in the mixture. Under constant matric suction, the unsaturated shear strength of the mixture improves with the increase in sand content. However, the predictive accuracy of the Fredlund shear strength formula gradually diminishes as the sand content increases.

Experimental study on critical state behaviour of unsaturated silty sand under constant matric suctions

There are relatively few experimental studies on the hydromechanical and critical state behaviours of unsaturated silty sand. In this paper, suction-controlled, consolidated, drained triaxial shear tests were conducted on silty sand under saturated (net confining pressures of 100, 200, 300 and 400 kPa) and unsaturated (suctions of 30, 100 and 200 kPa and net confining pressures of 100, 200 and 300 kPa) conditions. For unsaturated silty sand, the effects of dry density, matric suction and net confining pressure on the hydromechanical and critical state behaviour were investigated, and the critical state parameters were obtained. The main findings for unsaturated silty sand included shear shrinkage at low dry density (1·6 g/cm3) and dilatancy at higher density (1·96 g/cm3). The peak shear strength was positively correlated with dry density, matric suction and net confining pressure. The critical state volumetric strain and critical state degree of saturation were negatively correlated with dry density and matric suction but positively correlated with net confining pressure. A new dilatancy equation that includes the state parameter for unsaturated silty sand was established. A unique critical state line passes through the origin in the effective stress [Formula: see text]–deviator [Formula: see text] plane, and its slope is the saturated critical state stress ratio [Formula: see text]. In the lnp*–v plane ([Formula: see text] is the specific volume), the critical state lines corresponding to different matric suctions are parallel.

Effect of wetting stress paths on mechanical behavior and instability of unsaturated soil in stress state space

In current study, a test program is designed to observe the influence of wetting stress paths on behavior and instability of unsaturated soil in stress state space. The mechanical behavior was studied by performing three test series in constant matric suction plane. In first series, water infiltration was performed together with shearing starting from zero (0 kPa) deviatoric stress. In second series, first water was infiltrated at zero (0 kPa) deviatoric stress then samples were sheared in drained pore water pressure conditions. In third series, water was first infiltrated at zero deviatoric stress then specimens were sheared in undrained pore water pressure conditions. Instability was studied in constant shear stress plane by first shearing soil up to predefined level of deviatoric stress then infiltrating water keeping deviatoric stress constant on the specimen. This research also describes how unsaturated soil behavior and instability is varied with variation in net confining stresses when suction is decreased from initial measured value to start water infiltration with different wetting stress paths and their effect on shear state and failure surface in stress state space. The test results revealed that the movement of stress paths was independent of net confining stress, matric suction and shearing conditions employed in the study.

Fractional-order bounding surface model for unsaturated soils under cyclic loading

Modelling of unsaturated soil stress–strain relationships under cyclic loading with constant matric suction is a topic that is poorly addressed by the current literature. Using the theoretical framework of the bounding surface plasticity and Barcelona basic model (BBM) combined with the concept of fractional accumulated strain rate, a bounding surface model for accurately describing the unsaturated soil stress–strain relationship with constant matric suction is established here. The fractional accumulated strain rate is introduced to consider the influence of cyclic loading history. The movable mapping centre rule is used to describe the hysteresis characteristics of the dynamic stress–strain curve during the unloading process. The shape of the dynamic stress–strain curve can be described reasonably by considering the cyclic loading process as the three processes of initial loading, unloading, and reloading. The cyclic loading test data of unsaturated silt, sand, and lean clay with constant matric suction is used for parameter calibration and model validations. Results show that the proposed model can accurately capture the dynamic stress–strain relationship of unsaturated soil with constant matric suction.

True triaxial mechanical properties of unsaturated loess in foundation pit engineering

Loess is a typical structural unsaturated soil. Because the matric suction of unsaturated loess changes with water migration, the properties of unsaturated soil clearly change. Therefore, the matric suction is considered the main factor affecting the properties of unsaturated loess. Generally, soils are subjected to three principal stresses in practical engineering. To investigate the properties of unsaturated natural loess under complex stress states, a suction-controlled true triaxial apparatus with a rigid-flexible boundary was applied to test unsaturated loess under true triaxial loading. Four sets of suction-controlled tests, including isotropic consolidation followed by monotonic shearing, were carried out, and the matric suction was controlled in the true triaxial tests. The compression yield characteristics of loess were investigated under different matric suctions. The influence of the matric suction and intermediate principal stress parameter were analysed based on the test results. Consolidated drained tests of loess with constant matric suctions were performed under a constant net mean stress and deviatoric stress with different intermediate principal stress parameters. Stress-strain curves and failure envelopes of loess were also presented. The stress-strain-strength properties of unsaturated loess are related to the matric suction and intermediate principal stress parameter under true triaxial stress conditions. The earth pressure of the Q3 loess stratum of the loess foundation pit was analysed based on a strength criterion of unsaturated loess with matric suction.

Effect of Suction and Confining Pressure on Shear Strength and Dilatancy of Highly Compacted Silty Sand: Single-Stage versus Multistage Triaxial Testing

Abstract Evaluating short- and long-term performance of engineered earthen structures and slopes requires proper understanding of the behavior of compacted and highly compacted soils. However, limited studies exist in the literature of unsaturated soil mechanics experimentally testing highly compacted soils, which often possess complex shear strength and dilatancy characteristics. The main objective of this study is to investigate the effect of suction and confining pressure on shear strength and dilatancy of a highly compacted silty sand through multistage and single-stage triaxial testing. Multistage triaxial testing offers a time and cost efficient approach that can reasonably represent field conditions where soils experience fluctuations in external loading conditions (e.g., flood loading/unloading). This article presents the results of consolidated drained multistage and single-stage triaxial tests on highly compacted silty sand under saturated and unsaturated conditions. Soil specimens were isotropically consolidated at a constant matric suction of 0 (saturated), 20, 50, and 95 kPa under net confining pressures of 50, 100, and 200 kPa. Unsaturated specimens consistently exhibited higher shear strength in multistage and single-stage tests compared to saturated specimens. Peak state lines for all unsaturated tests were plotted and found to be near parallel to the saturated peak state line. All tested specimens exhibited dilatancy following a small initial contraction. Although the dilatancy during shearing in multistage tests was comparable to that in the single-stage tests, the cumulative volumetric strain resulted in a net increase in the specimen volume in the case of unsaturated tests, and a net decrease in the case of saturated tests. This difference in volumetric strain behavior resulted in a lower peak shear strength of the saturated specimens tested using multistage testing compared to single stage but a higher peak shear strength in the case of the unsaturated specimens.

Experimental Study on the Mechanical Behaviour of Natural Loess Based on Suction-Controlled True Triaxial Tests

Loess is mostly distributed in an unsaturated state in nature, and the complexity of the engineering properties of unsaturated soil is mainly due to the existence of matric suction. Therefore, matric suction must be considered in investigating the mechanical properties of unsaturated loess. However, soils are most often subjected to three principal stresses with different magnitudes in practical engineering. For the sake of examining and discussing the mechanical behaviour of unsaturated natural loess under a complex stress path, a suction-controlled true triaxial apparatus with a rigid-flexible boundary is used to test unsaturated natural loess under a complicated stress path. Four trials of isotropic consolidation tests are conducted on natural loess under suction-controlled conditions via the true triaxial apparatus. The consolidation yield characteristics of the natural loess under different matric suctions are investigated. Forty-eight trials of consolidated drained true triaxial tests under suction-controlled conditions are conducted on unsaturated natural loess to examine and discuss the influence of the matric suction and intermediate principal stress parameter (b-value). The consolidated drained trials are performed under a constant net mean stress and a constant matric suction with different intermediate principal stress parameters (b-values). Stress-strain curves and failure envelopes of the natural loess are also presented. The results indicate that the stress-strain-strength response of unsaturated natural loess depends on the matric suction and intermediate principal stress parameter under true triaxial conditions.

A Suction-Controlled Ring Device to Measure the Coefficient of Lateral Soil Pressure in Unsaturated Soils

A suction-controlled ring device has been developed to continuously measure the coefficient of lateral soil pressure in deformable unsaturated soil samples from the at-rest to the active condition under application of increasing vertical pressure and controlled matric suction. The device incorporates a thin aluminum specimen ring equipped with horizontal strain gages for recording the lateral soil strains. In addition, a sensor recording water volume changes is utilized to continuously monitor the degree of saturation of the soil sample during tests. The matric suction within the soil texture is controlled using the axis translation technique. In order to verify the performance of the ring device, twenty suction-controlled tests under application of five different constant matric suctions have been performed on two different unsaturated soil types, namely Firouzkouh Clay and Sand-Kaolin mixture, each with two different initial densities. During tests, the lateral soil pressure and corresponding lateral strains have been continuously recorded from the at-rest to the active state of the soil. Accordingly, the coefficients of the lateral soil pressure for the at-rest and the active states of the soil are calculated based on the experimental data and by considering the effective stress concept in unsaturated soils. Finally, the observed responses have been analyzed and the effects of initial density and matric suction on the ratio of lateral to vertical soil pressure have been discussed.

Study on Treatment of Loess Landslide Based on Nanosilica and Fly Ash Composite Stabilizer Filling Fissures

Frequent occurrence of landslides has seriously threatened the infrastructural projects in the Loess Plateau, China. At the beginning, with development of the landslide, several fissures were generated along the internal weak surface. Then, under the constant influence of rainfall and irrigation, the fissures were expanded and connected, which formed a consecutive sliding surface, consequently leading to the occurrence of the landslide. Therefore, the key to treat the landslide is to prevent the expansion and connection of fissures in the potential slip zone inside the slope. In this paper, an old landslide, which occurred at an early stage of the construction of the Lvliang Airport, was selected as a case study. A new type of grouting material was used to fill the fissures and reinforce the loess in the slip zone, so as to study the effect of material content on the slope stability. With current methods, loess specimens were taken from the slip zone inside the Lvliang Airport slope. Based on the GDS triaxial test system, a series of laboratory tests were performed on the undisturbed loess and the remolded which contained the stabilizer, including triaxial shear tests at constant matric suctions and wetting tests at constant deviator stresses. Moreover, the dichotomy in mathematics was chosen to find the threshold of the material content. The test results showed that the cohesion and the internal friction angle of the loess obviously increased after grouting. The failure behaviour of the loess along the wetting path was dependent on the material ratio and the stress level. Meanwhile, the critical state line of the slope, grouted with materials of different ratios, was compared by fitting the regression equation. It was found that the threshold value of the stabilizer content was between 15% and 20%, which was the most suitable ratio for improving slope stability with relatively lower cost.

Mechanisms of wetting-induced loess slope failures

Frequent occurrence of landslides induced by rainfall or irrigation has seriously threatened the urban and rural development in the Loess Plateau, China. The increase in pore water pressure has been identified as a key factor for understanding wetting-induced loess slopes failures. However, experimental studies are limited regarding the increase in pore water pressure of the collapse loess from an initial negative value until failure occurs under constant total stress condition. An old landslide with progressive retreat development at an early stage of the construction of the Lvliang Airport was selected as a case study. Field surveys including exploration wells and boreholes revealed very fresh sliding shear planes and clearly visible cracks, suggesting the creeping movement of the old landslide. In case of heavy rain or long-term rainfall, this old landslide may be resurrected, threatening the stability of the airport site. To examine the mechanism of the failure induced by wetting for this unsaturated loess landslide, loess specimens were taken from the field, followed by performing a series of laboratory tests, including triaxial shear tests at constant matric suctions and wetting tests at constant deviator stresses. The test results revealed that the wetting-induced deformations of the loess included volume and shear deformations, reflecting compression and shearing behaviour induced by wetting. The failure behaviour of the loess along a wetting path was dependent on the stress level and the loss degree of matric suction as well as the hydro-mechanical path, and could be well described by the linear form of the Mohr-Coulomb strength theory. On this basis, the threshold value of the stress level was identified, which could be used to judge whether the wetting-induced failure of the loess occurs. The threshold value of matric suction at failure was also identified to analyse the loss degree of matric suction from stable conditions to failure. The mechanism of the failure of the soil due to wetting revealed from the present study could interpret the rainfall-induced landslide in unsaturated loess.

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．

Matric Suction Creep Characteristics of Reticulated Red Clay

Matric suction creep is the creep of unsaturated soil caused by loading constant matric suction. Matric suction creep characteristics of reticulated red clay with constant net confing pressure and step-loading matric suction was studied using stress-controlling unsaturated triaxial test apparatus. Test and analysis results show that: (1) with constant net confining pressure, axial and radial strain of the clay caused by creep due to matric suction increases obviously; (2) the increments of axial strain of the clay, caused by creep due to matric suction under constant net confining pressure, is not the same as that of radial strain, and the absolute value of radial strain is bigger than that of axial strain; (3) creep strain produced by the same amount of matric suction decreases as the increasing of the matric suction during step loading creep, which means that matric suction makes the clay hard; and (4) the creep strain-time curves of both axial and radial creep can be simulated very well by exponential decay function, and the parameters of the function are matric suction depend obviously.

Tensile and shear failure behaviour of compacted clay – hybrid failure mode

Tensile and shear behaviour of unsaturated soil have been investigated separately in the past. However, there is no comprehensive study on the transition from tensile to shear failure in compacted clayey soils. A series of uniaxial tensile, direct shear and triaxial compression tests were conducted on compacted Regina clay specimens to investigate the strength mobilisation and failure mode under different loading paths. It was observed that the matric suction increases the apparent cohesion and tensile strength of the material. At a given constant matric suction, the compacted clay displays a transition from tensile to shear failure with increasing confining stress which can be approximated by a bilinear failure envelope encompassing the transitional behaviour. The friction angles in tensile regime are greater than those in compressive regime, which can be explained by the transitional change in failure surface morphology from tensile ruptured to shear smoothened one.

Characterization of Compacted Silty Sand Using a Double-Walled Triaxial Cell With Fully Automated Relative-Humidity Control

A fully automated relative-humidity (auto-RH) control unit has been adapted to a newly implemented double-walled triaxial cell to test unsaturated soils under higher total suction states via the vapor-pressure technique, allowing for direct measurement and control of the relative humidity inside the pores of the test soil. The triaxial system is also suitable for implementing the axis-translation technique. With the operational and fully integrated servo-controlled triaxial system, a series of conventional triaxial compression (CTC) tests were conducted on identically prepared specimens of compacted silty sand under constant total suction states of 20 MPa and 300 MPa, induced and controlled via the automated auto-RH control unit. The suitability and reliability of the integrated system was demonstrated by closely repeatable results obtained from the series of suction-controlled CTC tests. Suitable shearing rates for suction-controlled testing of compacted silty sand, via both axis-translation and relative-humidity-based techniques, were also empirically assessed through a series of strain-/suction-controlled tests. The latter were conducted at different axial loading rates (% axial strain per unit time) under either constant matric suction (0.5 MPa) or constant total suction (300 MPa). In both cases, the most suitable shearing rate was identified as the maximum rate for which the test soil continued to be subjected to a constant matric or total suction throughout the entire shearing stage.

Study on the stress path dependency of collapse behavior of Gorgan loess implementing unsaturated oedometer devices

Collapsible soils such as loess are particular types of problematic soils, whose collapse potential have caused severe damages to the buildings and structures. Loess is susceptible to large and sudden volume reduction upon wetting in constant normal stress or upon increasing normal stress in constant water content or a combination of these two stress paths. In this research both of these collapse induced paths: wetting and stress paths are investigated on undisturbed specimens of a Loess to observe the effect of various parameters on the collapse behavior of the tested soil., Seven controlled matric suction unsaturated oedometer tests have been conducted on undisturbed specimens taken from loessial soil of Gorgan, a city in Golestan province in Iran to study the collapse mechanisms of this type of soils. Two identical sets of unsaturated oedometer apparatuses designed and developed at Sharif University of Technology, with the capability of controlling matric suction have been utilized. In these tests, the matric suction, vertical net stress, volumetric water content, and vertical deformation of the samples have been controlled and recorded. Tests have been performed under two separate stress paths: constant matric suction with varying vertical net stress (loading and unloading) and constant vertical net stress with varying matric suction (wetting and drying). Obtained results show the high collapse potential of the tested soil and significant effects of stress path, matric suction, and vertical net stress on the collapse behavior of the tested soil.

Hydraulic and Mechanical Behavior of Unsaturated Silt: Experimental and Theoretical Characterization

AbstractA series of experiments were performed to investigate the hydraulic and κk(Sr)=bk(1−SrSr−Srirr)1/dk, R=DR, WTmechanical behavior of an unsaturated silt obtained from the Yellow River Delta. To identify the key factors that influence the soil-water characteristics (SWC), an experimental program was designed to study the SWC of the soil at different void ratios and vertical pressures. It was shown that an that that apparent hysteresis phenomenon occurred on the SWC curves when the soil experienced drying/wetting cycles. It was also shown that the influence of void ratio on the SWC was significant, and the stress state influenced the SWC only through changing the void ratio. Isotropic compression tests were conducted at different constant matric suctions, using a suction-controlled double cell triaxial apparatus. The soil under shearing showed apparent dilatant and softening behavior, in a way similar to medium to dense sand. In addition, the hardening effect of matric suction was significant, and bo...

Chemo-hydro-mechanical behaviour of compacted Boom Clay: joint effects of osmotic and matric suctions

An experimental programme involving osmotic and matric suction controlled tests is carried out to investigate the effects of increasing pore fluid concentration, chemical cycling (salinisation/dilution) and matric suction on the liquid retention properties, shear strength and volume change behaviour of Boom Clay. This Tertiary Clay formation has been thoroughly investigated as a possible host material for the disposal of Eurobitum bituminised radioactive waste (BW) in Belgium. Within this context chemical impact and partially saturated conditions are studied in a combined way to cover a wide range of in-situ states induced by underground construction, ventilation and diffusion of a large amount of sodium nitrate (NaNO3) (contained in the bituminised waste). Statically compacted specimens are prepared at different sodium nitrate concentrations or are exposed, at constant vertical stress, to sodium nitrate salinisation or pore water dilution. An aggregated structure with a double porosity network is induced on salinisation and subsequent compaction even at high water contents. An increase in pore fluid concentration enhances the aggregation of the microstructure and reduces the liquid retention properties of the material. Results of oedometer tests performed under constant matric suction and increasing osmotic suction show shrinkage of the sample, a decrease of the compressibility and an increase in the preconsolidation stress. Shear strength results have been interpreted using a single constitutive stress (average skeleton stress) to take into account matric suction and chemical effects on degree of saturation and on microstructural changes. A unique linear shear strength failure envelope has been obtained for the tests performed at different osmotic and matric suctions. The modification of the liquid retention behaviour of the material induced by salinisation has proven to be a crucial aspect for the interpretation of the obtained results.

Assessing the Hydro-Mechanical Behavior of Collapsible Soils Using a Modified Triaxial Test Device

Unsaturated loessial Aeolian deposits tend to experience significant volumetric compression when subjected to loading. This behavior is generally attributed to their open, unstable soil fabric and their weak inter-particle bonding forces, which together yield a soil void structure that is susceptible to collapse. The current study examines two possible mechanisms of pore collapse in a loessial soil: pore collapse induced by an increase in net confining stresses under constant matric suction conditions, such as what occurs when a new structure is constructed on top of a collapsible soil deposit, and pore collapse induced by a change in matric suction conditions under a constant net confining stress, such as what occurs when a collapsible soil deposit beneath an existing structure experiences significant wetting due to a large precipitation event. To accomplish this task, an innovative test approach was used to assess the hydro-mechanical behavior of a highly collapsible loessial soil. The test setup incorporates a set of electronic pressure regulators coupled with three electronic pressure sensors to measure and control the applied pressures, and two high-precision digital volume change measuring devices to measure changes in the volume of the specimen and its degree of saturation. Using this approach, “undisturbed” loess specimens are subjected to either isotropic compression at a constant matric suction, or wetting-induced collapse at a constant mean net stress. Results indicate that the hydro-mechanical behavior of collapsible soils is considerably stress-path dependent. For the same values of mean net stress, the deformation measurements of specimens subjected to isotropic compression were often larger than those subjected to wetting-induced collapse. For the isotropic compression tests, it was shown that the soil water retention curve of the soil depends on the mean net stress. Less sensitivity to the mean net stress was observed for specimens subjected to wetting-induced collapse.

Modelling of shear stiffness of unsaturated fine grained soils at very small strains

The shear modulus at very small strains (less than 0.001%) is an important parameter in the design of geotechnical structures subjected to static and cyclic loadings. Although numerous soil models are available for predicting shear modulus of saturated and dry soils, only a few ones can predict shear stiffness at very small strains of unsaturated soils correctly. In this study, a few unsaturated soil models are evaluated critically and compared with a newly developed model. This newly proposed model is verified by using measured shear modulus at very small strains for three different low plasticity fine grained soils available in the literature. It is found that this new model can predict shear modulus at very small strain resulting from an increase and a decrease in mean net stress at constant matric suction for low plasticity fine grained soils. Moreover, this model is able to give a reasonably good prediction on shear stiffness at very small strain during wetting of a collapsible unsaturated soil. In addition, the newly proposed model is illustrated to capture a consistent trend with experimental data of shear stiffness at very small strain for non-collapsible soils obtained during drying–wetting cycles. This evaluation revealed that the newly proposed model has better predictive capabilities than some earlier formulations of the same simplicity. In addition, the proposed model with fewer parameters has similar predictive capability as compared with a more complex model.