Construction Technology of HDPE Geomembrane in Sewage Treatment Plant

1 Project overview

With the improvement of people's quality of life, environmental protection projects have gradually increased. In recent years, our company has constructed the wetland project of Yongji Sewage Treatment Plant. HDPE geomembrane is used as the anti-seepage layer in this project. The thickness of the geomembrane is 03mm, and the total amount is 29,000 square meters. During the construction, QC activities were carried out, technical public relations were organized, and the key technologies of construction were mastered, and good results were achieved.

The wetland project of Yongji Sewage Treatment Plant started on April 20, 2007, and the project was completed on October 20, 2007. After three years of use, the quality is reliable, and the economic and social benefits are obvious.

2 Process principle

Make full use of the excellent anti-seepage effect of HDPE geomembrane and the corrosion resistance of acid and alkali, salt, oil and alcohol, and has excellent puncture resistance, etc., combined with the double seam welding connection process and reliable detailing practices, so as to achieve the entire project. Good anti-seepage effect.

3 Construction process

Construction preparation → geomembrane laying → aligning, aligning → laminating and shaping → wiping dust → welding experiment → welding → testing → repairing → re-inspection → acceptance → geotextile laying → geotextile sewing → maintenance and repair → acceptance.

4 Main operating points

4.1 Construction preparation

1) Before laying the HDPE film, the construction unit, the design unit and the supervision unit should conduct a comprehensive inspection and acceptance of the base, and the construction can only be carried out after the symbol design requirements.

2) The base surface should be dry, compacted evenly (the dry density of the soil base should not be less than 1.4t/m³), flat (the flatness error should not exceed 20mm), no cracks, no obvious sharp protrusions, no mud, no depressions, vertical depth There should be no tree roots, rubble, stones, steel head, glass chips within 25mm. Where there are springs or groundwater, corresponding drainage measures should be taken. The pits and ponds have been backfilled and compacted by local earthwork.

3) Organize pre-job training for operators to ensure that operators are certified to work.

4.2 Laying of geomembrane

1) Before laying the geomembrane, do the cutting analysis, and draw the laying sequence and cutting diagram. Before the geomembrane is cut, its relevant dimensions should be accurately measured, and then cut according to the actual size. Generally, it is not suitable to cut according to the size shown in the figure. It should be numbered one by one and recorded in detail on the special table.

2) When laying, it should be carried out in dry and warm weather as much as possible; try to minimize the seams, and save raw materials as much as possible under the premise of ensuring quality; when laying on flat ground, use tractor or manual spreading, and use geomembrane on the slope. The unfolding should start from the bottom and extend to the high position, and use the winch to lay.

3) The lap width between the film and the film is 100mm±20mm (depending on the welding machine used). In corners and deformed sections, the length of joints should be shortened as much as possible. Except for special requirements, on slopes with a slope greater than 1:6, there should be no horizontal angle less than 45° within 1.5 m from the top of the slope or the stress concentration area. of welds.

4.3 Geomembrane welding

1) After the geomembrane is unfolded, it should be welded in time. The double-track thermal opportunity is used for welding. The welding process is: adjust the pressure → set the temperature → set the speed → weld lap inspection → film loading machine → start the motor → pressure welding.

2) Before the formal welding, a 1.0 m×0.3 m sample must be tested on site. When the sample is welded, the lap width is not less than 10cm, and the peeling and shearing test is carried out on site with a tensile machine. After the sample is qualified, it is easy to use the adjusted speed, pressure and temperature for formal welding. The date and time are indicated on the sample. , ambient temperature.

3) Before welding, the sand, soil and other dirt on the membrane surface must be removed to ensure that the membrane surface is clean, and the joint between the membrane and the membrane can be laid flat before welding.

4) The adjacent geomembrane welds should be staggered and overlapped as much as possible. The joints formed between the membrane blocks should be T-shaped, not cross-shaped, and the intersection of the vertical and horizontal welds should be strengthened by extrusion welders. When the length of the film is not enough, it needs to be spliced ​​in the long direction, and the horizontal seam should be welded first, and then the longitudinal seam should be welded.

5) During the welding construction, 3 to 4 people are required to cooperate, 1 person holds the machine, 1 person cleans the surface to be welded, 1 person holds the power supply and observes the quality of the welding seam at the same time, marks the problem, and 1 person is behind repair.

6) T-shaped seam treatment: first weld the horizontal seam and then the longitudinal seam. When welding the vertical seam, the horizontal seam should be removed first, and then the heat welding machine should be used to weld here. In addition, the base material should be used to fill the scar for the T-joint, and the size of the scar can be 300mm×300mm. The right angle of the scar should be rounded.

7) Use an extrusion welding machine to repair and weld special nodes and local damage.

4.4 Inspection and repair of finished geomembrane

1) After the geomembrane is welded, visually inspect the appearance and weld quality of the geomembrane laid. Air pressure test is carried out for each weld of hot melt welding, vacuum test is carried out for each weld of extrusion welding, and a 1000mm×350mm sample is taken for each 1000m weld for strength test and destructive test.

2) After the inspection is completed, all the inflatable perforations made during the inspection should be filled by extrusion welding immediately. For the repaired welds, vacuum leak testing is performed, and for reworked welds, samples must be sampled for testing until they are qualified.

4.5 Anchorage at the end of geomembrane

1) The anchoring trench should be excavated at the position required by the design before the geomembrane is placed.

2) The geomembrane should be anchored in the surrounding natural environment. An anchoring trench of not less than 800mm×800mm shall be excavated at the edge of the off-slope not less than 800mm, the membrane shall be buried, and the earth shall be compacted.

3) After the HDPE geomembrane is laid, backfill the material with plain soil and compact it manually.

4.6 Geotextile laying

1) The laying of the geotextile shall be carried out after the acceptance of the geomembrane.

2) Before the geotextile is cut, its relevant dimensions should also be accurately measured, then cut according to the actual terrain, and numbered one by one, and the relevant information should be recorded on a special form. The laid geotextile must be dry and tidy, and the surface must not have oil stains, splashed chemicals or chemical spots that damage the geotextile.

3) When laying geotextiles, the stress requires minimum seams, and materials are saved as much as possible under the premise of ensuring quality. When the geotextile is sewn, the lap width is 75mm±15mm. The seams are arranged parallel to the maximum slope, i.e. by the direction of the slope. When laying, it should be laid from top to bottom, the lap direction should be adapted to local conditions, and the joint should be laid along the slope. The laying of the deformed section at the corner is the same as the laying of the geomembrane. Avoid damaging the geotextile or geomembrane when laying.

4.7 Geotextile connection

1) Each piece of geotextile must be connected on the day of laying. There should be no oil stains, debris, etc. at the joints.

2) If the length of the damaged part on the slope exceeds 10% of the length of the coil, the geotextile should be removed and replaced with another coil. On the base, if the length of the damaged part exceeds 10% of the length of the coil, the geotextile should be cut off and repaired by sewing with the same material.

3) When connecting the geotextiles, use sutures of the same quality as the geotextiles, and the sutures should be made of materials with stronger resistance to chemical damage and ultraviolet light irradiation.

4) The sewing of the fabric adopts a portable sewing machine. 3 people are also required to cooperate during construction, 1 person holds the machine, 1 person arranges the surface to be sewn, and 1 person holds the power supply. When sewing, the stitch distance is about 6mm, and the connection surface should be moderately tight and naturally smooth to ensure that the membrane and the fabric are jointly stressed.

5) One end of the geotextile must be anchored on the slope, and then the coiled material is lowered along the slope to ensure that the geotextile is kept in a taut state. All geotextiles must be pressed with sandbags. The sandbags will be used during laying Reserve to lay the top layer of material.

6) After the geotextile is laid, it should be permanently anchored in time according to the anchorage requirements of the geomembrane.

5 Quality Control

5.1 Quality Standards

1) The construction of geomembrane and geotextile is strictly in accordance with GB5029-98 Technical Specification for Application of Geosynthetic Materials, CJJ 113-2007 Engineering Specification for Anti-seepage System of Domestic Waste Sanitary Landfill and SL/T231-98 Polyethylene (PE) Geomembrane Technical specifications for anti-seepage engineering.

2) Geomembrane quality inspection and inspection before entering the site. Including sampling inspection and on-site appearance inspection, 10,000 square meters are regarded as one batch, less than 10,000 square meters are regarded as one batch, and 3 rolls are randomly selected from each batch of products. Before the on-site welding of the geomembrane, the appearance quality is checked. It mainly depends on whether there is a melting point or leakage point on the membrane surface, whether the joints of the manufacturer are firm, whether the cloth is uniform, and whether the edges are smooth and wrinkle-free. use.

3) The welding seam of the geomembrane should be neat and beautiful, and there should be no slip welding or jump welding. After the construction of geotextile stitching is completed every day, the construction team leader must check the number of needles to avoid damage to the geotextile.

4) Welding shall not be carried out during rain or snow, or when there is moisture or dew in the joints, or when there is a strong wind of level 4 or above, and construction shall not be carried out when the temperature is lower than 5°C.

5) Construction personnel should wear soft-soled shoes during laying, so as not to damage the geomembrane, and pay close attention to fire prevention; artificial folds should be avoided. After laying in winter, they should be tightened and flattened as much as possible, and moderately relaxed during laying in summer.

6) After the geomembrane is laid, walking on the geomembrane, handling tools, etc. should be minimized. Any objects that can cause harm to the HDPE membrane should not be placed on the membrane or carried on the membrane to avoid accidental damage to the membrane. .

7) The backfill soil shall not contain silt, tree roots, humus, stones larger than 75mm in diameter, garbage or other sundries, and its soil quality and water content must comply with the design requirements and construction specifications.

8) It is strictly forbidden to disturb the soil at the bottom of the ditch during the excavation of the anchoring ditch: the ditch bottom is flat, there is no loosening of soil, and the slope of the side slope shall be specified in the design and construction technical regulations.

6 Benefit Analysis

1) Economic benefit analysis: If artificially compacted clay is used as the anti-seepage layer, the comprehensive cost will reach about 80 yuan/㎡~90 yuan/㎡; if the HDPE membrane anti-seepage technology is used, the comprehensive cost will be about 45 yuan/㎡. The two are more economical to use HDPE film.

2) Social benefits: The construction method is simple, the construction period is short, the project can be put into use as soon as possible, and the social benefits are improved. HDPE membranes are used in environmental protection and energy saving projects such as landfills and sewage treatment plants, making environmental protection and the quality of environmental protection and energy saving technology to a new level.

3) Technical benefit analysis: HDPE geomembrane construction technology is one of the ten new technologies, and the application of this project has laid a foundation for the further promotion and application of this technology.