Economic pressure, the desire for green solutions, and intensification of climate extremes have converged to create a need for better methods to effect soil stabilization. Fortunately, a proven technology exists that addresses issues associated with these conditions and provides a more stable cover solution for landfill covers, lagoons, stormwater containment basins, and other geomembrane-covered systems. Soil, aggregate, and concrete protective covers over geomembranes can be secured against gravitational, hydrodynamic, and seismic forces using a geocellular confinement system.
Soil and aggregate are commonly used as a protective cover over geomembranes on slopes of 3H:1V or less. When slope gradients are greater than 3H:1V, soil and aggregate covers are typically unstable and not used. In arid areas, cover depth may range from 3 inches to 6 inches. In climatic conditions that support vegetation, cover depth may range from 6 inches to 24 inches or greater where the final depth is a function of the characteristics of the desired vegetation. Regardless of cover depth, if an extreme rainfall event occurs that is 10-, 20-, or 30-percent greater than what would typically be expected, soil mass increases, assumed friction angles decrease, and 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.
A residue disposal area at a pulp paper mill in Bahia , Brazil , illustrates failure of a typical unconfined soil cover exposed to heavy rains and its subsequent repair with the Presto Geoweb cellular confinement system. For many years, the mill dumped eucalyptus 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.
Stated objectives for the remediation included the following:
- prevent rain and wind dispersion of residues and contaminants,
- prevent infiltration of contaminants into the aquifer,
- prevent contaminated residues from coming in contact with personnel and animals, and
- develop an aesthetically pleasing landscape.
A solution was then developed considering the objectives as well as the future need to use the dumpsite for additional waste.
Remediation work started in late 2005 with grading of the residue to a stable slope of 3H:1V. This slope-grade also guaranteed the stability of the residue and allowed for placement of additional waste on top of the existing residue.
After the earthwork was completed, confinement work began in January 2006. First, a gas-collection system was installed. Next, a 60-mil-thick, textured, high-density polyethylene (HDPE) geomembrane was installed. A 6-ounce-per-square-yard (oz/yd 2 ) nonwoven geotextile was installed over the geomembrane as a protection layer and subsequently covered with 24 inches of soil. Within the soil cover, a subsurface drainage system was installed for removal of infiltrating rainwater. The drainage system used 4-inch-diameter, perforated PVC pipe installed directly over the geotextile, 33 feet on center, allowing the groundwater to pass through the pipe collection system and discharge into a surface channel at the toe of the slope.
Failure of the soil cover
In this region of Brazil , the rainy season starts in April and typically lasts to September. Heavy rain may also occur in other months. In mid-June 2006, 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 (Figure 1). Furthermore, at some locations along the slope-crest, the geotextile tore, exposing the geomembrane.
Contributing factors to the slope cover failure were a relatively low (however within acceptable norms) interface friction angle between the geotextile and the textured geomembrane; increased load from the saturated soil; and seepage forces caused by water flow within the relatively thick soil cover layer. The failure involved 12,900 square feet (ft 2 ) of cover soil and 19,400 ft 2 of geotextile. Although the geomembrane was not yet affected by the tear in the geotextile, a repair was urgently needed to prevent damage.
Cover repair with a geocellular system
After 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 (Figure 2):
- slope heights from 66 feet to 115 feet,
- a 3H:1V (18.4-degree) slope angle,
- a critical interface friction angle between the sand and geomembrane of 15 degrees,
- a total slope soil-cover of 24 inches,
- a cover anchoring system to resist all potential loading,
- a self-sustainable vegetated cover,
- the need for a drainage system that would quickly drain extremely saturated soil, and
- long-term functional life.
The new cover consisted of a sand-infilled, perforated Geoweb drainage layer installed directly over the textured HDPE geomembrane (Figure 3). The cell depth of the Geoweb layer was 3 inches. 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:
- increased friction angle at the critical interface with the geomembrane; and
- provided a critical system drainage layer.
In some areas, because of the characteristics of the final 21-inch-deep topsoil layer, a 6 oz/yd 2 nonwoven geotextile filter/separation layer was placed over the sand-infilled Geoweb layer. In other areas, this geotextile layer was not used because the final soil cover had characteristics compatible with the sand drainage layer.
The Geoweb sections were structurally anchored at the slope crest using high-strength polyester tendons with a long-term design strength of 3,000 pound-force (lbf). The required strength considered the fully covered and saturated soil mass over and in the Geoweb sections. For the 115-foot-long slopes, the analyses determined that five polyester tendons per Geoweb section were required. Tendons were secured to a 4-inch-diameter pipe deadman buried 18 inches 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, construction 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 clips at required downslope intervals. Proper design analyses to determine tendon and load transfer requirements, as well as geocell seam strength, are critical 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.
Results of the cover repair
Construction work took place from October to December 2006. The system has been through the 2007 and 2008 rainy season and has performed exceptionally (Figure 4).
Daniel F Senf, P.E., CPESC , is past international business development manager, Presto Geosystems; Bill Handlos , P.E., CPESC , is director, Presto Geosystems; Mauricio Abramento, Ph.D. , is with CEG Engenharia; and Marcos Mello Rocha Campos, MSc , is with GEOKLOCK Consulting and Environmental Engineering.
This article is based on a paper , “Failure of a landfill cover and remediation using geocells,” presented at the First Pan American Geosynthetics Conference & Exhibition, March 2008, in Cancun, Mexico. The original paper was co-authored by Abramento; Campos ; Claudia Vasquez Bastias, Fiberweb Bidim; and Daniel Senf , P.E., CPESC, Presto Geosystems.