Concrete dams |
The main advantage of CFRDs is the significantly smaller dam volume than a comparable ECRD as the upstream and downstream dam slopes are generally steeper than those of ECRDs. ECRDs also require large quantities of core materials, which may not be readily available at some sites.
The failure of the Gouhou CFRD may be
considered as representative for the internal erosion failure mode and the failure of the Taum Sauk CFRD as representative for the overtopping failure mode. In 2014 the 90m high Tokwe Mukosi CFRD in Zimbabwe, which stores a reservoir with 1.75km3
,
Above and below: Figure 4. Damage to vertical joint of Zipingpu CFRD caused by the 2008 Wenchuan earthquake in China (above) and the repair of a damaged vertical joint (below). During the earthquake the reservoir volume was about 30% of that due to normal water level (photos courtesy Xu Zeping)
nearly failed during a severe flood in February 2014, when the dam was still under construction and the upstream concrete face was not yet built. Severe seepage occurred as shown in Figure 3 as no waterproofing membrane was in place. With some delays the dam was completed and was commissioned in 2017. This important case study shows that excessive seepage through the rockfill can endanger the safety of CFRDs and that they may not be inherently safe as claimed by the designers of these dams. Of course, this also applies to rockfill dams with other types of upstream membranes, such as asphaltic concrete membranes or geomembranes or asphalt core rockfill dams. Regarding the load bearing behaviour of CFRDs or other dams with an upstream waterproofing membrane, the transfer of the water load to the foundation rock is much more efficient than in the case of ECRDs, asphalt core rockfill dams or dams with a concrete core wall.
However, in this paper, the focus will be on safety aspects of CFRDs and the weakest element in CFRDs is the concrete face, which must be watertight. In this discussion, the benchmark will be conservatively designed CFRDs, which may be considered to be the most resilient embankment dam type. But we must keep in mind that all embankment dams are vulnerable to overtopping and, therefore, as a prerequisite these dams must have adequate flood discharge outlets. Today, the safety flood for embankment dams is the PMF (probable maximum flood), which must be released under the following assumptions: The power plant is out of operation, i.e. no discharge through power waterways. The outlet (spillway opening or low-level outlet) with the largest discharge capacity is closed. In dams with several spillway openings more than one opening has to be assumed to be closed. The maximum water level in the reservoir shall not exceed the top of the impervious core of ECRDs. For CFRDs the reservoir level may be up to the dam crest. Special criteria may apply to account for waves in the reservoir and storm surges in large reservoirs.
The present discussion also applies to saddle dams, whose failure has similar consequences to the failure of the main dam. If the same safety criteria have to be applied to the levees of run-of-river power plants as for saddle dams, this is not addressed in this paper because levees may be considered to be a kind of saddle dam. We must also recognize that a dam with a long economic and safe life requires a good design, good quality of all construction works, regular maintenance and continuous safety monitoring.
Hazards affecting CFRDs The hazards affecting the safety of all types of dam
were discussed by Wieland (2023). The main hazards relevant for CFRDs can be classified as follows:
1. Natural hazards: Flood hazard (overtopping of dam, sedimentation of reservoir or blocking of spillway by floating debris). Earthquake hazard (inelastic deformations of dam body due to ground shaking or deformations in the dam footprint).
Mass movement at dam site (damage of dam crest and concrete slabs). Mass movement into reservoir (impulse wave causing overtopping of dam). Water waves in reservoir (damage of riprap at the upstream face of ECRDs, local destabilisation of reservoir slopes). Temperature effects (cracking of concrete slabs and corrosion of reinforcement).
2. Man-made hazards: Design errors, low quality construction works and lack of or faulty maintenance, etc. Terrorism and acts of war.
42 | July 2023 |
www.waterpowermagazine.com
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