In undertaking drainage design for soil-based cover systems, it’s vital to consider not just the thickness of the soil cover but also the soil cover permeability and drainage layer transmissivity.
Drainage layer transmissivity is related to water flow through cover soil due to rainfall precipitation less surface runoff, soil water storage, and to a lesser extent, evapotranspiration (surface evaporation and transpiration through plant media). When the cover soil reaches full saturation, infiltration through the soil is essentially equivalent to the soil saturated permeability.
However, precipitation expressed as a daily value in the HELP Model isn’t appropriate for use in transmissivity calculations. Separate analysis done for drainage design utilizing 100-year storm rainfall Intensity Duration Frequency curves with durations measured in 5- to 30-minute intervals should be used for subsurface drainage layer design as well. This is important because those short duration intervals in intense storms can cause significant damage in saturated soils.
The water flowing through the cover soil across a slope is found through an equation in which the permeability of the cover soil is multiplied by the length of the slope. For hydraulic continuity, the water following through the soil cover must equal the water that flows out of the drainage layer. That required flow is calculated by a formula that takes into account the slope gradient and the required drainage layer transmissivity, allowing for capacity loss due to geotextile intrusion into the geonet, biological and chemical clogging, and geonet “creep.”
In selecting drainage media for final cover solutions during preliminary design, the required design transmissivity can be qualitatively determined by selecting the index transmissivity (found on all data sheets) and dividing it by reduction factors for the aforementioned geotextile infiltration, geonet creep, biological and chemical clogging along with a factor of safety. For final design, the allowable transmissivity of the selected geocomposites can be quantitatively determined using transmissivity and creep test data.
The drainage media for cap systems offered by Agru America include conventional geocomposites, but feature the Integrated Drainage System. Agru’s Geocomposite is designed to replace one to two feet of traditional aggregate while providing drainage under site loads, gradients and site conditions. It is available in both single-sided and double-sided products with geonet core thicknesses from 200 mil to 300 mil and fabric ranging from 4 oz. to 12 oz.
The geotextile is bonded to the geonet with a hot knife application, allowing for high bond strength without the reduction of transmissivity values of other processes.
Integrated Drainage System (IDS) – Industry’s New Capping Standard
Agru’s HDPE and LLDPE IDS eliminates the need for a separate geonet or geocomposite drainage, which can mean significant cost savings and greater slope stability. With over 85 MSF installed across the US, this proven system provides unmatched interface shear strengths and transmissivity values in excess of 300 mil geocomposite material, while also providing cost savings when compared to most traditional capping applications.
