Railway Projects at Ghana Case Study

1800nos gabion boxes were supplied for railway works in Ghana, where the line crosses steep, rain-softened lateritic ground with little room on either side of the track. The gabions were used for retaining walls along the formation and an access track, holding back slopes that had been prone to slips.

Product – Gabion Boxes – 1800nos
Location – Ghana

What gabions are

Gabions are wire mesh cages filled with stone or other inorganic material such as broken concrete, and in architectural work sometimes glass rock. The name comes from the Italian word for a large cage. The mesh is usually galvanized or PVC-coated steel wire, and occasionally stainless steel or polymer. Stacked and laced together, filled gabions form walls that are heavy enough to act as gravity structures yet flexible and permeable. How long a gabion lasts depends mainly on the wire and its coating. Double-twisted hexagonal mesh gabions are specified to standards such as ASTM A975 and, in India, IS 16014.

They are used in road and railway works, slopes and retaining walls, river and shoreline erosion control, military works, foundations, landscaping, mining, ports and airports, and as architectural fences and facades.

Problem

The Ghana railway line runs through varied terrain. In several places space was limited and the side slopes were very steep, and the construction schedule was short.

Building and widening the track required large cuttings in lateritic and basaltic rock. Exposed laterite softens badly under intense tropical rain: it loses cohesion and strength when saturated and becomes prone to slides and slips. The cuttings therefore needed retaining walls and rockfall protection.

Ballast under the sleepers spreads load and gives the track resilience. The ballast shoulder slopes outward from the sleeper ends, so the formation needs enough width on each side to hold it, and under constant traffic ballast tends to work its way out of the track. On narrow, steep-sided sections, retaining walls provide that width.

Solution

A gabion box wall was designed and built to hold the site boundary along the length of a new development’s access road. The gabions were filled with crushed concrete and faced with stone fill.

Gabion walling was also used to build an access track alongside an existing station. When the old station is demolished and the new one built, a piling rig will use this track to install the new foundations. The gabion walls are built and backfilled on both sides of the line, which reduces the risk of the embankment slipping.

Why gabions suit this kind of site

• Drainage: water passes straight through the stone fill, so little hydrostatic pressure builds up behind the wall and extra drainage behind the face is often unnecessary. That matters on laterite, which weakens when it stays wet. A non-woven geotextile is normally placed between the backfill and the gabions so soil does not wash into the stone.
• Flexibility: the wire mesh lets the wall settle and deform on uneven or shifting ground without losing strength, which a rigid concrete wall cannot do.
• Corrosion resistance: a well-bonded zinc coating protects the wire from atmospheric corrosion; PVC coating adds protection where the wall is in contact with water or aggressive soil.
• Environment: gabions use stone with little processing and typically have lower CO2 emissions than a concrete retaining wall of the same size. With care, vegetation can grow in the wall and blend it into the surroundings.
• Durability: filled gabions withstand floods, heavy rain and soil pressure.

Related: rockfall protection and other case studies.