Geocomposites: Drainage Types, Testing, Design and Applications

Quick Answer

A geocomposite is a factory-made product that joins two or more materials, at least one of them a geosynthetic, so that each layer does one job. The main types are drainage geocomposites (an HDPE geonet, cuspated sheet or geospacer core with nonwoven geotextile filters on one or both faces), reinforcement composites (a geogrid bonded to a nonwoven), geosynthetic clay liners (GCLs) (bentonite between geotextiles or on a geomembrane) and paving interlayers for asphalt overlays. A drainage geocomposite is chosen on its in-plane flow capacity: the flow per metre width at a stated normal stress, hydraulic gradient and boundary condition, measured to ISO 12958 or ASTM D4716. Transmissivity is that flow divided by the gradient, and it is strictly valid only for laminar flow. Datasheet index values come from short tests between plates and are for comparison. Design uses a 100-hour performance test under site conditions, divided by reduction factors for geotextile intrusion, creep, and chemical and biological clogging (GRI-GC8, ASTM D7931). The result must exceed the required transmissivity, which is the inflow per metre width divided by the gradient, by a factor of safety of 2 to 3.

A typical drainage enquiry reads: "Drainage geocomposite, 6 mm, 2,000 m², best price." The buyer has compared datasheets on one number, the transmissivity at 20 kPa. Nothing says where the product goes: behind a retaining wall, under a metre of cover soil on a landfill cap, or beneath tens of metres of waste.

Those settings differ by more than an order of magnitude in normal stress, and the in-plane flow of the same product changes with stress, gradient, the soil or membrane pressing on it, and time. A drainage layer is specified from the flow it must carry in service, after the losses service imposes. This guide sets out how.

What a geocomposite is

A geocomposite combines materials so that each does what it does best. In a drainage composite the core carries water along its plane and holds its thickness under load, the filter keeps soil out of the core while letting water in, and on some products a film on the back face protects a wall or basement. Across the family, geocomposites drain, filter, separate, reinforce, contain liquids, protect geomembranes and relieve stress in asphalt overlays.

The case for a geocomposite over a granular layer is a thin, factory-made product with measured properties, quick to place and free of the site fines a gravel drain can pick up. That case is only as good as the numbers: the product must carry the design flow, under the design load, for the design life.

Types of geocomposites

GCLs. A US EPA fact sheet (EPA 530-F-97-002) describes a GCL as a thin layer of processed clay, typically bentonite, bonded to a geomembrane or held between two geotextiles. It notes that manufacturers typically use 1 lb/ft² (about 4.9 kg/m²) of bentonite, and that saturated bentonite typically has a hydraulic conductivity below 1 × 10⁻¹¹ m/s (1 × 10⁻⁹ cm/s). Needle-punched and stitch-bonded GCLs add internal shear resistance for slopes. The bentonite must stay dry until it is covered, and it needs enough cover soil to stay confined.

Paving interlayers. EN 15381 sets the characteristics of geotextiles and related products for pavements and asphalt overlays, covering the reinforcement, stress-relief and barrier functions. A paving fabric works only when it is saturated with the right quantity of bitumen tack coat, which is why EN 15381 includes a bitumen-retention test.

For separation and filtration fabrics on their own, see our guide to geotextiles; for reinforcement grids, our guide to geogrids.

How a drainage geocomposite carries water

Flow capacity and transmissivity

The property that matters is in-plane flow: the volume of water the product carries along its plane, per second, per metre of width, at a given normal stress and hydraulic gradient. ISO 12958-1 reports it in litres per second per metre.

Transmissivity (θ, in m²/s) is the in-plane flow per metre width divided by the hydraulic gradient. For a granular layer it equals permeability times thickness (θ = k × t). Two points follow from how cores behave:

• Transmissivity assumes laminar flow. Open cores such as geonets can carry non-laminar flow at higher gradients, so flow does not rise in proportion to gradient. ASTM D7931 notes that transmissivity decreases as the gradient increases, and ISO 12958-1 prefers the term "water flow capacity" for that reason. A value measured at a gradient of 1.0 does not scale down in a straight line to a gentle slope.
• The gradient comes from the geometry. Where water runs parallel to a slope, the gradient is the fall per metre of flow path: the sine of the slope angle. In a wall drain the water falls vertically and the gradient is 1.0.

What reduces flow in service

• Compression and creep of the core. Normal stress squeezes the core, and over time it creeps thinner. Columnar and cuspated cores can also fail in combined compression and shear, which ISO 25619-1 tests separately.
• Geotextile intrusion. Soil pressure pushes the filter into the core's channels and shrinks the flow area.
• Chemical and biological clogging. Minerals precipitate from leachate or hard groundwater, and biofilm grows in the core and filter.
• Filter clogging or blinding. Fines build up on or in a filter that does not suit the soil. One published set of checks asks for a geotextile 10 to 100 times more permeable than the soil, with an opening size no more than two to three times the soil's d85.

Testing: index versus performance

ISO 12958-1 (2020) introduced the split between index and performance tests and, with ISO 12958-2, replaced ISO 12958:2010. Its apparatus covers gradients of at least 0.1 and 1.0, with the specimen between foam rubber or smooth rigid plates. Foam presses into the filter much as soil does; rigid plates do not, so rigid-plate results overstate the flow against soil. ASTM D4716 seats the specimen for 15 minutes by default; ASTM D7931 asks for 100 hours at the design stress and slope gradient.

Designing a drainage layer

The design runs in four steps. Steps 2 and 3 are the allowable-flow method of GRI-GC8 and ASTM D7931; ASTM D7931 leaves the required flow and the factor of safety to the designer:

1. Required flow. Inflow per metre width (the rate arriving per square metre times the length draining to the outlet), then θreq = q ÷ i.
2. Performance flow. The 100-hour value, θ100, at the design stress and gradient, with the actual soils or membranes on each face.
3. Allowable flow. θallow = θ100 ÷ (RF_GI × RF_CR × RF_CC × RF_BC), the reduction factors for long-term geotextile intrusion beyond the 100-hour seating, creep, and chemical and biological clogging.
4. Factor of safety. FS = θallow ÷ θreq. ASTM D7931 sets no value; Giroud and co-workers recommend 2 to 3, and New York's landfill regulations require 3.

Default clogging factors published by Koerner and in GRI-GC8, as quoted in a geonet technical paper:

Creep depends on the core and the stress. One geonet study reports RF_CR of about 1.1 to 1.2 below 50 kPa, rising to nearly 2 above 700 kPa. Take the creep factor from the maker's creep data at your stress, not from a generic table. Where the 100-hour test already has the site soil on each face, some designers take RF_GI as 1.0.

Worked example: drainage layer on a landfill cap

A cap slope is 1V:3H and 50 m long. The geocomposite lies on the cap geomembrane under 1 m of cover soil. Assume:

• Inflow percolating through the cover soil: 1 × 10⁻⁷ m/s, about 8.6 mm a day. On a real project it comes from a water balance, or from the cover soil's permeability as an upper bound
• Normal stress: 1.0 m × 18 kN/m³ = 18 kPa
• Reduction factors: RF_GI 1.0, RF_CR 1.2 (low stress), RF_CC 1.2 and RF_BC 3.5 (the top of the cap ranges)
• Factor of safety: 2

The calculation:

• Slope angle: arctan(1/3) = 18.4°, so i = sin 18.4° = 0.316
• Inflow per metre width at the toe, conservatively over the full 50 m slope length: 1 × 10⁻⁷ × 50 = 5.0 × 10⁻⁶ m³/s per metre (0.005 l/s, or 18 l/h)
• Required transmissivity: 5.0 × 10⁻⁶ ÷ 0.316 = 1.58 × 10⁻⁵ m²/s
• With FS 2, the allowable transmissivity must be at least 3.16 × 10⁻⁵ m²/s
• Product of reduction factors: 1.0 × 1.2 × 1.2 × 3.5 = 5.04
• Required 100-hour performance transmissivity: 3.16 × 10⁻⁵ × 5.04 = 1.59 × 10⁻⁴ m²/s, a flow of 0.050 l/s per metre at a gradient of about 0.32 and 18–20 kPa, with cover soil above and geomembrane below

With FS 3 the requirement rises to 2.39 × 10⁻⁴ m²/s. The toe drain must also take the whole slope's flow: along a 150 m toe, 5.0 × 10⁻⁶ × 150 = 7.5 × 10⁻⁴ m³/s, or 0.75 l/s, has to reach the collector pipe and outlets.

What the example shows. A datasheet index value of 5 × 10⁻⁴ m²/s at 20 kPa looks about three times the requirement, but index values come from short tests between plates, which overstate flow against soil, and are often quoted at a gradient of 1.0 rather than the slope's 0.32. Ask for the 100-hour result under your boundaries before counting any margin. Biological clogging dominates here (3.5 of the 5.04 product); under a leachate collection layer, chemical clogging and creep take over.

Is it equivalent to gravel?

A granular drain's transmissivity is θ = k × t. A 300 mm layer with k = 1 × 10⁻³ m/s gives 3 × 10⁻⁴ m²/s; with k = 1 × 10⁻² m/s, 3 × 10⁻³ m²/s. A claim that a geocomposite "replaces" a given thickness of gravel means nothing without both numbers: the gravel's permeability and the composite's long-term flow at the design stress and gradient.

Applications and what governs each

• Landfill caps: drainage over the cap geomembrane stops pore pressure building in the cover soil and destabilising the slope; biological clogging and interface shear govern
• Landfill base liners and leak detection: several hundred kilopascals plus leachate chemistry and biology; creep and chemical clogging govern
• Retaining walls, abutments, basements and tunnels: sheet drains relieve water pressure, discharging at the base to a free outlet, normally a perforated pipe; see our guide to HDPE pipes
• Roads: fin drains along pavement edges, including jointed concrete pavements (see our dowel bars guide); reinforcement composites over soft subgrades; paving interlayers in overlays
• Green roofs, podium decks and sports fields: low stress, high flow, fine filters
• Water and wastewater sites: drainage under lagoon liners and tank bases at the plants in our guide to wastewater treatment equipment, discharging to pipes and manholes

Installation essentials

• Face and direction: filter to the soil, back film to the structure, and the main flow direction of a geonet down the slope; mark both on each roll
• Joints: shingle laps downslope, tie adjacent cores, and lap or sew the filter over the joint so soil cannot enter the core
• Cover promptly: limit sunlight exposure, keep plant off the exposed product, and push cover soil up slopes from the toe
• Outlets: a free, protected outlet sized for the total flow; a blocked toe turns a drain into a reservoir
• GCLs: keep them dry and place the cover soil during installation, as the maker specifies

Common Mistakes

1. Designing from the index value. Index flow is measured over minutes between plates; flow under soil, over years, can be several times lower.

   Prevention: Specify 100-hour performance flow at the design stress, gradient and boundaries, then apply reduction factors.

2. Testing at the wrong gradient. Flow in open cores does not rise in proportion to gradient, so a value at 1.0 misstates flow on a gentle slope.

   Prevention: Test at the slope's gradient (the sine of its angle), or bracket it.

3. Ignoring creep at high stress. Cores lose thickness under sustained load, and cuspated cores can buckle under compression and shear.

   Prevention: Use compressive creep data (ISO 25619-1) at the design stress, not short-term compressive strength.

4. Leaving out clogging in leachate. Mineral and biological growth can cut flow severalfold.

   Prevention: Apply the chemical and biological reduction factors for the application, and a factor of safety of 2 to 3 on top.

5. No outlet, or an undersized one. Water backs up and pore pressure rises behind the wall or in the cover soil.

   Prevention: Size the collector pipe and outlets for the full flow and protect them from blockage.

6. A filter that does not suit the soil. Large openings let soil into the core; a filter that is too tight blinds.

   Prevention: Check retention and permeability against the soil grading.

7. Forgetting shear on slopes. The weakest interface, whether soil to filter, filter to core, core to geomembrane or within a GCL, sets the slope's stability.

   Prevention: Specify interface shear and ply adhesion tests with the actual materials.

Ordering checklist

• Application: function, location, and what touches each face
• Design data: inflow, slope or wall geometry, normal stress, design life, fluid (water or leachate)
• Flow requirement: in-plane flow or transmissivity at a stated stress, gradient, boundary and seating time (100 hours for design)
• Core: type, thickness, polymer, and creep data at your stress
• Filters: one or both faces, mass, opening size and permeability matched to the soil, puncture and tensile strength
• Composite: ply adhesion, and interface shear data for slopes
• GCLs: bentonite type and mass per unit area, bonding method, flux
• Quantity: roll width and length plus laps; 150 mm side laps on 4 m rolls add about 4%, so 2,000 m² of cover needs about 2,080 m² before end laps and waste
• Documents: test certificates per lot, roll labels, and the declaration of performance and CE marking where a European application standard applies

Supply from Kasko Makine

Kasko Makine supplies geocomposites and the geosynthetics used with them:

• Geocomposite systems: drainage geocomposites (geonet or geospacer core with filters on both faces), gas venting composites, geosynthetic clay liners, erosion-control mats, and edge and strip drains. Cores are HDPE geonets, cuspated HDPE sheets, PP geospacers and cylindrical-channel mats, 5 to 25 mm thick, with nonwoven polypropylene or polyester filters, in roll widths of 1 to 4 m. The page cites EN ISO 12958 and ASTM D4716 for in-plane flow testing, and states that standard drainage geocomposites are stocked for immediate dispatch
• Geocomposite for road construction: geocomposites for roads and civil engineering, for drainage, filtration and separation layers and for soil stabilisation and reinforcement
• Geotextiles, woven and nonwoven: polypropylene or polyester, 100 to 1,000 g/m², in roll widths of 2, 4 and 6 m
• Geogrids, uniaxial and biaxial: uniaxial (PET or HDPE) and biaxial (PP) grids, with triaxial grids on request

Engineering support: Send the application, inflow or catchment data, slope or wall geometry, normal stress and the materials on each face of the drainage layer. We will review core and filter options against your data and quote geocomposites, geotextiles and geogrids together.

Send your inflow, slope, normal stress and boundary materials for a geocomposite proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Geotextile: Types, Functions and Applications in Civil Engineering
• Geogrid: Uniaxial, Biaxial and Triaxial Types for Soil Reinforcement
• HDPE Pipes: Properties, Grades and Specifications
• Manholes and Manhole Covers: EN 124 Load Classes, Chambers and HDPE Manholes
• Wastewater Treatment Equipment: Screens, Clarifiers, Aeration and Dewatering

Frequently Asked Questions

Q: What is a geocomposite?
A: A geocomposite is a factory-made product that combines two or more materials, at least one of them a geosynthetic, so that each layer performs one function. Examples are drainage geocomposites with a geonet or cuspated core and geotextile filters, geogrids bonded to nonwovens, geosynthetic clay liners that use bentonite as a barrier, and paving interlayers placed under asphalt overlays.

Q: What is a drainage geocomposite made of?
A: A drainage geocomposite has a core that carries water within its plane, usually an HDPE geonet, a formed cuspated sheet or a polypropylene geospacer, with a nonwoven geotextile filter bonded to one or both faces. The filter keeps soil out of the core while letting water in. Sheet drains for walls and basements may also carry a plastic film on the back face.

Q: What is transmissivity and how is it tested?
A: Transmissivity is the in-plane flow per metre width divided by the hydraulic gradient, in square metres per second. It is measured to ISO 12958 or ASTM D4716 at stated normal stresses and gradients. Short index tests between plates compare products; design needs a performance test with site soils and membranes and a 100-hour seating period, because flow falls with stress, intrusion and time.

Q: What reduction factors apply to drainage geocomposites?
A: GRI-GC8 and ASTM D7931 divide the 100-hour flow by reduction factors for long-term geotextile intrusion, creep of the core, chemical clogging and biological clogging. Published default clogging factors for landfill caps are 1.0 to 1.2 for chemical and 1.2 to 3.5 for biological clogging. Creep factors depend on stress: one geonet study reports about 1.1 to 1.2 below 50 kPa and nearly 2 above 700 kPa.

Q: How do you calculate the required transmissivity of a drainage layer?
A: Multiply the inflow rate per square metre by the length draining to the outlet to get the flow per metre width, then divide by the hydraulic gradient. On a slope the gradient is the sine of the slope angle. For example, 1 × 10⁻⁷ m/s over a 50 m slope at 1V:3H needs 1.58 × 10⁻⁵ m²/s, before reduction factors and a factor of safety.

Q: What is a geosynthetic clay liner?
A: A geosynthetic clay liner is a factory-made hydraulic barrier: a thin layer of bentonite held between two geotextiles by needle-punching, stitching or adhesive, or bonded to a geomembrane. A US EPA fact sheet notes that manufacturers typically use about 4.9 kg/m² of bentonite, and that saturated bentonite typically has a hydraulic conductivity below 1 × 10⁻¹¹ m/s. Keep it dry until covered.

Q: Can a drainage geocomposite replace a gravel drainage layer?
A: It can when its long-term in-plane flow at the design stress and gradient, after reduction factors and a factor of safety, matches what the granular layer would carry. A granular layer's transmissivity is its permeability times its thickness, so any claimed equivalence depends on the gravel's permeability. Compare calculated flow capacity, not thickness or a datasheet index value.