Expansion and construction of anti-seepage barrier system for YPEP geomembrane landfill site

1. Project overview

This project involves the expansion and construction of an impermeable barrier system at a municipal waste landfill in Liuyuan County, Shaoguan City, Guangdong Province. The landfill, located on a hilly area, has a design total capacity of about 1.5 million cubic meters, which allows it to be classified as a medium-sized landfill. The geological conditions of the site are characterized by typical southern red soils and weathered rock layers, and groundwater levels fluctuate significantly depending on the season, influenced by rainfall patterns.

The main task and goal of the project is to create an absolutely reliable engineering impenetrable barrier. This barrier should completely isolate toxic and hazardous leachates resulting from waste decomposition from the surrounding soil and groundwater, preventing secondary pollution of the regional ecological environment, particularly water sources in the Pearl River Basin. After extensive evaluation, a single-layer, impermeable, 1.5mm high-density polyethylene (HDPE) geomembrane roofing system with a smooth surface was selected for the project. It is complemented by a non-woven geotextile protective layer and a gravel drainage layer to form a complete, integrated solution for imperviousness, drainage and protection.

2. Design solution

The anti-seepage protection system is developed based on the principle of “multi-layer protection and fail-safe reliability”, the core of which is a sealed anti-seepage layer formed from a high-density polyethylene (HDPE) geomembrane.

2.1 Selection of materials and performance requirements

Main waterproofing layer:
The main waterproofing material is a high-quality smooth HDPE geomembrane with a thickness of 1.5 mm. This choice is due to its outstanding performance characteristics: firstly, the coefficient of water permeability of the material is less than 1×10⁻¹² cm/s, which provides a significantly higher degree of waterproofing compared to traditional clay screens; secondly, HDPE has excellent chemical resistance and is able to withstand the effects of complex acid-base components and heavy metal ions contained in the filtrate for a long time; thirdly, high tensile strength and elongation at break allow the material to adapt to possible uneven settlement of the landfill base, preventing rupture due to excessive local deformation; Finally, the material has excellent resistance to environmental and stress cracking, ensuring that structural integrity is maintained under long-term stress.

Protective layers:
Needle-punched non-woven geotextile made of polyester fiber with a density of 600 g/m² is laid on top and bottom of the geomembrane. The bottom geotextile is placed on a leveled clay base and acts as a shock-absorbing and protective layer, preventing sharp objects from puncturing the membrane. The upper geotextile covers the geomembrane, protecting the main impervious layer from mechanical damage during the subsequent laying of the gravel drainage layer.

Support and drainage layer:
The base consists of a carefully compacted clay layer (compaction rate ≥95%) to provide stable support. A 30 cm thick layer of crushed stone of a fractionated composition is laid over the protective geotextile of the membrane, which serves as the main channel for collecting and discharging filtrate.

2.2 Design solution

The anti-filtration protection system is formed from bottom to top in the following order:compacted clay base → bottom non-woven geotextile (shock-absorbing protective layer) → 1.5mm thick HDPE geomembrane (main anti-seepage layer) → top non-woven geotextile (protective layer) → gravel drainage layer.

In areas with slopes, a composite anti-seepage structure concept is used, which ensures the anti-slip stability of the geomembrane on inclined surfaces.

3. Installation and construction

Construction quality is critical to the effectiveness of the impermeable system. This project strictly adheres to a standardized "preparation-lay-welding-inspection-protection" process, with on-site construction supervision at key stages.

3.1 Preparation before construction

Preparing the base:
This stage is the foundation for all subsequent work. The base of the pit and slope surfaces must be leveled, compacted and cleared of any sharp objects such as tree roots, construction debris, stones or protruding ends of reinforcement. Internal and external corners of the base should be smoothed to rounded arcs (radius > 50 cm) to prevent stress concentration. Laying of the membrane can begin only after joint acceptance of the base by representatives of the civil construction contractor, technical supervision and the customer with the execution of a signed certificate of compliance.

Weather conditions:
Welding work must be carried out at temperatures above 5°C, wind speeds of no more than 4 on the Beaufort scale and in the absence of rain or snow. In extremely low temperature conditions, welding machines must be preheated. At high temperatures, the membrane should be laid with a small margin (sagging) to compensate for thermal expansion.

3.2 Laying and welding of HDPE geomembrane

Styling:
Before cutting, it is necessary to accurately measure and number each section of the membrane. When laying, the number of welds should be minimized and placed parallel to the steepest slopes of the site (i.e. from top to bottom) to facilitate joint control and reduce stress. During installation, the membrane should be manually tensioned and smoothed to prevent wrinkles, and then immediately secured with sandbags to prevent wind movement.

Overlapping and welding:
Connections between the membranes are made using the lap joint method, and the width of the overlap must strictly be at least 10 cm. Welding is performed with a two-track hot wedge welding machine, ensuring continuity of waterproofing. Before daily official welding begins, a test weld of a 0.9 m x 0.3 m sample must be performed on site and pull-out and shear tests must be carried out. Only after successfully passing these tests is it possible to continue welding using the established parameters (temperature, pressure, speed). Particular attention should be paid to the cleanliness of the seam area: the presence of dust, oil, moisture or silt is unacceptable. All T-joints and cross-joints must be reinforced with an extrusion welding gun to ensure a perfect seal.

3.3 Quality control of welds

All welds are subject to 100% inspection using a three-stage method: visual inspection, pneumatic pressure testing and destructive testing.

Visual inspection:
It is checked that the seams are uniform, smooth, without under-welds or signs of over-burning.

Pressure test:
Compressed air at a pressure of 0.15–0.2 MPa is supplied to the cavity formed by the double-track weld. The weld is considered to have passed the test if the pressure remains stable for 5 minutes or decreases by no more than 20%.

Destructive testing:
One weld sample is randomly selected for every 500–1,000 meters of welding. Tensile and shear tests are conducted on-site. The test is considered successful if failure occurs outside the weld line.

3.4 Anchoring and coating

Along the perimeter, the membrane is fixed in pre-dug anchor trenches, which are then filled with clay with layer-by-layer compaction. Once all welding and inspections have been completed, the top protective geotextile and gravel drainage layer must be immediately installed in accordance with the design sequence. When laying gravel, light equipment should be used with a reverse dumping method. It is strictly prohibited to move or turn equipment directly over an unprotected geomembrane.

4. Summary of the key benefits of HDPE geomembranes

Compared to traditional barrier materials (such as compacted clay) and other types of geomembranes, HDPE geomembranes demonstrate irreplaceable comprehensive benefits in modern environmental projects represented by this initiative.