Without the use of 3D geocell technology, soil and aggregate are commonly used over geomembranes on slopes of less than 3H:1V. When slope gradients are greater than 3H:1V, soil and aggregate covers need to be stabilized with a geocell system to avoid soil movement and potential failure. In arid areas, cover depth may range from 3 to 6 in. In conditions that support vegetation and cover, depth may range from 6 to 24 in. or greater where the final depth is a function of the vegetation’s characteristics. Geocells are available in depths from 3 to 8 in., allowing for sufficient coverage to protect the geomembrane from UV degradation, punctures, water seepage and water infiltration.
Regardless of cover depth, if an extreme rainfall event occurs that is 10%, 20% or 30% greater than expected, the soil mass increases, assumed friction angles decrease, and the factors of safety for soil stability drop to a point where failure of the cover occurs and exposure of, or damage, to the geomembrane results.
Geocell technology prevents the cover material from sliding, even under heavy rain or snow loads, and reduces the amount of lifetime maintenance required to protect the geomembrane liner.
Landfills are a common structure requiring a geomembrane. A residue disposal area at a pulp paper mill in Bahia, Brazil, suffered a failure of the existing unconfined soil cover due to heavy rains. The area was repaired with the Presto Geoweb 3D soil stabilization system. The 3D structure confines fill material in its network of interconnected cells, keeping the fill stable and resistant to movement.
For many years, the mill dumped eucalyptus tree trunks and bark, lime, contaminated soils and other waste on the site creating a large pile of environmentally damaging materials. Pressure from regulatory agencies, as well as obvious environmental issues, motivated the mill to take action.
A solution then was developed considering the four objectives and the future need to use the dumpsite for additional waste.
The remediation work started with the re-grading of the residue to a stable slope of 3H:1V. This grade also guaranteed the stability of the residue and allowed for the placement of additional waste on top of the existing residue.
After the earthwork was completed a few months later, confinement work began. First, a gas collection system was installed. Next, a 1.5 mm (60 mil) thick, textured, high-density polyethylene (HDPE) geomembrane was installed. A 6 oz/sq yd non-woven geotextile was installed over the geomembrane as a protection layer and subsequently covered with 24 in. of soil. Within the soil cover, a subsurface drainage system was installed to assist with removing infiltrating rainwater. The drainage system used 4-in. diameter perforated PVC pipe installed directly over the geotextile 33 ft on center, allowing the groundwater to pass through the pipe collection system and discharge into a surface channel at the toe of the slope.
In this region of Brazil, the rainy season starts in April and lasts through September. Due to climate change, heavy rains may also occur in other months. Only six months later after a heavy rainfall, the soil cover on the 3H:1V slope failed at the geotextile-geomembrane interface, and a large volume of soil slid down the slope from crest to toe. Furthermore, at some locations along the slope crest, the geotextile tore due to the sliding soil, exposing the geomembrane.
Factors in the failure included a relatively low interface friction angle between the geotextile and the textured geomembrane, increased load from the saturated soil and seepage forces due to water flow within the relatively thick soil cover layer. The failure involved 12,900 cu ft of cover soil and 19,400 cu ft of geotextile. Although the geomembrane was not yet affected by the tear in the geotextile, an urgent repair was needed to prevent damage.
After a complete analysis of the failure, several alternatives were considered for the repair. However, use of the Geoweb slope cover system best addressed all critical details. The following parameters were considered critical for the redesign of the soil cover system:
The new cover consisted of two layers to make up the 24 in. cover, a 3-in. sand-infilled, perforated Geoweb drainage layer installed directly over the textured HDPE geomembrane, topped with 21 in. of soil. In areas where the original geotextile had not failed, the Geoweb sections were installed directly over that geotextile layer. The use of a sand drainage layer provided two benefits:
The Geoweb sections were structurally anchored at the slope crest using high-strength polyester tendons with a long-term design strength of 3,000 lbf. The required strength considered the fully covered and saturated soil mass over and in the Geoweb sections. For the 115-ft-long slopes, the analyses determined five polyester tendons per Geoweb section were required. Tendons were secured to a 4-in. diameter, solid wall, PVC pipe deadman buried 18 in. below the final grade at the crest of the slope. The tendon strength design factor-of-safety was 5.0, and it included partial factors of safety for creep, knots, constructions damage, chemical and biological durability and overall uncertainties. All present and potential sliding forces within the Geoweb system were transferred to the tendons by means of special load transfer ATRA clips at required downslope intervals. Proper design analyses to determine tendon and load transfer requirements, as well as geocell seam strength are precritical factors to the overall long-term performance of the cover system.
During construction, the tendoned and perforated Geoweb material provided a stabilizing “formwork” that prevented sand flow during installation during rainfall, as well as allowing for rapid drainage of the saturated sand infill. The tendoned Geoweb system is a unique way to create a structurally stable sand drainage layer directly over a geomembrane.
Construction work took place over a three-month period. The system has performed exceptionally well through multiple rainy seasons. Technological advancements provide improved methodologies and materials so sustainable, long-term solutions can be developed for some of the most challenging site situations. The Geoweb system proved to be a technically sound, low environmental impact solution with high cost-benefit for this project.
Bill Handlos, P.E. is director for Presto Geosystems. Handlos can be reached at [email protected] Sam Justice, P.E. is a civil design engineer at Presto Geosystems. She can be reached at sam. [email protected]
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The Storm Water Solutions staff invites industry professionals to nominate the water and wastewater projects they deem most remarkable and innovative for recognition in the Annual Reference Guide issue. All projects must have been in the design or construction phase over the last 18 months.
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