Geotextile selection

Use proper criteria as specified by the design to determine the thickness of a sub-base and/or capping layer. If there are no local criteria and/or the existing guidelines are inappropriate, other, more simplified procedures may be used.

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.

As a general guideline, see the following to select and install the most appropriate grade of TYPAR Geotextile. These guidelines do not replace the experience of contractors familiar with geotextile installation and/or more in-depth or rigorous design specifications.

Strength and moisture content of the sub-grade layer
The strength and moisture content has the greatest impact on proper selection of the correct TYPAR Geotextile grade. Review and investigate the site thoroughly to assess these conditions. If no field data is available, use ‘5’.

Proper selection of grade
The grade of thegeotextile should be sufficient to withstand installation damage. Select more robust grades for lower sub-grade strength and/or large stone size.

Installation
Clear installation area of large objects, including tree stumps and boulders. Rough terrain such as steep undulations and rutting in excess of four inches should be filled and leveled. Kill strong perennials with weed killer, leaving other vegetation undisturbed if not detrimental to the design. Very soft soils may be aided by the presence of surface vegetation.

Unroll TYPAR geotextiles directly on sub-grade, overlapping edges between 12 – 36”. Use a larger overlap on softer sub-grades. Combine sewing and overlapping for a more cost effective solution where sub-grade strength is particularly low. 

Do not drive vehicles directly on exposed geotextile. Restrict construction traffic to areas of the geotextile that have been covered with sub-base and compacted to 95% proctor.

Sub-base selection and placement 
Sub-base intended forpermanent usage must be compacted and well-graded, capable of transporting water and resistant to degradation. Grading bands recommended for compactable granular materials are shown.

Loading requirements and required sub-grade strength will determine appropriate sub-base thickness. When calculating required thickness, a variety of factors should be taken into account including anticipated axle load during construction and during intended use. Increase sub-base thickness by 10-20% on bends or where an inferior sub-base material is used. 

Blade sub-base forward over the geotextile and grade down to required un-compacted depth. Standard practice with a sufficiently firm sub-grade is to place and compact sub-base layers to six inches. Refer to the AASHTO Green Book for further guidance.

When working with a soft sub-grade, place at least 12 inches (20 inches with exceptionally soft sub-grade) of lightly-compacted sub-base in one lift and then overlay with an additional, thinner layer of material that has been better-compacted.

Different techniques may be required for poorly graded, heavier traffic loads or very low CBR sub-grades. For example, rutting and pumping may occur with heavy compaction and a very soft clay sub-grade. In this instance it may be necessary to increase the thickness of the initial layer, allow sufficient time for the layers to consolidate and then place thinner layers while applying heavier compaction.

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