Weak rock mass strength estimation is a long-lasting challenge associated with geotechnical engineering due to its complex nature and limited definition. Weak rock masses normally refer to low strength, highly fractured decomposed and tectonically disturbed rocks which have properties intermediate from brittle rocks to ductile soils. Since the behavior of weak rock mass has not been fully understood, it is a common practice to apply existing empirical approaches, which are developed for competent rock masses influenced by joints, to determine their mechanical properties. This paper reviewed the current empirical approaches, and detailed weak rock mass strength calculations based on rock matrix, joint layout, joint condition and external factors. The limitations associated with these methods are discussed, and suggestions are provided for the selection of suitable methods.

1. Introduction

Determination of weak rock mass properties is a significant challenge in geotechnical engineering. In general, weak rocks are considered to be the transitional material between competent rocks and soil, therefore, their behavior converges to competent rock at its upper bound and soil at the lower bound. Despite significant amount of research, methods to estimate in-situ behavior and strength of weak rock masses remain to be relatively fragmented and incomplete. The difficulty of determining their behaviour is mostly caused by the complex nature and inadequate definitions. Different origin and alteration process of weak rocks result in variant properties that inevitably influence their overall behaviour. Hence, it is important to understand the differences in their property that inherited from both previous phase and alteration process and to adopt suitable approaches to estimate their strength according to these features.

In practice, the term weak rock commonly refers to both young sedimentary rocks with low compressive strength and heavily altered hard rock with intense structures [1–3]. Based on the origin and geological alterations, weak rock can be classified as young sedimentary rock, weathered competent rock and tectonically disturbed competent rock as shown in Fig. 1. Young sedimentary rocks such as mudstone and claystone contain poor lithification and weak particle cementation. The strength of them can be described by the ISRM definition of weak rocks with uniaxial compressive strength (UCS) being 0.5 MPa to 25 MPa [2, 3]. Weathered competent rocks such as sandstone can also be considered as weak rock. During prolonged exposure, some rock mass components start to break down and crack along pre-existing micro fractures. As a result of weathering, the well-developed, interconnected defect fabric deteriorates the integrity of the rock mass, thus lead to a reduction of the overall mechanical strength. This type of rock is well represented in Rock Mass Rating (RMR) and Geological Strength Index (GSI) classification systems as poor quality rocks with ratings lower than 25 and 20 respectively or less than 0.1 in Q system. In practice, there is a tendency to consider tectonically disturbed competent rocks, which preserves limited original structures formed in lithification, as weak rock mass [3]. Due to destruction of original structure during folding and shearing, it's common to observe widely existing intensive fractures. Thus, this type of rock has very low mechanical properties similar to other types of weak rock masses. Marinos and Heok's study of flysch in 1998 provides a good example of such weak rock [4–6].

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