Project Title: Developing Constitutive Relationships for the Properties of Unsaturated Bentonite Buffers under High Temperature

Project Sponsor: DOE NEUP

Collaborators: JS Chen, LianGe Zheng, Hao Xu

Description: The objectives of this project are to characterize the effects of high temperatures (up to 200 °C) on the mechanisms and material properties governing coupled heat transfer, water flow, and volume change in unsaturated, compacted granular bentonite, and to understand and simulate the multiphase hydration process of bentonite buffers in deep geological repositories with closely spaced waste packages or Dual Purpose Containers. The project tasks will include a combination of element-scale testing to measure bentonite material properties under high temperatures, tank-scale testing to capture the coupled processes during bentonite hydration under high temperatures, development of quantitative relationships to represent the experimentally-observed thermo-hydro-mechanical behavior, and numerical simulations of bentonite buffer hydration involving high temperatures for different initial densities and rates of water supply from the host rock. In the element-scale tests, different thermo-hydro-mechanical paths will be applied to compacted bentonite specimens having different initial conditions (density and degree of saturation) in thermal triaxial cells to collect data needed to define the new constitutive relationships. 

Project Title: EAGER: Solar Thermal Soil Improvement over Different Depths

Project Sponsor:  NSF CMMI 1941571

Description: This EArly-concept Grant for Exploratory Research (EAGER) project addresses the geotechnical engineering research needed to assess the feasibility of using solar thermal energy to improve the mechanical properties of soft soil deposits over different depth ranges. Specifically, heated fluid collected from solar thermal panels circulated through closed-loop geothermal heat exchangers in the subsurface is used to induce thermal volumetric contraction and a corresponding increase in shear strength of a targeted zone of soil. Arrays of geothermal heat exchangers in vertical and horizontal configurations will be investigated to improve soil over different depth ranges and areal distributions. Advantages of this approach are that soil improvement can be gained in a targeted manner using renewable energy, after which the geothermal heat exchangers can be used for long-term underground thermal energy storage, yielding cost savings when compared to available soft soil improvement technologies. The research plan seeks to better understand fundamental processes governing the thermal volume change of soft soils over different depth ranges and to improve constitutive models for soft soils needed in advanced computer simulations, addressing the NSF mission "to promote the progress of science." If feasible, solar thermal energy and geothermal heat exchangers will be important tools for the cost-effective improvement of challenging soft soil deposits encountered in civil infrastructure projects, offshore or river sediments, mine tailings dams, and coal ash impoundments. 

Project Title: Interaction of MSE Abutments with Superstructures under Seismic Loading

Project Sponsor:  Caltrans

Collaborators: Pat Fox, Benson Shing

Description: Geosynthetic reinforced soil (GRS) bridge abutments are widely used in transportation infrastructure, and provide many advantages over traditional pile-supported bridge abutments, including lower cost, faster and easier construction, and smoother transition between the bridge beam and approach roadway. However, the adoption of this technology in areas with high seismicity like California is pending until their seismic deformation response is better understood. The objective of an ongoing study funded by Caltrans and a FHWA pooled fund project is to characterize the seismic response of GRS bridge abutments using both shake table tests and numerical simulations. A series of five shaking table tests were performed by Yewei Zheng and the Powell Laboratory staff to investigate the seismic deformation response of half-scale GRS bridge abutments. The tests permit evaluation of the effects of bridge load, reinforcement spacing and stiffness, and shaking direction, and results show that reinforcement spacing and stiffness have the most significant effects on the deformation response. Shaking in the longitudinal direction also resulted in considerable facing displacements in the transverse direction, which indicates the importance of considering three-dimensional (3D) effects. The shaking table data is being used to validate 3D numerical simulations, which will be used to further understand the effects of different design details on the seismic deformation response of GRS bridge abutments that are needed to improve the seismic design guidelines for this type of structure. 

Project Title: Prediction of Seismic Compression of Unsaturated Backfills

Project Sponsor: PEER

Description: The goal of this study is to develop, implement and validate a new effective stress-based elasto-plastic constitutive model that can predict the evolution in seismic compression of unsaturated backfill soils in transportation systems (e.g., highway embankments, bridge abutments, earth retention systems, etc.). Seismic compression is defined as the accrual of permanent contractive volumetric strains in soils during earthquakes and has been recognized as a major cause of seismically-induced damage in earth structures. Accurate predictions are challenging for unsaturated soils, as the degree of saturation and matric suction (the difference between pore air and water pressures) will change during volumetric contraction and will affect the effective stress and dynamic soil properties (e.g., the shear modulus, damping ratio). Generation of pore air and water pressures depend on the bulk fluid modulus and on the initial degree of saturation in the soil. This study seeks to depart from commonly-used semi-empirical approaches for seismic compression prediction by developing an effective stress-based elasto-plastic constitutive model that can capture the impacts of initial degree of saturation and density on the deformation response. 

Project Title: Variation of at-Grade Tracks Due to Earthquake-Induced Ground Failure

Project Sponsor:  BART

Collaborator: Kenneth Loh

Description: The goal of this project is to provide BART with innovative cross-level variation sensor technology and experimental data from full-scale shaking table tests that can be used to calibrate and validate models used to predict the amount of cross-level variation that may occur in a rail system after an earthquake. It is proposed to perform a series of shaking table tests using the indoor shaking table at the University of California, San Diego (UCSD) Powell Structural Laboratory.

Project Title: Ground Improvement-Based Protection of Transportation Infrastructure: Validation of PBE via Centrifuge and Numerical Modeling

Project Sponsor:  PEER

Collaborator: Tara Hutchinson

Description: The goal of this proposal is to utilize ground improvement strategies to design shallow foundations for bridge, viaduct, or rail superstructures that have a controlled plastic hinging response during earthquakes. Our premise for this work is the long history of well-documented evidence indicating that under-designed foundations can provide substantial energy dissipation and re-centering of structural systems (including much past research supported by PEER). Deterrents to realizing these benefits in engineering practice however are the detrimental impacts of the kinematics of the footing itself, i.e. transient and permanent settlements and rotations. Clearly since footings are at the base of structures, their maximum and residual seismic deformations will be translated to the superstructure should they be allowed to yield or uplift. While this issue occurs in shallow foundations supporting buildings; it is ever more important in transportation structures, where long spans are required and highly flexible single or multi-column bents must be utilized.

Project Title: CAREER: Thermally Active Geotechnical Systems in Reinforced, Unsaturated Soils

Project Sponsor:  NSF CMMI 1054190

Description: The major research goal of this project is to understand the fundamental issues involved in using heat to improve the properties of unsaturated soils. Four research thrusts are being followed by the students working on this project: (1) understand the variables affecting thermally-induced flow of water in unsaturated soils, including the zone of influence and the time required for this process; (2) understand the impact of temperature on the shear strength and volume of unsaturated soils under low suction magnitudes (nearly saturated conditions); (3) understand the impact of temperature on the shear strength and volume of unsaturated soils under high suction magnitudes (nearly dry conditions); and (4) understand the impact of temperatures on the deformation response of geosynthetic reinforcements used in thermally active  geotechnical systems. In all four thrusts, experimental approaches are being used to understand the fundamental behavior of compacted, unsaturated silt. The results are being used to extend and validate constitutive models originally developed for water-saturated soils. The research thrusts on high and low suctions are being pursued separately due to the different mechanisms of volume change in these conditions, and because of the difference in experimental methodologies to control suction in dry conditions (vapor flow technique) and nearly saturated conditions (axis-translation technique). The findings from these research thrusts are being integrated to evaluate the feasibility of using near-surface geotechnical systems (retaining walls, embankments) as heat sinks for industry or buildings, while at the same time taking advantage of the spurious heat to improve the long-term mechanical response of the geotechnical system. The major educational goal is to develop a communication training program for geotechnical engineers that permits them to communicate their research findings to audiences with different levels of technical background. The outcome of the educational goal is to disseminate research findings to a wide audience and to provide new geotechnical engineers with the skills necessary to communicate the need for innovative solutions in practice. 

Project Title: SEP Collaborative: Pathways to Scalable, Efficient and Sustainable Soil Borehole Thermal Energy Storage Systems

Project Sponsor: NSF CMMI 1230237

Collaborators: Ning Lu, Shemin Ge, Kate Smits, Adam Reed

Description: The overall goal of this project is to understand the fundamental multi-physics processes, engineering challenges, environmental impacts, and implementation strategies for soil borehole thermal energy storage (SBTES) from renewable energy sources in the shallow subsurface (20 to 50 meters). We are investigating the injection of heat generated from solar-thermal installations into the vadose zone through properly spaced, closed-loop borehole heat exchangers so that the thermal energy may be accessed at a later point for direct use in building heating or electricity generation. The unique thermo-hydraulic properties and coupled heat, water, and vapor flow processes in unsaturated soils are being explored to enhance heat transfer in the vadose zone in a similar manner to that in a heat pipe, leading to more heat transfer than that obtained by conduction. The unsaturated soil surrounding the borehole array also acts as an insulator to minimize lateral loss of stored heat. 

Specific research goals are to: 

• Characterize the nonlinear transport properties of unsaturated soils from the field and laboratory tests, and of thermal grouts used to backfill the boreholes

• Evaluate coupled water, vapor and heat flow processes and potential environmental impacts in laboratory-scale simulations within densely-instrumented soil tanks

• Validation and establishment of scalable numerical models (Tough2 and COMSOL) to examine the long-term operation, efficiency, and environmental impact of SBTES systems

• Construct a field-scale test facility of closed-loop heat exchangers in boreholes beneath a thermo-hydraulic barrier to evaluate the efficiency of heat injection and withdrawal for different borehole array geometries and fluid flow patterns in the heat exchanger tubing

• Explore approaches to enhance the heat exchange efficiency (injection and withdrawal)

Project Title: Soil Structure Interaction in Geothermal Foundations

Project Sponsor:  NSF CMMI 0928159

Collaborators: Hon-Yim Ko, Moncef Krarti, Richard Regueiro, Tad Pfeffer

Description: This project developed an understanding of soil-structure interaction mechanisms in energy piles through a combination of element scale testing, centrifuge modeling, numerical simulations, and field testing. Thermal oedometer tests helped understand the permanent strains encountered during cyclic heating and cooling for different initial stress states (Vega et al. 2012; Vega and McCartney 2014) and temperature-controlled true-triaxial tests helped understand the impact of stress-induced anisotropy on the thermal expansion of soils in different directions (Coccia et al. 2011; Coccia and McCartney 2012). Centrifuge testing revealed the role of radial thermal expansion on the side shear resistance of soils (McCartney and Rosenberg 2011), as well as the impact of end constraint boundary conditions on the distributions of stresses and strains in energy piles (Stewart and McCartney 2012; Khosravi et al. 2012; Stewart and McCartney 2013; Goode and McCartney 2014a, 2014b; Goode et al. 2014; Goode and McCartney 2014). The numerical models developed and validated in this study include heat transfer analyses (Rouissi et al. 2012), a load transfer analysis for prediction of thermally induced axial stress/strain predictions (McCartney et al. 2014), as well as a poro-thermo-elasto-plastic finite element model for evaluation of the impact of heat and water flow processes on energy piles (Wang et al. 2012, 2014a, 2014b). This project also supported the instrumentation and monitoring of a pair of energy piles in a building in Denver, CO (McCartney and Murphy 2012; Murphy and McCartney 2014). The intellectual merit of the project consisted of the new centrifuge modeling approach that provided data to validate new soil-structure interaction tools and verify data collected from instrumented field sites, and the broader impacts include outreach presentations and involvement of undergraduate students and minorities. A total of 1 PhD, 5 MS, and 4 undergraduate students worked on this project. This study led to a keynote paper (McCartney 2011) and a book chapter (McCartney 2013). 

Project Title: Full-Scale Implementation of Energy Foundations

Project Sponsor:  DoD ESTCP

Collaborators: Karen Henry

Description: The objective of this project sponsored by DoD ESTCP (project EW-201153) was to construct a new building at the US Air Force Academy with several geothermal foundations having different designs. Data is currently being collected to demonstrate the feasibility of using geothermal foundations to heat and cool buildings, and to compare the energy efficiency of a ground-source heat pump system with a conventional heating and cooling system. The results from this project are being used to enhance the design methodologies used for geothermal foundations. As the importance of energy dependence for buildings is a growing priority for the Department of Defense (DoD), new technologies are being developed in order to reduce the need of external energy requirements in all aspects of building operations.  Geothermal foundations are building foundations that serve the dual purpose of supporting building loads as well as providing access to geothermal energy. Tubing is embedded into the concrete of the deep foundation system that allows energy to be exchanged with the ground as a means of building heating and cooling. 

One of our specific objectives is to understand soil-structure interaction in full-scale geothermal foundations. As a foundation element is heated and cooled, it will expand and contract, which can lead to load distribution within the foundation that could potentially compromise the structural integrity of the building. Although no significant structural issues have been reported in previous research, analytical tools are needed to validate designs with respect to load distribution in the foundation element. Since the soil conditions at every site vary, an in-situ testing device has been developed to determine site-specific design parameters that could be implemented in a computer model in order to more accurately predict load distribution patterns in the foundation during heating and cooling. This research aims to further advance tools to aid in the safe design of geothermal foundations in order to have more widespread usage with the DoD and throughout the world.

Project Title: Integrated Experimental - Computational Multiscale Immersed Particle-Continuum Approach to Modeling and Simulation of Multiphase Soil Failure Mechanics under Buried Explosive Loading

Project Sponsor: ONR MURI

Collaborators: Richard Regueiro

Description: A recent project sponsored by a MURI grant from the Office of Naval Research (ONR) (grant N00014-11-1-0691) has focused on understanding the mechanical behavior of unsaturated soils under conditions related to buried explosives. Woongju (MJ) Mun is studying the compression behavior of unsaturated soils under isotropic stresses up to 160 MPa. Jenna Svoboda, Mehmet Can Balci, and Thayza Teixeira have studied the impact of shearing rate on the shear strength of unsaturated soils. MJ also worked with Fabricio Valente to study the compression behavior of sand-clay mixtures. We are collaborating with researchers from other universities to integrate our work on soil behavior into the prediction of blast pressures and ejecta distribution related to a buried explosive. 

Several previous researchers in our group have focused on understanding the fundamental properties governing the deformation response of unsaturated soils. Ali Khosravi and Majid Ghayoomi performed a series of tests to characterize the role of stress state on the dynamic shear modulus of unsaturated compacted soils. Ali Khosravi developed an approach to control the suction and track changes in degree of saturation within a soil specimen inside a resonant column-torsional shear cell, which permitted an understanding of the impacts of hydraulic hysteresis on the shear modulus. Ali worked with Chris Lynch and Nahed Alsherif to perform a series of staged triaxial tests on unsaturated, compacted silt, which helped us define the suction-stress characteristic curve for the silt. This information was being used to complement Ali's research findings on the definition of a single-value effective stress in unsaturated silt. Nahed performed a series of triaxial compression tests on unsaturated silt at high pressures and high temperatures. She used the vapor flow technique to control the suction magnitude in compacted silt specimens, and has observed a complex combination of hardening during suction application and softening during heating.  Majid Ghayoomi evaluated the seismic response of partially saturated sand layers using a novel geotechnical centrifuge test. He used steady-state infiltration of water to control the degree of saturation in a sand layer, then performed cyclic shaking tests. Majid used this setup to provide validation data for a semi-empirical model to predict the seismic compression of sand layers having different degrees of saturation. 

Project Title: Large Scale Measurement of Internal and Interface Shear Strength of Tire Derived Aggregate

Project Sponsor:  CalRecycle

Collaborators: Pat Fox

Description: The California Department of Resources Recycling and Recovery (CalRecycle) has been working for more than 15 years to develop technology to promote the use of Tire-Derived Aggregate (TDA) in civil engineering applications in California.  During that time, CalRecycle has designed and constructed numerous successful project incorporating TDA. To complete the designs for these projects, material properties are needed for TDA. One of the most important of these properties is the shear strength, as it is critical in the design of the light-weight fill applications (e.g., embankments, light rail foundation, landslide repair, and retaining walls). 

The shear strength of TDA has been typically determined through standard soil testing procedures (e.g., ASTM D3080). This is generally acceptable for crumb rubber and some types of small size (Type A) TDA material with a particle size range of 3 to 4 inches according to ASTM D6270. For large-size (Type B) TDA, with a particle size of 3 to 12 inches according to ASTM D6270, standard soil testing devices are too small to achieve the required strain/displacement for peak strength measurement. Type B TDA requires less processing and is therefore more cost effective than Type A TDA for earth fill applications. However, because larger strength testing devices generally have not been available until recently, designs using Type B TDA have been based on conservative estimates of shear strength, potentially making TDA less competitive as an alternative fill material for construction projects. To address this concern in a previous testing program for CalRecycle reported by McCartney et al. (2016), UCSD constructed a large combination direct shear/simple shear machine to measure shear strength properties for Type B TDA. TThe details of the machine in these two modes are reported in detail in Fox et al. (2018). Further, the direct shear/simple shear machine was used to perform a comprehensive testing program on the internal shear strength of Type B TDA and concrete-Type B TDA interface shear strength, reported in detail by Ghaaowd et al. (2017), and the cyclic shearing response of Type B TDA, reported in detail by McCartney et al. (2017). The shear strength data obtained from these studies is applicable for the design of field projects involving beneficial reuse of this material in reinforced walls, slopes, and embankments, while the cyclic shearing response of Type B TDA is applicable for seismic analyses of TDA structures.