Geomembranes in PICP
Permeable interlocking concrete pavement (PICP) systems can be designed and constructed to accommodate three drainage conditions:
- Complete infiltration of water into to the high infiltration rate soil subgrade with no underdrains;
- Partial infiltration of water into low infiltration rate soil subgrade with some outflow through underdrains;
- No infiltration into the soil subgrade with all outflow exiting through underdrains.
- All conditions have similar surfacing, and base/subbase reservoirconstruction. No exfiltration designs, however, use a geomembrane on the sides and bottom of the base/subbase reservoir to contain stormwater and prevent it from infiltrating into the soil subgrade. Commonly called an impermeable liner, Figure 1 illustrates a typical PICP design using such a membrane.
A no infiltration design with a geomembrane is typically used in the following conditions:
- The soil has very low permeability, low strength, or is expansive;
- High depth to a water table or bedrock;
- To protect adjacent structures and foundations from water;
- When pollutant loads are expected to exceed the capacity of the soil subgrade to treat them.
By storing water in the base/subbase and then slowly draining it through pipes, the design behaves like an underground detention pond with the added benefit of reducingcontaminants. A no infiltration retention design may be used for water harvesting. The water may be piped to an underground cistern for reuse on site. Harvested rainwater reduces landscaping water requirements and in some cases it can be used for gray water within buildings.
Geomembranes are a class of geosynthetic fabricated to create a sheet barrier that is relatively impermeable and is installed to prevent the flow of liquid or gas across that barrier.
Geomembranes can be manufactured from a range of polymers including polyvinyl chloride (PVC), chlorosulfonated polyethylene (CSPE), chlorinated polyethylene (CPE), or, more recently, polypropylene (PP), ethylene propylene diene monomer (EPDM), high-density polyethylene (HDPE) and linear lower density polyethylene (LLDPE), very flexible polyethylene (VFPE). Each of these polymers is unique and provides varying levels of resistance to acids, alkalis or petrochemicals. Some polymers can also function in extreme heat or cold. Normally, the surface of a geomembrane is smooth, but some sloped applications can benefit from a textured surface that provides greater friction with the adjacent geotextiles or soil.
Geomembranes come in a range of thicknesses depending on the polymers and the manufacturingprocess. For example, HDPE geomembrane is typically available in 40, 60 and 80 mil (1.0, 1.5 and 2.0 mm) thicknesses and in a range of roll widths. Geomembranes have different engineering properties depending on polymer type, thickness and manufacturing process. Typically the nominal thickness, density, tensile strength, tear resistance, dimensional stability and puncture resistance are provided in manufacturers’ literature and referenced in product specifications.
Geomembranes for PICP are typically fabricated on the job site and this requires cutting, fitting and seaming to create waterproof joints. Different seaming techniques are used depending on the polymer,environmental conditions and project requirements. Materials like EPDM and PVC are routinely seamed using adhesive or double-sided tape. Before two panels are joined, the areas to be joined are usuallycleaned and primed. HDPE and other polymers are typically welded together with extrusion welders or hot wedge welders. Seams for all materials should be field-tested to ensure their integrity, especially around underdrains penetrating the geomembrane. For smaller projects, it might be possible to have the supplier prefabricate the geomembrane to meet site requirements. Prefabricated geomembranes are typically delivered to the site folded on a pallet.
When preparing a site for a geomembrane application, remove rocks, roots, and other sharp objects from the subgrade that may damage the geomembrane during installation, aggregate compaction,or use. Such areas should be filled with dense-graded aggregate and compacted before placing the geomembrane over them. A layer of non-woven geotextile is commonly used to protect one or both sides of the geomembrane. The thickness of the geotextile is typically selected based on the materials placed next to the geomembrane and the importance of preventing puncturesof the geomembrane. Figure 2 illustrates a green alley in Richmondwith a geomembrane that is protectedby a non-woven geotextile before placing and compacting the subbase aggregate.
When designing a no infiltration PICP system, there are many factors that must be considered in selecting the geomembrane and protection materials. For most projects, consultation with an engineer familiar with the design of a geomembrane is recommended.