ABSTRACT:

Quantifying the strength and deformability characteristics of rock and rock masses is essential in geotechnical engineering, since the risks, costs, performance, and safety of underground works depends on a realistic prediction of rock mass behavior. However, there are unavoidable accuracy limitations in forecasting behavior due to inherent inhomogeneties and the statistical nature of rock mass parameters. There are also uncertainties related to parameter measurements. It is therefore necessary to establish a general procedure for the consistent and coherent geotechnical design of underground structures, which is traceable and auditable. This paper summarizes a laboratory testing philosophy that is consistent with the Austrian Guideline for Geomechanical Design of Underground Structures, which has found significant application in engineering practice. Specific examples of this philosophy are also reviewed.

1. INTRODUCTION

Evaluating rock mass behavior is an essential aspect of geotechnical engineering. The accuracy of behavior evaluations is however limited, often involving an optimization process with multiple technical and nontechnical variables. To facilitate the optimization process in complex geologic situations, a multi-disciplinary approach involving geologists, geophysicists, and engineers is advantageous. In characterizing the rock mass, consistent and coherent procedures should be adopted, allowing continuous updating an integration of exploration and design work phases. The parameters necessary for rock mass characterization should be relevant to the anticipated rock mass behaviour, and are therefore project specific. Characteristic parameters that can strongly influence the behaviour of some rock mass types are shown in Figure 1, taken from the Austrian Guideline for Geomechanical Design of Underground Structures [1]. The Austrian Guideline recommends a procedure for rock mass characterization that commences with establishing a geological model. This is an essential first step for characterizing the rock mass. The model is utilized as a guide to establish exploration and lab testing protocol. The procedure continues by identifying geomechanically relevant (key) parameters for each rock mass type (Figure 1). Rock mass types are then established, the number being dependent on project-specific geological conditions and the design process stage. The key parameters selected should correlate to the anticipated rock mass behavior and reflect rock mass properties having significant influence on construction means and methods [2]. For example, abrasivity parameters for bit and disk wear, or chemical parameters for corrosion, may represent key parameters for some projects. Information related to the identified key parameters should be updated, as necessary, during exploration, design, and construction project phases. Even if the parameters have highly statistical properties, the corresponding behaviour types can be aided with the use of probabilistic methods All input parameters for geomechanical models (e.g. finite element and distinct element methods) should be defined prior to commencing a laboratory testing program. Particularly for weak or highly fractured rocks, the acquisition and preparation of samples for strength tests often results in a biased selection of stronger samples due to difficulties in specimen preparation. One must therefore attach great importance to the exploration and sampling protocol to ensure that representative samples, including the influence of singular geological features and weak zones, are available for laboratory testing.

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