Table 1. Basic information of typical rockfill dams built on overburden layers in China. Technical problems requiring special attention in the design and construction of rockfill dams over thick overburden layers include, but are not limited to, the following aspects: Shear strength and deformability of load-bearing layers. The shear strength of underlying foundation layers influences the overall stability of the dam, while the deformability of these layers controls not only the deformation of the dam but also the deflection of the cutoff wall, if used. The inhomogeneity of foundation materials can result in differential and incompatible deformation within the dam and may ultimately lead to threatening cracks.
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Concrete faced rockfill dams in China 10 September China now has 39 concrete faced rockfill dams in operation. Since a number of CFRDs have been constructed comprising compacted rockfill, with a semi-pervious filter grading face zone, monolithic face slab, dowelled and grouted toe slab plinth , and multiple water stop perimeter joint.
The first modern CFRD, the By 39 dams of this type had been completed in China, 14 of which have a height over 70m. In addition, there are 35 dams under construction including Tianshengqiao, known as TSQ-1 and 20 dams, all over m high, at the planning stage.
The CFRD has been used for many different purposes and different sizes of reservoir, from small irrigation projects to large reservoirs on main rivers, as well as for large pumped storage power projects. Construction began in and is to be completed this year. The increased use of CFRD mainly results from its low cost and short schedule, as compared to the earth-core rockfill dam.
In practice, the river was blocked for the TSQ-1 project in December , and diversion tunnels were plugged off in December The first unit started generating power in December The critical path for generation was not the construction of the dam but of the power facilities.
At present the highest dam in the world is at Aguamilpa, which is m high, but the main embankment material at that site is river gravel, which is less compressible than the limestone rockfill for TSQ-1 and Shuibuya. There are many design and research institutes in China engaging in this new type of dam. The Design guide and Code were used to reflect the state-of-art in this field both from Chinese practice and from other countries.
The design guide and code are critically necessary in China. As is well known, the 70m Gouhou dam was completed in and failed in One of the main causes was the lack of the provision of an internal chimney drain in the sandy gravel fill. Leakage from the joint between the crest parapet and face slab appeared at the downstream slope and caused piping of the material. Meanwhile, upstream undermining destroyed the gravel support underneath the face slab.
The slab collapsed due to reservoir water pressure, and as a consequence the dam was overtopped. The overtopping caused the dam to fail within two hours. According to the designer, the dam was completed too early to meet the Design guide. The main features of the Code are briefly described below. Both types of figures are specified in the Code. Learning the lessons of Gouhou dam, the importance of seepage control has been emphasised in the Code.
The Code requires the cushion zone material to be internally stable, and have self-filtering properties. The interfaces of the cushion zone and transition zone, and of the transition zone and main rockfill zone, should meet filter criteria. The grading of cushion zone 2A has been specified in the Code by our laboratory test. The specified grading for zone 2B is the same as that for downstream fine filter for an earth-core rockfill dam.
In our opinion, leakage is an economic problem and of no dam safety concern in the case of CFRDs. If erosion takes place it can result in additional settlement of face slab, and if the face slab is destroyed important leakage occurs. Conversely, leakage can be a dam safety problem for a concrete faced gravel-fill dam, if the seepage is improperly controlled.
Checks on the face slabs of two dams, when the reservoir was emptied, revealed some honeycomb concrete and hairline cracks parallel to the perimeter joint, found at abutment slabs near the perimeter joint.
These may be structural cracks. Before the reservoirs were emptied there were no signs to indicate problems, as the leakage is insignificant and clear.
We conclude that the defects in the face slab might be unavoidable, but leakage can be controlled at a negligible level, provided the cushion material is internally stable, dense and has low permeability. Cushion zones of 1m and 3m have both been used in China, and are specified in the Code. For the 1m option the cushion zone is placed by backhoe, while for 3m width the cushion zone can be spread by bulldozer.
In our experience the backhoe is helpful for preventing segregation, as it also has a remixing function. The backhoe can also remove oversized rocks at the interface of cushion zone and transition zone that do not meet filter criteria, before placing cushion material against the transition zone.
As mentioned, structural cracks parallel to the perimeter joint have been found in steep abutment face slabs. The Code requires placing of a zone of increased modulus with material compacted in thinner layers and with more passes to make the change in support more gradual to reduce bending stresses in the face slab.
As regards quality control, compaction parameters such as layer thickness, capacity of vibratory roller and number of passes, and water volume added, are used for quality control and modification of the compaction parameter.
For very high dams, such as TSQ-1, six passes of the Sealing of the joints is very important. Long term two year tests using with pressures up to 3.
The Code recommends, for low dams: one bottom copper water stop and a primary water stop for all joints; for high dams: bottom copper water stop and surface mastic sealing; and for very high dams bottom copper, surface mastic sealing and fly ash cover. Construction technology We have a half-year long rainy season and another half-year long dry season, so meteorological conditions in China are very favourable for river handling.
Two alternative handling methods have been developed. In the first, the river can be blocked off by cofferdams at the beginning of dry season, and a temporary dam section constructed before the start of the next rainy season, to retain the flood by stabilising the cushion zone of the dam.
In this option, the highest temporary dam section, at 72m, was for the Baixi dam. With a high temporary dam section, the cost of river handling can be reduced a great deal by increasing the velocity in the diversion tunnels and by routing floodwaters via the newly created reservoir to cut down the peak flow.
Dam construction can be continuous in this method, since the river is blocked off. For the second alternative, in the first dry season the foundation river overburden is tripped away. Meanwhile, the river portion plinth and overflow cofferdams are completed, as is the protection rockfill blanket cover on the river bed.
The dam construction in the river portion is stopped until the beginning of the second dry season. The temporary dam section constructed in the second season is then used to retain the flood. The first alternative is often used where the river deposit is shallow or could be left for the dam foundation. The second is often used where the river deposit is thick and must be removed. Of course, the temporary dam section for the second alternative will be higher than the first one.
For example, the temporary dam section of TSQ-1 project was m high, but the construction period will be longer. The option mainly depends on natural conditions. Upstream stabilisation A unique roller-compacted mortar has been developed for upstream face stabilisation and used mostly in China. The roller compacted mortar is completed together with the cushion zone slope-compaction in a single job.
After trimming the upstream slope, the slope is compacted by roller using two passes without vibration and four passes with vibration. A second trimming is conducted and a cm thick layer of mortar added comprising The surface is compacted using two passes of a roller without vibration, followed by four passes with vibration , then finally by two passes without vibration. Of course, vibration is only applied and counted for up-slope passes.
As regards slipforming, the most advanced slipform for face slab concreting is an unrailed slipform. The slipform slides on the running surface on the slab side-forms or on the concrete surface and can simplify the construction work by eliminating the use of triangular starter bays.
The first unrailed slipform was developed and used in China for the face slab placement of the Xibeikou dam in March The m wide slipform weighs 4. The slipform was designed for a 12m wide face slab. The wooden side-forms, in sections 2m in length, can be handled manually. The slipform is operated by two separate 5t winches on the crest and is transported by mobile crane from bay to bay. The concrete is delivered by chute from crest. In our practice, the average slipform production is m2 face slab per month and China has developed slipforms with a width of m.
All the dams under construction will use unrailed slipforms to place face slabs. Related Articles.
Papers by Keyword: Concrete Face Rockfill Dam (CFRD)
The results show that permanent deformation will increase gradually with the increasing of peak value of seismic acceleration. In the coupled analysis, the paper focused on hydrodynamic pressures in the reservoir zone, dynamic response and pore water pressure in the structure zone. The result shows that the dynamic response of added mass model is greater than that of potential-based fluid model. The porous medium of alluvium deposit is of great significance in performing soil liquefaction analysis and reservoir-dam-foundation system. According to the Baixi project, a 3D numerical model was built including the concrete face, filled rock, rock foundation and peripheral joint. Then, based on the deformation monitoring data during the construction and water storage period, the Duncan E-B model parameters were back analyzed.
Concrete faced rockfill dams in China
Dams can be formed by human agency, natural causes, or even by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their size height , intended purpose or structure. By structure Based on structure and material used, dams are classified as easily created without materials, arch-gravity dams , embankment dams or masonry dams , with several subtypes. Arch dams Gordon Dam , Tasmania , is an arch dam. In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal hydrostatic pressure between vertical cantilever and arch action will depend upon the stiffness of the dam in a vertical and horizontal direction.
List of tallest dams
Concrete faced rockfill dams in China 10 September China now has 39 concrete faced rockfill dams in operation. Since a number of CFRDs have been constructed comprising compacted rockfill, with a semi-pervious filter grading face zone, monolithic face slab, dowelled and grouted toe slab plinth , and multiple water stop perimeter joint. The first modern CFRD, the By 39 dams of this type had been completed in China, 14 of which have a height over 70m. In addition, there are 35 dams under construction including Tianshengqiao, known as TSQ-1 and 20 dams, all over m high, at the planning stage.