A broad diversity of biological organisms and systems interact with soil in ways that facilitate their growth and survival. These interactions are made possible by strategies that enable organisms to accomplish functions that can be analogous to those required in geotechnical engineering systems. Examples include anchorage in soft and weak ground, penetration into hard and stiff subsurface materials and movement in loose sand. Since the biological strategies have been ‘vetted’ by the process of natural selection, and the functions they accomplish are governed by the same physical laws in both the natural and engineered environments, they represent a unique source of principles and design ideas for addressing geotechnical challenges. Prior to implementation as engineering solutions, however, the differences in spatial and temporal scales and material properties between the biological environment and engineered system must be addressed. Current bio-inspired geotechnics research is addressing topics such as soil excavation and penetration, soil–structure interface shearing, load transfer between foundation and anchorage elements and soils, and mass and thermal transport, having gained inspiration from organisms such as worms, clams, ants, termites, fish, snakes and plant roots. This work highlights the potential benefits to both geotechnical engineering through new or improved solutions and biology through understanding of mechanisms as a result of cross-disciplinary interactions and collaborations.

This study involves an evaluation of the design and construction process for a soil-borehole thermal energy storage (SBTES) system installed in a sandy-silt deposit. A series of simplified numerical simulations were performed to understand the role of different variables on the heat storage in the SBTES system. The results indicate that soils with lower thermal conductivity have less lateral heat loss, and that arrays with smaller borehole spacings permit more concentrated storage of heat at higher temperatures.

Ahead of Print articles will move to that issue's Table of Contents. When the article is published in a journal issue, the full reference should be cited in addition to the DOI.

Soil-borehole thermal energy storage (SBTES) systems are used for storing heat collected from renewable sources in the subsurface so that it can be used later for space or water heating. Heat sources such as solar thermal panels generate heat during the day with a greater energy generation during summer months, so SBTES systems permit storage of the abundant and free thermal resource (Sibbitt et al. 2012, McCartney et al. 2013). SBTES systems function similarly to geothermal heat exchange systems, where a carrier fluid is circulated through a closed-loop pipe network installed in vertical boreholes backfilled with sandbentonite. Different from boreholes in conventional geothermal heat exchange systems, the boreholes in SBTES systems are spaced relatively close together (1-2 m) in an array to concentrate heat in the subsurface (Claesson and Hellström 1981). SBTES systems are a convenient alternative to other energy storage systems as they are relatively inexpensive, involve storage of renewable energy (solar thermal energy), and are space efficient as they are underground (Başer and McCartney 2015a). Despite the successful use of SBTES systems in community-scale applications (Sibbitt et al. 2012; Nussbicker-Lux 2012; Bjoern 2013), there are still opportunities for engineers to improve the performance of SBTES systems by considering the role of the hydrogeological setting in the subsurface. A goal of this study is to understand the benefits of installing SBTES systems in the vadose zone, the layer of unsaturated soil or rock near the ground surface that may extend to depths greater than 10 meters in some locations. The unsaturated porous material in the vadose zone has a lower thermal conductivity than when saturated, limiting the transient spreading of heat away the subsurface heat storage system (Choi et al. 2011). The 3 volumetric heat capacity of soils in unsaturated conditions is lower than in saturated conditions but is still greater than in dry conditions. For example, the volumetric heat capacity of a silty soil is 2.5 MJ/m 3 K for saturated conditions, 2.0 MJ/m 3 K for a degree of saturation of 0.5, and 1.2 MJ/m 3 K for dry conditions (Baser et al. 2016d). One challenge is that the modes of heat transfer in unsaturated porous materials are more complex than when dry or water-saturated. Specifically, in addition to coupling between the thermal and hydraulic properties of unsaturated soils and the effects of temperature on fluid properties (e.g., Lu and Dong 2015), the modes of heat transfer in unsaturated soils include a combination of conduction, convection due to the flow of pore water in liquid and vapor forms under thermal and hydraulic gradients, and latent heat transfer due to phase change. Several studies have developed models to capture these different mechanisms of coupled heat transfer and water flow in unsaturated soils, and have applied them to problems associated with radioactive waste repositories (e.g., Ewen and Thomas 1989; Thomas and Sansom 1995; Gens et al. 1998; Gens et al. 2009), soil-at...

Although siting of geothermal energy storage systems in the vadose zone may be beneficial due to the low heat losses associated with the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in soils surrounding geothermal heat exchangers may play important roles in both the heat injection and retention processes that are not considered in established design models for these systems. This study incorporates recently-developed coupled thermo-hydraulic constitutive relationships for unsaturated soils into a coupled heat transfer and water flow model that considers time-dependent, nonequilibrium water phase change and enhanced vapor diffusion to study the behavior of geothermal energy storage systems in the vadose zone. After calibration of key parameters using a tank-scale heating test on compacted silt, the ground response during 90 days of heat injection from a vertical geothermal heat exchanger