ETL 1110-1-189
14 Feb 03
Table 9
Geosynthetic Overlap Requirements For Use Pavements
Step 1: Determine Design Subgrade Soil Strength and Geosynthetic Applicability
CBR < 1.0
1.0 < CBR < 4.0
CBR > 4.0
3 ft
2 ft
1 ft
6.2 Geogrid Installation
Prior to installation of the geogrid, the site should be cleared, grubbed, and excavated to the
design grade if not done previously for geotextile placement. As noted in Section 6.1, small
sections of geogrid can be used to cover roots, stumps, or stalks prior to geotextile installation to
reduce the potential for fabric puncture. Also noted in Section 6.1, stiff geogrids have been
successfully used as retrievable walking platforms to facilitate placement of the geosynthetic
separation and reinforcement layers. In this application, the stiff geogrid is unrolled along the
outer edges of the traffic lane to provide construction personnel with a temporary walkway to
unroll and place the reinforcement materials.
The geogrid should be rolled out by hand, ahead of backfilling, directly on the subgrade or
geotextile (if required) for design aggregate thicknesses less than 14 inches. For design
aggregate thicknesses in excess of 14 in., the geogrid should be placed in the middle of the
aggregate layer. For shallow-depth soft subgrades (CBR > 0.5), the geogrid is laid in the
direction in which the aggregate will be spread. For deep or very soft subgrades (CBR < 0.5),
the geogrid is laid transverse to the direction of the traffic lane. Table 9 should be used to
determine the appropriate overlap requirements, and the overlaps should be placed such that the
spreading of the aggregate base will not disturb the overlap distance. The geogrid can be held in
place using staples, pins, or sand bags for windy conditions.
6.3 Aggregate Placement
Vehicles must not be driven on the unprotected geosynthetics prior to aggregate placement.
For very soft subgrades (CBR < 1.0), an effective construction procedure is to install the
aggregate thicker and narrower than the design. Thus, the aggregate should be placed in the
center of the traffic lane at a depth greater than the final design grade. The weight of this
material will begin to displace excess water from the subgrade and begin subgrade consolidation.
This results in an increase in the overall subgrade strength beneath the traffic lane. After the
required amount of aggregate for the roadway and shoulders has been placed in the center of the
traffic lane, the excess aggregate can be spread laterally to the shoulders of the roadway until the
design grade is achieved. The aggregate material can then be compacted to the design thickness.
Care should be taken to spread the aggregate in the direction of the geosynthetic overlaps.
For subgrade CBR strengths between 1.0 and 3.0, the full design aggregate thickness should
be placed in one lift. Thinner lifts of aggregate run the risk of damaging the geosynthetics or
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