Contractors building a distributor road serving major housing developments on the outskirts of Inverness, brought in geotechnical and construction specialists Maccaferri to design a complex reinforced soil structure spanning a drainage culvert in a deep, steep sided gulley.
The Parks Farm distributor road links the housing development of Milton Leys and Wester Inshes with the greater sub-urban area of Inverness. The road also provides access to a new development of 380 houses for local developer, Tulloch Homes, as well as a new primary school and a district centre.
At the edge of the development, the road had to cross two 10.0m deep, tree lined gulleys. The site was heavily constricted with difficult access and contained an existing stone bridge which had to be retained.
According to Ian Will, senior design technician for HGA Consulting Engineers, the depth of the gulleys and the limited space made a conventional earth slope solution impossible. "There was simply insufficient space to use a battered earth embankment over the culvert. This factor, together with the complex curves involved meant that a steep sided, reinforced soil structure was the only cost effective solution," he said.
Geotechnical specialists Maccaferri, conceived a composite reinforced soil structure comprising a 10.0m high Green Terramesh embankment facing system in combination with Enkagrid Pro 90 and 60 geogrid reinforcement, sandwiched at 0.6m vertical increments between layers of compacted site-won granular backfill.
At the core of the structure is a 3.2m deep mass of lightweight, Cordek Filcor high density EPS [Expanded polystyrene] blocks which provides physical mass to the structure and, at the same time, reduces the weight of overburden placed on the 8.4m diameter corrugated steel arch culvert.
Green Terramesh is an environmentally friendly reinforced soil system from Maccaferri, made from PVC coated and galvanised, double-twist wire mesh. The mesh forms both the soil reinforcing geogrid tails and the slope face, all in one unit.
The geogrid tails are sandwiched between layers of compacted granular fill, effectively reinforcing it and allowing the slope structure to stand steeper, withstand greater loads, and take less land than an unreinforced slope.
Behind the front face of the unit is a factory fitted, heavy welded panel and a bio-degradable coir erosion control blanket. The welded panel and its support brackets hold the face at the correct angle during back-filling operations, removing the need for external formwork and shuttering, saving on installation costs and time.
According to Maccaferri, this makes the installation of Green Terramesh many times quicker than traditional wrapped face soil reinforcement structures and gives it a crisper, more regular face finish.
The coir matting behind the front face stops the backfill from spilling out between the apertures in the mesh facing. About 250mm depth of topsoil is installed directly behind the face of the unit to provide the nutrient reservoir for rapid establishment of vegetative cover.
This green face finish also satisfied local planning requirements for an environmentally sympathetic engineering solution. Commenting on the construction method, Ian Will said: "As well as giving us a sound geotechnical solution to the road building challenges, the Maccaferri system gave our client clear environmental, engineering, time and cost benefits."