--- title: "What Is a Geocomposites Drainage Layer and How Is It Sized?" date: 2023-11-30 author: "Pawan Khatri" url: https://oceangeosynthetics.com/what-is-a-geocomposites-drainage-layer/ --- November 30, 2023 | [Geocomposites](https://oceangeosynthetics.com/category/geocomposites/), [Insights](https://oceangeosynthetics.com/category/insights/) # What Is a Geocomposites Drainage Layer and How Is It Sized? A geocomposites drainage layer is a thin sheet that carries water, leachate or gas sideways along its own plane. In the common form, a ribbed HDPE geonet core is bonded on one or both faces to a nonwoven filter geotextile. The geotextile lets liquid in and keeps soil out; the core provides open channels for the liquid to run to an outlet. A sheet 5–8 mm thick can stand in for a granular blanket many times its thickness, which saves aggregate, haulage and space. The catch is that its flow capacity falls under load and over time, so it has to be sized for the pressure and conditions it will see, not for the number on the first line of a datasheet. ## Where a geocomposites drainage layer is used The same product shape turns up in very different structures: - **Landfill base liners.** Above the geomembrane, collecting leachate and carrying it to the sump. Between two liners, as a leak detection layer. - **Landfill caps.** Above the barrier layer, draining rainwater that soaks through the cover soil so it does not build pressure on the slope. - **Behind retaining walls and abutments.** Relieving water pressure on the back of the wall in place of a graded stone filter. - **Highway and railway formations.** Draining the formation and cutting off capillary rise from a wet subgrade. - **Under pond liners.** Venting gas and draining groundwater that would otherwise lift the sheet. Our article on [geomembranes for landfills and ponds](https://oceangeosynthetics.com/geomembranes-for-landfills-and-ponds/) explains why that matters. - **Basements, tunnels and roof gardens.** Draining against a structure, though here a [dimple drain board](https://oceangeosynthetics.com/products/dimple-drain-board/) is often the alternative. A bare [geonet](https://oceangeosynthetics.com/products/ocean-geonet/) is not a geocomposite. It becomes one once a geotextile is bonded to it, and that bonded filter is what lets it sit directly against soil. ## Transmissivity: the number that sizes the layer Water flowing through a geotextile (across it) is described by permittivity. Water flowing along a drainage layer is described by transmissivity, θ, which is the in-plane hydraulic conductivity multiplied by the thickness. Laboratories report it in two ways: - **Transmissivity** in m²/s, from ASTM D4716. - **In-plane flow rate** in litres per second per metre width at a stated hydraulic gradient and normal stress, from EN ISO 12958. Both depend heavily on three test conditions, and a quote is only comparable to another when all three match: 1. **Normal stress.** The core thins under load, and flow drops with it. Ocean’s data sheets show this: the 5.5 mm MoRTH grade gives 0.55 L/m·s at 100 kPa and 0.45 L/m·s at 200 kPa, both at a gradient of 1. 2. **Hydraulic gradient.** A vertical wall drain works at a gradient close to 1. A landfill base sloping at 2% works at about 0.02, and flow capacity at that gradient is far lower. 3. **Boundary materials.** Soil or a geomembrane pressing on the filter pushes it into the core channels. A test between steel plates hides this loss, so ask for results with the actual soil or sheet against each face. ## From lab flow to allowable flow A short laboratory test measures a fresh sample over a few hours. In service, the layer carries load for decades, the geotextile slowly creeps into the core, and minerals or bacterial slime can build up in the channels. GRI-GC8, the Geosynthetic Institute’s method for drainage geocomposites, turns the lab value into an allowable value by dividing it by reduction factors: | Reduction factor | What it covers | | --- | --- | | Intrusion | Geotextile pressed into the core channels by the adjacent soil or sheet | | Creep | Long-term thinning of the core under sustained load, from compressive creep tests | | Chemical clogging | Precipitates such as calcium carbonate or iron deposits from leachate or groundwater | | Biological clogging | Bacterial growth, most severe in landfill leachate | The designer then applies an overall factor of safety on top. Leachate collection layers get the largest clogging factors because landfill leachate carries both minerals and bacteria; clean groundwater behind a wall gets much smaller ones. ### Worked example (illustrative) The numbers below are illustrative assumptions, not design values for any project. A cantilever wall retains 5 m of silty sand. The designer estimates seepage reaching the back of the wall at 0.02 litres per second per metre run. The drain runs vertically, so the gradient is about 1, and the lateral earth pressure at the base is under 100 kPa. - Lab flow for the 5.5 mm grade at 100 kPa, gradient 1: 0.55 L/m·s. - Assumed reduction factors for clean groundwater: intrusion 1.2, creep 1.4, chemical clogging 1.2, biological clogging 1.3. Product: about 2.62. - Allowable flow: 0.55 ÷ 2.62 ≈ 0.21 L/m·s. - Factor of safety against the design inflow: 0.21 ÷ 0.02 ≈ 10. That margin is comfortable. Repeat the same arithmetic for a landfill base at a 2% gradient with leachate clogging factors, using flow data measured at that low gradient and the real waste load, and the answer can be a thicker core or a granular layer instead. ### Comparing with a granular layer To compare a geocomposite with a gravel or sand blanket, convert the blanket to transmissivity: its hydraulic conductivity times its thickness. For example, a 150 mm sand layer with an assumed conductivity of 1 × 10⁻⁴ m/s has a transmissivity of 1.5 × 10⁻⁵ m²/s, or 0.015 L/m·s at a gradient of 1. A geocomposite replaces it only if its long-term allowable value, at the same stress and gradient, is at least as high. On a regulated landfill, the equivalence also has to be accepted by the pollution control board. ## Other checks that decide the choice - **Filter compatibility.** The geotextile’s apparent opening size (ASTM D4751) must hold back the adjacent soil while letting water pass. See [does water go through geotextile fabric](https://oceangeosynthetics.com/does-water-go-through-geotextile-fabric/) for how retention and permeability are matched. - **Interface shear on slopes.** On landfill side slopes and caps, the geocomposite sits between a geomembrane and cover soil. Its friction with each, measured to ASTM D5321, often controls slope stability more than the flow does. - **Ply adhesion.** The bond between core and geotextile must survive handling and shear on the slope. - **Chemical resistance.** HDPE cores and PP filters suit most leachates and groundwater, but aggressive industrial effluent should be checked against the polymers. ## Ocean drainage geocomposite grades Ocean’s [geocomposite materials](https://oceangeosynthetics.com/products/geo-composites/) are a geonet core laminated on both faces with a nonwoven polypropylene filter. The product page lists two standard grades: a 7.2 mm railway geocomposite with its flow measured at 20 kPa, for formation drainage and capillary cut-off, and a 5.5 mm, 700 GSM geocomposite to MoRTH specifications, tested at 100 and 200 kPa for walls, abutments and deep fill. Project examples include a [drainage geocomposite 5.5 mm in an Odisha landfill](https://oceangeosynthetics.com/drainage-geocomposite-55-mm/), [700 GSM geocomposite on railway work in Gadchiroli](https://oceangeosynthetics.com/railway-infrastructure-maharashtra-700-gsm/) and the [BCGCL project at Jharsuguda](https://oceangeosynthetics.com/drainage-geocomposite-bcgcl-project-jharsuguda-odisha/). ## Installation points that protect the flow 1. Lay rolls with the core running in the direction of flow, downslope on slopes. 2. Overlap adjacent rolls so the filter geotextile flap of one covers the edge of the next, and tie the cores where the specification calls for it. 3. Fold or seal the geotextile over exposed edges so soil cannot enter the core. 4. Connect the layer to a real outlet: a perforated pipe, a sump or weep holes. A drain with no exit fills and stops working. 5. Place cover soil from the bottom of the slope upwards, and keep tracked plant off the sheet until enough cover is in place. ## Frequently asked questions ### What is the difference between a geonet and a drainage geocomposite? A geonet is the bare ribbed HDPE drainage core. A drainage geocomposite is that core with a filter geotextile bonded to one or both faces, so it can be placed directly against soil without the core clogging. ### Which test standard measures geocomposite flow? ASTM D4716 reports transmissivity, and EN ISO 12958 reports in-plane flow rate in litres per second per metre width. Both must state the normal stress, hydraulic gradient and boundary materials used. ### Can a drainage geocomposite replace a gravel layer in a landfill? It can where its long-term allowable flow, after reduction factors for intrusion, creep and clogging, at least matches the granular layer at the same stress and gradient, and the regulator accepts the equivalence. ### Which Ocean grade suits a retaining wall? The 5.5 mm, 700 GSM MoRTH geocomposite, whose flow is measured at 100 kPa and 200 kPa. Check the required flow and pressure against the project specification before ordering. ## Sizing a drainage layer for your project? Send us the application, design inflow, normal stress and slope, and we will check them against our grades. [Contact Ocean Non Wovens](https://oceangeosynthetics.com/contact/) to discuss your drainage layer. 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