GEOSYNTHETICS AS DRAINAGE AND FILTER LAYERS

The main functions of geotextiles and related products used in drainage systems are filtration, separation and drainage.

In drainage systems, geotextiles act as a filter. In geotechnical applications, a geotextile filter has two main functions: it allows water to penetrate and it retains (holds back) soil, not soil particles. The filter should allow fine particles carried by the water to pass through. If fine particles were unable to pass through the filter, collimation would occur. Two criteria are taken into account when designing filters: permeability and retention capacity.

The two criteria are opposing, but not contradictory. The geotextile material should have a characteristic pore size that is small enough to retain soil and large enough to allow water to flow freely and for the finest soil particles carried by water to pass through the filter.

DRAINAGE

In drainage, geotextiles are used as drainage between the overlying soil and the drainage material. Some geosynthetics (e.g. drainage geocomposites) have drainage functions. Applications include:

• Horizontal drainage (often referred to as longitudinal drainage in traffic engineering), in which geotextiles surround an easily permeable granular backfill that fills an excavated trench,
• Surface (layered) drainage,
• Ribbed drainage, an arrangement consisting of a geotextile filter and a core (mesh, embossed film, etc.) that conducts water.

GEOTEXTILE MATERIALS USED AS FILTERS IN DRAINAGE SYSTEMS

One of the first factors to evaluate a geotextile material as a filter is the structure of the material, particularly its effect on the size and distribution of pores in the material.

In this respect, geotextiles can be divided into three categories:

• Thin non-woven,
• Thick non-woven,
• Woven.

Thin non-woven geotextiles are materials with a thickness of less than 1 mm. They are usually constructed of a thin continuous yarn that is initially laid to form a loose net. This net is then either crushed with hot rollers, which causes the yarn to weld at the points of contact, or it is stitched through a set of hooked needles. Needling causes mechanical entanglement of the fibres, fixing them together. As a result of the random distribution of contact points, the pore openings have a more varied size than in the case of fabric.

Thick non-woven geotextiles are usually felts, composed of chopped fibres. They are initially laid as a loose fleece and then needled. The product is often finished on rollers to give it a relatively smooth surface. Due to their thickness, these geotextiles can be treated as three-dimensional filters. As with thin geotextiles, their pore dimensions can vary over a wide range.

Woven geotextiles contain two components: a warp running longitudinally and a weft running across the fabric. Materials woven using typical techniques have an orthogonal structure, with relatively regular holes, evenly distributed over the surface of the material. The components of the fabric are usually flat ribbons, ribbon yarns or round continuous yarns.

WORKING WITH THE GEOTEXTILE FILTER

Criteria for geotextile filters depend on flow conditions. In typical drainage applications, there is an established unidirectional laminar flow.

As water flows through the soil-geotextile system, the coarser soil grains create (can be empty) vaults above the openings of the geotextile material. This allows the finer particles adhering to the material to flow through the pores of the geotextile material and be washed away by the flowing water. If there is no soil suffusion, i.e. migration of very fine soil particles through the pores of the soil matrix, the system stabilises very quickly and there is no change in the soil structure away from the filter. A reverse filter is formed above the surface of the geotextile material.

A reduction in flow velocity can often be observed during the initial formation of the vault network. This is due to the pores of the geotextile material being blocked by larger soil grains. Blocking does not mean plugging, but only partially covering the holes.

Both fabrics and non-woven fabrics show some decrease in the transverse permeability of the system until a steady state is formed. Steady state conditions can only be achieved if the geotextiles are designed correctly, i.e. appropriately for the layout. They should be sized so that the hydraulic flow is established to allow for a network of vaults and, if possible, a filter zone in the ground.