BW How-To

The performance of roads, parking lots, loading docks and other high traffic areas can be vastly improved through the separation and stabilization of the roadway structure.  Geotextiles serve as a permeable stabilization layer, preventing the aggregate and sub-grade soils from intermixing while allowing the passage of water.  They also enhance the structural properties of the subgrade and aggregate to minimize the cost of the road structure.

**Note: These are considered to be general guidelines, appropriate for common construction conditions.  They are not intended to supersede any information found on MSDS or Manufacturer’s Technical Data Sheets.  These are also not intended to be construction documents.  Consult local ordinances and/or engineers before building.  For site-specific or design requirements that may require modification to these tips, please contact your local Brock White representative for more information.**

Prepping the Subgrade:

Initially, the site should be cleared for tree stumps, large stones, and other sharp objects that could puncture the fabric.  This step should be performed regardless of subgrade strength.

For roadways, this typically involved the removal of vegetation, roots, and topsoil.  Though in localized soft or otherwise unsuitable subgrade areas, it may require excavation and backfilling with a select material.

Geotextile Placement:

The fabric should be rolled out onto the subgrade beginning at a point that allows easy access for construction equipment.  On very soft subgrades (CBR<1) that fabric layout and aggregate placement should begin on firm soil on the site perimeter to establish an “anchor point”.  Do not drag the geotextile across the subgrade.  It should usually be laid in the direction of construction traffic; however, specific projects may alter this layout.  Geotextile panels should be overlapped both side-to-side and end-to-end, in the direction of the aggregate placement.

The recommended overlap, depending on the subgrade strength, can be found in the table below:

Aggregate Placement:

Aggregate is then placed and spread on the fabric.  Soil, rocks or pins should be used to anchor the leading edge of the fabric to prevent it from lifting during placement of the first aggregate lift.  Aggregate is typically back-dumped onto the geotextile as the truck should not drive directly on the fabric.

Lift thickness should not be less than 6 inches and the first lift should be as thick as necessary to limit rutting to less than 4 inches.  During spreading, the bulldozer should blade into the load and slightly upward to prevent stressing the fabric.  Follow this procedure for each load until the fabric is completely covered.  Observe the aggregate layer for rutting to determine which areas may need additional aggregate for good stability.

Aggregate Compaction:

The aggregate must be compacted as required by the project specifications.  It should be initially compacted by “walking” the tracked bulldozer back and forth over the aggregate while waiting for the next load.  Construction traffic will then compact the aggregate until reasonable stability is obtained.  Finally, a vibratory compactor is used, first without vibrations for several passes and then with full vibration.  Any weak areas found during the final compaction usually indicate inadequate aggregate thickness.  Do not grade these ruts down; rather fill with additional aggregate and compact to the specified density.

Construction Monitoring:

It is important that the construction conditions and process be monitored.  If the actual subgrade has lower strength than that assumed for design, the structural section design thicknesses must be re-evaluated.  Observing rutting of the aggregate layer, as noted above, can pinpoint weak subgrade areas, allowing design adjustments to be made on site.  Aggregate base placed for a new pavement may also be monitored for weak areas and corrected before the pavement layer is placed.