One of the best solutions for remediation of existing dams is RCC (Roller Compacting Concrete) due to its economical and construction time. The application of RCC in rehabilitation of dams has different parts and some important parts of remediation in dams include erosion and scour protection of slopes, crests, stilling basins of embankment, repairing breaches and seepages, increased the hydraulic capacity for service and emergency spillways and construction of downstream gravity sections for seismic strengthening and increased sliding stability of concrete gravity, arch, and buttress dams. The following cases histories describe selected applications of RCC in a dam remediation project. The organization that has great investigation on RCC rehabilitation is U.S. Army Corps of Engineers, which prepares a list of advantages of RCC technique: Costs: In comparison with conventional dams, RCC can decrease the costs in ranging from 25 to 50 percent. Rapid Construction: In today’s world time is an important factor. RCC’s finishing time is typically 1 to 2 years less than regular concrete dams. Spillways: Constructing the spillway in main part of dams is costeffective which by comparing to the embankment dams that require using spillways in an abutment.
Application in Construction of New Dams
Despite the fact that RCC is so economical, seepage is the fundamental problem faced with RCCs. In some cases, it may cause problems and might be acceptable by appearing of the seepage in the downstream of a dam. The RCC dams are not uniformed as expected to be, because of some natural characteristics of construction procedures such as permeability which may cause to segregation. The other considerable problems of RCC dams are high permeability due to dry concrete that contains low amount of cement and leads to low density zones, the permeability between lifts, and the high risk of joint’s separation on upstream which will develop by thermal reaction. To solve the above mentioned problems, using concrete with high cementations content decrease permeability of lift joints. However, it can affect thermal cracking in plain concrete. The high cementations concrete in RCC will complicate construction due to controlling the temperature of mix, placing, and lift control to preventing cold joint. The seepage can be the main problem of RCC dams which can determinate the dams by leaching out of cement by the seeping water. Bursting the Camara RCC dam built in 2002 in Brazil revealed that seepage may have terrible effects. Consequently, to reach a sophisticate system in upstream, geomembrane and integral concrete of upstream and RCC can be the turning part of solution. The concrete of upstream facing is make by internally vibrated concrete placed at the same time by RCC, and will enrich upstream concrete facing. Moreover, the placement of concrete requires higher care and different mix design from the body of RCC dam, and also needs vertical joints for water penetration. Using artificial materials is the other part for protecting upstream permeability which can be named by watertight sealing element and embedded water-stops. These instruments have negative influence on time and cost during the construction procedure through entire dams. In order to overcoming this problem, selecting geomembrane system has significant benefits in terms of cost and schedule which can provide upstream impermeability. Some of the other advantages of geomembrane will be presented below:
1. Reducing cement content,
2. Reducing or omitting pozzolan,
3. Reducing fly ash,
4. Omitting the temporary system for cooling,
5. Reducing bedding mix for lift surface
6. Omitting or reducing of contraction joint in upstream,
7. Reducing the coast of horizontal joints treatment
Design Principles :
The first designing method for RCC dams is according to the concrete with low cement content. This has benefits such as lower temperature in contrast to conventional concrete, subsequently decreasing the tensile stresses (less than 120 kg/m3), as well as the water cement ration in less than ordinary concrete due to compacting by roller. Designing and analysis a RCC dam by using uplift force is directly involved with lift joint, permeability, placements methods to prevent segregation, and techniques used at upstream facing for water tightness. The uplift reduction is therefore a function of the water tightness of the upstream facing and of the efficiency of the drains. The uplift can be reduced in dam foundation using a tight connection for foundation sealing system such as cut-off wall or grout curtain. The presence of a perimeter plinth at the heel of the upstream face with a short grouting curtain which will bond the plinth to the rock and will intersect the grouting curtain from the gallery will reduce the remaining possible infiltration paths of water from the reservoir, which basically is:
1. Through permeable foundation
2. Through the bottom part of defective vertical joints
3. Through horizontal lift joints not perfectly treated 3.
It is worth to mention that using upstream geomembrane sealing can enhance the RCC mix design by decreasing the amount of cement and pozzolan. The below table present the data of cement and pozzolan content using a geomembrane (Table 1). The geomembrane technique for RCC is divided in two parts:
1. Exposed sealing system
2. Covered sealing system.
The exposed sealing system Exposed system: at this method, geomembrane is installed on the upstream which is exposed to reservoir’s water and environment (Figure 1). The covered sealing system Covered system: In this method, geomembrane uses as formwork by casting on precast panels. The geomembrane in covered sealing system permanently embedded, however just on the open joints, geomembrane remain exposed to water of reservoir (Figure 2) 4. RCC Stepped Spillway Using stepped spillways in RCC dam has a great advantage, which can dissipate the energy of water through the spillways slop 5 (Figures 3 and 4). Comparison of the Two Systems In the both solutions, geomembrane prevents the water penetration over entire surface and makes RCC waterproof through all lift joints. The geomembrane is the best water-stop for covering the face, otherwise in conventional techniques for water-tightening the RCC dams try to use PVC water-stops at construction joint. The advantages of the geomembrane usage is avoiding water infiltration in lift joints, reducing design constraints, such as conventional concrete layers on the upstream face, special paste treatment of joints or bedding mixes. Therefore, it leads to place the RCC mix through the dam’s crosssection without placing bedding mix on the joints and concrete at the upstream face. However, in some cases the bedding mix may need to be used to increase the shear strength and stability of dam. Moreover, on the upstream face, the bedding mix may be required just for preparing a slight thickness for touching formwork. Its use was mainly dictated by the need to achieve good compaction of the RCC mix at the upstream concrete. The projects of RCC dams, which used geomembrane instead of bedding mix, are the dams of Riou (France), Balambano (Indonesia), Concepcion and Nacaome (Honduras). The exposed solution provides some additional technical and economic advantages as compared to the covered solution 5. One of the fundamental principles usage of face drainage system appears at the design stage and throughout of the geomembrane service life in dams. Face drain system, are accepted by many experts, due to reduction abilities at the design uplift in the part of designing. The advantages of face drainage system during service life of dams are revealed in the following paragraphs: Monitoring: In order to control the quantity of drainage water, the face drainage system lets the performance of waterproofing system be monitored by measuring fluctuations with respect to an