John S. McCartney

Centrifuge Modeling of Soil-Structure Interaction in Energy Foundations

AbstractThis study presents a centrifuge modeling approach to characterize the transient thermomechanical response of energy foundations during heating-cooling cycles to provide data for calibration and validation of soil-structure interaction models. This study focuses on the response of a scale-model energy foundation installed in an unsaturated silt layer with end-bearing boundary conditions. The foundation response was assessed using embedded strain gauges and thermocouples. Other variables monitored include foundation head displacements, soil surface displacements, and changes in temperature and volumetric water content in the unsaturated silt at different depths and radial locations. Measurements during the initial heating process indicate that the thermal axial stress is greater near the toe of the foundation as a result of the restraint associated with mobilization of side shear resistance along the length of the foundation. The thermal axial strains were close to the free-expansion thermal strain...

Impact of Hydraulic Hysteresis on the Small-Strain Shear Modulus of Low Plasticity Soils

AbstractExperimental studies have observed that the small-strain shear modulus (Gmax) of unsaturated soils measured during hydraulic hysteresis has a greater magnitude during imbibition than during drainage when plotted as a function of matric suction. To capture this behavior, a semiempirical model was developed to interpret the impacts of the stress state and hydraulic hysteresis on Gmax of low plasticity soils. Different from previous empirical relationships for Gmax, this model incorporates elastoplastic constitutive relationships, which integrate the effects of mean effective stress and hardening because of either plastic changes in volume or changes in the degree of saturation. The effective stress is defined as the sum of the net normal stress and the product of the effective saturation and matric suction, facilitating integration of the soil-water retention curve parameters into the model. An experimental testing program involving measurement of Gmax of compacted silt during hydraulic hysteresis w...

Critical Review of Thermal Conductivity Models for Unsaturated Soils

Although it is well established that heat conduction in unsaturated soil depends on liquid saturation, there are several models available to consider the changes in thermal conductivity during drying and wetting. The key factors affecting thermal conductivity of unsaturated soil are evaluated through a critical examination of these different models and their development. Depending on the principles and assumptions employed, these models are categorized into three groups: mixing models involving series/parallel elements; empirical models where thermal conductivity values at dry and saturated states are used; and mathematical models based on phase volume fractions. Experimental data for different soils are used to assess the quality of prediction for these models. It is found that all the existing models do not realistically account for pore structure or interface properties, and thus are not capable of properly predicting thermal conductivity as a function of liquid saturation. A conceptual model based on soil–water retention mechanisms, is proposed to overcome the pitfalls of the existing models and can be used to establish quantitative thermal conductivity models for variably saturated soils in the future.

Centrifuge Permeameter for Unsaturated Soils. I: Theoretical Basis and Experimental Developments

A new centrifuge permeameter was developed with the specific objective of expediting the measurement of the hydraulic characteristics of unsaturated soils. The development, theoretical basis, and typical results associated with using the centrifuge permeameter for concurrent determination of the soil-water retention curve (SWRC) and hydraulic conductivity function ( K function) of unsaturated soils are presented in this paper. Components developed for the centrifuge permeameter are described, including the centrifuge, permeameter, water flow control system, and instrumentation used to concurrently and nondestructively measure the infiltration rate (flow pump and outflow transducer), volumetric water content (time domain reflectometry), and matric suction (tensiometers) in flight during steady-state infiltration. A companion paper focuses on definition of the SWRC and K function for a clay soil using the procedures described in this paper. While conventional geotechnical centrifuges are used to reproduce t...

Impact of effective stress on the dynamic shear modulus of unsaturated sand.

The dynamic shear modulus of soils is needed to predict soil behavior in response to cyclic loading. Even though the effective stress has been shown to have a significant impact on the dynamic modulus of water-saturated and dry soils, its effect on the dynamic shear modulus of unsaturated soils has not been evaluated. Specifically, studies on the dynamic response of unsaturated soils have characterized variations in small-strain shear modulus (Gmax) as a function of the degree of saturation or matric suction alone. In contrast, this study evaluates the use of the suction stress characteristic curve to characterize the impact of mean effective stress (’m) on the dynamic shear modulus of unsaturated sand. A fixed-free resonant column test device was adapted with a hanging column setup so that the small-strain dynamic shear modulus could be measured for sand specimens under different confining pressures and matric suction values. Trends between the small strain shear modulus and effective stress for unsaturated sand were found to be different from those reported in the literature, where Gmax varied linearly with the square root of ’m.

Centrifuge Modeling of End-Restraint Effects in Energy Foundations

AbstractThis study presents the results from physical modeling experiments on centrifuge-scale energy foundations in dry sand and unsaturated silt layers. These experiments were performed to characterize end restraint effects on soil-structure interaction for energy foundations in different soils and include tests on foundations with semifloating and end-bearing toe boundary conditions and free-expansion and restrained-expansion head boundary conditions. Two scale-model energy foundations having different lengths were constructed from reinforced concrete to simulate end-bearing and semifloating conditions in soil layers having the same thickness. The foundations include embedded thermocouples and strain gauges, which were calibrated under applied mechanical loads and nonisothermal conditions before testing. The variables measured during the experiments include axial strain and temperature distributions in the foundation, temperature, and volumetric water content measurements in the soil, vertical displace...

Coupled Axisymmetric Thermo-Poro-Mechanical Finite Element Analysis of Energy Foundation Centrifuge Experiments in Partially Saturated Silt

The paper presents an axisymmetric, small strain, fully-coupled, thermo-poro-mechanical (TPM) finite element analysis (FEA) of soil–structure interaction (SSI) between energy foundations and partially saturated silt. To account for the coupled processes involving the mechanical response, gas flow, water species flow, and heat flow, nonlinear governing equations are obtained from the fundamental laws of continuum mechanics, based on mixture theory of porous media at small strain. Constitutive relations consist of the effective stress concept, Fourier’s law for heat conduction, Darcy’s law and Fick’s law for pore liquid and gas flow, and an elasto-plastic constitutive model for the soil solid skeleton based on a critical state soil mechanics framework. The constitutive parameters employed in the thermo-poro-mechanical FEA are mostly fitted with experimental data. To validate the TPM model, the modeling results are compared with the observations of centrifuge-scale tests on semi-floating energy foundations in compacted silt. Variables measured include the thermal axial strains and temperature in the foundations, surface settlements, and volumetric water contents in the surrounding soil. Good agreement is obtained between the experimental and modeling results. Thermally-induced liquid water and water vapor flow inside the soil were found to have an impact on SSI. With further improvements (including interface elements at the foundation-soil interface), FEA with the validated TPM model can be used to predict performance and SSI mechanisms for energy foundations.

Centrifuge Permeameter for Unsaturated Soils. II: Measurement of the Hydraulic Characteristics of an Unsaturated Clay

This paper presents the hydraulic characteristics of an unsaturated, compacted clay, including its soil-water retention curve (SWRC) and hydraulic conductivity function ( K function), determined using a new centrifuge permeameter developed at the University of Texas at Austin. A companion paper describes the apparatus, its instrumentation layout, and data reduction procedures. Three approaches are evaluated in this study to define the SWRC and K function of the compacted clay under both drying and wetting paths, by varying the inflow rate, the g level, or both. For imposed inflow rates ranging from 20 to 0.1 mL/h and g levels ranging from 10 to 100 g, the measured matric suction ranged from 5 to 70 kPa, the average volumetric water content ranged from 23 to 33%, and the hydraulic conductivity ranged from 2 × 10 − 7 to 8 × 10 − 11 m / s . The SWRCs and K functions obtained using the three different testing approaches were very consistent, and yielded suitable information for direct determination of the h...

Effects of infiltration and evaporation on geosynthetic capillary barrier performance

This study includes an experimental investigation of the transient movement of water in unsaturated soil layers underlain by a geocomposite drainage layer (GDL) during cycles of infiltration and evaporation. The distribution in volumetric water content with depth in a soil column having a height of 1350 mm underlain by a GDL was measured during transient infiltration. The capillary break effect was observed to affect the soil up to a height of 500 mm above the GDL, with an increase in volumetric water content up to 20% above that expected for the case of infiltration under a unit hydraulic gradient. Due to the long duration of this test (2000 h), a shorter 150 mm high soil column was also evaluated to investigate the soil–GDL hydraulic interaction during cycles of infiltration and evaporation. The capillary break was observed to have re-established itself after infiltration was stopped and the soil near the interface dried. The suction and volumetric water content measured in the soil at breakthrough were...

Thermo-Mechanical Characterization of a Full-Scale Energy Foundation

This paper documents the thermo-mechanical characterization of an energy foundation constructed as part of a new single-story building at the U.S. Air Force Academy. During a 39 hour heating test, the foundation increased in temperature by 21.7 °C, while the sandstone subsurface only increased in temperature by less than 1 °C at a distance of 1.22 m from the center of the foundation. The distributions in thermal axial stress and displacement in the foundation were calculated from the measured thermal axial strain values, and a maximum compressive thermal axial stress of 5.2 MPa was observed near the middle of the foundation, and a maximum upward displacement of 1.55 mm was observed at the head. The thermal axial stresses and displacements measured were well within acceptable limits for structural elements.