The concept of filling baskets with stones to create ‘gabion-like’ retaining structures is well established, having been introduced by the Egyptians over 2,000 years ago. Today, Maccaferri Ltd, specialists in retaining structures, erosion protection and soil reinforcement, has refined the basic concept to supply gabion engineered solutions to meet the most challenging of modern environments.
Gabions supplied by Maccaferri have been used to retain chalk cuttings excavated during the construction of the newly completed Roadchef Motorway Services Area (MSA) on the M3 at Winchester. An impressive expanse of gabion wall up to six metres in height extends for up to 340m adjacent to the southbound on-slip and northbound off-slip access roads. An additional section of Green Terramesh® soil reinforcement supports the southbound off-slip.
Working closely with the Highways Agency, consulting engineers Woods Warren were responsible for the initial design solution. As the proposed service area was in an area of ancient woodland, local conservation groups were keen to preserve the natural environment. The use of hard engineering features including reinforced concrete retaining walls had been ruled out at the public inquiry. An alternative solution was required which would fulfill a number of key criteria.
Firstly, the design must conform to Highways Agency requirements for strengthened earthworks, namely a minimum design life of 60 years. It should also offer a compact solution confined to a build footprint within the existing highway. Finally, it must meet concerns of local environmentalists for a ‘natural’ engineering solution.
Gabions could fulfill all these demands.
Commenting on the decision to approach Maccaferri to develop a design based around gabion walls, Roger Frost of Woods Warren said: “Maccaferri gabions are a well proven product, they carry BBA certification and can be built to feature a steep face. In addition, the company offers an excellent service and is very quick to respond. Their engineers worked with us to develop a design for gabion retaining walls which would meet all our concerns.”
Wembley-based contractor PJ Carey (Contractors) Ltd was awarded the construction contract (on a Contractor Design basis for the retaining walls). Its tender was based on the gabion and Green Terramesh soil reinforcement solutions proposed by Maccaferri, following cost comparisons with other solutions, including soil nailing.
All running lanes of both carriageways of the M3 remained open during the construction phase. The gabion walls, with a footprint of just 3.5m, were built from within the ‘hard shoulder’ area. The lack of working room and other site logistics dictated that cutting the chalk slopes took place by day and the gabion walls were installed at night in a 24 hour ’round the clock’ operation.
PC Construction, a gabion and soil reinforcement specialist sub-contractor, installed the gabion walls. A total of 5,500m³ gabions were installed at an average rate of construction of 140m³ gabions per night. The excavation and Green Terramesh soil reinforcement system were both carried out by main contractor PJ Carey (Contractors) Ltd.
Landscape architects Macgregor Smith recommended a natural stone finish for the gabion walls. This will be allowed to ‘green up’ naturally with local vegetation and avoid importing alien species into a conservation area.
Box gabions are rectangular units fabricated from a double-twist, hexagonal mesh of PVC and galfan coated steel wire. Galfan is a five percent aluminium/zinc mischmetal which provides enhanced corrosion protection characteristics when compared to conventional galvanising. The standard size for each gabion is 2m x 1m x 1m with a fixed diaphragm at 1m, but alternative sizes may be specified to meet precise specifications.
Supplied folded flat, units are laced together using continuous 2.2mm diameter lacing wire, or alternatively high tensile steel fixing rings can be attached using a special pneumatic assembly tool. Once assembled, each unit is filled with natural stone to create a retaining structure which is in sympathy with the surrounding environment.
Green Terramesh units are formed from a single sheet of hexagonal, double-twist woven steel mesh. When installed, it is formed into three sections, a horizontal soil reinforcement section, a facing section and a horizontal return top section. Triangular support brackets provide added rigidity and allow easy installation of the grid. These may be customised to match the precise angle of the slope to be reinforced. The Green Terramesh system is a quick and easy system to install as it does not require formwork to achieve the steep, sloping face.
The UK arm of the Maccaferri Industrial Group, headquartered in Italy, Maccaferri Ltd specialises in retaining structures, soil reinforcement, erosion protection, pavements, bio-engineering, basal platforms, waste containment and rockfall engineering. It supplies a very wide range of products including gabions, geotextiles, geogrids and geomats. In addition to a full range of products for retaining structures, soil reinforcement and erosion protection, the company offers an in-house design and consulting service and a design and construct service.