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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, | |
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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
INTRODUCTION
Nirali Dam is a 70 feet high homogeneous earthen dam. A reservoir with gross storage of 416 acre-feet has been created on the upstream due to construction of the dam Niralidam.mpg. The dam was constructed in 1967. The main purpose of the project is to harness the water of Niralir river for irrigation as well as water supply schemes. The maximum height of the dam in the central spill portion is 68.5 feet whereas on the flanks the maximum height in the non-spill portion is 70 feet. The earthen embankment is homogeneous in nature and has been constructed from well compacted clayey silt. A toe drain (Fig. 1) has been incorporated in the dam section to keep the phreatic line down and safely discharge the seepage.
Figure 1. Comparison of upward and downward hydraulic forces
Figure 2. Typical cross section of the earthen embankment
The topography of area represents typical features of a plateau i.e., deep channel ravines surrounded by rolling hills. The site geology is simple. Recent alluvial deposits cover the entire area. The alluvium overlies Sandstone, siltstone and shale. Sandstone is ridge forming. One of such ridges is crossed by the river to form a gorge. The spillway is situated across one such gorge. The alluvial deposits have almost horizontal dip but the rocks of the Siwalik age dip 35°S to 40°S and their strike is EW. The rocks exposed at the site are sandstone, bluish gray, friable, medium grained and thickly bedded. There are a few lenses of clay that have resulted in cavities due to weathering. There exists a fault a few feet upstream of dam center line.
Figure 2a. Cross section at spillway
Figure 2b. Plan view of spillway
The spillway cross section is shown in the Figure 2a and the plan view in 2b (Spillway from downstream.mpg) and (Spillway from upstream.mpg). It is a peculiar structure. Instead of constructing a solid concrete section the designers opted for a sand embankment and over it the concrete slabs were placed. A stilling basin has also been provided at the toe of the spillway for energy dissipation. The spillway structure is resting over a fault. The project documents are silent whether the fault is dead or alive. There is no indication of any analysis performed to evaluate movements along fault due to construction of a 70 ft high structure. Possibility of movements along fault, therefore, cannot be ruled out. Instruments were, however, not installed to monitor movements along fault and it can only be speculated that the fault moved due to additional load of structure. Any such movement would result in cracking of concrete slabs thus providing free access to the overflowing water to erode the sand embankment. The cross sectional drawing for the spillway indicates that lower slabs are resting on sand embankment which is supported by silt type foundation soil. Saturation of such foundation material because of water impoundment can result in a reduction in bearing capacity and settlement of the spillway structure. It was hypothesized that cracks in concrete slabs were formed due to foundation settlement. Field and lab testing was done in order to test this hypothesis. Another problem that can be visualized concerns the construction of the sand embankment itself. Experience indicates that it is very difficult to properly compact sand on slopes. Similar problem has recently been observed at Ghazi Barotha project main channel where special YO-YO compactor arrangement had to be employed to achieve desired compaction of filter sand. It is possible that at Nirali the sand embankment was not compacted properly and it settled during project operation thus resulting in cracking of the concrete slabs. During dam inspection by the research team these cracks were critically examined. The repair work to concrete in the top slab suggests that the cracks were wide enough to provided free access to spillway water to enter sand embankment and transmit full upward hydraulic pressure on the lower slabs.
FIELD and LABORATORY TESTING RESULTS
Detailed in situ soil and concrete testing was done at a number of locations by conducting SPT and rebound hammer tests. Disturbed and undisturbed soil samples were also collected. The Project completion drawing indicate that the upper slabs of concrete spillway rest on sand embankment that in turn rests on friable sand stone whereas the lower slabs rest on sand embankment that was constructed on silty sand bed. The characteristics of this silty sand bed were explored to evaluate settlement potential of this soil. There was only one location on the left abutment that could be used for installing the boring equipment. Accordingly standard penetration test was conducted on this material located from the ground at a depth of 3 m. The number of blows required for 12 “ penetration was 17. This blow count yields ultimate bearing capacity of 2.5 kg/cm2. The results of unconfined compression test conducted on shelby tube samples retrieved from this depth yield ultimate bearing capacity of 2.25 kg/cm2 which is in close agreement with results of SPT. Subsequent sieve analysis and Atterberg limits established that the soil is silty sand ( SM ) with approximately 12% fines. This type of material would not experience long term consolidation and accordingly these tests were not undertaken. Schmidt rebound hammer tests were conducted on the concrete slabs. The results are summarized in Table 1. It can be seen that there is considerable variation in the rebound values.
Table 1. Results of Schmidt rebound hammer test conducted on the concrete slabs
FIELD TEST RESULTS
The region is frequented by collapsible and expansive soils and it was speculated that cracking of spillway slab and its eventual failure occurred due to either soil collapse upon saturation or because of expansion. The spillway geology however indicates that the upper slabs of concrete spillway rest on friable sand stone whereas the lower slabs rest on silty sand bed. There is no possibility of settlement of sand stone except for possible movement along the mapped fault. Field and laboratory testing of silty sand also establishes that its bearing capacity is far in excess of load imposed by the structure and would not experience consolidation. The non destructive tests on concrete, however, reveal that the concrete is highly variable. Incorporation of 40% plums may have further contributed towards concrete variation. Such variation can lead to thermal cracking of concrete and field observations do indicate that the concrete experienced cracking prior to failure of structure. Low range of rebound values for the old concrete compared to the newly laid concrete suggest that concrete of poor quality was used in the original construction.
FAILURE MECHANISM
The site staff indicated that the concrete slabs had narrow cracks prior to ultimate failure of spillway. These cracks could have been formed due to movement along the fault or due to highly variable nature of concrete panels resulting in thermal cracking. Temperature steel was not provided in the design to circumvent this problem. The concrete panels were supported on a sand embankment. Accordingly during flood when the water started to flow over the glacis it found its way into the cracks and saturated the sand embankment. This resulted in transmittal of high pore pressures on the concrete panels. Figure 3 shows variation of hydrostatic pressure under the concrete panels when the water level in reservoir is 2 feet above the spillway crest. It can be seen that under such situation the uplift force is almost four times the downward force due to self-weight of slabs. This difference is greatest for the lower slabs and no wonder they were washed out. Subsequently the entire sand embankment was washed away leaving large caverns under the upper slabs. These required expensive remedial measures by concrete injection and replacement of damaged slabs.
Figure 3. Comparison of upward and downward hydraulic forces
CONCLUSIONS
The following conclusions were drawn from this study:
· Inadequate site investigations were carried out and the critical spillway structure was constructed over a fault. It was not established whether the fault is active or dead.
· A peculiar design was adopted whereby the concrete panels were placed on top of a sand embankment instead of constructing a solid concrete gravity type dam in the river section. The designer 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 if the concrete slab cracks then 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, therefore, concluded that the design was faulty and shall not be adopted for other projects.
· Plum concrete was used in design resulting in considerable variation in the properties of concrete. This can easily give rise to thermal cracking. Furthermore no thermal reinforcement was provided to check thermal cracking of concrete.
Reference
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