1 Main construction methods of HDPE impermeable membrane

1.1 Construction process

Before laying the HDPE impermeable membrane, first draw the laying and welding layout diagram of the HDPE impermeable membrane. The process is: plan according to the actual terrain size of the site → cut the membrane and transport it to the corresponding position → press the operation Program laying, welding → apply for acceptance after passing the self-inspection, and prepare for the next process.

1.2 Principle of electrofusion welding

Using the principle of electrothermal melting, the temperature change is accurately controlled. When the electrothermal stick moves forward, the two axles and wheels move forward together to press the two membranes together.

1.2.2 Key points of electrofusion welding

After turning on the machine, carefully observe the temperature rise displayed by the indicator, so that the equipment is fully preheated to about 190°C. When inserting the film into the welding machine, the lap size should be prepared and the action should be quick. During welding, the welding personnel should pay close attention to the condition of the welding seam. When adjusting special faults in time, they should stop the machine in time to avoid scalding the membrane. Auxiliary personnel must also always monitor whether the power supply voltage of the welding machine is within (220±11)V, otherwise, it should be stopped immediately for maintenance.

2 Construction methods for special parts

The equipment foundation of chemical equipment has the characteristics of large volume, large quantity, deep foundation and complex underground pipe network, which brings great difficulties to the laying of anti-seepage membrane. For special parts, corresponding measures should be taken according to local conditions.

2.1 Treatment of the slope foot of the pipe channel

When the film is applied at the intersection of the slope toe, bend and field bottom slope of the pipe canal slope, the foundation is required to be smooth and smooth during the bend, and no negative slope can appear. When laying the membrane, try to relax as much as possible, so that the membrane cannot be suspended.

The key points of construction in this case: in irregular areas such as corners, it should be cut into an "inverted trapezoid" with a small upper width and a lower lower. If the ratio is not accurately grasped, it will cause the membrane surface on the slope to "bulge" or "hang in the air".

2.2 Construction method of metal pipe side wall through membrane

Construction points: First, anchor the membrane on the slope around the side wall through the membrane metal pipe to the slope with shooting nails, put a 1mm thick HDPE anti-seepage membrane pad under the shooting nails, and press the membrane cover in advance (with 1mm thick HDPE anti-seepage membrane). ). Then, according to the size of the metal pipe, cover the film-penetrating part---the length is greater than 200mm and the thickness is 2.0mm HDPE film sleeve, and then according to the film size of the film sleeve and the anchored film around the metal pipe, a circular tube shoe is made and welded by extrusion. The inner and outer edges of the tube shoe are welded to the slope membrane and the metal pipe membrane respectively, and finally the exposed part of the membrane sleeve is fixed and compacted with galvanized steel wire.

2.3 Fixation of the anti-seepage membrane on the slope top of the pipe channel

The anti-seepage material of the underground pipe network canal and the plain concrete on both sides of the canal are anchored by flat steel strips of 4mm × 40mm × 120mm, and anchored by expansion bolts, and the interval of expansion bolts is 400mm. In order to ensure that the expansion bolts are not corroded, a 1.0mm thick HDPE impermeable membrane is used for extrusion welding and sealing.

3 Welding quality testing methods

There are two kinds of welding quality inspection for HDPE impermeable membrane: non-destructive inspection (leak detection test) and destructive inspection. The double weld formed by hot melt welding adopts the inflating method, and the single weld formed by extrusion welding adopts the electric spark method to detect the leakage.

3.1 Inflatable method detection

Seal both ends of the weld cavity of the impermeable membrane to be tested. Insert the needle into one end of the cavity after the detection device is connected. Start the air pump, input the high-pressure gas to reach 0.15~0.2MPa, and stop the pressurization. There is no significant drop in pressure within 1 to 5 minutes, and the other end is opened, and the gas in the cavity disappears, indicating that the entire weld has passed the test. If the pressure gauge drops significantly within 1 to 5 minutes, it proves that the weld needs to be repaired or reworked.

3.2

Using the characteristics of HDPE impermeable membrane as an electrical insulator, when the instrument scans the parts with holes or holes connected to the ground, bright sparks are generated.

Testing procedure: Embed fine copper wire before extrusion welding. When testing, connect to the power supply, and use the detector to scan at a height of about 30mm from the weld. If there is no spark, the welded seam will be welded, and if there is a spark, it means that there is a loophole in this part, and it needs to be reworked and re-tested.

3.3 Destructive detection

Whether it is hot-melt welded double weld or extrusion welded single weld, the strength inspection of the weld, including shear strength and peel strength, must be carried out. The strength inspection is carried out according to the frequency of testing a group of welds (at least five samples per group) every 1000 meters, and is designated by the owner or supervisor or randomly sampled for testing. Doubtful locations without prior notice to the impermeable membrane installation contractor.

4 Conclusion

HDPE anti-seepage membrane buried in the ground is not easy to be damaged, long life, good anti-seepage effect. At present, it is gradually applied in petrochemical projects. Although the on-site construction is complicated and the initial investment is relatively large, it has a significant effect on environmental protection and is worthy of promotion.