Bio-inspired Methods for Reduction of Penetration Resistance in Granular Materials
Numerous geotechnical applications such as pile installation or CPT exploration require large rigs to generate reaction forces for the penetration of geotechnical elements. These rigs can increase financial and environmental impacts and pose accessibility challenges to engineering projects. Bio-inspiration can be used to identify geometries of penetrometers to reduce the penetration resistances, which may enable the use smaller rigs that would reduce the related economic and environmental impacts of geotechnical activities. This research aims to identify the attributes of organisms that make them efficient burrowers and evaluate the application of these attributes for geotechnical engineering activities. Particularly, this work focuses on the attributes of penetrometer apex angle and geometric asymmetry. Evaluation of the bio-inspired penetrometer, in terms of the generated tip resistance, is accomplished by performing Discrete Element Method (DEM) simulations where each geometry is penetrated into a specimen contained in a calibration chamber. In this research, both shallow and deep penetration conditions are considered. Shallow penetration conditions are defined as penetration in an unconfined specimen at normalized depths ratios (Z/D, depth to probe diameter) smaller than 7, while deep penetration conditions are defined as penetration in a confined specimen subjected to a vertical stress of 100 kPa and a horizontal stress of 50 kPa. Results show that an apex angle close to that of a honeybee stinger, 30°, minimizes the tip penetration resistance for shallow penetration. Results additionally show that an apex angle of 15° reduce penetration resistance for deep penetration conditions. However, this reduction in penetration resistances at deep conditions was smaller than that achieved in shallow conditions. With DEM, it is possible to monitor the forces and position of each individual particle, thus allowing for close examination of the soil failure mechanisms generated by each tested probe geometry. The results show that probes with small apex angles displace particles horizontally and create increases in horizontal stress at locations near the probe tip. In contrast, probes with large apex angles displace particles vertically down, creating an increase in vertical stresses below the probe tip. The asymmetric probes simulations showed no reduction of penetration resistance compared to their symmetric counterparts. The asymmetric tip may also contribute to an imbalance in the forces acting on the probe, which may cause the probe to not penetrate the substrate vertically. The mechanisms explored using DEM can also help develop an understanding for future improvements of probe geometry to achieve more efficient penetration during in-situ tests and construction activities.