Concrete dams |
Comparing vulnerabilities of CFRDs and ECRDs
Concrete-faced rockfill dams (CFRD) have been the preferred dam type for high embankment dams for almost fifty years. They are claimed to be the safest dam type by their proponents. Here, Martin Wieland discusses the vulnerabilities of CFRDs and provides a comparison with earth core rockfill dams (ECRD) – the dam type which was replaced by CFRDs
Below: Figure 1. Detail of transition in crest elevation between concrete dam section in the foreground and rockfill dam in the background to prevent overtopping of the rockfill dam section (Kenering dam in Malaysia, a composite concrete gravity dam with two rockfill dams at abutments)
IN RECENT YEARS CONCRETE-FACED rockfill dams (CFRDs) have been built in increasing number as an economic alternative to conventional earth-core rockfill dams (ECRDs). Several of these dams already completed or under construction have heights exceeding 150m. CFRDs are also built in seismically active regions. They have been claimed to be the safest dam type by their developers. Today, such claims are also made for asphalt core rockfill dams. The two basic failure modes of embankment dams are progressive erosion due to overtopping and internal erosion. Protection against overtopping can be provided by an adequate freeboard, by widening and strengthening the crest, by wave walls on the crest, etc. In composite dams consisting of a concrete section and embankment sections or dams with saddle dams, the overtopping risk of the embankment dam and the saddle dams, which are usually embankment dams themselves, can be reduced by increasing their height (Figure 1) so that in the worst case the concrete section of a composite dam will be overtopped dam, but not the vulnerable embankment dams. Maximum overtopping depths of up to about 2m may be accepted for concrete dams. However, the sliding stability of overtopped dams must be checked. This may be a safety problem for small gravity dams, but usually not for high concrete dams. In storage schemes comprising a main dam and saddle dams, one of the saddle dams could be used as a fuse plug
or emergency spillway and the crest elevations of the other dams forming the reservoir should be higher than that of the fuse plug. Using adequate filters embankment dams can
be protected against internal erosion. Due to seismic deformations or slope failure the filter may be damaged. Dams with thin filters, whereby many of them are older dams, are vulnerable to such deformations.
As dams may be subjected to many different
hazards from the natural and man-made environments as well as site-specific and project-specific hazards (Wieland, 2023), which may affect dams in different ways, it is too soon to maintain that certain dam types are safer than others. For example, a comparison of different dam types (CFRD, ECRD, gravity dam and arch dam) subjected to the multiple hazards associated with strong earthquakes, i.e. ground shaking, movements of faults or discontinuities in the footprint of dams and rockfalls at the dam site, was made by Wieland (2016). This qualitative comparison showed that for the abovementioned seismic hazards the best solution would be a conventional ECRD, as CFRDs would be vulnerable to fault movements, rockfalls on the face slab and ground shaking due to the different deformational and strength properties of the concrete face and the rockfill. However, the damage to the face slab due to rockfalls could be prevented by rockfall nets and other protective measures and the problems caused by the different deformational behaviour of the concrete face and the rockfill could be overcome by using rockfill with a high modulus of elasticity and by providing adequate vertical and horizontal joints in the face slab. Because of the high permeability of rockfill, CFRDs should not fail if leakage through the face slab occurs, which also applies to asphalt core rockfill dams. However, in order to achieve a high stiffness of the rockfill, the grain distribution must include a fraction of relatively fine particles, which influences the permeability of the rockfill and a water table may develop near the base of the dam, depending on the amount of leakage through the concrete face, i.e. mainly through the joint sealing system. The primary objective of this paper is to discuss the main factors, which could damage the concrete face, the joints between concrete slabs, waterstops and the plinth, and to provide recommendations on how these damages may be prevented or minimized. One must also keep in mind that like other dams, CFRDs are prototypes, therefore generalizations are
40 | July 2023 |
www.waterpowermagazine.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53
