Holding the River Steady: Ocean Double-Twisted Gabions at a Lock-Cum-Bridge in Malappuram, Kerala

A lock-cum-bridge at Malappuram in Kerala needed bank protection and scour defence that could withstand a monsoon-fed river’s force. Ocean Non Wovens supplied 190 units of Ocean Hexagonal Woven Double-Twisted Gabion for the works, used for bank protection, retaining and scour defence around the structure.

Project Snapshot

A lock-cum-bridge does two demanding jobs at once. It regulates the flow and level of a river while carrying road traffic across it, so the structure and the banks around it are permanently exposed to moving water, changing levels and the erosive force of a monsoon-fed river.

On such a project at Malappuram in Kerala, the banks and approaches needed protection that could stand up to that environment without cracking or being undermined.

Why Gabions Are Used in River and Water Structures

Kerala rivers are short, steep and fed by an intense monsoon, so they rise quickly and carry a great deal of energy. Around a fixed structure like a lock-cum-bridge that energy concentrates: the flow accelerates through the regulated opening, scours the bed at the foundations, and attacks the banks immediately upstream and downstream.

If the river undermines the toe of a bank or the base of an approach, the whole structure is at risk.

The traditional answer was mass concrete or stone masonry, but rigid protection has real weaknesses here. Concrete is impermeable, so it traps water pressure behind it, and brittle, so when the river scours the bed at its foot or the ground settles, it cracks and can fail.

Gabions work with the river rather than against it, being flexible, permeable and built from stone, which is why they have become a standard tool for river training, bank protection and retaining structures in hilly, high-rainfall regions.

What a Hexagonal Double-Twisted Gabion Is

Double-Twisted Mesh Construction

A gabion is a box made of steel wire mesh, filled on site with hard stone and stacked to form a structure. The Ocean gabions used on this project are made from hexagonal woven mesh in which each pair of wires is twisted around the other twice, a construction known as double-twisting.

In a double-twisted mesh, if a single wire is damaged or cut, the twist stops the mesh from unravelling, so the box keeps its shape and strength. A simple welded or single-twist mesh does not have this safeguard and can open up at a single break.

Wire Coating and Standards

The wire itself is heavily coated to resist corrosion, with thick zinc or zinc-aluminium galvanising and often an additional polymer coating for submerged conditions.

Filled with sound stone and laced securely to its neighbours, each gabion becomes a heavy, monolithic unit, and a wall of them behaves as a single flexible gravity mass. In India this mesh is covered by IS 16014, and internationally by EN 10223-3 and ASTM A975, which set the wire diameter, mesh size, coating weight and tensile requirements that give engineers confidence in the product.

Why Gabions Were Chosen Over Rigid Alternatives

For a lock-cum-bridge on a monsoon river, the qualities that matter most are flexibility, permeability and durability, and gabions provide all three where concrete struggles.

• Flexibility: When the riverbed scours or the foundation settles, the structure deforms and redistributes load rather than cracking.
• Permeability: Water that seeps into the bank drains straight out through the stone fill instead of building up the pressure that pushes over so many rigid walls.
• Local materials: Built largely from locally sourced stone, gabions make good use of material available near many Kerala sites, reducing cost and the need to haul in cement and reinforcement.
• Speed of construction: They need no formwork or curing and can be assembled and filled by hand, which suits river sites with difficult access and a short dry-season window. For a structure that must be ready before the next monsoon, that speed is valuable.

The Engineering Challenges at the Site

Several conditions shaped the design:

• The river carries high monsoon flows, so velocities around the regulated opening are severe and the risk of bed scour at the toe of the banks and near the abutments is constant.
• The banks are often soft and become saturated during the rains, which raises pore-water pressure and makes them prone to slipping.
• Changing water levels, from full regulation to drawdown, subject the banks to repeated wetting and drying and to the pressure changes that accompany a falling river.
• The high rainfall keeps the ground wet for much of the year and demands free drainage in any protection structure.
• The works had to be completed within the limited dry period.

How the Gabions Perform

Retaining and Bank Protection

As retaining and bank-protection structures they hold the approach embankments and riverbanks in place with a heavy gravity mass that resists the push of the soil behind. As revetment along the water-facing slopes they armour the bank against the current and wave action that would strip it away.

At the toe and around the abutments, gabion aprons resist and adapt to scour. Because the mattress is flexible it settles into a scour hole and keeps protecting the bed rather than being undermined as a rigid apron would be.

Drainage and Vegetation

Throughout, the permeability of the stone fill relieves the water pressure that builds in a saturated bank, so the structure drains itself and stays stable through the seasons.

Over time, silt collects in the voids and vegetation establishes in the face of the gabions, which binds the structure further and blends it into the riverscape. The combined effect is durable protection of the banks, the approaches and the foundations of the lock-cum-bridge.

The Technical Basis

Gravity Stability, Flexibility and Permeability

Three properties explain why the system works:

• Gravity stability: The weight of the stone-filled units resists both the earth pressure behind a retaining wall and the drag and lift of the flow along a revetment, with walls built to a batter or in steps.
• Flexibility: The structure deforms and conforms to ground movement and scour without the brittle failure of concrete.
• Permeability: Free drainage through the stone prevents the build-up of pore-water pressure.

Mesh Integrity and Filtration

Underlying all of this is the double-twisted mesh, whose non-ravelling behaviour keeps the units intact even if a wire is damaged, and the corrosion-resistant coating that gives long service life in a wet, sometimes submerged environment.

Where a gabion retains fine bank soil, a geotextile filter is placed behind it to let water pass while holding the soil back, protecting the bank from within.

Installation and Site Practice

The performance of a gabion structure depends on sound construction. The key steps are:

1. Units are delivered folded, then assembled and laced together with the correct tying wire so adjacent boxes act as one.
2. The foundation is levelled, and a geotextile filter is laid where the structure will retain soil.
3. The gabions are filled with hard, well-graded stone sized for the mesh, packed by hand at the exposed faces to minimise voids.
4. Internal connecting wires are placed in the taller units to stop the stone bulging the faces.
5. Each course is laced to the one below, and walls are stepped or battered as the design requires.

Careful filling and lacing separate a gabion structure that lasts for decades from one that sags. Common mistakes to avoid:

• Undersized or poorly graded stone
• Loose lacing
• Skipping the filter layer behind the wall

Long-Term Performance

A gabion structure is built to last, and its durability rests on the coated wire and the stone fill. The heavy galvanising and any polymer coating protect the wire against the corrosion that constant moisture would cause, giving long service life even where the units are periodically submerged.

The stone fill is inert and needs no maintenance, and because the structure is flexible and self-draining it does not suffer the cracking and pressure failures that afflict rigid walls.

As silt and vegetation take hold, it becomes more stable and more natural in appearance with age. Maintenance is minimal, usually no more than occasional inspection, and the system avoids the repeated repair that scour and water pressure force on conventional protection.

Sustainability

Gabions are among the more environmentally sound options for river protection:

• Filled largely with local stone, they cut the transport and carbon associated with cement and concrete and use no formwork or curing.
• Their permeability keeps the natural movement of groundwater intact.
• Their capacity to trap silt and support vegetation lets them green over and integrate into the river environment rather than sealing it in concrete.
• Built to last for decades with little maintenance, they reduce the need for repeated reconstruction.

About Ocean Non Wovens

Ocean Non Wovens supplied 190 double-twisted gabion units for the bank protection, retaining and scour defence works at this lock-cum-bridge.

As an Indian manufacturer and supplier of gabions, non-woven and woven geotextiles, geocells, geogrids, geobags and HDPE liners, the company produces materials to Indian and international specifications.

Ocean Non Wovens supports river training and bank protection, retaining structures, highways and bridges, airports, railways, water resources and environmental engineering, industrial developments and defence infrastructure across India. For gabion works, slope protection, reinforcement, filtration or containment on your next project, Ocean Non Wovens can supply the right product for the site conditions.