To address its need for additional landfill space, the Hong Kong Environmental Protection Department contracted for a 35-million-metric ton, 61-hectare (151-acre) expansion of an existing waste-disposal facility in the North East New Territories (NENT).
The NENT Landfill is one of three municipal and construction waste landfills serving Hong Kong. Plans for its expansion included accepting nearly 3500 metric tons per day of waste (400 truckloads) for 16 years. During its lifetime, the authorities would strictly observe continuous monitoring of leachate, landfill gas, groundwater quality, surface water, noise, and other operating parameters. At closing, waste height would reach 140 meters (460 feet). Thereafter the facility would be maintained for 30 years.
The Hong Kong Environmental Protection Department awarded the design, construction, operation, restoration, and aftercare to Far East Landfill Technologies, Ltd. (Hong Kong). Excavation activities began in mid-1996.
Standard practice in most areas of the world requires that solid- or hazardous-waste landfills, lagoons, and reservoirs use geomembranes to contain liquids. These very-low-permeability liners have demonstrated excellent performance. However, they are susceptible to damage from the angular drainage stone that commonly cover them. Heavyweight nonwoven geotextiles, such as Geotex 3201, can serve as a cushioning barrier to protect the geomembrane against puncture, impact, abrasion, and the tear damage during installation and throughout the life of the facility.
The first step in the NENT expansion was to install a 6-mm (240-mil) geosynthetic clay liner (GCL) above the groundwater drainage layer. Following that, the Serrot Corporation installed a 2-mm (80-mil)-thick, high-density polyethylene (HDPE) geomembrane. The designers had estimated that the 140-meter (460-feet) height of waste would pressure the geomembrane with 1,500 kPa (217 lb/sq in.).
The engineers had also factored in construction loading, soil gradation, maximum particle size, landfill geometry, and cost. From their calculations, they predicted that a 1200-g/m2 (35-oz/sq yd) geotextile would provide sufficient security against geomembrane damage. Serrot laid 95,000 sq yd (79,420 m2) of geotextile cushion, which was unrolled on the HDPE liner, overlapped, and sewn with heavy-duty polypropylene thread. Subsequent installation of a 500-mm (20-in.) granular leachate collection layer, consisting of drainage stone, produced no damage to the geomembrane.
Click here to view the cross section of landfill design showing geotextile with respect to other substrates Courtesy Synthetic Industries.
Synthetic Industries, the geotextile’s manufacturer, had provided extensive manufacturing quality control (MQC) test results for its product. Nevertheless, GeoSynthetic Consultants (Australia) took construction quality assurance (CQA) samples prior to the textile’s installation. These would serve for various index and performance tests by U.S.-based Advanced Terra Testing. The tests included a geo-static deformation that simulated the weight of the final landfill height on the geomembrane. The tests showed that Geotex 3201 exceeded deformation-test requirements while enhancing the HDPE liner’s puncture resistance.
The Geotex 320, besides performing well during installation, allowed for additional landfill capacity compared with thick protective soil layers. This proved that ultra-heavyweight nonwoven geotextiles are an economical alternative in waste-containment facilities.
This case study is adapted from one produced by Synthetic Industries, Geosynthetic Products Division, 4019 Industry Drive, Chattanooga, TN 37416. Tel: 423-899-0444, 800-621-0444; Fax: 423-899-7619.