Geosynthetics

Geotextiles are permeable fabrics placed in or on the ground. Non-woven grades separate, filter and protect. Woven grades add tensile strength. The grade is selected for the job: mass per unit area, tensile strength, puncture resistance and permittivity are matched to the soil and the load. Overlaps and fixing details decide how well the fabric performs once it is covered.

Geogrids are open grids whose apertures interlock with the surrounding fill. The grid takes tensile load that the soil cannot. They are used as reinforcement in retaining walls, steep slopes and embankments over weak ground. Long-term design strength allows for creep, installation damage and the environment, and these factors are applied to the short-term strength.

A biaxial geogrid has the same strength in both directions. It is placed within or beneath granular base layers on weak subgrades. It confines the aggregate, reduces the base thickness needed and spreads wheel loads. Typical uses are haul roads, yard areas and low-volume roads.

A geomat is a three-dimensional mat with open space for soil and roots. It protects the surface against rain and flow while vegetation establishes, and the roots then anchor into the mat. Geomats suit embankments, cut slopes and channel linings where grass is the finished surface.

Geocells are three-dimensional honeycomb panels, opened out on the ground and filled with soil, aggregate or concrete. The cell walls confine the fill, which raises its stiffness and holds it on a slope. Applications include slope protection, channel lining, load support on soft ground and retaining structures.

Band drains are pushed vertically into soft clay on a grid. Water in the clay then travels a short horizontal distance to the drain, instead of a long vertical distance to the surface, so consolidation settlement completes in months rather than years. A working platform and a drainage blanket on the surface complete the system.

Thick, non-woven geotextile mats go beneath armour stone, gabion mattresses and geomembranes. They filter the soil, protect the layer above from the ground and cushion against puncture by stone. Mat weight is chosen for the stone size and the drop or placement method.

Coir and other natural fibres are woven or needled into mats and rolls. They hold the soil surface and retain moisture during the first growing seasons, and they degrade as vegetation takes over. They suit lower-risk slopes and banks where a permanent synthetic layer is not required.

A geodrain is a drainage core wrapped in filter fabric. It collects water behind retaining walls, basement walls and abutments, and carries it to an outlet. The core keeps its flow capacity under load. Compared with a stone drain it is thin and quick to install.

On a low-CBR subgrade, grids and fabrics improve the performance of a flexible pavement. A grid reinforces the base, a fabric separates the base from the subgrade, and a paving fabric under an overlay slows reflective cracking. The details are set against the traffic and the subgrade strength.