SLOPE STABILITY ANALYSIS OF GIDABO EARTHEN DAM WITH AND WITHOUT GEOGRID.

Abstract

Dams are structures constructed for various purposes like flood control, navigation, water source, recreation, power generation, irrigation etc. Nowadays embankment dams are being widely constructed worldwide more than concrete dams in areas where the bearing capacities of foundations are poor and in wide stream valleys. Earth–fill types are more suited than rock-fill types when the foundation condition is even so weak. The results of standard penetration tests of the boreholes drilled along the Gidabo dam axis indicated that the foundation condition is more suitable for earth-fill dam. The problem with such dams is that they are highly susceptible to slope failure under such loading conditions: at the end of construction, steady state seepage under high water level, and reservoir rapid drawdown (RDD). To prevent such failure, their slopes are usually designed flatter. This in turn consumes huge volume of construction materials. In this paper, the means of reducing the construction materials is investigated. This was achieved by incorporating the tensar uniaxial geogrids as reinforcement. The analysis was carried out by Finite Element Method via Shear Strength Reduction technique using the finite element tool “PLAXIS 8.2”. Two earthen dam models of same materials but with different slopes were prepared to clearly show the effectiveness of geogrid reinforcement inclusion in slope stability enhancement. The dam construction was carried out by means of staged construction. The flatter dam model was constructed successfully and found stable for all loading conditions considered. But, the steeper dam model was failed just on the first phase of construction. Then, geogrids were used to stabilize it. The effects of varying both the geogrid axial stiffness (EA) and the number of its layers revealed direct proportionality with the computed factors of safety. The final analysis which was made by the geogrid with EA of 64.1kN/m in four layers indicated good stability for the end of construction and steady state seepage loading conditions, but poor stability for RDD. The safety factor for the RDD was then improved by adding one more layer of the geogrid at u/s portion of cofferdam where failure surface was observed. The cofferdam on this project is a permanent part of the main dam. In their nature geogrids having coarser aperture sizes, effective only when applied to coarser materials because they enhance stability by interlocking the surrounding earth materials. In this study, the reinforced steepened dam model saved the gravel fill volume of about 200m 3 /m width of the dam when compared to the unreinforced flatter one.