When the Tennessee Department of Transportation (DOT) initiated a road widening project at the intersection of I-75 and Highway 153 in Chattanooga, it became clear that the expanded portion of the road extended into a wetlands area.
The road would need additional support to minimise settlement in that area. The intersection is a major artery, therefore it was critical that the project be completed quickly and with minimal interruption of traffic.
Tennessee DOT considered three options. One was over-excavation and replacement with suitable material. Another was lime stabilisation. The third option was a woven geosynthetic separating the soft soil from the stone sub-base. This solution was chosen as being less time consuming and more cost efficient, and its high tensile modulus served a duel purpose in that it also stabilised the stone sub-base.
Synthetic Industries' Geotex 315ST was selected because it met the Tennessee DOT specification. Furthermore, Geotex 315ST met the stringent requirements of the American Association of State Highway Transportation Officials M288-96 specification. Given the aggressive stone layer to be placed on it, a robust woven geosynthetic was specified.
For construction, the site was cleared and a smooth subgrade prepared. The geosynthetic was then unrolled over the soft soil area and wrinkles were removed. Embankment material was pushed on to the geosynthetic to avoid damage from direct dumping.
In 1997 serious cracks were found in the sea wall at Kinmel Bay in North Wales. For Conway Borough Council this was an unwelcome reminder of events in nearby Towyn where the sea wall breached in 1992 causing extensive flooding.
Work at Towyn to rebuild the sea wall and construct a revetment was extended to take in Kinmel Bay. Client Conway County Borough Council awarded a contract to Jones Brothers.
Following repair of the sea wall, work began on a wave absorption revetment. Existing timber groynes were first removed and the beach was excavated to the proposed formation between 1m and 3m depth, through sand, shingle and existing rock armour.
A layer of Geofabrics HP6 geosynthetic was then laid. The material was required to resist point loads from the armouring rock as well as having high permeability and filtering efficiency. It also had to absorb deformation in areas around rocks, without rupture.
Altogether 47,600m2 of HP6 was used on the project. A bonus was the fact that the material is supplied in 6m wide rolls and given the requirement for 1m overlaps, wastage was minimised and laying relatively speedy. Contracts manager Brian Roberts confirms this: 'Geofabrics are very easy to lay and this helped us save valuable time, which is essential when carrying out inter-tidal work'.
Once placed, the geosynthetic was covered by a 600mm layer of bedstone sized 250mm down. This was then topped by rock armour in two sizes: from 1t to 3t and from 3t to 6t. This was brought in from local quarries. Originally estimated to take 40 weeks, the contract was in fact completed in just 28.