HIGH PRESSURE ROCK MASS GROUTING OF UNDERGROUND TUNNELS AND CAVERNS IN HARD ROCK ENVIRONMENT
Grøv, E. Chief Scientist/Professor SINTEF/NTNU, Trondheim, Norway
Woldmo, O. Managing Director, Normet. Norway
Alitinform №6(49) 2017 г. 52-69 p.
Rock mass grouting has become an important aspect in tunneling and underground excavation, particularly whilst executing such work in urban areas with a highly developed surface infrastructure and also in areas which due to various reasons are sensitive to fluctuations in ground water levels. The public focus on tunneling work has increased during the last decades, not only project cost and schedules are scrutinized carefully, but also the consequences caused by ground water lowering on the surroundings such as flora and fauna, building settlements, etc. This article presents some guidelines on how to approach these situations taking into account rock mass grouting as a main measure to deal with the ground water in rock tunneling. Use of sensitivity analysis is one tool at hand to identify the vulnerability of the surroundings with respect to ground water draw down caused by tunneling activities.
The primary objective is to make the tunnel tight enough for its purpose. In addition this article will include two more objectives to have in mind when planning and executing rock mass grouting; namely that grouting is all about water control — not water proofing, with the focus on prevention rather than cure suggesting pre-excavation grouting being preferred to post-grouting.
The rock mass is a significant barrier in itself. However, it is a discontinuous material, ranging from an impervious medium to highly conductive zones. A standard procedure in Norwegian tunneling includes pre-grouting for the purpose of reducing the permeability as well as consolidating the rock mass. This procedure has developed from the early tunneling projects in the city of Oslo, through unlined, high pressure water tunnels for hydropower projects, oil and gas storage and subsea rock tunnels to the current generation of urban tunneling. This article lists briefly the various reasons for such groundwater control, and provides a description of the modern concept for pre-grouting for underground tunneling as developed in Norway during the last decades. Further, some project examples will be provided to describe the current practice in Norwegian tunneling.
The concept is based on utilizing the self-supporting capacity of the rock mass, supplemented by use of rock bolts and sprayed concrete to establish permanent rock support, whilst water control is achieved by use of pre-grouting of the rock mass. Pre-grouting (“prevention”) is the preferable method to post-grouting (“cure”). Post-grouting being a method applied subsequent of the tunneling excavation is often an intricate, time consuming and costly process.
Rock mass grouting, pre-excavation grouting, hard rock, water control, cement based