(1) Plane layout of composite geomembrane laying

The facing surface of the dam is paved with composite geomembrane as the anti-seepage of the dam body. First, a trapezoidal groove is excavated along the foot of the dam and around the dam body, and then a C 15 concrete section with a bottom width of 0.75 meters, a top width of 1.5 meters and a height of 2 meters is poured . For the water wall, the geomembrane is laid from the top of the dam to the foot of the dam and around the dam body, and embedded in the cut-off wall. Excavate a solid trench at the top of the dam to fix the geomembrane, excavate an anti-slip tooth groove in the middle of the upstream dam slope to embed the geomembrane, and then backfill it with clay for compaction. At the junction with the water discharge tunnel, the geomembrane is embedded in the side wall concrete around the stilling shaft of the water discharge tunnel.

(2) Excavation of cutoff wall

The siltation in front of the dam of Guankoutian Reservoir is relatively serious. The original base elevation of the reservoir dam is 1545.6m. After many years of operation, the siltation elevation is 1553.5m, and the thickness of the silt layer in front of the dam is 7.9m. The dead water level of the reservoir is 1554.0m. If the cutoff wall is excavated from the foundation surface, the reservoir will still have a water level storage capacity of 8.4m. Setting up a temporary construction cofferdam requires a large amount of work and drainage is difficult. Therefore, the cutoff wall will be excavated from 1554.0m , using the dam body soil layer as the bearing layer, the maximum thickness of the original dam body soil layer as the bearing layer is 6.4m, the bulk density is 17.1 kN/m 3 , the saturated bulk density is 18.4 kN/m 3 , the internal friction angle is 16.3 °, and the cohesion force is 32.6 kPa, soil particle specific gravity Gs=2.7, soil void ratio e=1.05, permeability coefficient K=7.8×10 -5 cm/s.

(3) Composite geomembrane blanket design

1) Selection of geomembrane

According to the relevant regulations in the "Technical Specifications for the Application of Geosynthetics" (GB50290-2014), the thickness of the selected geomembrane should not be less than d/5 (d- is the maximum particle size of the geomembrane in contact with the protective layer), so the final choice of geomembrane The thickness is 0.6mm. It is a two-cloth-one-membrane PE composite geomembrane with a specification of 600g/m 2 /PE0.6mm/600g/m 2 and a width of 4m×50m. Its technical parameters are as follows:

serial number Specifications project 0.8(mm) Test Execution Standards
1 Film thickness deviation (%) -9  
2 Quality deviation (%) -10  
3 Width deviation (%) 0.5 GB/T17642-2008
4 Breaking strength (KN/M)≥ 15.5 GB/T17642-2008
5 Elongation at break (%) 65 GB/T17642-2008
6 Tear strength (KN)≥ 0.56 GB/T17642-2008
7 CBR bursting strength (KN)≥ 2.4 GB/T17642-2008
8 Resistance to hydrostatic pressure (Mpa)≥ 0.8 GB/T17642-2008
9 Permeability coefficient cm/s ≤ 10-12 _ GB/T17642-2008
10 Exterior qualified GB/T17642-2008

The geomembrane is laid on the dam slope and the bank slope of the upstream face, the area is large, and it is a slope, so the stability of the geomembrane and the protective layer should be calculated, and the calculation formula is as follows:

K=f/tgα

f—The coefficient of friction between the PE geomembrane and the protective layer on the membrane (0.55)

α—dam slope angle, 15º (1:4.0); bank slope 22° (1:2.5)

After calculation, when the protective layer of the geomembrane is a fine sand cushion, the anti-sliding safety factors K=2.04 and K=1.38 are greater than 1, and the laid geomembrane can meet the anti-sliding requirements.

2) Construction of geomembrane

The composite geomembrane is used as the horizontal anti-seepage of the dam body. The geomembrane is laid from the top of the dam to the upstream dam foot and the cutoff wall around the dam body. The bank slope geomembrane is embedded in the bank slope concrete side wall. Excavate the buried ditch at the top of the dam to fix the geomembrane, and excavate a non-slip cog in the middle of the upstream dam slope to embed the geomembrane. The large sample of the buried ditch and the non-slip cog is shown in the figure "GZ-PD-GKT-Modified -02".

In order to lay the geomembrane, the gravel layer and slope protection blocks on the dam slope should be removed, the gravel and sharp stones contained on the surface of the dam body should be removed, and the surface should be leveled. Lay a layer of 15cm thick fine sand cushion first, and then lay the geomembrane. In order to protect the geomembrane, lay a 10cm thick sand cushion on it, and the sand particle size should not exceed 0.5mm. Then lay prefabricated concrete on the sand cushion layer, the thickness of the prefabricated concrete slab is 8cm, which can be used not only as the weight of the geomembrane, but also as a slope protection function.

①Geomembrane detection

The composite geomembrane must comply with the national standard specification "Geosynthetics Nonwoven Composite Geomembrane" (GB/T17642-2008). The purchase of composite geomembrane materials must be a product of a reputable manufacturer, and must have a test report from an authoritative unit certified by the state, and must have a factory certificate; performance and characteristic indicators and instructions for use, and entrust a third party to test the product and pass the test. available later. When selecting the width of the composite geomembrane, the membrane joints should be minimized under the premise of meeting the thickness requirements, and the geomembrane transported to the site should be used up on the same day.

② Cushion construction

In order to lay the geomembrane, the gravel layer and slope protection blocks on the dam slope should be removed, the gravel and sharp stones contained on the surface of the dam body should be removed, and the surface should be leveled, and a layer of fine soil cushion with a thickness of 15cm should be laid , the compaction coefficient is greater than or equal to 95%, and the ruins layer will not be damaged during the construction process.

③Technical requirements for geomembrane laying

  1. When laying, the membrane should be properly relaxed, and a margin of 3%-5% should be reserved. Made into raised wavy slack ways. To adapt to temperature changes and foundation subsidence. And avoid artificial hard folding damage.
  2. For the laying of the composite geomembrane on the slope, the joints shall be parallel to the direction of the dam axis and shall be laid in order from top to bottom . Slope bends shall have membranes and joints connected.
  3. The composite geomembrane should be naturally relaxed and firmly attached to the support layer, and should not be wrinkled and suspended.
  4. Cut and splice according to the required size, and the connection between the two pieces should be smooth and wrinkle-free.
  5. The composite geomembrane should be laid and pressed to prevent the wind from blowing. Before the protective layer is covered, a sandbag weighing 20-40 kg should be placed at the corner of the membrane every 2-5 meters.
  6. In order to ensure the convenience of construction and the quality of splicing, the composite geomembrane should be as wide as possible to reduce the amount of splicing on site, and cut it in the unit as needed.
  7. When laying geomembrane, it should be laid when the outdoor temperature is above 5 degrees Celsius and the wind force is below level 4 without rain or snow. In case of rain or snow, construction should be stopped and the laid composite geomembrane should be covered with color strip cloth or plastic cloth.
  8. During the laying process, workers should not wear hard-soled shoes or shoes with spikes. Do not place heavy objects directly on the composite geomembrane. Do not knock stones on the composite geomembrane and any construction work that may cause damage to the geosynthetics. At the same time, check the appearance of the film at any time for defects such as damage, pitting, and holes.
  9. If there is any defect or damage after the film is found, it should be repaired with fresh base material in time. Each side of the repaired scar should exceed the damaged part by 10-20cm.

④Construction of protective layer

After the composite geomembrane is laid and the welding test is qualified, the protective layer should be laid in time. The laying speed of the protective layer should match the speed of the film laying. The laid composite geomembrane should not be damaged during the laying of the protective layer. Lay a 15cm thick fine sand cushion on it, and then lay a 10cm thick sand cushion, and lay prefabricated concrete on the sand cushion. Slope protection effect. When constructing the cushion on the composite geomembrane, first fix the composite geomembrane, and pour concrete from one end of the composite geomembrane to the other end evenly to prevent wrinkles.

⑤Geomembrane welding

According to the technical requirements, the geomembrane is welded by hot-melt welding, and the TH-515 geomembrane welding machine is used. Before welding, wipe it with a clean gauze or cotton cloth or use a hair dryer to blow the joints of the welding seam, so that there is no water, no dust, no dirt. When welding, double seam welding is adopted, and the width of each seam is 10mm to avoid defects such as missing welding, burns and wrinkles. The composite geomembrane is cut neatly to ensure sufficient lap width. When there are voids, shrinkage and distortions during construction, remove them and re-weld them. After the geomembrane is welded, cover it with colored strips to avoid direct sunlight to prevent damage.

⑥ Construction of special parts

The composite geomembrane is used as the horizontal anti-seepage of the dam body. The geomembrane is laid from the top of the dam to the upstream dam foot and the cutoff wall around the dam body, and embedded in the cutoff wall. Bury the anti-wave wall at the top of the dam to fix the geomembrane, and excavate three anti-skid grooves in the middle of the upstream dam slope to fix the geomembrane. For the part where the geomembrane is connected to the stilling well, the geomembrane is first embedded in the concrete side wall of the stilling well. At the connection between the geomembrane and the intercepting wall, the stilling well, and the anti-wave wall, the embedded length of the geomembrane is 20cm.

⑦ Quality inspection and acceptance during construction

Before each layer of composite geomembrane is covered, it is carefully inspected and accepted: visually check whether the seams are not scalded , whether there is no obvious bulge or wrinkle when laying flat, and whether the laying is smooth.

The weld seam of the composite geomembrane avoids false welding and missing welding. Take a group of 1000m2 samples for weld tensile strength test, and the weld strength is not less than 80% of the base metal. And the fracture of the specimen shall not be at the fractured seam.

⑧Geomembrane construction process