Pumps & Systems, March 2008
Soils with high concentrations of clay and silt can wreak havoc on construction sites, drain pipes, sewers and pumps. Users working with soils might want to consider geotextiles as a preventive maintenance alternative.
Soil erosion isn't just a problem for farmers, Niagara Falls and the Grand Canyon. Most construction projects have to contend with the effects of water on excavations-not only as a consequence of rainfall runoff but also from ground water intrusion. The mud common to most jobsites at one point or another-tracking over roads and clogging drainage ditches and storm sewers-has lead to the need for more effective control mechanisms that are "user friendly" and low cost. Textiles have been used for centuries to try to filter out the mud and let the water drain away; some original wellpoints used lamp wick and burlap to separate the water from the soil.
In an effort to keep the mud "corralled," geotextile silt fences are required installations as a normal construction practice to allow the water to flow off-site without carrying away the soils. Geotextiles, because of their diversity, light weight, strength and adaptability are finding a prominent place in water-soil management.
Geotextiles generally fall into two categories, woven and non-woven. Woven textiles have a uniform pattern and can be tailored to a specific open area ratio. These are generally thin single layers and are very strong and tear-resistant. Non-woven geotextiles have no pattern and the fibers intertwine to produce thickness. The fabric is generally thicker and less tear-resistant than woven fabrics.
With woven geotextiles, the larger the soil particles to be controlled, the more open the weave. Woven fabrics have great strength and are common now for ground cover-holding grass seeds and the soil in place, giving the grass time to root and stabilize slopes. More sophisticated approaches weave the grass seeds and matting into the fabric so that the ground cover can be rolled out like a carpet on the bare earth. With the addition of a grass-green color, the ground cover gives the appearance of having already germinated.
Silt fences are woven in a graduated weave tighter at the bottom than at the top. This reduces weight and puts the filtering benefit at the ground level where it's needed. Silt fence material is trenched in to anchor the fence against high velocity runoff, keeping the soil and debris inside and letting the water get outside.
Similar woven geotextiles work wonders in dewatering applications as wraps around well screens and sump pit screens. The textile can be sewn into sleeves or tubes to be slid over the well screen or sump pit framework. This technique allows the user to compensate for well screens with slots that are too large or for sump pit piping with large perforations.
By adding the geotextile sleeves, the user can select a finer grade of filter pack or gravel pack more closely suited to the surrounding soils, i.e., use a finer weave geotextile in silty and clayey soils. These soil types wreak havoc on construction sites, plugging up drain pipe, sewers and wearing out pumps. Municipalities plagued with the problem of cleaning out storm and sanitary sewers plugged with sediment from new construction sites have levied heavy fines on the contractor at fault, making geotextiles an attractive preventive maintenance alternative.
Non-woven geotextiles tend to be more adaptable to variable soil conditions, particularly in areas of glacial development where the soil profile changes from random deposits of very fine silts, clays and sands to coarse sands, gravels, cobbles and boulders. The non-woven material has thickness which reduces the tendency for soil "bridging," where soil particles interlock to "dam" up the open channels in the filter and block water flow.
Over the last 20 years, the strength and tear resistance of non-woven geotextiles has been greatly improved, allowing for installation in long segments. Installed vertically to depths of 60-ft and deeper, geotextile assemblies create "water wicks" removing water from unstable soils to induce natural compaction. This passive application is permanent and protects new structures from upheaval and settlement. These geotextile wicks allow the soil to adjust to changing water table levels without removing fine particles of soil in the process.
Non-woven geotextiles attached to the proper support structure or framework are used routinely today on below ground building walls to channel ground water and rainwater runoff away from the walls to collection piping, thus eliminating seepage into the building basements. The same principle is being applied to temporary and permanent construction dewatering systems.
This relatively new approach produces fast, clean results, eliminating turbidity in the discharge water and-with the proper installation-the need for using a filter sand or gravel around wells or wellpoints. This innovation gives the contractor the ability to adjust to changing ground water conditions with less guesswork, faster installation, lower labor costs, less down time and happier environmental managers.