Relationship between soil structure and mechanical behavior for an expansive unsaturated clay

Chapter 18 - Pile behavior modeling in unsaturated expansive soils

Studying the wetting-induced softening behavior of unsaturated soils is important for understanding rain-induced slope failures in unsaturated soils. However, experimental studies on expansive soils are limited. This study investigates the wetting-induced softening characteristics of an unsaturated expansive clay via suction-controlled triaxial tests on recompacted and natural specimens. It was found that the wetting-induced swelling of the soil included a significant component of plastic strain and its magnitude decreased with the net confining stress. When a soil specimen was subjected to wetting at a constant deviator stress, a threshold value of soil suction was identified after which the axial strain increased greatly. This value tended to increase with the applied net stress ratio. After the threshold suction, the wetting process for recompacted specimens could be divided into a yielding process associated with volumetric strain hardening/softening and a failure process in which the volumetric behavior depended on the applied net stress ratio. The natural specimens exhibited a brittle failure like a heavily overconsolidated soil during wetting at a constant deviator stress and continuous volumetric dilation occurred after the threshold suction. The wetting-induced softening soil behavior revealed from the present study could interpret the rain-induced progressive slope failure in unsaturated expansive soils.

This study focuses on the advancements of characterization of unsaturated expansive soils which would allow better prediction of its swelling and mechanical behaviours. The paper has been divided into three parts. Firstly, a novel model is developed by considering two micro-soil parameters, soil-specific surface area and internal pore size distribution, to predict the natural swelling in expansive clayey soils. The model was validated by comparing the predictions with experimental results for eight soils. Secondly, the paper deals with the diffused-double-layer (DDL) theory-related electrostatic forces from individual clay minerals of expansive soils and their influence on soil swelling. A DDL-based model was also developed, which was validated for swell prediction for fourteen expansive soils. A good correlation was observed for both models when the results were compared with experimental results. Finally, the paper demonstrated the use of vapour pressure technique to control suction beyond 5 MPa for performing suction-controlled repeated load triaxial (RLT) test at high suction state. This novel technique mitigates the limitation of axis-translation technique for maintaining suction till the air-entry-value (AEV) of the ceramic disk. These new models and unsaturated laboratory testing techniques would aid in better characterization of expansive soils in their unsaturated state. This would also provide accurate prediction of swell and stiffness behaviours of soils, thereby reducing the uncertainties in the design of civil infrastructure built on expansive soils.KeywordsUnsaturated soilsSwellingExpansive soilsResilient modulusHigh-suction state

A thermo-chemo-electro-mechanical framework of unsaturated expansive clays

Proceedings of geotechnical sessions at the 2001 Annual Civil Engineering Conference, held in Houston, Texas, October 10-13, 2001. Sponsored by the Shallow Foundation and Soil Properties Committee of the Geo-Institute of ASCE. This Geotechnical Special Publication contains 14 papers that discuss the effects of trees and moisture movements on various lightly loaded foundations on expansive clays. Papers review current state of the practice in foundation design, maintenance, and remediation as it relates to vegetative effects and expansive clays on foundations. Topics include: tree root damage to buildings; predicting volume changes in expansive soils resulting from environmental factors including vegetation; drought; expansive clay soil problems and solutions throughout the world; soil-structure interaction; field test program to obtain geotechnical properties; depth of wetting and active zone in expansive soils; slabs on expansive clay with vegetation considerations; volume change coefficient of expansive clay; in-situ modification of active clays for shallow foundation remediation; cracking of pavement due to drying shrinkage; soil suction measurements; and data base on under-slab moisture contents in Texas.

This study aimed to evaluate the influence of the addition of coconut (coir) fibers on the microstructural, hydraulic and mechanical behavior of an unsaturated compacted soil. Specimens were molded and compacted, forming composites with 0%, 0.1%, 0.5% and 1% fiber in relation to their dry mass. The characterization of pores from the soil and fiber soil mixtures was performed by the Mercury Intrusion Porosimetry tests. Suction values were obtained through the filter paper method and soil water retention curves were adjusted with the Durner model due to the bimodal behavior. Tensile strength values were obtained from the indirect tensile strength test (Brazilian tensile test) for specimens with different suction values. It was found that the increase in fiber content in the material lead to a non-linear increase in macropores, which affected both the hydraulic and mechanical behavior of the soil. Furthermore, the shape of the soil water retention curve was preserved, but there were changes in the values of first and second air entry and residual suction. The tensile strength was negatively influenced, reaching a reduction of about 30% in the situation with higher fiber content. However, for higher levels, the behavior of the soil changed from brittle to ductile, increasing the supported deformations.

Expansive soils in unsaturated state cause problems to several civil engineering structures due to its transient volume change behavior. Present study is focused on evaluation of swelling and collapsible characteristics of unsaturated compacted expansive soil specimens and their relationship with suction. A series of volume change swell pressure tests and double oedometer tests were conducted at varying degree of saturation to evaluate swelling pressure and collapse potential of Bharuch expansive soil respectively. Suction measurements of soil were conducted by employing WP4C Dew point Potentiameter at different water content. Presence of higher negative pore pressure induced higher swelling pressure, which was observed to vary exponentially with total suction. Results indicated significant influence of soil suction on soil fabric arrangement, which substantially affected collapse behaviour of expansive soil. Collapse potential of Bharuch expansive soil was observed to decrease with increase in water content due to reduction in suction of soil indicating metastable state of soil at lower degree of saturation.

In the southwest of China, there are widely distributed expansive soils. However, to save costs and manage the speed of construction, these shallow expansive soils are often filled with subgrade materials. Therefore, it is necessary to clearly understand the mechanical behaviors of unmodified shallow expansive soils. Current research on the mechanical behaviors of shallow expansive soils is mainly focused on shear and compressive strengths but rarely on the tensile strength since general tests are costly, time consuming, and difficult to conduct. Therefore, uniaxial tensile, unconfined compression and direct shear tests were carried out to study the mechanical behavior of shallow unsaturated expansive soils under different saturation degrees, and the tests analyzed the change mechanism of its mechanical behavior. The following were found: (1) with an increase in saturation degree, the uniaxial tensile strength, unconfined compressive strength, shear strength, cohesive force, and internal friction angle first increased and then decreased; (2) when the saturation degree increased from 18.7% to the saturation degree corresponding to the peak, the uniaxial tensile strength, unconfined compressive strength, cohesive force, and internal friction angle increased by about 11 times, 3.24 times, 2.34 times, and 0.52 times, respectively; (3) when the saturation degree increased from the saturation degree corresponding to the peak to 80.3%, they decreases by about 42%, 51.4%, 36%, and 50%, respectively; (4) with the increase in dry density, the saturation degree corresponding to the peak of uniaxial tensile strength gradually increased, while the saturation degree corresponding to the peak of unconfined compressive and shear strength did not significantly change.

A model considering solid-fluid interactions stemming from capillarity and adsorption mechanisms in unsaturated expansive clays

A mechanical approach for unsaturated expansive soils considering double porosity has been developed based on the porous media theory. In this approach, the adsorbed and the capillary water, as well as the micropores and macropores, are two distinct phases. An interaggregate stress considered as the work-conjugate of the macrostructural strain increment has been defined. Both physicochemical and capillary effects of the pore water have been introduced at the macroscopic level. Other work-conjugate variables relevant for the constitutive modeling of double-porosity unsaturated media have also been identified, consisting of the modified suction as conjugate of the increment of the macrostructural degree of saturation and the microstructural effective stress as conjugate of the microstructural volumetric strain increment. A hydromechanical model for unsaturated expansive clays taking into account the interaction between the micro- and the macrostructures in expansive clays can thus be built. Based on the bounding surface concept, an anisotropic loading – collapse yield surface has been introduced to reproduce the three-dimensional mechanical behavior. To analyze the model capabilities, two series of laboratory tests consisting of multiple wetting and drying cycle tests on Boom clay and triaxial tests on Zaoyang (ZY) expansive clay were simulated. The comparisons between numerical and experimental results show that the model can reproduce with reasonable accuracy the mechanical behavior and the water retention characteristic of unsaturated expansive clays.

Construction on weak expansive soils is challenging due to their low bearing capacity and high-volume susceptibility under moisture fluctuation. The uplift pressure from expansive clay can induce significant swelling pressure on foundations, but on the other hand, shrinkage of clay can result in substantial foundation settlement during dry seasons. This differential movement of the foundation can distress the superstructure of a building, resulting in serviceable and ultimate limit state failures. The current approach to dealing with foundations in expansive clay soils is to construct a rigid slab that can withstand the anticipated movement or a pier-type approach using engineering design principles or a normal-type slab based on the fill being placed under controlled conditions. We introduce a capping layer under the foundation to control moisture fluctuation and increase the bearing capacity of the foundation using a recycled glass-based stabilization approach. A prototype foundation was constructed in the laboratory using an optimum stabilization mix design that was derived using glass and other sustainable additives. Slab movements and soil conditions were monitored over a 6 month period under simulated dry/wetting moisture fluctuations and operational loads. Results revealed that the performance of the foundation under controlled conditions was outperformed by the capping layered foundation during service loadings including seasonal moisture fluctuation. The outcomes from this research will have a significant impact on improving foundation performance in expansive soils, as well as proposing a sustainable foundation construction process using recycled glass waste.

Inclusion of expansive clay behaviour in coupled thermo hydraulic mechanical models

This research presents a series of compression load tests conducted on a single pile embedded in saturated and unsaturated expansive clayey soil. Soil water characteristics curve (SWCC) and initial soil suction (total and matric) are measured using the filter paper technique to study the influence of soil suction on the pile and tip capacities. Soil pressure gauge is used and embedded in the soil under the pile at a distance of 2D. The results showed that the ultimate pile capacity is increased when the initial degree of saturation decreases. In addition, the tip capacity is increased when the degree of saturation decreases. Then, the sharing ratios of the ultimate tip and shaft capacities to the ultimate pile capacity are calculated. The results have shown that the sharing ratio of the ultimate tip capacity to the ultimate pile capacity of a single pile is increased with the decrease in the degree of saturation while the ratio of the shaft capacity to the ultimate pile capacity is decreased.

Expansive soil is the most predominant geologic hazard which shows a large amount of shrinkage and swelling with changes in their moisture content. This study investigates the macro-mechanical and micro-structural behaviours of dredged natural expansive clay from coal mining treated with ordinary Portland cement or hydrated lime addition. The stabilised expansive soil aims for possible reuse as pavement materials. Mechanical testing determined geotechnical engineering properties, including free swelling potential, California bearing ratio, unconfined compressive strength, resilient modulus, and shear wave velocity. The microstructures of treated soils are observed by scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy to understand the behaviour of the expansive clay blended with cement and lime. Test results confirmed that cement and lime are effective agents for improving the swelling behaviour and other engineering properties of natural expansive clay. In general, chemical treatments reduce the swelling and increase the strength and modulus of expansive clay, subjected to chemical content and curing time. Scanning electron microscopy analysis can observe the increase in formation of particle clusters with curing period, and x-ray diffraction patterns display hydration and pozzolanic products from chemical particles. The correlations of mechanical properties and microstructures for chemical stabilised expansive clay are recommended.

By combining the pressure plate, saturated salt solution–vapour equilibrium, and WP4C instrument methods, a soil-water characteristic curve (SWCC) for expansive soil at all suctions during drying‒wetting cycles was obtained, and expansive soil hysteresis and volume changes were analysed. Microstructural scanning electron microscopy (SEM) and pore size distribution (PSD) tests were conducted on samples along the wetting path to study microstructural evolution of expansive soil in canal embankments at all suctions. The expansive SWCC at all suctions exhibits hysteresis due to the nonuniform pore distribution, contact angle hysteresis, and trapped air. The energy required for the suction balance differs between the wetting and drying paths. The pore volume of saturated expansive soil is the largest, and the volume decreases with increasing matric suction. Same-void-ratio expansive soils have different degrees of saturation, indicating a significant coupling effect between saturation and void ratio (specific volume) with changes in matric suction. Expansive soils have dual-pore structures with inter/ inter- and intra-aggregate pores. The boundary between the inter/intra-aggregate pores is 0.2 μm. The intra-aggregate pore volume distribution is consistent at all suctions. During initial wetting, the dominant pore diameter decreases and the macropores close. During intermediate wetting, the pore distribution remains relatively unchanged. During final wetting, the pore distribution changes and aggregate expansion deformation and soil expansion occur. Due to differences in the liquid phase properties of macro- and micropores, the pore structure and distribution of expansive soil undergo significant changes with variations in hydromechanical properties.