Discrete Element Modeling of Bio-Inspired Soil Penetration Processes for In-Situ Testing Probes

摘要

Soil penetration activities, such as site investigation, pile driving and tunneling, are ubiquitous and fundamentally important in civil engineering. These penetration activities are energy-intensive and responsible for considerable environmental disruption due to the usage of large equipment required to provide reaction force. Bio-inspired geotechnics has received significant attention in recent years due to the growing need for making construction processes more sustainable. This research investigates the anchor-tip strategy inspired by earth and marine worms, razor clams, and caecilians and the circumnutation strategy inspired by plant roots. Throughout this dissertation, it is shown that using the bio-inspired strategies can facilitate soil penetration by reducing the mobilized penetration resistance and generating anchorage reaction forces.Discrete element modeling (DEM) is used to simulate the bio-inspired soil penetration process of an in-situ testing probe in granular soils. Different soil conditions are examined in the DEM simulations, including soil density and magnitude of overburden stress. Simulations were performed on specimens confined to constant stress levels using servo-controlled algorithms to model deep penetration conditions. Simulations were also performed on unconfined soil specimens under gravity to model shallow soil conditions. The simulated anchor-tip strategy consists of radially expanding of a probe section or sections (i.e. anchor(s)) and subsequently displacement of the probe tip to deeper locations. The global and meso-scale responses of the probe-soil system are analyzed to shed light on the working mechanisms of this strategy. Specifically, the expanded anchor serves as a integrated anchorage system to provide the reaction force needed for penetration. The anchor expansion leads to reduction in tip resistance by altering stress states around the tip. The effects of a number of aspects of the anchor and tip geometry, as well as of soil depth, are explored. The effects of soil density on the anchor-tip strategy are also highlighted. Due to the complexity of anchor-tip strategy, three simulation strategies employed to control the probe's motions: displacement-controlled algorithm, velocity controlled algorithm with force limits and force-controlled algorithm. Among these three strategies, the first one is the most simplified one while the last one best approximated the motion used by the model organisms.The simulated circumnutation inspired motion (CIM) consists of helical movements of the probe tip accompanied by downward penetration of the entire probe. The probe forces, torque, mechanical work and particle contact orientations are analyzed and discussed. The results indicate that CIM leads to a decrease of penetration resistance by altering the contact orientations near the tip from the vertical to the horizontal direction. However, this reduction in penetration resistance comes at a cost of increased torque and in most conditions an increase in rotational work. A comparison of the CIM and rotational penetration (RP) strategies shows that the CIM mobilizes smaller penetration forces and requires less total mechanical work to penetrate the same distance as the RP. The effects of the CIM velocities and probe geometry are also examined.The understanding of the probe-soil interactions during the bio-inspired penetration processes lays the foundation for the development of innovative soil penetration tools and techniques to increase the efficiency of construction activities. For example, studies on bio-inspired probes can guide the design of future lightweight penetration devices. Such probes could reduce the energy consumption during transport and the challenges associated with limited accessibility in certain sites, such as congested urban areas and outer space bodies. These studies show that by learning from nature, more efficient solutions can be developed for geotechnical engineering applicat