Post Peak Behavior of Rocks and its Numerical Simulation Considering Softening

Maji, V. B. (Indian Institute of Technology Madras) | Dharani Raj, S.V. (Indian Institute of Technology Madras) | Sai, M. Partha (Indian Institute of Technology Madras)

OnePetro 

Abstract

The post peak behavior of rocks has a significant influence on its strength and deformational characteristics and is crucial in many applications like underground coal mining. A clear understanding of post peak behavior of coal is necessary for economical and safe underground coal extraction. In this study, laboratory experiments and corresponding numerical modeling is done to understand the post peak behavior of a coal and coal pillar. A series of laboratory tests are conducted using coal samples and also artificially prepared gypsum samples with varying material strength. A numerical model is developed in FLAC3D and simulations are run with strain softening model to capture the post peak responses of the material. This model allows to attain the peak strength following the Mohr-Coulomb behavior and once it attains the peak, the strength parameters are softened with respect to plastic strain that the material experienced using a piecewise linear functions. The softening parameters are selected in such a way that a realistic behavior could be achieved. With the validated model, several parametric studies such as influence of dilation, confinement, and friction angle are performed. Understanding the influence of the post peak parameters gave a full extent of the usage of material performance in the numerical model for better design of underground excavations. The numerical model is subsequently extended to design coal pillar for a underground mines is briefly discussed.

1. Introduction

Design of supporting rock pillars in underground excavations specially applications like mining is based on the maximum pillar strength as well on the post-failure behavior. The complete stress-strain behavior of the pillars play an important role for those pillar stability. Around the underground structures there is possibility of formation of plastic regions. Hence, the design steps together with support systems expected to be accommodative in accordance with the existence of such regions. To estimate the parameters related to the post-failure of supporting pillars, large scale in-situ compression tests are needed to be conducted, which is quite difficult and expensive. In this study, post peak characteristics of coal samples and artificially prepared gypsum samples are obtained using MTS servo controlled testing machine available in the Department of Civil Engineering, IIT Madras. The same experiments are also numerically modeled in FLAC3D and attempt was made to capture the post peak response. Simulation were done by moderating various parameters namely cohesion, friction angle and dilation. Mohr-Coulomb Strain Softening model is used as the decay of strength parameters with respect to the plastic strain to obtain the softening behavior (Itasca FLAC3D manuals, 2008). With the validated model, parametric studies are done in order to understand the influence of various parameters on post peak responses.