Water Retention Characteristics of Swelling Clays
By Kannan R. Iyer and D. N. Singh
ABSTRACT: Initial state of soil (viz., slurried, intact or compacted state) influences the soil water retention characteristics (SWRC), which in turn affects the unsaturated soil behaviour. Few studies have investigated the effect of initial state of soil on their SWRC, and such studies are rare for swelling clays. In this context, drying- and wetting- path SWRCs have been developed, in the present study, for intact and reconstituted specimens of swelling clays, by employing Dewpoint Potentiometer (WP4C®) and Environmental Chamber in tandem. The wetting-path SWRC has also been developed by controlled water sprinkling method. From the study, influence of initial water content has been observed to be higher on drying-path SWRCs as compared to wetting-path SWRCs of the clays. Further, the drying-path SWRCs for intact and reconstituted specimens converge beyond certain stage of drying. The study suggests the utilisation of reconstituted specimens for studying behaviour of intact clays in relatively dry state.
KEYWORDS: Intact and reconstituted states, Soil-water retention characteristics, Initial state of soil, Drying- and wetting- paths, Swelling clays, Dewpoint Potentiometer
Water Retention and Unsaturated Hydraulic Behaviors of a Biochar-modified Silt
By Abraham C.F. Chiu, B. Qiao and Y. Xiao
ABSTRACT: Biochar has been used to modify the soil cover of municipal solid waste (MSW) landfill to mitigate methane emission. To model the coupled water and reactive gas transport in unsaturated soil cover, water retention curve and unsaturated permeability function of biochar-modified soil cover are required. A laboratory study is presented to investigate the effects of biochar content (BC) and void ratio on the water retention and unsaturated hydraulic behaviors of a biochar-modified silt. Biochar contains high internal porosities and it exhibits negative surface charge. Adding biochar to the silt alters the microstructures of modified soil. The pore size distributions measured by the mercury intrusion porosimetry indicate that the modified silt contains more micro-porosities than the untreated silt. Results of modified evaporation test showed that water retention capacity increases with increasing BC. In other words, the modified silt can hold more water for a given suction. On the other hand, the modified silt exhibits a lower saturated permeability and also a lower rate of change in permeability with respect to suction. Despite the modified silt is less permeable than the untreated silt in the low suction regime, it becomes more permeable after drying to the high suction regime.
KEYWORDS: Biochar, Silt, Water retention curve, Unsaturated permeability, Pore size distribution
Simplified Model for Heat Transfer in Unsaturated Soils Considering a Nonisothermal Thermal Conductivity Function
By R.A. Samarakoon, and J.S. McCartney
ABSTRACT: Heat transfer in unsaturated soils occurs primarily due to conduction, convection of pore fluids in both liquid and vapor forms, and latent heat transfer. Due to the complexity involved in simulating this coupled problem, this paper investigates a simplified model for heat transfer in unsaturated soils using a conduction analysis with a nonisothermal thermal conductivity function. Specifically, a relationship between the apparent thermal conductivity and degree of saturation that indirectly incorporates the effects of heat transfer due to convection and water phase change through temperature effects was defined based on experimental observations, and the governing equation for conductive heat transfer was reconsidered to account for the variation in nonisothermal thermal conductivity with respect to space and time. An axisymmetric analysis for horizontal heat transfer in a soil layer from a line heat source was performed using the simplified heat transfer model, and results were compared with a conventional isothermal conduction analysis. Further, a comparison of simulated soil temperatures from the simplified heat transfer model with measured temperatures from an experimental study on heat transfer in unsaturated silt shows a good match, indicating that the simplified model may be used for preliminary analyses of problems involving monotonic heating.
KEYWORDS: Apparent thermal conductivity, Heat transfer, Nonisothermal conditions, Unsaturated soil
A Simple Approach to Monitor Soil Moisture Dynamic in Vapour Equilibrium Cell
By Y. Lu, Z. Shang, Hamayon Tokhi and Hossam Abuel-Naga
ABSTRACT: Vapour equilibrium technique (VET) is usually used to determine the water retention curve of soils. It is a suction-controlled technique where the soil specimen is allowed to reach the suction equilibrium in a desiccator where the relative humidity is controlled by a chemical solution. The suction equilibrium condition is deemed to have been reached when its soil moisture content does not change with time. Therefore, VET requires continuous monitoring for moisture content of the testing soil without disturbing the water vapour exchange process. The aim of this study is to present the electrical resistivity (ER) method that can be used to measure the moisture content of the specimen in VET without disturbance. The ER method is able to monitor the changes of soil moisture with time, and to successfully determine the time at which the soil moisture reaches the equilibrium state. The total suction equilibrium condition was validated independently with the suction measurements using a WP4C water potential meter. The results show good agreement between the total suction equilibrium condition determined by the proposed ER method and the WP4C measurements.
KEYWORDS: Vapour equilibrium technique, Unsaturated soil, Electrical resistivity, Water retention
A Classification Tree Guide to Soil-water Characteristic Curve Test for Soils with Bimodal Grain-size Distribution
By L. Zou and E.C. Leong
ABSTRACT: Soil-water characteristic curves (SWCCs) can be unimodal or bimodal. However, insufficient SWCC data points may cause a bimodal SWCC to be erroneously interpreted as a unimodal SWCC. Suggested suction levels to determine SWCC in ASTM 6836-02 (2008) are excessive and can be reduced if the type of SWCC that a soil may have can be identified prior to the SWCC test. Bimodal grain-size distribution is a pre-requisite for soils to have a bimodal SWCC but not all soils with bimodal grain-size distribution have bimodal SWCCs. Models have been proposed to estimate bimodal SWCC of soils with bimodal grain-size distribution. However, the criteria used by these models identify soils with bimodal SWCC are not accurate. In this paper, a classification tree to identify bimodal grain-size distribution (GSD) soils with bimodal SWCC is proposed so that SWCC tests can be better planned to obtain sufficient data for correct interpretation of the SWCC. The classification tree was developed using a database of 226 soils with bimodal GSD. An independent data set consisting of 60 SWCCs from extant literature was used to evaluate the classification tree and the criteria proposed by others. The classification tree was shown to outperform the criteria proposed by others for identifying bimodal GSD soils with bimodal SWCC. Recommendation on suction levels for SWCC tests to obtain unimodal and bimodal SWCCs was made for the test methods in ASTM 6836-16 (2016).
KEYWORDS: Unsaturated soil, Grain-size distribution, Soil-water characteristic curve, Bimodal, Unimodal, SWCC test, Classification tree
Backpressure Saturation Effects on the Mechanical Behaviour of a Quasi-Saturated Compacted Residual Soil
By G.G. Carnero-Guzman and F.A.M. Marinho
ABSTRACT: The use of saturation methods in triaxial tests is a common practice to obtain the strength parameters of the soil in effective terms. However, these methods may influence the results obtained in the laboratory negatively. For instance, undesirable volumetric variations in the sample may be created depending on the saturation stages applied to the samples. Furthermore, these methods commonly require large backpressure values to saturate samples even if their saturation corresponds to the “quasi-saturated state”. This quasi-saturated state (related to saturation values above 90%) is commonly found in the engineering practice for fills of embankment compacted above the optimum water content. At this state, the soil is expected to behave as a saturated soil and the suction of the soil tends to be zero. This paper studies the effect of two saturation processes in a residual soil from São Paulo, Brazil, compacted in the quasi-saturated state. CIU triaxial tests were performed with fully saturated and quasi-saturated samples. Both processes lead the samples to different wetting paths and volumetric changes that, as a result, influenced the pore-water pressure development and the effective strength parameters.
KEYWORDS: Quasi-saturated state, Pore-water pressure development, Saturation laboratory methods, Residual soil
Induced and Inherent Anisotropies of Saturated and Unsaturated Soil Shear Properties
By H. Toyota, B. N. Le and S. Takada
ABSTRACT: Anisotropies of shear properties were examined for saturated and unsaturated soils. Induced anisotropy arises when an anisotropic stress is applied to a soil. Using a hollow cylinder torsional shear apparatus, cohesive saturated and unsaturated specimens were anisotropically consolidated in different directions. Then the shear strengths under an undrained condition for saturated soil and a constant water content condition for unsaturated soil were measured using those specimens. Inherent anisotropy develops from the orientation of soil particles during sedimentation. Specimens with different depositional angles were extracted from sand samples deposited in an inclined container. Finally, the shear properties under a drained condition for saturated sand and a constant suction condition for unsaturated sand were evaluated using the triaxial apparatus. The experimental results indicate that anisotropy, especially of the shear strength, is present for both the cohesive soil and the sand. However, shear strength anisotropy is lesser for the cohesive soil in the unsaturated condition than in the saturated condition.
KEYWORDS: Anisotropy, Cohesive soil, Matric suction, Sand, Torsional shear, Triaxial test, Unsaturated soil
Probability and Statistics Approach for Determining Pore Size Distribution of Coarse-Grain Soil
By R. Kitamura and K. Sako
ABSTRACT: The basic scheme of microscopic mechanical model is proposed to analyze the various mechanical behaviors of unsaturated coarse-grain soil. Physical quantities required for the proposed model are grain size distribution, soil particle density, void ratio and water content. The probability theory and inferential statistics are applied to relate the macroscopic physical quantities used in the conventional soil mechanics to the microscopic physical quantities in the proposed model.
KEYWORDS: Unsaturated soil, Microscopic model, Probability, Grain size distribution, Void ratio, Water content
Elastoplastic Modelling of Hydro-mechanical Behaviour of Unsaturated Soils
By J.R. Zhang, D.A. Sun and W.J. Sun
ABSTRACT: The development of constitutive models for unsaturated soils is briefly reviewed, and the state-of-the-art of elastoplastic constitutive models for unsaturated soils is summarized. This paper introduces an elastoplastic model that couples hydraulic and mechanical behaviour for unsaturated non-expansive and expansive soils. The two coupled elastoplastic constitutive models are illustrated with laboratory tests to show their application and performance. Hydraulic hysteresis in the soil-water characteristic is modeled as an elastoplastic process with the elastic region of the degree of saturation. The effect of change in degree of saturation on the stress-strain-strength behaviour and the effect of change in void ratio on the soil-water characteristics are taken into consideration in the model in addition to the effect of suction on the hydraulic and mechanical behaviour.
KEYWORDS: Unsaturated soil, Elastoplastic model, Stress-strain relation, Soil-water characteristic, Coupling
Interpretation of Desiccation Soil Cracking in the Framework of Unsaturated Soil Mechanics
By H. Al-Dakheeli and R. Bulut
ABSTRACT: Cracks evolve in expansive soils when subjected to drying and shrinkage processes. The underlining driving mechanism is the development of the tensile stress in perpendicular direction to the crack length. The tensile stresses are generated from the restrictions applied on the soil from shrinking freely. When the tensile stress exceeds the soil tensile strength, cracks initiate and propagate. The soil shrinkage, generated tensile stress, and tensile strength are governed by the effective stress regime existing in the soil microspores. All these mechanisms require the tracking of the soil cracking phenomenon from the internal stresses developed in the microspores system. Understanding the stresses that are developed in the soil microspores system is a challenging task. It is mainly due to the complexity of the physicochemical interactions taking place in the soil pores. This paper presents understanding and modelling the soil cracking by relying on unsaturated soil mechanics. Based on the laboratory results from desiccating expansive soils subjected to restricted shrinkage, this paper demonstrates that the crack growth in soils occurs mostly in the unsaturated condition for initially saturated soils. Restricted shrinkage tests are carried out using restrained ring testing method to induce cracks in the soil specimens. The results demonstrate that a crack for initially saturated soils first initiates at suction close to the air-entry value. Free shrinkage tests are also conducted to predict the soil shrinkage curve. The results from restrained ring tests are explained in terms of the soil-water characteristic curve and soil shrinkage curve.
KEYWORDS: Desiccation cracks, Unsaturated soil mechanics, Restrained ring testing method, Shrinkage curve
Field Response of an Instrumented Dyke subjected to Rainfall
By A. Jotisankasa, S. Pramusandi, S. Nishimura and S. Chaiprakaikeow
ABSTRACT: A field study was undertaken of an instrumented dyke on soft Bangkok clay in Pathumthani, Thailand. The studied site was characterised using dynamic cone penetration tests, field vane shear tests and Spectral Analysis of Surface Waves geophysical tests. The porewater pressure, suction, moisture content and rainfall were continuously monitored over the rainy season in 2017. The soft clay nearer the dyke had higher strength than in the zone further away due to consolidation. The upper 0.5 m of dyke fill material made up of silty and clayey soils were found to experience drastic suction changes, reaching 1800 kPa towards the end of a drought and abruptly reducing to 20-40 kPa within a day upon the onset of the rainy season. Such large and abrupt changes of the suction are likely to have aggravated the surface cracking and hence the dyke movements. In contrast, the response at 3 m depth from the dyke shoulder was almost insensitive to the short-term rainfall patterns. Vertical movement of the dyke surface showed compression-swelling phenomena, probably due to the combined effects of drying, collapse-on-wetting and swelling and suggested that some movement is recoverable. This was not the case for horizontal movements, which exhibited constant outwards cumulative displacement.
KEYWORDS: Dyke, Slope, Pore-water pressure, Suction, Rainfall, Instrumentation
Simplified Shear Deformation Method for Analysis of Mechanical Behavior of a Single Pile in Expansive Soils
By Y. Liu and S.K. Vanapalli
ABSTRACT: Seasonal volume changes associated with wetting and drying conditions due to infiltration and evaporation of water significantly influence the mechanical behavior of expansive soils. For this reason, rational design of foundations in expansive soils has been a challenge for the geotechnical engineers. Pile foundations have been found to be more suitable in carrying loads from the superstructure to the expansive soil below alleviating stability and deformation problems in comparison to other types of foundations. However, there is a lack of simple procedures in the literature to estimate how the load transfers from pile to the soil, which is the key information required in the rational design of pile foundations in expansive soils. In this paper, the conventional shear deformation method is modified to estimate the load transfer mechanism from pile to expansive soil taking account of the influence of infiltration and evaporation of water. The proposed approach is based on the mechanics of unsaturated soils and can be extended for single pile considering the influence of volume change behavior in expansive soils. In addition, parametric analysis undertaken in the present study suggests that the pile diameter and pile length significantly influence the mechanical behavior of piles in expansive soils. Geotechnical engineers can use the proposed method in routine design of foundations for expansive soils as the complexity associated with the estimation of pile mechanical behavior in expansive soils is significantly reduced.
KEYWORDS: Expansive soil, Unsaturated soil, Pile, Shear deformation method
Validation of Foundation Design Method on Expansive Soils
By K.C. Chao and J.D. Nelson
ABSTRACT: Nelson et al. (2015) presented design principles for foundations on expansive soils. The design principles consider free-field heave throughout the design life of a structure as the basis for foundation design. The design principles also consider water migration in the vadose zone, and the time required for subsoil wetting over the design life of the structure. This paper presents a method to validate the foundation design method presented in Nelson et al. (2015). The validation was performed using detailed long-term data obtained on a building constructed on expansive soils at the Denver International Airport, Denver, Colorado, USA. Water migration in the vadose zone and heave of floor slabs and drilled pier foundations were monitored over the time period from 2000 to 2016 and extended to a 25 year period (1991 – 2016) beginning at the end of construction. Water content profiles were modeled using VADOSE/W software, and heave of slabs and piers were computed using the design method presented in Nelson et al. (2015). The depth of wetting and changes in water content were used to compute heave according to the design method. Calculated heave was compared to the survey data. It was shown that the design method was capable of predicting heave to within 30 percent of the measured heave over a 25-year period.
KEYWORDS: Expansive Soils, Heave Prediction, Foundation Design, Water Migration Modeling
