| Unlined pressure tunnels are used to convey water to hydroelectric powerhouses. Usually each project has a forebay reservoir, an intake, a pressurized tunnel, a surface or underground powerhouse and a tailrace tunnel or channel. This thesis deals with the design of unlined pressure tunnels. These were first driven by the Norwegians in hard granitic rock masses, which would constitute the ideal material for this type of structure. However the increase in demand for electrical energy has pushed the industry to implement projects in sites with less favourable geological conditions all over the world. For each pressure tunnel, there are normally three different types of sections along its length: the unlined section, the semi-lined section and the lined section. The unlined pressure tunnel is pervious to a certain extent even if the rock is hard and non-porous and the rock mass is of good quality. Unlined tunnels are thus prone to hydraulic jacking. Semi-lined tunnels use unreinforced shotcrete and/ or concrete whereas fully lined tunnels are defined as those with a steel liner cast in concrete and completed with some annulus grouting to fill voids in the steel-concrete-rock mass system around the tunnel. This type of lining is totally impervious but is very expensive to build. Thus, unlined pressure tunnels, being the most economical, play an important role in the economics of a hydroelectric power project.;The FLAC 2D analyses show that the presence of topographic features has a great influence on the minimum stress distribution. Also, that Norwegian criterion is not adequate when the tunnel diameter is large compared to the rock cover. Finally, the presence of structural features near the tunnel, greatly change the minimum stress distribution and thus renders the Norwegian criterion inadequate in those cases.;Based on these results, a design methodology is proposed for use at the preliminary stage in the form of a factor of safety against hydraulic jacking to be used in the Norwegian criterion. Limitations on the use of the criterion are presented. The thesis presents recommendations to orient investigations and for final tunnel and liner design.;In this thesis, an extensive review of literature is conducted and most published unlined pressure tunnel design guidelines against hydraulic jacking are reviewed. The thesis places emphasis on the preliminary phase when no field measurements are available. The spatial distribution of the minimum stress required to control hydraulic jacking around unlined pressure tunnel is studied. The topographic effects, the rock cover-to-tunnel diameter ratio, the presence of geological feature are all investigated using FLAC 2D code. Minimum stress results are compared to the minimum stress given by the well-known Norwegian design criterion, and a correction factor called, Cover Alteration Ratio (CAR) is introduced. Cases where the Norwegian criterion is not adequate are identified and it is proposed to deal with these situations by adopting a larger factor of safety. |
