Numerical assessment of plasticity development and energy expenditure of ant-like microtunnelling

This study aims to investigate the potential mechanical benefits of the excavation technique adopted by Harvester ants, that dig tunnels in multiple passes, progressively augmenting the diameter of their shafts along the entire tunnel length during each pass. Bio-inspired abstraction is employed to extract the governing principles of ant tunnelling, characterized by progressive tunnel diameter augmentation, and a proof-of-concept study is conducted to establish a foundation for the development of deployable engineering designs. The analysis is based on circular unsupported tunnels of 10 cm diameter excavated in moist sand deposits. The results can be directly applicable to small exploration robots and microtunnelling devices while also providing insight for larger scale unsupported tunnels in other soil types. The geometry of the excavation-induced plastic zone and the required excavation force, torque, and energy are analysed for a single-pass and a five-stage excavation process. Both excavation techniques are simulated in 2D with the Finite Element Method (FEM). Stress magnitudes calculated with the FEM are introduced in analytical expressions to evaluate the total jacking force, torque, and energy required to excavate an elementary length of tunnel. Over 640 combinations of soil elasto-plastic parameters and cavity depth-to-diameter ratios are analysed. The geometry of the plastic zone that develops over successive excavation passes exhibits a higher degree of complexity than during a single-pass excavation, particularly at greater depth — as reflected by fractal dimensions and Minkowski functionals. Staged excavations lead to more localized plastic deformation zones for soils with low apparent cohesion, high depth-to-diameter ratio, high internal friction angle, and low dilation angle. Emulating the staged excavation process of Harvester ants can reduce the force, torque, energy and power that are needed for microtunnelling compared to more traditional single-pass excavation processes, particularly for low apparent cohesion soils and deep cavities. The key to power and energy savings lies in the incremental formation of localized plastic zones around the tunnel face, which is accompanied by stress redistribution and arching. This lowers the lateral earth pressure that must be supported at the excavation front, the normal stress that controls the shear strength against the applied torque, and the radial stress normal to the sides of the excavating device which controls the frictional force that must be overcome to advance the tunnelling device.