Compacted Clay Specimens Research Articles

Fracture process zone of mode I in compacted clay through digital image correlation

To study the generation characteristics and length variation of the soil fracture process zone (FPZ), mode I fracture tests were conducted on compacted clay specimens with different moisture contents and dry densities. Combined with the digital image correlation (DIC) method, the displacement fields of prefabricated crack tips at different loading stages were obtained. The results showed that the mode I fracture toughness (KIC) increased and then decreased as moisture content increases, and increased linearly with dry density. As the load increased, the FPZ generation rate accelerated. The FPZ length at peak load ranged from 7.92 mm to 12.54 mm and continued to increase after peak load. The FPZ length calculated with the Schimdt model was lower than that measured by the DIC method. Finally, the differences in the FPZ lengths of compacted clay and rock-like materials were discussed.

Study on Osmotic Consolidation and Hydraulic Conductivity Behavior of an Expansive Soil Inundated with Sodium Chloride Solution

Canals are a very imperative source of irrigation for the agricultural sector in India. Seepage causes major water loss in canals, and hence, the installation of liners becomes necessary. Compacted clay soils are commonly used as liners in the canals. This structure will most probably be subjected to salinization and desalinization cycles throughout its life. Because of the interaction between the pore liquid and clay particles, physico-chemical influences considerably impact the behavior of clay barriers. In this paper, the effect of interacting fluid on volume change, consolidation parameters, and hydraulic conductivity of compacted clay soil is investigated with the help of a one-dimensional consolidation test. The compacted clay specimens were immersed alternatively with distilled water (DW) and sodium chloride (NaCl) solutions (SW) at constant loading of 10 kPa, which replicates the load conditions in the field canal due to 1 m head of water and incremental loading as per IS 2720 part 15 standards. The experimental results proved that there is a percentage volume change increase of about two times for each stage inundated with 4M NaCl solution than its preceding stages inundated with distilled water at constant loading of 10 kPa. The consolidation rate was accelerated with 4M NaCl solution than the normal consolidation at incremental loading. The permeability coefficient in the salt water-induced sample increased by 217% more than the distilled water-induced sample at incremental loading. Therefore, the soil specimen subjected to alternate salinization and desalinization cycles significantly affects the volumetric and consolidation behavior, leading to decreased life of clay barrier structures.

The Effect of Initial Water Content and Density on the Swelling-Shrinkage and Cracking Characteristics of Compacted Clay

The swelling-shrinkage and cracking characteristics of compacted clay under the coupling effect of initial conditions are rarely studied. The dry-wet cycle test of compacted clay with varying initial water contents and densities was performed in this study; the size and cracking conditions were investigated. The results showed that when the initial moisture content was 21% and the dry density was 1.65 g/cm3, the longitudinal expansion amount of the compacted clay was relatively small. However, it was rather large when the initial water content was 17% and 19%. Under the same dry density, the final vertical shrinkage ratio of the sample with a water content of 21% was the smallest, while that of the samples with a water content of 23% and 25% was rather big. Under the same water content, the final vertical shrinkage ratio decreased with the increase in dry density. After three wetting-drying cycles, compacted clay with a dry density of 1.65 g/cm3 and an optimal initial water content of 21% produced the fewest cracks and had the lowest cracking factor (CF) (only 7.58%). The compacted clay specimens with the dry densities of 1.55 g/cm3 and 1.60 g/cm3 had rather significant cracking at the same initial moisture content of 21%. The mercury intrusion porosimetry (MIP) test demonstrated that in the first two dry and wet cycles, the distribution of large pores decreased and that of tiny pores increased. After the third cycle, the distribution of small and medium pores decreased slightly. The results of this study will provide theoretical guidance for selecting cover soils in landfills.

Shrinkage behaviour of slurry-consolidated and compacted clay soil

This research investigates the shrinkage behaviour of clay specimens subjected to different hydraulic paths. The volume change and microstructure of desiccated clay specimen directly dried from reconstituted slurry and compacted states, and the desiccated clay specimens obtained by subjecting the as-compacted clay specimen to cyclic wetting and drying process were compared. The experimental results showed that the initial microstructural difference between reconstituted slurries and compacted specimens influences their shrinkage behaviour. The compacted clay specimens retained the bimodal structure even after subjecting to cyclic wetting and drying, unlike the reconstituted slurry, which showed a predominant unimodal structure at the end of drying process. This is due to the absence of large macropores in the desiccated clay specimen dried from reconstituted slurry state, which is evidenced from the mercury intrusion porosimetry studies, scanning electron microscopy and nano-focus X-ray-computed tomography imaging experiments.

Suitability of swelling and collapse theory proposed based on virgin compression surface

This study investigates the suitability of the swelling and collapse theory proposed based on a constitutive virgin compression surface (VCS) developed within the modified Monash Peradeniya Kodikara (MPK) framework. The modified MPK approach incorporates net stress, void ratio, and moisture ratio as state variables to interpret the swelling-collapse behavior of unsaturated soils. The soil selected for this research was an expansive Quaternary age basaltic residual clay located in Victoria, Australia. The experimental program included testing on unsaturated compacted clay specimens and clay specimens stabilized with lime at the optimum lime content (OLC). The OLC was obtained based on the swelling potential. Static compaction tests were conducted on untreated and lime-treated samples to establish the VCS and to propose the relationship between moisture ratio and net stress. Next, 1-D compression and consolidation laboratory tests were carried out to investigate and compare the mechanism of collapse and swelling based on well-established suction-based theories and the moisture content-based approach of this research. The swelling and collapse response obtained through both theories were very close which verified the suitability of the moisture content-based approach, being the Modified MPK framework. This study also investigated the application of suction-controlled experimental data extracted from the relevant literature within the modified MPK framework. The novelty of this study is proposing and validating a framework for conversion of the results between two different partial theories to interpret the volumetric behavior of expansive clay. Finally, a new method is proposed to estimate the swelling-collapse state of a sample through wetting without the need to establish the VCS.

Effect of pore fluid and wet-dry cycles on structure and hydraulic conductivity of clay

Compacted clays in landfill applications are subjected to both physico-chemical changes and wet–dry cycles. Physico-chemical interactions occur at the microstructural level between the clay particles and chemical constituents such as brine solutions or leachates generated from landfill waste. Furthermore, the volumetric changes during wetting and drying also cause microstructural changes to compacted clays. The microstructural changes due to the physico-chemical interactions and wet–dry cycles are reflected at the macrostructural level and govern the macro-behaviour of compacted clays. Therefore, this paper brings out the combined effect of the interacting fluid and wet–dry cycles on volume change, microstructure and hydraulic conductivity of compacted clay soil. To achieve this, the compacted clay specimens were inundated with distilled water and sodium chloride (NaCl) solutions during wetting cycles. The experimental results showed that the ability of clay particles to swell at microstructural level and decrease the size of macropores in compacted clay was completely lost with 4 M sodium chloride solution and with increase in wet–dry cycles. Thus, the hydraulic conductivity of compacted clay specimen inundated with 4 M sodium chloride solution was unusually high at the end of the second and further wetting cycles even when subjected to higher effective confining pressures.

Influence of freeze-thaw cycles in the presence of a supplementary water supply on mechanical properties of compacted soil

In seasonal frozen regions where the groundwater is relatively shallow, the upward migration of groundwater during the freezing process will change the moisture content of the subgrade soil after freeze-thaw (F-T) cycles. This study attempted to investigate the deterioration effect of F-T cycles in combination with available groundwater migration on the mechanical properties of compacted clayey soil. For this purpose, the compacted clay specimens were frozen and thawed from the top to bottom and external water was supplied from the bottom of the samples. Specimens with different initial states (i.e. different initial dry densities and initial moisture contents) were subjected to different number of F-T cycles. The changes in moisture content and frost heave after the F-T cycles were measured. The failure strength and elastic modulus of the soil then were determined using unconsolidated and undrained triaxial compression tests. The results of the study demonstrated that water replenishment of the soil during the freezing process had considerable influence on the mechanical properties of soil, and the availability of a supplementary water supply could accelerate the development of stable state in the failure strength and the elastic modulus of soil. Moreover, the reduction rates of failure strength and elastic modulus reached 85% and 92% after 7 F-T cycles, respectively. The degradation effect of the F-T cycles on the mechanical property of soil can be reduced by enhancing the dry density or by moderately raising the moisture content of soil. Finally, the fitted models describing the combining effects of F-T cycles, initial dry density, initial moisture content and confining pressure on the failure strength and elastic modulus of soil were developed and verified by the experimental data to be rational and reliable.

Physico-Chemical Effects on Shrinkage Behavior of Compacted Expansive Clay

AbstractPhysico-chemical effects have a significant impact on the behavior of clay barriers due to the interactions between the pore fluid and clay particles, and they pose a great challenge because the efficiency of clay barriers and cover systems may be altered. Therefore, this paper highlights the effect of physico-chemical factors on shrinkage behavior of compacted expansive clay. To achieve this, the compacted clay specimens were inundated with distilled water, NaCl, and CaCl2 salt solutions in separate oedometer cells at a vertical pressure of 6.25 kPa and allowed to swell. The swollen specimens were gradually shrunk until the specimens attained constant mass, and the changes in void ratio and water content were monitored during drying. The experimental results showed that magnitude of induced osmotic suction and type of pore fluid had a significant impact on shrinkage behavior of compacted clay specimens due to the changes in soil structure. The average pore size of compacted specimens significantl...

Compression mechanisms of unsaturated clay under high stresses

This paper investigates the compression behavior of unsaturated clay under mean stresses up to 160 MPa and different drainage conditions. A new isotropic pressure cell was developed that incorporates matric suction control using the axis-translation technique, and a high-pressure syringe pump operated in displacement-control mode was used to control the total stress and track specimen volume changes. In addition to presenting results from characterization tests on the cell, results from a series of isotropic compression tests performed on compacted clay specimens under drained and undrained conditions are presented. These results permit evaluation of the hardening mechanisms and transition points in the compression curve with increasing effective stress. As expected, specimens tested under undrained conditions were much stiffer than those tested under drained conditions. In the drained tests, the rate of compression was sufficient to permit steady-state dissipation of excess pore-water pressure except under the highest stress ranges. Suction-induced hardening was observed when comparing saturated and unsaturated specimens tested in the drained compression tests. In both the drained and undrained compression tests, the range of applied stresses was sufficient to cause collapse or dissolution of the air voids (pressurized saturation) and convergence of the virgin compression lines for unsaturated specimens with that measured for saturated specimens. A gradual transition to full-void closure was observed at high stresses when the compression curves were plotted on a natural scale, but the shapes of the compression curves at high stresses were not consistent with conventional soil mechanics models when plotted on a semilogarithmic scale. The results from this study provide insight into how constitutive models for unsaturated soils can be extended to high stress conditions for drained and undrained conditions.

Crystalline and Osmotic Swelling of an Expansive Clay Inundated with Sodium Chloride Solutions

Compacted expansive clays swell due to crystalline swelling and osmotic/double layer swelling mechanisms. Crystalline swelling is driven by adsorption of water molecules at clay particle surfaces that occurs at inter-layer separations of 10–22 A. Diffuse double layer swelling occurs at inter-layer separations >22 A. The tendency of compacted clay to develop osmotic or double layer swelling reduces with increase in solute concentration in bulk solution. This study examines the consequence of increase in solute concentration in bulk solution on the relative magnitudes of the two swelling modes. The objective is achieved by inundating compacted expansive clay specimens with distilled water and sodium chloride solutions in free-swell oedometer tests and comparing the experimental swell with predictions from Van’t Hoff equation. The results of the study indicate that swell potential of compacted expansive clay specimens wetted with relatively saline (0.4, 1 and 4 M sodium chloride) solutions are satisfied by crystalline swelling alone. Comparatively, compacted clay specimens inundated with less saline solutions (0.005–0.1 M sodium chloride) require both crystalline and osmotic swelling to satiate the swell potential.

Influence of Osmotic Suction on the Soil-Water Characteristic Curves of Compacted Expansive Clay

Unsaturated clays are subject to osmotic suction gradients in geoenvironmental engineering applications and it therefore becomes important to understand the effect of these chemical concentration gradients on soil-water characteristic curves (SWCCs). This paper brings out the influence of induced osmotic suction gradient on the wetting SWCCs of compacted clay specimens inundated with sodium chloride solutions/distilled water at vertical stress of 6.25 kPa in oedometer cells. The experimental results illustrate that variations in initial osmotic suction difference induce different magnitudes of osmotic induced consolidation and osmotic consolidation strains thereby impacting the wetting SWCCs and equilibrium water contents of identically compacted clay specimens. Osmotic suction induced by chemical concentration gradients between reservoir salt solution and soil-water can be treated as an equivalent net stress component, ( pπ ) that decreases the swelling strains of unsaturated specimens from reduction in ...

Swell–compression behaviour of compacted clays under chemical gradients

The present study examines the effect of inflow of sodium chloride solutions on the swell–compression behaviour of compacted expansive clays under a range of external loads. Inflow of sodium chloride solutions reduced the swell magnitudes and pressures and even caused the compacted clay specimens to experience compressive strains in oedometer tests. The axial strain difference of specimens inundated with sodium chloride solutions and distilled water at a constant effective stress predicted the osmotic consolidation strains of a saturated clay specimen from an increase in osmotic suction in the pore fluid. Specimens exposed to larger osmotic suction gradients (Δπ) are predicted to experience larger osmotic consolidation strains. The predicted osmotic consolidation strains exceeded the experimental osmotic consolidation strains. The free-swell and load method and the swell under load procedure predicted different swell pressures owing to differences in strain contributions at the microstructure and macrostructure levels. The increase in osmotic suction in pore water apparently acts as an equivalent net stress (ρπ) that favours a reduction in swell potential of the compacted clay specimens due to a reduction in reversible swelling strains from an increase in net stress and irreversible macrostructural component that decreases because the distance of the stress point to the load–collapse (LC) curve decreases.Key words: compacted clays, chemical gradients, osmotic consolidation, suction, swell.

Role of direction of salt migration on the swelling behaviour of compacted clays

Osmotic suction arises in clay soils due to the presence of dissolved salts in soil water. Variations in dissolved salt concentration of soil water impact the osmotic suction balance between soil water (of clay soil specimens) and external reservoir that gets adjusted through diffusion of salts and osmotic flow of water molecules. Available theoretical and experimental studies in literature mostly focus on the behaviour of clay samples reconstituted from slurries. Compacted clays differ from clay samples reconstituted from slurries as they are characterized by both matric suction and osmotic suction. Compacted clays are typically exposed to osmotic suction gradients in landfills and brine ponds where compacted clay liners are in contact with leachate/brine solutions. Upon exposure to solutions, diffusion of salts dissipate osmotic suction gradients, while, absorption of salt solutions in the partly saturated void spaces dissipate matric suction of the compacted clay specimens. The impact of dissipation of matric suction and osmotic suction gradients on the swelling behaviour of compacted expansive clays forms the focus of the present study. Experimental results illustrated that the direction of salt diffusion during dissipation of osmotic suction difference has a significant bearing on the swelling behaviour of compacted clay specimens.

Deformation Characteristics of a Compacted Clay in Collapse Under Isotropic and Triaxial Stress State

Deformation behavior of an unsaturated soil in collapse was studied using a modified triaxial test apparatus. Three kinds of wetting test were conducted on compacted clay specimens, (i) wetting tests under isotropic stress state, (ii) wetting tests under constant shear stress, and (iii) repeated wetting during shear test under a constant mean net stress. Collapse occurred later in comparison to isotropic stress state conditions under triaxial stress state when shear stress was kept constant. This phenomenon is due to the decrease of the coefficient of permeability, which relates to the changes associated with bulk water to meniscus water. The relationship between void ratio change and increase in water content observed under the different stress states tested showed similar behavior. In the repeated wetting during shear test, a continuous decrease in void ratio was observed with water absorption and drainage. This phenomenon is also related to the change of the bulk water to meniscus water. The secondary wetting process is observed in the experimental testing program discussed in this paper. The presently available constitutive models do not take into account the secondary wetting process. The research studies presented in this paper provide more insight into the collapse behavior.