Abstract

This paper presents the details of the approach to numerical modelling of the subvertical rock slopes resulting from the deep excavations associated with the foundations and basements for Hinkley Point C, Nuclear Power Station, in Somerset, UK. The ground model, which was based on an extensive ground investigation, indicated that the rock mass comprised thinly bedded mudstones and occasional limestones of the Blue Lias Formation. The structural geology of the site consisted of two major faults with five minor faults. These faulting resulted in highly fractured rocks with closely spaced discontinuities with unfavourable orientations and dip angles relative to the slope face. The design methodology for the project included Discrete Element Method (DEM) of numerical modelling using UDEC, a commercial software suitable for modelling fractured rock behaviour. As part of the development of the analytical model, a number of simplifications in terms of modelling, needed to be undertaken to enable development of a series of models that adequately reflected the likely ground conditions during excavation, but that could be run effectively under the usual constraints of a commercial project. One of such key simplifications relates to rock joint spacings. Modelling the rock joints spacings as measured on site in UDEC would result in extremely long computation time. It was necessary to scale up joint spacing in the model without compromising on the accuracy of the results. Series of sensitivity analyses were carried out to assess the effects of joint spacing on the slope stability analysis. From these analyses, an appropriate joint spacing scale up factor was derived. The output from the modelling included assessments of likely displacement of the rock slopes during excavation and as part of the construction works for the basements a programme of extensive monitoring was undertaken to verify the results of the modelling. The paper presents the ground model as interpreted from the ground investigation, the sensitivity analysis undertaken to help develop the approach for the analytical model and the results of the numerical modelling. It then goes on to compare these with the actual ground conditions revealed as excavation was undertaken, and the deformation recorded on the rock slopes during excavation. The paper goes on to present conclusions with respect to the modelling similar rock masses effectively using numerical modelling techniques.

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