Stability problems in new road cuts have resulted in high additional costs for the Swedish National Road Administration. Despite the fact that the contractors follows the same blasting regulations (Vagverket, 1994), the rock slopes excavated by these contractors are damaged in different ways. This article deals with a case study on how blast design influenced the stability of four roads cuts in Sweden. The purpose of the study is to suggest methods on functional demands and estimations of rock slope stability. One economical and practical criterion on rock slope stability is the sum of visible boreholes. There has to be more studies on rock slope conditions in various geomechanical conditions, to achieve practical rock mechanical methods for rock slope stability analysis. By introducing effective regulations combined with the knowledge of rock mechanical excavation methods one can increase the knowledge and safety of the rock slopes.
Stability problems in road cuts have resulted in high maintenance costs for the Swedish National Road Administration. The stability problems are mainly caused by structural failure due to natural joints (geological structures) and low rock mass strength. Blasting may break bridges of intact rock so that two joints in the virgin rock mass becomes one large-scale discontinuity, which can act as a slip surface. Blasting sometimes gives unfavourable geometries and changes of the stress state in the slope. Furthermore, blasting induced vibrations may also create new cracks and propagation of pre-existing joints. Freezing of water in the slope and growth of roots can generate an inner pressure in all kinds of discontinuities. The propagation of these fractures may then lead to stability problems in the slope. To avoid most of these stability problems, the rock slope has to be excavated using the best methods, considering the rock mass behaviour and the geological conditions.
Four road cuts have been studied along the E6-road, in the southwest of Sweden (Eriksson, 1999). In this study, the effects from different blasting techniques used by the contractors on the final slope stability were investigated. Roek mass classification using RMR (Rock Mass Rating, (Bieniawski, 1976)), showed that the rock mass quality of all nine rock slopes was similar. Despite the fact that the contractors followed the same blasting regulations (Vagverket, 1994), the rock slopes excavated by the different contractors were damaged in different ways.
One important factor governing the rock slope stability is the geological structures. Structural failure modes such as plane, wedge and toppling failure are typical for small scale slopes (Sjöberg, 1996; Hoek et al., 1981).
One way of estimating the rock slope condition is to estimate the total length of visible boreholes at the slope face and calculate the visible borehole length divided into the total borehole length ratio. The visible boreholes are an indication that the borehole pressure has not been high enough to destroy the boreholes completely.