Spiling bolts are often used as a pre-support measure for tunnelling in an extremely weak rock mass. However, this application is based mainly on experience without a clear description of how spiling bolts work. This may be the first time spiling bolts have been modelled using pile elements in FLAC3D. The models are able to describe a more correct picture of the supporting action of spiling bolts by including the bending moment for the bolts. The models show a great improvement of the roof stability when the bolt ends are fixed. Forces and deformations of the bolts are also studied. The models demonstrate the “umbrella” effect of the spiling bolts. The model shows that some input parameters for the “pile elements” spiling bolt are difficult to quantify. These parameters are only possible to obtain from in situ tests. References have to be made so that input parameters, to some extent, can be obtained.
1.1 Cave-in problem A serious cave-in problem was encountered in the headrace tunnel of Buon Kuop hydropower project, located in the central part of Vietnam. The longitudinal section is presented in Figure 1. The headrace tunnel consists of two parallel tunnels, each 9 m in diameter, in young sedimentary rocks. The tunnels are excavated by normal drill and blast methods. The second tunnel face is about 70 to 100 m behind the first one. Headrace tunnel length is about 4.8 km, and the excavation was carried out through three accesses: from the intake, from the downstream end and from an adit in the middle. The cave-in problem happened in the first headrace tunnel at chainage 34+70, where the excavation was progressing from downstream and hit a weakness zone of about 15-20 m width.