A CASE-STUDY OF CUA_DAT CFRD IN VIETNAM

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1 A CASE-STUDY OF CUA_DAT CFRD IN VIETNAM Giang Pham Hong, Michel Hotakhanh, Nga Pham Hong, Hoai Nam Nguyen, Abstract:Dams have been taken an important role in time and surface redistribution of water for socio economic and environmental purposes in Vietnam. Cua_Dat is a multipurpose scheme under construction in Vietnam. The main dam will be the highest CFRD in Vietnam (H= 119m, L= 1023m, V = 10hm 3 ). The dam will be constructed in 5 years ( ). The river diversity with an overtopping of the dam under construction while big flood in 2007 is described. The 3-dimensional stress and displacement analysis with the ANSYS software results in the arrangement of double, upper and downer layers of reinforcing bars in the facing concrete slabs. Brief on the Cua_Dat Dam. Cua_Dat dam is located in the middle of the basin of the Chu River, which is the biggest tributary of Ma River, which one of major rivers in the northern central.. The project had been waiting investment for many years It has kicked off again since 1997 and the dam has been newly located a little bit upstream with some construction disadvantages but the resettlement would be much more reduced and feasibly realized. The dam site is 230km far from Hanoi southward. The reservoir storage about 1.45 billion m 3 is designed with main functions as followings: (i) flood control; (ii) water supply for beneficial area of 37,000ha with a population of 2.5 million; the irrigated area will be 87,000ha; the discharge supplied to industrial production is about 8m 3 /s; (iii) hydropower generation with capacity of 97MW. The main CFRD dam is 119m high and 1023m long (Fig.1) and accordingly, has a body volume of 10million m 3. Apart from that, the project also composes (Fig.2): a reinforced concrete spillway, which consists of 5 gates with the dimension of 11m 17m, discharging a flow of 11,600m 3 /s at frequency of 0.01%. two tunnels with diameter of 9m, one of them is 820m long used as temporary diversion tunnel during construction period and entire reservoir discharge, the other with 600m long is to convey water into the hydropower plant. several supplementary dams with height ranging from 20m to 41m. additional irrigation canal system for 37000ha. 1

2 Fig.1 Designed cross section of the Cua_Dat CFR D Face Concrete Slabs of the Cua_Dat CFR D The construction duration is 5 years ( ). The project implementation involves number of domestic experts and contractors. Downstream view of the dam 1 Some foreign experts from China, Canada, France, helped with advices. The most difficulty that the project construction has to

3 cope with is rather complicated geological structure in the foundation of the main dam. Thus, in order to construct the dam foundation, it has been had to dig out a large grouting into 40m deep and constructing volume of surface layers of soil and badly weathered rock as much as 7millions m 3 along. It has been implementing many seepage prevention measures, such as: The average flow discharge is 89 m 3 /s at Fig.2. Layout of the main CFRD and other headwork a bentonite trench under the upstream toe of the concrete face slab; using highly qualified seepage prevention additives for the concrete face slab together with a geo-membrane below; creating cutoff footing layer on foundation surface; fulling the cushion compacted clay zone between coffer dam and block of the upstream toe and bottom slab. River Diversion Difficulties are also met with the river diversion because of big peak flow discharge in flood season (about 3 months per year). frequency 75% in comparison with the peak diversion one is 7520 m 3 /s at frequency 1%. The 20 year-flood, adopted as the construction flood, has a peak inflow discharge of 1230 m 3 /s during the dry season and 5050 m 3 /s during the wet season. In order the diversion work has been carried out by some steps to accelerate the construction work and make it more safe. The first low coffer dam was along the main stream channel to treat the large alluvial plain. The subplementary coffer dam has been located next to the diversion tunnel intake. To minimize the cost of the critical stage of the construction (wet season of 2

4 Fig.3 The cross section of CFRD under construction in the 2 nd year facilitates the peak flood diversion The first moment of flood overtopping in October 2007 Serious erosion of the dam portion in channel due to big flood over the design criteria the 2 nd year) by only one tunnel (L = 820m, diversion structures, it was admitted to D=9 m) with an overtopping of the upstream/ divert the flow. The cross section of CFRD 1

5 under construction in the 2 nd year facilitates the peak flood diversion (fig.3). Laboratory tests (scale 1/40), taking into account the zoning and materials of the three dams, have been performed in order to check their stability and to measure the pressure and velocity of the flows at different parts of the structures. These tests have permitted to verify the resistance of the dams and to define precisely the most suitable protections for their crest, upstream/downstream faces and toe during the overtopping. Different solutions have been contemplated (concrete face slabs, gabions, placing the rockfill by longitudinal steps,..). The tests carried out indicate that this diversion method is possible, so it leads to significant cost and delay savings compared to usual methods (high upstream cofferdam with several large diameter tunnels). The main coffer dam near to the upstream toe will also combine with the cushion compacted clay zone. In October 2007 a very big flood came with the discharge of more than 7000m 3 /s. It s bigger 37% than the designed one. The overtopping flow made about 30% of the dam portion in channel erosed seriously. So the downstream slope of the dam portion must be strengthened sufficiently to be safe in big flood overtopping. The stepping stone gabions seemed to be rather weak. Even in that disadvantageous case the expenditure to restore the erosed dam portion is about 20% of the cost of a diversifying tunnel only. 3 Displacement & Stress Analysis of Reinforced Concrete Face Slabs The displacement & stress analysis of reinforced concrete face slabs has been implemented by use of the Finite Element Method with the ANSYS software. The slabs have been divided into shell elements. The rockfill body has been divided into solid elements (Fig.4). The computation has been done in the case of static loading without gravity force. The slabs are divided into 121,000 shell elements and the dam body together with its foundation are divided into 847,000 solid elements. Numerical results of the second stage are showed in fig.5. Fig.4. Element network in numerical analysis 2

6 Fig.5. Numerical results of displacements & stresses Displacement u X Displacement u Y Displacement u Z 2

7 Stressv intensity σ= σ 1 - σ 3 with σ 1, σ 3 max & min principal stresses Von Mises equivalent stress σ e Some remarks can be derived from the above numerical results of the displacement & stress analysis of the face slabs: - The tensile strength would ocur at the two dam portions on the left & right shoal sides. - The region, where the maximum u X and u Y values occur, is in the dam portion on the river channel. It is at the level about less than half of the height. - The inluence of the strong rock foundation deformation on the face slabs can be negligible. It s noted that the slabs behave as bending plates, not membranes. So the reinforcing bars should be put in two layers and designed as ones in cross section of bending reinforced concrete elements. 3 Conclusion Cua_Dat is considered as one of the most important dam in the Dam Development Program of Vietnam because its scale and efficiency. It is not only among the first CFRD but also the largest CFRD in Vietnam. Besides the geological characteristics of the foundation is complicated. The river diversion using flow overtopping met with big flood. The dam work in the channel portions has to be halted in flood season due to the overtopping diversion. The cracks in the lowest part of the face concrete slabs by water pressure in the dam body were treated with difficulties They all should considered as useful experiences of CFRD construction.

8 REFERENCES [1] Feasibility Studies of Cua_Dat Dam, Hydraulic Engineering Consultancy Company, Hanoi, Vietnam [2] M.Ho Ta Khanh, Pham Hong Giang.Design of the Cua Dat CFRD in Vietnam and innovative river diversion system Int. Symp. ASIA 2006 Water Resources & Renewable Energy Development in Asia, Bangkok, Thailand, Pham Hong Giang, Nguyen Hoai Nam. Displacement and Stress Analysis of the Reinforced Concrete Face of the Cua_Dat Dam, Int. Symp. ASIA 2008 Water Resources & Renewable Energy Development in Asia, Danang, Vietnam,