Flexible continuous sheets of one or more synthetic material which are relatively impermeable. Membranes have to be site glued or welded to ensure waterproof function.
Membranes are mainly used because they are impermeable to gases and liquids and protect the surrounding environment: for example, they might be used to avoid leachate from waste/contaminated land entering the surrounding soil and groundwater. Hence, the selection of membranes needs to take into account their long term durability in often very harsh chemical conditions and under potential multiaxial stresses and strains due to settlement and thermal variations.
Their required impermeability often dictates how they are installed: contiguous sheets of membrane cannot be simply overlapped or taped but require welding/gluing to ensure impermeability and specific tests on the seams need to be conducted. For example, when geomembranes are used to build water reservoirs, a water tightness test consisting of filling the structure with water for 24 hours needs to be conducted. Membranes must be protected from the risk of tear and puncture by granular materials with a separating layer of fine material or geotextile fleeces.
Very often, a comprehensive construction quality assurance (CQA) protocol is put in place during the construction phase of the works, to include material delivery inventories, documented storage conditions, non-destructive seam testing results and visual inspection reports for each element of the system during the installation of the components. Geomembranes might be used to wrap the sides and bottoms of excavations destined to form a tank for the containment of fluid/water.
Geomembranes might also be used with geocellular units for the construction of soakaways: they provide protection to the units from infiltration of soil, which would otherwise reduce the permeability of the geocellular structure, and avoid water leaking into the surrounding ground which could soften and settle as a result.
Geomembranes are used in conjunction with various system for the collection and drainage of the liquid or gas seeping through from the layers contained by the membrane, e.g. as liners in landfill and contaminated land applications.
Unpaved granular or permeable pavements might be used in some developments as a technique for stormwater control. Membranes beneath the pavements prevent water infiltration and the subgrade from becoming weak through saturation.This water may be collected and recycled in certain situations.
This membrane technique may be used in conjunction with geogrids which ensure that any horizontal braking forces developed at the pavement surface are distributed more evenly throughout the granular material - this avoids any excessive shear developing at the interface between granular material and membrane, which could damage the polymeric sheet and destabilise the pavement structure.