Analysis and Design of a Deep Subsurface Dam

A 3-D finite element (FE) numerical model was used to analyze structurally a proposed subsurface concrete dam to serve as a strategic water supply storage for the Holy city of Makkah, Saudi Arabia. The work was done mainly to show the behavior of the subsurface dam under the subjecting loads. The plastic concrete was chosen for the construction of the cut-off wall required for the reservoir. Three-dimensional finite element analyses were made for three thicknesses of the cut-off walls: 0.6 to 0.8 m, 0.8 to 1.0 m and 1.0 to 1.2 m for the alluvium depths of 30, 50 and 70 m, respectively. The change in soil rigidity with depth was taken into account. The vertical and horizontal boundary conditions were designed to simulate the proper behavior of the structure It was found that increasing the thickness of the wall improves the distribution of the mobilized passive pressure opposite to the water pressure acting on the upstream face of the cut-off wall, while, the vertical and horizontal stresses developed in the wall due to the applied loads increased. In addition, stresses in the wall increase incrementally with the increase of wall height. The maximum horizontal and vertical stresses developed in proportion to the 1/6 bottom height of the wall. In the lower part of the wall, the mix with rich cement content must be used. It is recommended also to use vertical reinforcement imbedded by the anchorage length in the lock of the wall inside the bedrock and extended 1/20 the wall height. Results showed that the two vertical boundaries in the FE mesh must be placed at minimum distance equaling twice the height of the alluvium deposits from the centerline of the cut-off wall. .