Abstract

This paper is a case study on the estimation of rock mass strength for jointed sandstone using Synthetic Rock Mass (SRM) modeling. The case study broadly follows the SRM approach described by Cundall et al. (2008). First the strength and elastic response of intact rock are quantified in laboratory experiments including Uniaxial Compressive Strength, Brazilian, and Tri-axial tests at confining pressures from 1, 5 & 10 MPa. Second, a Bonded Particle Model loaded in uniaxial compression is calibrated to the laboratory measured strength and deformation parameters. Third a three dimensional synthetic rock mass is formed through the introduction of joint sets in random locations and orientations, using the Smooth Joint Model (SJM). The SRM is then subjected to a range of stress states and stress paths to estimate the yield criteria and rock mass modulus for a range of fracture frequencies.

INTRODUCTION

Synthetic rock mass modeling (SRM) is a promising new technique combining discrete fracture network simulation (DFN) and bonded particle models (BPM). The aim of this paper is to apply SRM modeling to assess the impact of discontinuities on the large scale strength of Hawkesbury sandstone. The Hawkesbury sandstone forms the foundation for the city of Sydney, and hosts many significant civil and mining excavations including freeway tunnels, slopes, underground car parks, storage facilities, reservoirs, bridges, quarries, house foundations, longwall mines, and natural cliffs. Hawkesbury sandstone is also used as a building material for civil and mining structures.

Hawkesbury sandstone is a mid Triassic sedimentary quartzose sandstone, with a small proportion of feldspar, clay, and iron compounds such as siderite. Hawkesbury sandstone forms one of the upper strata in the Sydney basin deposit. The geological history of the basin is understood to involve sedimentation and a number of phases of earth movements. The underlying structure of the basin was laid down during the Permian and earlier geological periods (Figure 1) under marine and marshy conditions which produced the sandstone and siltstone formations and associated coal measures [1].

(Figure in full paper)

Since Triassic times the surface of the Sydney basin has been above sea level and deposition of sediments is thought to have been by a vast braided river system, similar to the modern Brahmaputra River of Bangladesh [2]. Over millions of years sands were consolidated into sandstone up to 200 metres thick.

LITERATURE REVIEW
Bonded Particle Models for Intact Rock

The Bonded Particle Model (BPM) is an implementation of the discrete element method, in which damage is represented as broken bonds, which form and coalesce into macroscopic fractures during loading [4]. BPM has been shown to reproduce many features of rock behavior including: elasticity, fracturing, acoustic emission, damage accumulation producing material anisotropy, hysteresis, dilation, post-peak softening and strength increase with confinement.

One advantage of BPM is that no assumptions are made about fracture location and failure mode. Cracking and sliding are emergent features in the method, and take place spontaneously, leading to a variety of mechanical behaviors and failure mechanisms.

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