River Close-off and Concrete Cut-off Wall Construction

The river close-off and the concrete cutoff wall construction are two key issues in building Phase-II rock-and-earth cofferdam.

River Close-off on the Main Channel

The TGP river close-off on the main channel features deep river, large inflow, intense construction, tight timetable, navigation during the close-off process, deep surface layer above the levee foundation, and other difficulties. The TGP dam is situated at the backwater region of the Gezhouba Reservoir, and during the close-off the maximum water depth at the riverbed was 60 m, a world record. It's crucial to prevent levee breakdown and keep the levee stable during the bank-off advance. So many hydraulic model tests, value calculations, and mechanism analysis were carried out to study this issue. The efforts led to conclusion that when water depth is reduced to 20 m, levee breakdown can be avoided. Accordingly, it was decided to adopt the program of pre-leveling, upstream single levee blocking, two directional bank-off advance, and downstream follow-up advance. Along with the gradual

inflow decrease, close-off levees successively advanced for preparation. At 3:30 pm, 8 November 1997, main channel was closed off successfully. World records were created in TGP close-off as follows: inflow of 8480 ～ 11600 m³ /s during the close-off, water depth of 60 m, and rock-and-earth dump intensity for the upstream and downstream bank-off advance in 24 hours totaling 194,000 m³ .

The main channel close-off design, and the construction technology research and practice were awarded with the first prize for national science and technology achievement in 1999.

Deep Water Cofferdam and Concrete Cutoff Wall

The TGP Phase-II cofferdam was built to guarantee the year-around construction in the foundation pit during the TGP Phase II construction period. The scale of Phase-II cofferdam construction ranks top in the world. Since the water depth reaches 60 m, approximately 2/3 of the cofferdam was constructed underwater and it's very difficult to do the work. The geological condition is complicated and not suitable for building cutoff wall. The field condition only allows taking weathered grit as fill material, so the structure is loose in underwater dumping, its physical indices is bad and deformation will develop. Therefore the construction has to be conducted within a low-water season. The biggest difficulty of the Phase-II cofferdam is the construction of concrete cutoff wall. Dumping weathered grit into deep water and building two layers of 74 m flexible concrete cutoff walls in the sand-gravel cofferdam were achieved with good quality, seepage was measured to be only 65 L/s, quite rare in similar works.

For the first time, centrifugal model test was conducted in the construction of cofferdam to define that the density of dumped grit in water depth of 60 m should be 1.75 ～ 1.85 t/ m³ , and underwater slope angle should be 27°. The critical technical problem of deep water cofferdam design was solved. And it's the first time to make use of weathered grit and construction wastes and successfully produce flexible wall material that meets design requirement of the cutoff wall of the deep water cofferdam. Advanced construction equipment and auxiliaries were manufactured to solve the problem of granite layer level difference of more than 30 m, slope level of 70° ～ 80°, bedrock with double-directional slopes, and difficulty to make trench for filling with weathered grit. A new record, monthly constructing cutoff wall of 13000 m² , was created. During dismantling, after all-around inspection, sampling, and analysis on the cofferdam and the cutoff wall, it was proved that the survey, research, design, and construction of the cofferdam are successful and serves as a reference to similar projects.