Bio-inspired geotechnical engineering: Principles, current work, opportunities and challenges

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Bio-inspired geotechnical engineering: principles, current work, opportunities and challenges

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

Coupled Heat Transfer and Water Flow in Soil-Borehole Thermal Energy Storage Systems in Unsaturated Soil Profiles

This research focuses on understanding the key variables affecting the performance of soil-borehole thermal energy storage (SBTES) systems installed in the vadose zone. In particular, this study seeks to understand how the coupled thermal and hydraulic properties of unsaturated geomaterials and coupled heat transfer and water flow processes may be exploited to enhance heat injection and heat retention in an array of geothermal borehole heat exchangers. Specifically, heat injection into unsaturated geomaterials may be enhanced through convection of liquid water and water vapor as well as through latent heat energy transfer associated with water phase change. Further, heat retention in unsaturated geomaterials may be enhanced by the reduction in thermal conductivity associated with the thermally-induced drying expected during heat injection. Multi-fidelity numerical models are employed in this study to consider the roles of different mechanisms of heat transfer during a cycle of heat injection and ambient cooling. A conduction-only model was found to be useful for evaluating scalability issues and for the geometric design of a full-scale SBTES system installed in Golden, Colorado, but required a low soil thermal conductivity to match collected field data. A numerical model the incorporates convection in the liquid and vapor phases as well as phase change was validated using the results from a full-scale SBTES system installed in an unsaturated rock layer in San Diego, California. The numerical model was calibrated using coupled thermo-hydraulic soil properties from element-scale experiments and parameters governing water vapor diffusion and phase change rates from tank-scale heat injection experiments. A comparison of the simulation results from this models with another model that does not incorporate vapor flow confirmed the significant effect of water vapor convection on the permanent drying of the subsurface during heat injection. The results from the validated numerical simulations and field experiments confirm the potential benefits of siting SBTES systems in the vadose zone. The validated numerical model considering convection in the vapor phase can be used in future studies to evaluate the evolution in the energy extraction efficiency over multiple heat injection and extraction cycles for SBTES systems in the vadose zone

Coupled Heat Transfer and Water Flow in Soil-Borehole Thermal Energy Storage Systems in Unsaturated Soil Profiles

This research focuses on understanding the key variables affecting the performance of soil-borehole thermal energy storage (SBTES) systems installed in the vadose zone. In particular, this study seeks to understand how the coupled thermal and hydraulic properties of unsaturated geomaterials and coupled heat transfer and water flow processes may be exploited to enhance heat injection and heat retention in an array of geothermal borehole heat exchangers. Specifically, heat injection into unsaturated geomaterials may be enhanced through convection of liquid water and water vapor as well as through latent heat energy transfer associated with water phase change. Further, heat retention in unsaturated geomaterials may be enhanced by the reduction in thermal conductivity associated with the thermally-induced drying expected during heat injection. Multi-fidelity numerical models are employed in this study to consider the roles of different mechanisms of heat transfer during a cycle of heat injection and ambient cooling. A conduction-only model was found to be useful for evaluating scalability issues and for the geometric design of a full-scale SBTES system installed in Golden, Colorado, but required a low soil thermal conductivity to match collected field data. A numerical model the incorporates convection in the liquid and vapor phases as well as phase change was validated using the results from a full-scale SBTES system installed in an unsaturated rock layer in San Diego, California. The numerical model was calibrated using coupled thermo-hydraulic soil properties from element-scale experiments and parameters governing water vapor diffusion and phase change rates from tank-scale heat injection experiments. A comparison of the simulation results from this models with another model that does not incorporate vapor flow confirmed the significant effect of water vapor convection on the permanent drying of the subsurface during heat injection. The results from the validated numerical simulations and field experiments confirm the potential benefits of siting SBTES systems in the vadose zone. The validated numerical model considering convection in the vapor phase can be used in future studies to evaluate the evolution in the energy extraction efficiency over multiple heat injection and extraction cycles for SBTES systems in the vadose zone

Undrained Shear Strength of Frozen Unsaturated Silts

This study focuses on investigation of the undrained shear strength of unsaturated frozen silts prepared at varying initial thermal and hydraulic conditions. The initial degree of saturation controls ice and unfrozen water contents at temperatures below depression point. The strength properties of frozen soils are highly influenced by ice and water contents which is highly coupled with thermal state of the soils. To evaluate the strength properties of frozen silts, a series of direct simple shear experiments were performed using Bonny silt prepared at different initial degrees of saturation under monotonic shear loading. Compacted silt samples at varying degrees of saturation were subjected to artificial freezing before shear loading and stress-strain curves were recorded during loading. Identical samples were prepared and sheared at room temperatures for comparison. The stress-strain behavior of frozen silts was observed to be significantly different than those of obtained at room temperatures where on an average the shear strength of the saturated frozen soils was higher by 150% in comparison to the shear strength of the same soil in saturated unfrozen condition. The undrained shear strengths for frozen soils were also observed to be affected by initial degree of saturation where the strength was observed to increase by 142 % when the initial degree of saturation was increased from 0.51 to 1.00. The results obtained from this study will be used in ongoing investigations of capacity of deep foundations in warming permafrost