Abstract
Engineering in weak rock using modern, computer-based design techniques is notoriously challenging because the laboratory tests and classification methods routinely carried out on such materials yield rock parameters. Such parameters are not directly compatible with the most widely used constitutive model of those computer-based methods – that is, the Mohr–Coulomb failure envelope. There is no direct relationship between Mohr–Coulomb (linear) and Hoek–Brown (non-linear) parameters, and marrying those two failure envelopes is notoriously difficult. When formulating a failure criterion for rock materials, based on rock and rock mass parameters, a correlation with the Mohr–Coulomb failure criterion has been proposed, which can be obtained from the computer program RocLab. One of the essential input parameters for such correlation is called the maximum confining pressure. This parameter directly affects the correlated shear strength parameters on the Mohr–Coulomb failure envelope and hence choosing it with care is the key to producing a safe, yet economic design. The present paper discusses the use of several methods for determining the maximum confining pressure to derive Mohr–Coulomb parameters for weak rock in slope and retaining wall applications. The suitability of each method considered is discussed against the results of back analyses, and design recommendations are proposed.
Highlights
The engineering behaviour of weak rocks with unconfined compressive strengths (UCSs) towards the lower end of the rock competency scale (defined as UCS < 15 MPa by Carter et al (2008)) is notoriously challenging to estimate accurately as such materials sit in an intermediate category, between rock and soil, often labelled as intermediate geomaterial (IGM)
With regard to slope and retaining wall applications, the behaviour of weak, weathered rock at the lower end of the competency scale is generally controlled by the overall shear strength of the rock mass, rather than by discontinuities, as is often the case for hard, more intact rocks
A: as proposed by Hoek et al (2002) Method A corresponds to the recommendation provided by Hoek et al (2002). This means that the following relationships are recommended to calculate the maximum confining pressure for slope application ð Þ 3
Summary
The engineering behaviour of weak rocks with unconfined compressive strengths (UCSs) towards the lower end of the rock competency scale (defined as UCS < 15 MPa by Carter et al (2008)) is notoriously challenging to estimate accurately as such materials sit in an intermediate category, between rock and soil, often labelled as intermediate geomaterial (IGM). In this example, the relationship proposed by Hoek et al (2002) to determine the maximum confining pressure for slope application was used. The authors propose to compare/benchmark the results of three different methods (selected based on what seems to be most commonly used in the industry) for defining the maximum confining pressure when deriving Mohr–Coulomb parameters from the Hoek–Brown failure envelope. Recommendations on these methods are provided in the final section. The general application corresponds to the value of UCS/4, which is the maximum recommended confining pressure when approximating the Hoek–Brown failure criterion with Mohr–Coulomb parameters
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