Abstract

Experiments at constant normal stiffness (CNS) are normally carried out to understand underground shear processes of rock joints. However, in many test setups the available space around the joint is limited implying it is not possible to measure the dilatancy directly over the joint. Therefore, the displacement transducers must be in locations where the risk is that additional displacements originating from deficiencies in the test system will be measured causing too low normal loads to be applied. Herein, this issue is investigated in a new 5 MN direct shear test setup. The system normal stiffness was found to be about 11 300 kN/mm derived from normal loading up to 4.5 MN using a steel specimen. The direct shear testing performance under the CNS configuration was evaluated using the steel specimen, which had a joint with a known angle of inclination. The normal load error at 3.9 MN (28 MPa) was 11%, but by application of the effective normal stiffness approach using the system normal stiffness as input the error basically could be eliminated. The results demonstrate the robustness of the setup designed for joint areas up to 400 x 600 mm with normal and shear loads up to 5 MN.

Introduction

Joints influence the stability of rock masses and the structural integrity of geotechnical structures. Joint shearing is a critical failure mode having an impact on the structural integrity of geotechnical structures [1]. Examples of factors influencing the shear strength are normal stress, infilling materials, surface roughness, compressive strength of the joint surface, joint matching and possibly the scale of the joint surface. However, in situ investigations are for both technical and economic reasons not always feasible [2]. Much of the research work relies therefore on data from laboratory direct shear testing which covers a broad range of topics. Examples among several others are morphological characterization by [3] [4], replica studies by [5] [6], development of shear strength models by [7] [8], experimental studies on shear behavior by [9] [10] and investigation of the shear strength of the interface between the rock and surrounding materials by [11] [12].

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