| Concrete dams
3. Project-specific hazards: Dam geometry and layout of face slab with plinth. Ageing of face slab (cracking and corrosion of reinforcement) and waterstops. Effect of inelastic static and seismic deformations of the dam body on in-plane stresses in the face slab, taking into account the arrangement and detailing of the joints. Temperature effects on face slab especially in drying-wetting zone of the face slab. Permeability of rockfill and vulnerability to internal erosion, etc.
Physical degradation of rockfill, collapse settlement, stiffness of rockfill, etc.
Modifications in dam body and repair works.
4. Site-specific hazards: Topography and geometry of dam body. Geology (permeable rock, clay interfaces in foundation, karst, etc.) and local site conditions (soil cover, liquefiable material, etc.).
Seepage in foundation and arrangement of grout curtain or cut-off wall. Effect of changes in the ground water table due to reservoir impounding on slope stability, weakening of rock or causing differential foundation movements, etc.
This list is quite exhaustive. Item (3) is mainly related to CFRDs and the face slab, whereas the other items apply to both CFRDS and ECRDs as well as to other types of embankment dam. However, these hazards – many of which are time-dependent – have different effects on CFRDS and ECRDs. In general, the safety implications of these hazards for ECRDs and other conventional embankment dams are quite well understood, but the long-term behaviour of CFRDS and other new types of dam is less well understood as they may not yet have experienced extreme or unusual events. Therefore, in future some dams may have to be upgraded and the design and safety criteria, etc. must be updated. These periodic safety reviews are standard practice in the dam industry. They also include the time-dependent effects of climate change hazards. It is not the intention to discuss each of the above
hazards and its effect on the safety of the face slab of CFRDs as every dam project is unique since in terms of dam safety the priority order of hazards varies.
The main safety aspects of the concrete face discussed in the subsequent sections are as follows: Observed damage in face slabs. Different deformational behaviour of rockfill and the reinforced concrete of the face slab. Detailing of joints in the face slab.
Different hazards may contribute to these items. Because earthquakes may produce the largest
stresses and deformations in all types of dams, emphasis will be put on this extreme load case in the subsequent sections. The seismic load combination also includes the usual static loads such as dead load, water load and silt load. Therefore, if a dam is structurally safe for the seismic load combination, it will also be safe for the static loads. This also applies to CFRDs. Wieland (2010) and Wieland and Brenner (2007) have discussed different seismic safety aspects for CFRDs. The present paper is basically an extension of these papers as new information has become available and several high CFRDs have been built or are under construction. In China, there are plans to build CFRDs with heights up to 300m, in which seismic performance and safety aspects will play an even greater role in the future. Extreme man-made effects such as terrorism and acts of war are not discussed. Face slabs could be damaged by such actions quite easily.
For the safety assessment of dams, the extreme seismic load combination must be analysed. This means that the recent failures of old water storage dams in the US and tailings dams in Brazil and Australia, which were attributed to static liquefaction, would never have been declared safe if the extreme seismic load combination had been considered in the safety analyses.
Observed damage of face slabs of
CFRDs and damage analysis Damage to face slabs has been observed at different CFRDs, mainly for very high dams. The main reasons for the damage, which also caused an increase in the seepage through the dam body, may be summarized as follows:
Leakage of joints in the face slab due to inadequate joint detailing and installation of waterstops. High in-plane stresses in face slab due to deformations of rockfill causing damage of joints and cracks in face slab, when the biaxial strength of the reinforced concrete slab is exceeded.
www.waterpowermagazine.com | July 2023 | 43
Above left and right: Figure 5. Damage of horizontal joint of Zipingpu CFRD caused by the 2008 Wenchuan earthquake in China (photos courtesy Xu Zeping)
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
