Simulation on mechanical behavior of cohesive soil by Distinct Element Method

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Abstract

Analysis in the dynamic mechanical behavior of cohesive soils subjected to external forces is very important in designing and optimizing terrain machines. Distinct Element Method (DEM) is an ideal method to analyze large discontinuous deformations of soil, but the conventional DEM model is difficult in simulating the complex behavior of cohesive soil. In order to simulate and analyze the behavior of cohesive soil accurately, the DEM mechanical model of cohesive soil with parallel bonds between particles was established by considering the capillary and the dynamic viscous forces induced by the presence of water between soil particles. During the excavation process by a bulldozing plate, the dynamic behavior of cohesive soil was simulated by DEM software PFC2D. The phenomena that the discrete particles were bonded into clusters initially, and the clusters were broken into smaller clusters or discrete particles during the excavation process, are consistent with the ruptures and separations of the actual cohesive soils subjected to external forces.

Introduction

DEM is a discontinuous numerical method based on molecular dynamics. It was developed and applied for analyzing rock mechanics by Cundall in 1971 [1]. It overcomes some of the disadvantages of Continuum Mechanics Method (CMM) such as Finite Element Analysis, which ignores individual unit characteristics, and relies on the highly simplified mechanical equations excessively. DEM represents great superiorities to CMM in analyzing discrete materials. Considering soil as a multiphase compound, it is discrete by nature. The soil which is cut or separated by soil engaging components is much more discrete, therefore DEM is an ideal method to analyze large discontinuous deformations of soil [2], [3].

Cohesive soils are very common we come across in agricultural operations and constructions. The analysis of the dynamic mechanical behavior of cohesive soils subjected to external forces is very important in designing and optimizing terrain machines. Cohesive soil contains water, and the presence of water can produce cohesion between soil particles, which makes the mechanical structure of these soils much more complex. The conventional DEM model is difficult in simulating the complex behavior of cohesive soil. In order to simulate and analyze the mechanical behavior of cohesive soil accurately, it is necessary to establish a DEM mechanical model of cohesive soil by considering the effects of water on the mechanical behavior of cohesive soil.

Section snippets

Effects of water on mechanical microstructure of cohesive soil

Interstitial liquid in agglomerate may exist in a pendular, funicular capillary or immersed state depending on the degree of saturation [4]. The water content of cohesive soil varies between that of dry soil and of saturated soil. Water distribution in cohesive soil is uneven and discontinuous, therefore interstitial water between soil particles in these soils exists in a pendular state. In this state, water is present in soil in the form of discrete liquid bridges, as shown in Fig. 1. Fisher

DEM mechanical model of cohesive soil

According to the above analysis on the microcosmic mechanical structure of cohesive soils, the mechanical behavior of cohesive soils is mainly influenced by the capillary and the dynamic viscous forces produced by the discrete dynamic liquid bridges between soil particles, besides the contact forces and the forces of friction at contacts between soil particles. Therefore, when the DEM mechanical model of cohesive soil is established to simulate the dynamic behavior of these soils by using DEM

Simulation and analysis of cohesive soil by DEM

The dynamic behavior of cohesive soils during the excavation process by a bulldozing plate was simulated by using the above established DEM mechanical model of cohesive soil via PFC2D. The initialization of the interaction between a bulldozing plate and cohesive soils is shown in Fig. 5. Fig. 5(a) is the complete model. The model is composed of different particle aggregate clusters with different colors, in which black particles are discrete particles without conglomerating. The phenomenon of

Conclusion

  • (1)

    Water distributes in cohesive soil unevenly and discontinuously. The interstitial water between soil particles forms discrete pendular liquid bridges which produce the capillary and the dynamic viscous forces between soil particles. These two forces are the main factors affecting the mechanical behavior of cohesive soil.

  • (2)

    The DEM mechanical model of cohesive soil with parallel bonds between particles was established by considering the capillary and the dynamic viscous forces induced by the

Acknowledgement

The project was supported by National Natural Science Foundation of China (Grant No. 50175045) and the China National Key Grant of Basic Scientific Project (2002CCA01200).

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