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Geotechnical Research Volume 2 Issue 4Range of confining pressures for the Hoek-Brown criterion Method considered for the determination of the range of confining pressuresExperience working on different projects, where Mohr-Coulomb parameters derived from the Hoek-Brown failure envelope were required, revealed an array of different methods used in the industry for selecting the value of maximum confining pressure when using the RocLab software.In this paper, 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.In order to compare the results obtained with the different methods considered, a set of common assumptions has been defined, matching what is normally required on civil engineering projects, as follows.■ A 20 kPa uniformly distributed load is applied at ground level. ■ An open-cut excavation of a rock slope with dewatering, the groundwater level is below the critical slip's shear plane and below the base of the excavation. Hence, the effective stress present along the critical plane of failure is equal to the total stress. ■ The excavation depth is 30 m and the excavation slope is identical in all models.■ Seismic loads have been considered by using the following horizontal and vertical coefficients, respectively: k h = 0·042 and k v = −0·021. ■ The stratigraphy has been split into three different layers of the same weathered rock material, with identical properties (as shown in Figure 4). This is to enable the derivation of different maximum confining pressures for each of these layers. It also allows results to be obtained which are independent of the strength properties of the materials considered, and direct comparison of the various methods used. ■ The software used to carry out the slope stability analyses is Slope/W from the Geoslope package.The sections hereafter detail the different methods considered in this note and how the minimum and maximum confining pressures are determined. mass and H is the height of the slope. Note that with this method, only the overall slope height is considered in deriving the maximum confining pressure; relative thicknesses of different stratigraphy units are not considered. In this method, the minimum confining pressure is taken as zero. figure also shows the evolution of the total stress with the slope height, which illustrates the non-linear nature of the reduction factor incorporated in Equation 3 compared with an effective stress calculation.The rest of the parameters are fixed and highlighted in Figure 2. The zone corresponding to the majority of civil engineering applications has also been highlighted.The red line plotted in Figure 2 corresponds to the General Application option of the RocLab program when selecting the maximum confining pressure to use. The general applicati...

The use of the finite element method to model excavations and tunnels in Dublin black boulder clay has, in the past, had limited success owing to the failure of the available constitutive models in commercially available software programs to adequately represent the essential features of a stiff soil. A stiff soil — lodgement till — underlies much of the city of Dublin, Ireland; consequently, the successful prediction of deformations in this soil arising from structural and infrastructural projects is of considerable importance. Research has shown that the stress–strain response of a stiff soil is complex and depends on many factors including stress history, stress level, and strain direction. These important features are included in the hardening plasticity small strain stiffness (HSS) soil model that is incorporated in the Plaxis V8.4 finite element code. This paper describes the field and laboratory methods that were used to determine the parameters for incorporation in this soil model and the validation of these parameters. These parameters are used to model the deformations around two excavations: a 4.5 m deep excavation with a vertical face, and a 10.7 m deep excavation with a face slope of 70°–75°. Good agreement was found between the predicted and observed deformations.