Engineering geological evaluation and support design for Borgundfjord sub-seatunnel, west coast Norway
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A new sub-sea tunnel from Sula (Veddevika) to Ålesund has been under consideration for almost 20years. New initiative from Bytunnelen AS and Sula municipal has resulted in new evaluations and afeasibility study of a sub-sea tunnel project under the Borgundfjord. This thesis is a continuation ofearlier project work of autumn, 2010. Engineering geological investigations and laboratory testing ofrock samples from the project area are used to assess important properties of the Borgundfjord subseatunnel project. Instability and water leakages in sub-sea tunnels can have disastrous consequences with potential loss of the entire tunnel construction due to the infinite water supply above. A total loss of a sub-seatunnel has yet to happen in Norway. Serious cases of water leakage have occurred in Norwegian tunneling history. The Oslofjord, Bjorøy and Frøya tunnel are examples of this. Comprehensive mitigation efforts were needed in order to keep up tunnel construction. These involved pre-injection grouting and freeze stabilization. Serious cave-in situations occurred in the Ellingsøy and the Atlanterhav tunnel. These cases were connected to unstable rock masses in weakness zones, near the low points of the tunnels. The tunnel heads needed a concrete plug in both cases, before careful blasting and support work could commence through the respective weakness zones. Planning phase engineering geological investigations are important in connection to tunnel placement, early cost predictions and construction time analysis. These assessments will often be based on numerous methods of investigations that clarify the general rock mass conditions of the project area and locations of possible weakness zones. An important investigation method, which is required in all sub-sea tunnel projects, is seismic measurement. Field investigations of the Borgundfjord tunnel project area was carried out over 2 days. Emphasis were on strike and dip measurements, tunnel portal locations and rock mass classifications using the RMR and Q-value system. The rock mass of the area has “fair” to “good” general quality. Different sub-areas of the entire project area show signs of 2 to 3 joint planes, together with the foliation plane in the rock mass. Three rock samples from the Borgundfjord project area were brought back to NTNU, for laboratory testing. The E-module, poissons ratio, sound velocity, point load and uniaxial compressive strength tests were carried out. The tested, intact rock parameters show signs of good quality rock. The Gneiss from Ålesund is a bit stronger than the Amphibolite and Eclogite, collected in Sula. Further evaluation on tunnel portal placement and tunnel alignments, together with results from field investigations form a basis for rock support amount estimations and cost analysis on two tunnel alternatives. A significant difference in length, cause the deeper tunnel alternative to get lower total costs. This, in addition to factors concerning potential leakage problems and issues of the tunnel portals, is the reason why this cheaper alternative is considered to be the best. Numerical modeling is useful when assessing the stability of an underground opening. The deepest point, in one of the tunnels, is modeled with and without the proposed rock support design. Values acquired from the laboratory tests were utilized in the analysis. The result of the modeling illustrate that the capabilities of the rock support will be sufficient in the critical weakness zone.