| Concrete dams
not be closed under seismic action. For this purpose, a three-dimensional dynamic analysis of the CFRD is required.
As the slopes of CFRDs are usually much steeper than those of ECRDs, the seismic deformations in the crest region of CFRDs (horizontal displacements) are much larger than in other types of embankment dams of similar height and with much flatter slopes. These deformations may create further damage in this part of CFRDs, although this has not yet been observed; but up to now only few CFRDs were exposed to strong ground shaking.
Consequences of damage to the face
slab of CFRDs The consequences of damage to face slabs, i.e. joint leakage and leakage through cracks in the face slabs, are an increase in seepage through the dam. The amount of leakage depends, on the one hand, on the crack width and the hydrostatic pressure, which is maximum at the base of the dam, and, on the other hand, on the seepage through the joint system. Leakages as high as several m3
/sec were observed
/sec and for the 202m high Campos Novos CFRD in Brazil the seepage reached 11m3
/sec. In the case of Zipingpu CFRD, which suffered some
face slab damage as shown in Figures 4 and 5, the maximum seepage depends on the reservoir level and was about 50 l/s for the maximum reservoir level. At the time of the earthquake the reservoir volume was only about 30% of that at the full supply level. Therefore, before the earthquake the seepage was about 10 l/s, which increased shortly after the earthquake to 19 l/sec. This means that the damage caused in the submerged lower portion of the dam was minor. In general, leakage through the cracked face slab and joints should not be a dam safety issue, but there are cases where excessive leakage has caused severe safety problems. One of the problems may be the use of low quality erodible rockfill. Damage to the face slab may be higher near the crest
due to increased seismic deformations in this zone. Special measures may be required, but the most effective means is by providing flatter slopes of the CFRD. In highly seismic zones the slopes could be 1:1.5 or even 1: 1.6, which is much flatter than a value of 1:1.3 used in zones of low seismicity. These high crest deformations are also a problem of other types of impervious elements (e.g., asphalt cores or concrete core walls), when slope failures near the crest could occur.
Conclusions The main conclusions of the discussion of different
safety aspects of CFRDs and their comparison with ECRDs are as follows: 1. CFRDs are vulnerable to overtopping like all types of embankment dams. There are several reasons for overtopping of dams such as inadequate spillway capacity or the malfunction of spillways and/or low- level outlets.
2. The face slab is vulnerable to deformations of the dam body, e.g. due to settlements after construction of the face slab and due to seismic deformations. Such deformations may cause cracking in the face
in dams with damaged face slabs or defect waterstops. For example, for the 92m high Vajiralongkorn (Khao Laem) CFRD in Thailand the maximum seepage was 2m3
slab or damage to vertical (and horizontal - if any) joints. To minimize deformation of the rockfill, it must have a high modulus of elasticity, i.e. rockfill must be well-compacted and have a proper grading of the particles.
3. It must be checked, if face slab leakage may create safety problems to the dam. Fine material could be washed out causing further dam deformations. In order to minimize deformations of the dam body, the rockfill must be well compacted and must have a high modulus of elasticity (or deformation modulus), which can only be achieved by well- graded rockfill, i.e. there must be particle breaks during compaction. Rockfill particles with very high strength like basalt or some other types of rock with uniform particle size will result in rockfill of low modulus of elasticity of, e.g., less than 50 MPa, whereas for high CFRDs values exceeding 100 MPa are needed. The breakage of rockfall pieces could mean that the rockfill will not drain as quickly as assumed in the design and could lead to a small rise in water level within the rockfill depending on the amount of leakage through the face slab.
4. The face slab, joint sealing and plinth may be exposed to falling rock near the abutments, and any damage to them may cause local seepage in that part of the dam.
5. The face slab (near the crest) could also be damaged by explosives or the impact of boats.
6. CFRDs could be damaged by strong earthquake ground shaking as in the case of the Zipingpu CFRD in China, during the 2008 Wenchuan earthquake. The detailing of the vertical joints has a major impact on the damage to face slabs under both static and seismic deformations. As the slopes of CFRDs are much steeper than those of ECRDs, the seismic deformations in the crest region of CFRDs (horizontal displacements) are much larger than in other types of embankment dam of similar height and with much flatter slopes. These deformations may create further damage in this part of CFRDs.
7. CFRDs are vulnerable to fault movements in the footprint of the dam and also to non-uniform foundation settlements, especially when the dam is not built on rock.
8. CFRDs may have a shorter lifespan than ECRDs, due to the corrosion of the reinforcement in the relatively thin face slabs. Ultimately, the reinforcement will start to corrode despite a concrete cover of the reinforcement of say 7-10 cm. Most vulnerable are cracked slabs and slabs near the crest, which experience several wetting-drying cycles or even frost cycles.
It may be concluded that face slab leakage is the main safety concern and there is a need to evaluate the effect of this leakage on the safety of the dam body, taking into account that the permeability may be substantially less than assumed in the design.
Bibliography
ASDSO (2005) Taum Sauk Project, Dam failures and lessons learned. Association of State Dam Safety Officials, Lexington, KY, USA,
https://damfailures.org/case-study/ taum-sauk-dam-missouri-2005/
Harrris M. (2014) Tokwe-Mukosi Dam flaws possibly linked to cost- cutting measures, Hydro Review, July 2014, Charlotte, NC, USA
Marulanda C.E., Marulanda A.P. (2008) Recent experience on design, construction and performance of CFRD dams. Proc. 6th Int. Conf. on Case Histories in Geotechnical Engineering, Arlington, VA, USA, 11-16.8.2008
Wieland M. (2010) CFRDs in highly seismic regions, Int. Journal Water Power and Dam Construction, March 2010
Wieland M. (2016) Criteria for the selection of dam types in areas of high seismicity. Proc. Symposium on Appropriate Technology to Ensure Proper Development, Operation and Maintenance of Dams in Developing Countries, ICOLD Annual Meeting, Johannesburg, South Africa, 20- 25.5.2016
Wieland M. (2023) How to deal with the many hazards affecting dam safety? Plenary lecture, 2nd Int. Conf. on Dam Safety Management and Engineering (ICDSME 2023), Kuala Lumpur, Malaysia, 16-17.3.2023
Wieland M., Brenner P. (2007) Earthquake aspects of concrete- face rockfill dams, diaphragm walls and grout curtains, Int. Journal Water Power and Dam Construction, May 2007
Zhang L.M., Chen Q. (2006) Seepage failure mechanism of the Gouhou rockfill dam during reservoir water infiltration, Soils and Foundations, Vol. 46, No. 5, Japanese Geotechnical Society
Note: This paper was originally presented at the XX Technical Dam Control International Conference
Author details
Martin WIELAND, Dr. sc. techn., Chairman of ICOLD Committee on Seismic Aspects of Dam Design, Dam Consultant, Dietikon, Switzerland and AFRY Switzerland, Zurich, Switzerland,
martin.wieland48@gmail.com
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