William J. Likos

Tensile Strength of Unsaturated Sand

A theory that accurately describes tensile strength of wet sand is presented. A closed form expression for tensile strength unifies tensile strength characteristics in all three water retention regimes: pendular, funicular, and capillary. Tensile strength characteristically increases as soil water content increases in the pendular regime, reaches a peak in the funicular regime, and reduces with a continuing water content increase in the capillary regime. Three parameters are employed in the theory: internal friction angle (at low normal stress) ϕt , the inverse value of the air-entry pressure α , and the pore size spectrum parameter n . The magnitude of peak tensile strength is dominantly controlled by the α parameter. The saturation at which peak tensile strength occurs only depends on the pore size spectrum parameter n . The closed form expression accords well with experimental water retention and tensile strength data for different sands.

Automated Humidity System for Measuring Total Suction Characteristics of Clay

A computer-automated experimental system for determining total suction characteristic curves using relative humidity control is de- scribed. The system is applicable in the relatively high suction range important for fine-grained materials (7000 to 700 000 kP a). The new system has several advantages over existing suction measurement techniques; most notably, it is fully automated, has a much broader me asurement range, is capable of determining both wetting and drying characteristics in significantly less time, and requires only one undisturbed sample for testing. Tests are conducted to evaluate the system response and illustrate its use in practice. Total suction characteristic curves are determined for four types of clay, ranging from highly expansive smectite to non-expansive kaolinite. Concurrent characteristic curves are determined usi ng the filter paper method for comparison. Practical applications are demonstrated in two ways. First, select results are analyzed to assess the sw elling potential of the four clays using an existing methodology based on total suction testing. Second, aspects related to the adsorption kinetics of expansive soils are eval- uated by analyzing the results with a first-order kinetic model.

PORE-SCALE ANALYSIS OF BULK VOLUME CHANGE FROM CRYSTALLINE INTERLAYER SWELLING IN Na+- AND Ca2+-SMECTITE

Water-vapor sorption experiments were conducted to quantify bulk volume change of compacted expansive clay specimens resulting from interlayer hydration and dehydration in the crystalline swelling regime. Effects of interlayer cation type and pore fabric are examined by comparing results for natural Na+-smectite and Ca2+-smectite specimens compacted over a range of initial bulk densities. Transitions in interlayer hydration states are reflected in the general shape of the sorption isotherms and corresponding relationships between humidity and volume change. Hysteresis is observed in both the sorption and volume-change response. Volume change for Ca2+-smectite specimens is significantly greater than for Na+-smectite over the entire range of packing densities considered. Loosely compacted specimens result in less volume change for both clays. Results are interpreted in light of a conceptual framework based on previous SEM and TEM observations of particle and pore fabric for Na+ and Ca2+ smectite at high suctions. A pore-scale microstructural model is developed to quantitatively assess changes in interlayer and interparticle void volume during hydration. Modeling suggests that the relatively small volume changes observed for Na+-smectite are attributable to a reduction of interparticle void volume as expanding quasicrystals encroach into surrounding larger-scale pores. Volume change hysteresis is attributed to unrecovered alterations in interparticle fabric required to accommodate the swelling process. The results provide new insight to address volume change upscaling, hysteresis, and the general evolution of bi-modal pore fabric during crystalline swelling.

WATER VAPOR SORPTION BEHAVIOR OF SMECTITE-KAOLINITE MIXTURES

An experimental program was conducted to investigate the water-vapor sorption characteristics of smectite and kaolinite mixtures. End-member smectite and kaolinite were slurry-mixed together at mass-controlled ratios corresponding to 0%, 20%, 50%, 70%, 80%, 90% and 100% smectite. Vapor desorption isotherms for the mixtures were measured at 24°C for relative humidity (RH) ranging from ∼95% to 0%.Results show that the amount of water adsorbed by the clay mixtures at a given RH increases systematically with increasing smectite content. Derivative analysis of the sorption isotherms shows evidence of transitions between the two-, one- and zero-layer hydrate-states for the smectite-rich mixtures. The transitions become less apparent as the smectite content decreases. Monolayer coverage, specific surface area, and heat of adsorption were estimated from the isotherms using BET theory. It is shown that monolayer coverage and specific surface for the clay mixtures can be reasonably approximated by weighted averaging of the end-member clay properties. General methodologies are presented for predicting the sorption behavior (i.e. soil-water characteristics) and effective specific surface area from measurements of the end-member sorption isotherms.

Hysteresis and Uncertainty in Soil Water-Retention Curve Parameters

AbstractAccurate estimates of soil hydraulic parameters representing wetting and drying paths are required for predicting hydraulic and mechanical responses in a large number of applications. A comprehensive suite of laboratory experiments was conducted to measure hysteretic soil-water characteristic curves (SWCCs) representing a wide range of soil types. Results were used to quantitatively assess differences and uncertainty in three simplifications frequently adopted to estimate wetting-path SWCC parameters from more easily measured drying curves. They are the following: (1) αw=2αd, (2) nw=nd, and (3) θsw=θsd, where α, n, and θs are fitting parameters entering van Genuchten’s commonly adopted SWCC model, and the superscripts w and d indicate wetting and drying paths, respectively. The average ratio αw/αd for the data set was 2.24±1.25. Nominally cohesive soils had a lower αw/αd ratio (1.73±0.94) than nominally cohesionless soils (3.14±1.27). The average nw/nd ratio was 1.01±0.11 with no significant depen...

Pore-Scale Model for Water Retention and Fluid Partitioning of Partially Saturated Granular Soil

AbstractA model for the water retention behavior of unsaturated granular soil is developed by extending the classic bundled cylindrical capillary representation of pore space to a geometry more closely approximating that of granular porous media. Expressions for pore-scale saturation are derived as functions of matric suction for a three-dimensional unit pore comprising angular pore space bounded by spheres in simple cubic packing order. Water retention curves are modeled by assigning a statistical distribution of pore sizes optimized to best match experimentally determined retention curves. A key model attribute is its capability to capture evolution of fluid partitioning along drainage or wetting paths by differentiating pore water retained as thin films adsorbed to particle surfaces, liquid bridges retained in wedge-shaped pores, and saturated pockets in relatively small pores. Interfacial surface tension, solid-liquid contact angle, wetting direction, and mineralogy (Hamaker constant) are treated as m...

POROSITY EVOLUTION OF FREE AND CONFINED BENTONITES DURING INTERLAYER HYDRATION

Methods for predicting the volume change and swelling-pressure behavior of expansive clays require detailed understanding of coupled interactions between clay microstructure and macrostructure under hydraulic, thermal, and mechanical loads. In this study a suite of water-vapor sorption experiments was conducted using compacted bentonites hydrated in controlled relative humidity (RH) environments maintained under free and constrained volume-change boundary conditions. Emphasis was placed on examining the influences of compaction and predominant exchange cation on the water uptake, volume change, and swelling pressure response. Densely compacted specimens exhibited greater volume changes under free swelling conditions and greater swelling pressures under fully confined conditions. Water uptake, volume change, and swelling pressure were all more significant for Colorado (Ca2+/Mg2+) bentonite than forWyoming (Na+) bentonite. Plastic yielding, evident as a peak in the relationship between swelling pressure and RH, was more evident and occurred at lower RH for the Colorado bentonite. This observation was interpreted to reflect the limited capacity for interlayer swelling in Ca2+/Mg2+ bentonites and corresponding structural collapse induced by the onset of water uptake in larger intra-aggregate and inter-aggregate pores. A semi-quantitative model for the evolution of clay microstructure resulting from interlayer hydration was considered to attribute the experimental observations to differences in the efficiency with which transitions in basal spacing translate to bulk volume changes and swelling pressure. Results provide additional insight and experimental evidence to more effectively model the mechanical behavior of compacted bentonites used as buffer or barrier materials in waste repository applications.

Quantification of Grain, Pore, and Fluid Microstructure of Unsaturated Sand from X-Ray Computed Tomography Images

A comprehensive series of three-dimensional x-ray computed tomography (XCT) imaging experiments was conducted to quantitatively assess the multiphase particle- and pore-scale properties of fine Ottawa (F-75) sand. The specimens were prepared to saturations ranging from approximately 5 % to 80 %. Specimens were doped with 10 % CsCl pore fluid solution and imaged using a monochromatic synchrotron x-ray source at energies below and above the Cs x-ray absorption k-edge to allow for high contrast between the solid, liquid, and air phases. Multiphase properties quantified from the XCT images included individual particle sizes and areas, as well as grain size distribution, pore shape and size distribution, water menisci distribution, solid, liquid, and gas surface areas, and particle contact coordination number. At low saturations, pore water is distributed primarily in the form of pendular rings and liquid bridges located between individual grains and in the smallest pore throats and bodies. A highly discontinuous water phase is evident as a large number of separately identifiable water units having very small volume. As the water saturation increases, the number of individual water units decreases; as expected, the average volume of these units increases significantly as the pore water coalesces into larger and larger units. Results obtained using SEM imaging and conventional geotechnical testing methods for particle-size distribution and soil–water retention were compared with those derived from analysis of the XCT images. Results compare very well in each case, typically within a few %. It is shown that the XCT is a reliable and non-destructive method to quantify pore-scale information vital to advance understanding of the hydrologic and mechanical behavior of unsaturated soils at the macroscale.

Modified Direct Shear Apparatus for Unsaturated Sands at Low Suction and Stress

Modifications to a conventional laboratory testing system are described for direct shear testing of unsaturated soils at relatively low matric suction and net normal stress. Matric suction ranging from zero (saturated) to about 10 kPa is controlled using a hanging column assembly (ASTM D6836). Net normal stress ranging from about 0.3 to 10 kPa is controlled by directly applying dead loads to the specimen via a series of aluminum top caps machined to varying thicknesses. Precise control of suction and normal stress within these ranges makes the apparatus ideal for examining the shear strength behavior of unsaturated sands, which are characterized by relatively low air-entry pressures and for which the influences of matric suction on mechanical response can be subtle. Soil-water characteristic curves are concurrently obtained during the shear testing program by measuring transient and equilibrium pore water drainage under the imposed suction changes. Testing procedures and recommended protocols are described. Results from a series of tests using saturated and unsaturated specimens of poorly graded fine sand are presented to demonstrate application and performance of the system. Relationships between shear strength and matric suction are non-linear and exhibit peak shear strength at matric suction within the range of the air-entry suction. High friction angles measured for the portions of the failure envelope at low matric suction and normal stress may indicate the effects of dilation on the strength development.

Constant Flow Method for Concurrently Measuring Soil-Water Characteristic Curve and Hydraulic Conductivity Function

A constant-flow laboratory testing method (CFM) is presented for concurrently measuring the soil-water characteristic curve (SWCC) and hydraulic conductivity function (HCF) of unsaturated coarse-grained soils. Two computer-automated syringe pumps are employed to control the volumetric water content of a specimen and to periodically impose constant volumetric flow rates through the specimen, respectively. Hydraulic conductivity (k) corresponding to each water content increment is determined from Darcys law by measuring the steady-state gradient induced by the applied constant flow. Matric suction is maintained by axis translation using elevated pore air pressure and high-air-entry ceramic disks. Diffused air bubbles are removed using unique passive bubble traps. SWCCs and HCFs are obtained for three sandy soil specimens, requiring about 25–35 days to obtain both functions for each soil. The range of the system is demonstrated for k from about 10−4 cm/s to 10−9 cm/s at corresponding matric suction between about 0 kPa (saturated) and 40 kPa. Results are validated by comparison with independent SWCC measurements obtained using Tempe cells and HCFs estimated using a statistical model.