3d finite element analysis of circular tunnel The case of genale dawa hydropower tunnel

Abstract

Tunneling is a recent practice in Ethiopia due to the fact there were no major engineering projects over the years. In this research, computing the stress and deformation of Genale Dawa Hydropower tunnel was the main task, In order to do this 2D FEA parametric study were developed over the opening diameter, ground water table, and model dimension. Consequently, the conducted 2D and 3D FEA were compared to the conventional empirical approaches. From the parametric study, appropriate FE model dimension ratio of the considered dimension to the opening diameter should be higher than 7 and 3 for the width and depth below the tunnel respectively. The effect of opening diameter to the crown displacement was found to vary with a linear relation of 0.0411m + 0.0053xD. Five 2D FEA has been carried out across the length of the tunnel in every 8 meter spacing using Plaxis 8.2. In addition, 3D FEA were computed using Plaxis 3D in which the sloppy overburden was replaced with a surcharge along the tunnel length. From 2D FEA result, the computed inward crown displacement of 0.0041m is almost the same result as the first plane of 3D FEA, 0.0038m. However, when moving inside the tunnel the 2D FE result has an increasing displaccement for the overburden increases but the 3D result found to decrease along each plane due to the confinement increases when moving inside from the tunnel inlet which is concluded to be incomparable. In another hand, analytical closed solution estimates the inward displacement much higher, 0.08769m. Comparisons between results from FEM and empirical approaches shows that the empirical approaches gave a exorbitant stress concentration around the tunnel. In another way the FEA result did not predict the observed rockfall for it is not modeled to analyze the structural instability. In order to treat unforseen structures of FEA result, it is important to observe on site assesment while excavation is going