Geotextile selection

The thickness of a sub-base and/or capping layer should be determined using appropriate national design criteria. Other more simplified procedures may be adopted if these do not exist or are inappropriate.

For example, the nomogram shown is for the design of unpaved roads and may be used to check initial layer thicknesses for a
paved road.

Where information is scarce, the following may prove useful for the selection and installation of the most appropriate TERRAM grade. These guidelines should not be used to replace more rigorous design and the experience of contractors familiar with the installation
of geotextiles.

The selection of the most appropriate TERRAM grade is largely dependent on the strength and moisture content of the subgrade. Site investigation should be used to assess these parameters. 5 may be used for guidance in the absence of field data.

The TERRAM grade must be sufficiently robust to resist installation damage. The lower the subgrade strength and the larger the stone, the more robust the grade needs to be. 

The area should be cleared of any large objects, such as stones and tree stumps, before geotextile placement. Ruts and sharp undulations in excess of 100mm should be filled and levelled. Strong perennial weeds, such as thistles, should be treated with weed killer. Other vegetation can be left undisturbed, if this is allowable and not detrimental to the structure. The presence of surface vegetation can actually aid construction with very soft soils i.e. CBR <10kn=\"\" m2.<=\"\" p=\"\">

TERRAM geotextiles can be unrolled directly onto a subgrade with adjacent and subsequent rolls overlapped between 300mm and 1000mm - the softer the subgrade, the greater the overlap. A combination of overlapping and sewing may be more economical where the subgrade strength is particularly low, or in other critical situations (see the TERRAM Jointing bulletin).

Vehicles and plant must not run directly on exposed textile. Construction traffic should be restricted to areas of textile which have been covered with sub-base and preferably compacted to the minimum required depth.

The sub-base must be well-graded, compactable and for permanent works, capable of transporting rising water and resistant to long-term degradation. Recommended grading bands for compactable granular materials are shown.

The sub-base thickness will depend on loading and on the strength of the subgrade. The thickness should take into account the maximum anticipated axle load, both during construction and in service, and should be increased by 10-20% on bends or where a slightly inferior sub-base is used.

Sub-base should be bladed forward over the textile and graded down to the required un-compacted depth. Typical practice with a firm subgrade is to place the sub-base in layers which are compacted to 150mm using a vibro-roller.  Further advice is provided in the Department for Transport’s Specification for Highways Works.

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With a soft subgrade it is prudent to place at least 300mm of lightly-compacted sub-base in one lift (500mm on an exceptionally soft subgrade) before overlaying this with a thinner layer of better-compacted material.

A very low-CBR subgrade, heavier traffic loadings, or a poorly-graded sub-base may require differing techniques. For example, heavy compaction with a very soft clay subgrade can lead to rutting and heave, and it may be necessary to increase the initial layer thickness and allow time for consolidation of the subgrade before the placement of thinner layers and applying more intense compaction.