Charbel Khoury

Effects of soil skeleton deformations on hysteretic soil water characteristic curves: Experiments and simulations

[1] Soil water characteristic curves (SWCCs) represent the relationship between suction and water content in unsaturated soils. The SWCCs exhibit hysteresis during wetting-drying cycles; however, the empirical expressions used to describe SWCCs have typically ignored the hysteresis. Additionally, the shape of the SWCC will vary depending on the void ratio of the soil and changes resulting from soil skeleton deformations, which may also show hysteretic behavior under various loading conditions. Therefore, it is important to investigate, both experimentally and theoretically, the relationship between soil skeleton deformations and the SWCC for different soils. There is limited information in the literature that examines, both experimentally and theoretically, the complex coupling between the soil skeleton deformation and SWCC behavior, and generally, this behavior is not well understood. This paper presents laboratory test results of SWCCs determined under different confining stresses on similarly prepared samples of a silty soil; drying, wetting, second drying, and scanning curves were obtained. The influence of soil skeleton deformations on SWCCs is inferred from the curves measured in an oedometer under different stress conditions. An elastoplastic phenomenological constitutive model based on the bounding surface plasticity theory was utilized to simulate the coupled mechanical-hydraulic behavior of measured results. This research demonstrates that the model is capable of predicting hysteresis in SWCCs and soil skeleton deformation and the coupling between the hydraulic and mechanical behavior of unsaturated soils.

Effect of Cyclic Suction History (Hydraulic Hysteresis) on Resilient Modulus of Unsaturated Fine-Grained Soil

This study explored the effect of hydraulic hysteresis on the resilient modulus (Mr) of subgrade soils. For that purpose, suction-controlled Mr tests were performed on compacted specimens along the primary drying and wetting and secondary drying and wetting paths. First, Mr tests were performed continuously on the same sample at various suction values along all of the paths (drying, wetting, etc.). A relationship between Mr and matric suction was obtained and identified as the resilient modulus characteristic curve (MRCC). MRCC results indicated that Mr increased with suction along the drying curve. Results along the primary wetting curve indicated a higher Mr than along primary drying and secondary drying curves. The second series of tests, referred to as virgin tests, were performed at suction values without subjecting the sample to previous Mr testing. Results indicated that Mr values were lower compared with results from the other type of test (i.e., with continuous Mr testing), and that Mr results were influenced by the continuous stress loading. However, Mr values along the wetting curves were still higher than for the drying curves for these virgin tests and therefore indicated that results were also influenced by hydraulic hysteresis (i.e., cyclic suction stress loading). New models to predict the MRCC results during drying and wetting were proposed on the basis of the soil–water characteristic curve hysteresis. The models favorably predicted the drying and then the wetting results with the use of the soil–water characteristic curve at all stress levels.

Modeling Resilient Modulus Hysteretic Behavior with Moisture Variation

AbstractThe changes in subgrade moisture conditions play an important role in the in-service performance of a pavement. These conditions are sensitive to environmental conditions, such as a rise in the water table, precipitation, freeze-thaw cycles, and wet-dry cycles, among others. Several studies have been conducted to develop relationships between the mechanical properties, namely, the resilient modulus (MR) of subgrade soils and moisture conditions. No studies, to the authors’ knowledge, have directly addressed the hysteretic behavior of MR with moisture conditions. This study evaluates the effect of moisture hysteresis on MR values of a subgrade soil following two different paths: (1) drying-wetting-drying path and (2) wetting-drying path. Initially, cylindrical specimens were prepared at approximately optimum moisture content and maximum dry unit weight. The moisture content in each compacted specimen was altered to achieve the target moisture content. Specimens were then tested for MR values. The l...

Influence of Hydraulic Hysteresis on the Shear Strength of Unsaturated Soils and Interfaces

The soil water characteristic curve (SWCC) portrays the relationship between soil suction and water content. This curve represents a fundamental behavior of unsaturated soils, from which many engineering properties (e.g., hydraulic conductivity, shear strength) can be estimated. However, the SWCC exhibits hysteretic behavior where a soil with different values of moisture content (e.g., due to seasonal variations) corresponding to drying and wetting paths can have the same suction. Hysteretic behavior (hydraulic hysteresis) can significantly influence the mechanical response of soils and soil-structure interfaces. This paper describes an investigation of the influence of hydraulic hysteresis during drying and wetting on the shearing response of unsaturated silty cohesionless soil alone and in contact with a rough steel counterface using a specially designed direct shear apparatus. For both soil and rough interface conditions, it was found that shear strength following a drying-wetting path was higher than for only the drying path, at similar suction and net normal stress. The magnitude of the difference in shear strength appears to correlate reasonably well with the difference in volumetric water content between the drying-wetting and drying paths at a given suction and net normal stress prior to shearing.

Soil Fused with Recycled Plastic Bottles for Various Geo-Engineering Applications

The emphasis of the present work is to study the mechanical properties of a new generation of geo-plastic products that can be used in various aspects of geo- engineering applications. The plastic-soil based geoplastic-material (PS) is produced by heating and mixing simultaneously recycled plastic bottles with soil at a specified temperature to melt the thermoplastic material so that a uniform fused mix is attained. This study is also directed to assess the potential of PS as an additive to stabilize open graded aggregate base. Laboratory results showed that PS properties varied with the types of soil. PS produced with sand exhibited unconfined compressive strength and indirect tensile strength values higher than PS produced with lean clay. Generally, specimens exhibited unconfined compressive strengths as high as ordinary concrete specimens. In addition, permeability and strength results of open-graded aggregates stabilized with PS indicated that such technology produces a drainable and a stable base for a better pavement performance with ecological and economic implications.

Variations of resilient modulus of subgrade soils with postcompaction moisture contents.

Environmental conditions such as temperature and moisture can significantly influence the performance of a pavement structure. These conditions vary within the pavement layers, degrading or enhancing the properties of pavement materials. The AASHTO 1993 design guide accounts for this influence by incorporating a single resilient modulus (MR) value that represents the effective roadbed resilient modulus; the revised AASHTO Mechanistic–Empirical Pavement Design Guide (MEPDG) uses models that predict changes in modulus due to moisture, temperature, or both. The effect of changes in postcompaction moisture content (i.e., wetting or drying) on the resilient moduli of subgrade soils is evaluated. The effects of wetting and drying were examined by conducting resilient modulus tests on specimens compacted at various initial moisture contents: optimum moisture content (OMC), 4% drier than OMC (OMC – 4%), and 4% wetter than OMC (OMC + 4%). Results showed that MR–moisture content (MRMC) relationships caused by drying exhibited higher values than the corresponding MRMC curves for specimens subject to wetting. The most significant finding was that changes in MR values depended on the initial compaction moisture contents; the MR values compacted at OMC exhibited a different drying and wetting trend than the ones compacted at OMC ± 4%. Finally, the MEPDG MR–moisture model was modified in this study to better predict the variations of resilient modulus with moisture changes.

Environmental influences on the engineering behavior of unsaturated undisturbed subgrade soils: effect of soil suctions on resilient modulus

Abstract This study evaluated the effect of soil suction on resilient modulus of undisturbed subgrade soil specimens collected from a section located on I-40 in Canadian County, Oklahoma. A total of 18 undisturbed specimens were tested for resilient modulus (<i>M_R</i>) and soil suction. AASHTO 2002 Design guide <i>M_R</i> predictive model was used and evaluated for its reliability in predicting <i>M_R</i> values. Findings from <i>M_R</i> and soil suction tests showed that <i>M_R</i> increased as total and matric suctions increased. The variation of <i>M_R</i> with both suctions revealed the same trends. Additionally, an existing <i>M_R</i>-stress-suction model was modified to evaluate the variation of <i>M_R</i> with suction and stresses. The modified model was a reliable predictor of <i>M_R</i> values of unsaturated subgrade soils.

Shear strength of unsaturated soil-geotextile interfaces.

Geotextiles are widely used in various applications including earthen structures, which are often built in unsaturated soil conditions. The design of these earthen structures is often dominated by the shear strength of the interface between soil and reinforcement (i.e. geotextile) layers. However, the unsaturated soil-geotextile interface interactions are not completely understood. This paper examines the shearing behavior of unsaturated soil-geotextile interfaces. Direct shear test results are used to define failure envelopes for unsaturated soil and soil-goetextile interfaces. Experimental results reveal a nonlinear relationship between the soil-geotextile interface strength and matric suction. The paper demonstrates that this non-linear failure envelope can be modeled using the Soil Water Characteristic Curve (SWCC) and saturated effective stress-strength parameters. The paper also compares the shearing behavior of unsaturated soil and unsaturated soil-geotextile interfaces.

Observations from Borehole Shear Testing in Unsaturated Soil

The Iowa Borehole Shear Test (BST) is an in situ device to rapidly determine a Mohr-Coulomb failure envelope in a borehole. Under the right conditions in clayey soils, the failure envelope from the BST may yield a friction angle and cohesion intercept in agreement with effective stress strength parameters determined from laboratory tests. Generally, this occurs when the in situ soil is in a nearly saturated condition and where shear-induced excess pore pressures are minimal, such as for stiff clays. For unsaturated soils, the interpretation of BST results is complicated by the presence of, and unknown variations in, soil suction during the test. This paper presents and discusses the results of BSTs and corresponding suction determinations in unsaturated clayey soils at two test sites. Results indicate that soil suction, in addition to other test variables, has a strong influence on failure envelope. Range of normal stress used in the test was also observed to have a strong influence on the resulting Mohr-Coulomb failure envelopes.

Laboratory and field assessment of pile run chat from the Tar Creek superfund site stabilized with class C fly ash and cement kiln dust

Abstract A combined laboratory and field study was undertaken to assess the suitability of pile run chat as a roadway base material. Chat is mine tailings from the abandoned mines in the Tri-State Mining District involving Oklahoma, Kansas and Missouri. The chat does not have any cohesion and consequently no compressive strength in unconfined form. In this study, pile run chat was stabilized with 10% class C fly ash (CFA) and 10% cement kiln dust (CKD), separately. Results from the laboratory study showed an appreciable increase in the unconfined compressive strength (UCS) and elastic modulus of pile run chat due to cementitious stabilization. CFA stabilized specimens exhibited higher strength than the CKD stabilized specimens. Results from the field tests, namely, Spectral Analysis of Surface Waves (SASW) and Falling Weight Deflectometer (FWD), showed that stabilized chat base layer perform well as part of a pavement structure. It was also found that the field modulus of elasticity, determined from both SASW and FWD, of CFA-stabilized chart base layer were higher than the corresponding values of CKD-stabilized chat base layer.