Year

2007

Degree Name

Doctor of Philosophy (PhD)

Department

School of Civil, Mining and Environmental Engineering - Faculty of Engineering

Abstract

Natural rock joints are normally filled with fine materials such as clay and silt which influence entire rock mass stability. Saturated infilled joints have contributed to the instability of rock mass during undrained shearing due to the build up of pore water pressure within the joints. It is probable that most of the discontinuities in nature will be in an overconsolidated or pre-loaded state. The shear response of infilled rock joints is generally controlled by the type and thickness of infill, joint roughness, drainage conditions, and the stress history. Based on the research work carried out at the University of Wollongong in the past, a shear strength model for unfilled and infilled joints was developed using the Fourier functions coupled with energy considerations adopting a hyperbolic technique. It was found that the hyperbolic constants were often sensitive to the types of infill, and the hyperbolic fit was not always accurate for infill such as graphite. In order to predict shear strength more accurately, a conceptual normalised shear strength model was recently developed based on two algebraic functions (Indraratna et al., 2005). Although this model conveniently predicted the shear strength with some accuracy, it required considerable extension to incorporate the degree of overconsolidation in relation to the development of pore water pressure. In this research study, an experimental investigation was carried out to study the effect of overconsolidation on the shear behaviour of saturated infilled joints. For this purpose, the high-pressure two-phase triaxial apparatus at the University of Wollongong was modified with the installation of a mechanical driving system to apply a constant axial strain to shear infilled joints under a given confining pressure. Extensive tests were conducted on saw-toothed (18 degrees asperity angle) joints under consolidated undrained conditions with pore water pressure measurement. Limited tests on planar and sheared sandstone joints were also conducted for comparison. Natural silty clay collected from a rockslide site was used as infill for the entire experimental program. The shear behaviour of saw-toothed joints was investigated for varying infill thickness to asperity height ratios (t/a) in the range of 0-5.0, and varying overconsolidation ratios (1, 2, 4, and 8), at confining pressures of 200 and 500 kPa. Accordingly, a mathematical model is presented for predicting the shear strength of normally consolidated and overconsolidated rough infilled joints. The proposed shear strength model evaluates the reduction in shear strength observed with increasing t/a ratios at varying OCRs. It highlights the role of the critical t/a ratio, beyond which no further reduction in shear strength occurs. This critical t/a divides the infilled joint behaviour into the quote asperity interference unquote and quote asperity non- interference unquote regions. In the region of asperity interference, the sum of two algebraic functions (i.e. An and Bn - represent joint and infill characteristics, respectively for OCR=n) models the decay of shear strength with increasing t/a. In the region of asperity non-interference, the shear strength is modelled purely with infill properties. The proposed model describes how the OCR influences the shear strength, development of pore water pressure, and the critical t/a ratio. This study extends our current understanding of the shear behaviour of infilled rock joints with potential applications in rock engineering such as rock slope stability and underground excavations in jointed rock.

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