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Geotechnical Evaluation of a Spillway Failure due to Peculiar Design of the Structure
Associate Professor, Civil Engineering Department, and Chief Instructor, Civil Engineering Department, | |
ABSTRACT |
Nirali dam is an earthen dam with a central concrete slab spillway structure. During floods of 1992 its spillway structure experienced severe distress. The concrete slabs of the sloping glacis were lifted up as water started to spill over the concrete structure. In the peculiar design of this structure a sandy embankment was constructed on top of sandstone/shale foundation. Concrete slabs were constructed on top of this embankment for safely passing the floods. It is reported that as soon as water started to spill, the lower slabs lifted up and the sandy embankment was washed away. After this incident the upper slabs just hung in the air and were heavily cracked. Subsequently remedial measures were undertaken and the caverns under the upper slabs were filled with pumped concrete and the lower portion of damaged spillway was reconstructed with fresh concrete. A research was recently conducted at National University of Sciences and Technology, Risalpur Campus, to determine the causes of this failure. This paper presents findings of this research study. Since problematic soils exist in the entire northern part of the country, this study aimed at investigating such problematic soils so as the practicing engineers can benefit from this information while planning and executing irrigation/storage structure projects in similar geologic environments. Based on site investigations, lab testing and subsequent analysis of data it is concluded that the spillway structure failed due to its peculiar and faulty design. Instead of constructing a solid concrete section the designers opted for a sand embankment and over it the concrete slabs were placed. This was probably done to economize the dam section. The designers ignored the fact that the concrete slabs may crack due to thermal variations, due to inadequate compaction of sand along the slopes or due to fault movement. Further more it was not visualized that in the event of concrete cracking the reservoir head will be fully transmitted under the concrete panels and the self weight of concrete will not be able to resist uplift hydraulic forces. It is concluded that during flood of 1992 water entered the sand embankment thus exerting high uplift pressures on the concrete panels and lifting them up. Subsequently the sand embankment was washed down.
KEYWORDS: dam spillway, uplift pressure, spillway failure