China now uses concrete faced rockfill designs for many dams. Chen Qian describes how early designs have been developed over the last ten years
The concrete faced rockfill dam (CFRD) has been greatly advanced in China during the last ten years. Its value to the hydro resources and electricity supply sectors is shown by the great investment in designing and constructing such dams.
So far more than 50 CFRDs have been built or nearly completed in China. Among these are Tianshengqiao 1, which has a dam height of 178m, and Shuibuya, which at 233m is the highest CFRD in the world.
The main reason that CFRDs have developed so rapidly in China is that they have advantages such as full use of local embankment materials, simpler construction, a shorter construction period, and a lower construction cost. CFRD dams are therefore more suited to both the engineering and the state conditions in China for water resources and hydro power.
Early CFRD designs
The development of the CFRD form in China has gone through a typical course of import, digest, practice and summation.
In ancient China stones were widely used for putting up dikes and dam construction. In modern times, China’s rockfill dam construction started after the 1950s. For example:
•Shizitan dumped rockfill dam, with a gravity wall, was constructed in Sichuan province in the 1950s.
•Maotiaohe 1 wooden face dumped rockfill dam was built in the 1960s in Guizhou province.
•Nanshui directional blasting rockfill dam with a clay diaphragm is in Guangdong province.
•Shifayu directional blasting rockfill dam with an asphalt concrete face is in Shanxi province.
By the 1970s China had constructed rockfill dams with an earth diaphragm that were more than 100m high. The Lubuge dam in Yunnan and the Shitouhe dam in Shanxi are good examples.
China’s earliest dumped CFRD is the Maotiaohe No 2 Baihua dam in Guizhou province, which was completed in 1966. The dam is 48.7m high, with an upstream slope of 1V:0.6H. A dry masonry cushion layer was placed on the upstream slope. A concrete plinth was placed in the dam toe, which was connected with the steel concrete panel slab (see diagram below). Since then, China has constructed several similar dumped CFRDs, including those at Nanshan and Sanduxi, but their heights do not exceed 50m. Typical is Kekeya, which was completed in 1982 in Xinjiang. It was built on a foundation of alluvial gravel with a dam height of 41.5m. Its base was treated with a concrete watertight diaphragm and was connected with the upstream panel slab using non-reinforced concrete and an arch connecting plate. The dam slope was nearly the same as that of any embankment dam.
This type of dam is now no longer employed, and the last dumped CFRD is Luocun Reservoir CFRD in Zhejian. Luocun is 57.6m high, and construction lasted from 1979 to 1990. The dam was basically designed according to traditional experience, although some local modifications were made on the face slab, waterstops, plinth, etc as the design was affected by updated CFRD technology during the later design period. The concrete face was placed by slipforms. The vertical joints are spaced 12m apart and there are no horizontal joints.
Since 1985 China has been developing projects using roller compacted CFRD.
The first CFRD of this type that was begun is the Xibeikou reservoir CFRD in Hubei province. The height of the dam is 95m. The first CFRD to be completed in this style is the Guanmenshan Reservoir CFRD in Liaolin province. This 58.5m high dam was completed in 1988. Now some 45 CFRDs have been completed and more than 40 more are under construction. Among these, by 1999 ten CFRDs had been completed or proposed with a dam height of more than 100m. The highest of these so far completed is 178m, and amongst those proposed the highest will be 233m.
These CFRDs have been built throughout China. The conditions of the sites chosen vary greatly, and as a result so do the experiences and problems encountered during construction. Chinese designers and constructors are capable of suiting measures to local conditions and completing satisfactory CFRDs: although CFRD construction began late in the country it has developed very fast, and China is among the countries that have built the most dams of this type. Characteristic of China’s recent CFRD designs are a thin layer of roller compacted rockfill, the use of slipformed face concrete, a narrow plinth, a well-graded cushion layer, and treatment of a deep and thick overburden.
A dam body can be of higher density, and deformation can be reduced by replacing a thick dumped layer with a thin roller compacted layer. This also allows for more adaptability in the use of embankment materials. The thickness of a dumped rockfill layer was not the same throughout the layer: it might vary from 18m to 30m or more. Within the layer three distinct regions develop: the fines would fill the pores of the upper layer after being flushed by water; the mid-layer forms a zone with the pores filled by mid-sized stones; the lower layer is of large stones. The porosity of the large stones of all dumped rockfill is 32-40%. The compressibility of the lower layer is very high.
The thick roller compacted rockfill (lift thickness <2m), by contrast, can reach a higher density, and the porosity of such a layer is generally less than 25% — in some cases less than 20%.The deformation modulus for different conditions is nearly 30-130MPa. Settlement of the rockfill has mostly finished during the construction period — settlement is less than 0.5% after dam completion (including impounding period). This makes the dam performance more reliable. In Chinese experience, settlement of dumped rockfill three years after completion is almost five times that for roller compacted rockfill; and the settlement of dumped rockfill 30 years after completion is almost eight times that for roller compacted rockfill.
Observations on CFRDs with a height of 100m show that a roller compacted rockfill would stabilise nearly three years after its completion; however, a dumped rockfill would still have some settlement after 30 years. The ratio of annual settlement is about 12.8:20.
The thick roller compacted rockfill has resolved the critical problems such as large deformation and discordance of a face slab resulting from an earlier dumped rockfill.
Slipforming face concrete results in a face slab divided into strips with vertical joints. Slipforming can be completed with or without horizontal joints depending on the construction conditions.
Joints between the strips do not use compressible filler: waterstops of many types are used instead, and rigid contact is maintained between strips, so as to avoid enlarging any deformation of a perimeter joint because of deformable fillers in the joints. The width of the strips is generally 12-16m. The width of the earlier triangular filling plate was placed by hand near the peripheral joint, resulting in a smooth level on the upper rim of the plate, and then a slipform is used to place the concrete. This type of formworking has not been adopted for the recent CFRD construction. The horizontal joints in the face are usually planned to take into account construction conditions, impounding ahead of time or flood control at the end of the dam construction.
For a mid height or low CFRD the horizontal joints cannot be planned. A horizontal joint can be treated as a construction joint without a waterstop, with steel passing through the face. Treatments such as cleaning and roughing are needed when concrete is placed continuously. The thickness of a face slab decreases as the concrete placing quality and formworking improve. The general formula used is t = 0.3+0.003H, where H is the design water depth in metres or the height of the dam at the point in question. The steel can be reduced except for special conditions such as cold regions, or edge or corner strips. The steel is generally 0.35-0.4% of the volume of face concrete; in some cases it may be as low as 0.3%. More work can be completed in a short construction period because slipforming allows balanced construction and continuous placement. In China a type of trackless slipform is in use. The slipform is used to place concrete with a variable thickness or width, or a triangular or turning face slab, while maintaining the dam body profile. At the same time, the conditions for placing concrete have been improved by replacing the dry masonry cushion layer with a crushed rock cushion layer under the face.
Changes in the plinth have resulted in great developments in CFRDs. Recently thin plinths have been employed: they are regarded as a watertight structure and a connecting element in the impervious face of a dam body, but not as an element carrying stress. The stability is not affected by applied forces such as the face’s sliding-down force, water pressure, rockfill pressure and so on. This greatly simplifies the structure of the plinth, as there is not a cutoff in the rock foundation. Nor is a plinth needed to support the face slab: only a thin plate as thick as the slab or slightly thicker, acting as a watertight plate and also a grout cover. There is therefore more flexibility in selecting the plinth base.
A weathered, soft or karst rock foundation can also be adopted as a plinth base in addition to traditional sound, groutable or erodible rock foundation — even a residual soil foundation can be used. Plinths on alluvial river beds are seen more and more now in China. This has made selection of a dam site and a dam axis much simpler. Construction is also easier: slipform can be adopted to place a longer strip of plinth as its profile is shaped. This method can be adapted and
has been used, for example, at Tianshengqiao 1.
The cushion layer directly under a concrete face has experienced several changes. From a hard cushion layer placement with large rubble, it has developed into homogenous crushed rocks and then a coarser graded crushed cushion layer. Now a fine graded cushion layer is commonly used. This layer’s function originally was to support and level the face slab. Now it serves as a second watertight layer and can limit inflow; it also operates as a filter, stopping the silty sand in a joint with an inflow, and healing the joint if leakage occurs in a face or from a joint.
A deep and thick overburden has been treated with a concrete watertight wall on which the plinth and dam body lie. The plinth and the concrete watertight wall are connected with a expansion joint, forming a complete watertight system. This type of construction is now commonly used in China.
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