Remodeling for ensuring structural safety
A necessity for remodeling to ensure structural safety may arise because
- the dam is pretty old and turns out to be structurally unsafe according to present practice
- increased inflow/outflow discharge requires a wider section of the spillway
- additional loads are introduced following a major earthquake
In the above cases, hydrologic and hydraulic safety considerations are also involved together with structural safety that generally requires strengthening of the dam cross section with buttresses and other similar measures. However, for spillway, widening of the cross section is the only alternative available. In addition to ensuring a good bond between the old and the new construction, such a modification would have serious implications on the hydraulics of the spillway and energy dissipating arrangements. Figure 1 depicts a possible remodeling of an existing spillway by way of flattening of the rear slope for widening of the section, which would indicate such implications.
Two case studies are discussed here to illustrate the aspects involved. The first case deals with an old spillway subjected to increased design inflow and concerns about the structural stability due to the increased height of the dam. The second case deals with a spillway requiring wider section due to additional load to be accounted for the increased seismicity.
Loch Raven Dam, USA
The Loch Raven dam, in Maryland, USA is a 96 year old structure with a 30 m high dam and an 88 m long spillway. It was originally designed for an inflow flood of 1189 cub met/sec (42000 cfs) and an outflow flood of 637 cub met/sec (22500 cfs) with a depth of overflow of 2.44 m (8 ft). The safety considerations resulted in a revised inflow flood equivalent to PMF of 6796 cub met/sec (240 000 cfs) and corresponding outflow of 4870 cub met/sec (172 000 cfs) with a depth of overflow of 8.84 m (29 ft). To accommodate this, the height of the dam was increased with the spillway crest elevation remaining unchanged. The spillway section was widened and the stilling basin was converted to a submerged slotted bucket, noting that excessive scour may result for the higher discharges. For further details, refer Redeveloping Loch Raven
Koyna Dam, India
The 103 m high Koyna dam in Maharashtra state, India was designed with conventional method as a rubble concrete gravity dam. Seismic forces considered were nominal corresponding to horizontal acceleration of 0.05 g constant over the height of the dam. No vertical component of earthquake was considered. After the earthquake of 6.5 magnitude, which occurred on 11 December, 1967, damaged portions of NOF dam were strengthened with concrete backing and buttresses. There was no damage to spillway section. Another earthquake of 6.3 magnitude occurred in the region in September 1993. Though, there was no damage this time, the authorities decided to adopt strengthening measures based on the analysis considering seismic forces corresponding to a Maximum Credible Earthquake of 6.8 magnitude. While the NOF portions already strengthened were found to be safe, the spillway section needed strengthening. The Pseudo static analysis carried out or the load combination (Res. At FRL+EQ+extreme uplift) indicated the necessity of increasing the rear slope of the spillway section from 1V:0.725H to 1V:1.1H. The flattening o the slope resulted in an encroachment into the stilling basin, requiring extension of the latter in the downstream direction. The final section evolved as a result of hydraulic and structural analyses is shown in Figure 2.
Bhave,A.P. and Joshi, S.G. (2006)- Evaluation of Existing Dams & Action Plan for Restoration Case study: Koyna Dam- Proc. A national Level Short Term Training Programme on Evaluation of Existing Dams and Action Plan for Restoraion, conducted by Sinhgad College of Engineering, Pune, India. Feb 2006.