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Monday , 24 January 2022

Asia’s biggest geogrid reinforced retaining wall coming up at Sikkim’s Greenfield Airport

In limited spaces, gabions, i.e. terramesh wall units are used as the facia. The facia and secondary reinforcement are to be monolithic and ready to use at site. This system was developed in the late 90s by Officine Maccaferri, Italy in order to analyse how a more efficient use of the reinforcements will work in favour of the overall project’s economy.

The maximum height of the cutting slope protected by gabions walls at the site will be approximately 100m. Toe walls and erosion control solutions on cutting side (West) consist of stone filled gabions as toe walls (3m high max), geo-textile as filter/separator behind gabion walls, erosion control coir blankets along finished slope above toe wall, gabion toe wall and overlying slope finished to corresponding angle of repose.

The slope is protected by erosion control blanket made of coir in order to prevent erosion and allow vegetation to grow. The erosion control blankets will be anchored to the finished slope with the help of ‘U’ pins.

Products

Pruducts used for the reinforced wall like Terramesh, Green Terramesh and ParaLink are indiginised and produced at the Ranjangaon factory of Maccaferri, near Pune, Maharashtra.

The Greenfield Airport being the first of its kind in India, it was a challenge to construct it at a remote land locked location. The airport contract is on construction basis. The volume of earth work was huge and the specifications were very stringent. To construct 80 metre high earth fill wall in layers of 0.30 metre itself was a challenge. SS Raju

Paralink is a high strength geogrid that acts as primary soil reinforcement. It guarantees high tensile strength, overall and compound internal stability of structure. Double Twist Hexagonal Steel Wire products (System Terramesh or Green Terramesh) act as facia & secondary reinforcement allowing for a better compaction and assuring the facing shape, stiffness and stability. When longer reinforcement lengths are required, Terramesh units with longer tail length have production difficulties. Longer units require multiple folds which make the handling of on-site material difficult. The use of high strength geogrid (Paralink), easier to stock at site in rolls, solves these problems. Modular facia of Terramesh and Green Terramesh ensure advantages like product uniformity, elimination of separate formwork, single construction unit and ease of maintenance at site.

ParaLink™ geogrids are planar structures consisting of a mono-axial array of composite geosynthetic strips. Each single longitudinal strip has a core of high tenacity polyester yarn tendons encased in a polyethylene sheath. These single strips are connected by non-resisting cross laid polyethylene strips which give a grid like shape to the composite, as shown in below figure 1.

Green Terramesh, GTM, constitutes a mechanically woven double twisted (Zind and PVC Coated) steel wire mesh, a biodegradable erosion control blanket and a welded mesh panel with loose steel tie rods. The steel rods are fixed at the top of the mesh panel. At the bottom, they are connected to a steel rod passing through secondary reinforcement while installation at the job site to form the required slope angle. A biodegradable 100% coconut fiber biomat is attached to the inner facing in order to prevent soil erosion at facia and allow vegetation to grow.

A Terramesh unit, TMS, is fabricated from galvanised and PVC coated double twisted steel woven wire mesh. Terramesh units are folded at the factory and transported to the job site in bundles where they are unfolded again, and the front panel, back panel and the diagphram are appopriately laced together to form a unit as shown in Figure 2. The facing element of a Terramesh unit is divided into two cells by a diaphragm positioned at approximately 1 m centres.

ParaMesh enables to build a vertical Reinforced Soil Wall with Terramesh as facia (Figure 3), or a steep Reinforced slope (upto 70m to horizontal) with Green Terramesh as facia. It is also possible to build a combinaton of vertical wall and steep slope to make opitmum utilisation of space. The photograph at the top shows a ParaMesh structure under construction at the Sikkim Aiport. Green cover has developed over the GTM units within two months of installation. The strucutre blends in well with the scenic beauty of Sikkim.

However, it is to be kept in mind that the weathered stones which appear to have good strength in dry conditions lose the strength significantly in water.

Storm Water Drainage

Since Sikkim receives very heavy rainfall, it was imperative to plan a foolproof storm water drain design. What needs to be kept in mind is that the runway strip is carved out of sloping hills of Pakyong Bazaar, that local people use water from the jhoras (natural streams) for their daily requirement (there is no other source of water) and that they need the maximum amount of water for their use. So, despite nine out of 11 jhoras within the site across the runway, the design team decided not to retain that water for airport use.

The crossing jhoras are channelised by six box culverts under the runway towards the downstream by providing catch water drains and stepped intercepting drains along the gabion cascades. Water from the cascades gets diverted outside the site through RCC box culverts. So, longitudinal drains along the runway strip, catch water drains on the graded area on the hill, designing of jhoras as step type drains, toe drains along the footsteps of RE walls and box culverts have been used to provide proper drainage system.

In order to avoid high water velocity and possible washout, the water from the hill slope was intercepted and diverted into chutes or natural hill side drains through strategically placed (by giving a gradient of 1 in 50 to 1 in 33 ) catch water drains. Longitudinal drains, constructed of dry rubble masonry with saucer, semi circular, trapezoidal, triangular or V-shaped in cross section, are provided on the road side at the foot of the hill slope to divert water from the portion of the hill slopes below the catch water drains. For draining silt and debris, material culverts are provided. Adequate protective works are required at the discharge point towards the valley side – stepped toe walls are provided to dissipate the kinetic energy of the discharged water of outfall.

Seismicity Precautions

Since the area is highly seismic, AAI carried out the entire design with the support of M/s Mott Mc Donald keeping the aspect of earthquakes in mind.

However, Gharpure explains, “Rigid structures are prone to cracks. This is not a rigid structure. It is built layer by layer and the fascia is of stone or vegetation. It is very flexible and better adaptable to earthquakes. It suits the size of the design. This zone 5 was designed by Mott Macdonald on the assumption that should an emergency occur, the design should be able to withstand seismicity. So, this aspect has been incorporated in the design.”

Green Efforts

Bioengineering techniques were followed on the site through vegetation insertion like staking and planting creepers with long roots. “Soil bioengineering techniques with woody types of vegetation need to be developed in maximum area due to their deep root system development,” says Gharpure. “Seeds of local plants shall be scattered on the fascia or mixed with the vegetative soil. Live stakes suitably selected by a local horticulture expert keeping the local climatic conditions in mind, should be horizontally placed between the fascia units at 1.5m c/c.”

Focus on revenue generating infrastructure, manpower issues and liquidity crunch are some of the major challenges in infrastructure development. Creating awareness is one approach that can help to highlight the importance of non-revenue generating infrastructure as well such as rock fall protection. We are doing a lot of promotional works for general and technical solutions that we offer. People are becoming more sensitive and understanding. To overcome the problem of manpower, we may move towards mechanised construction in future. — Ashish Gharpure

Tonnes of concrete and steel has been replaced with green solutions, thus reducing carbon footprint. Large chunks of land has been saved from being used for construction and even the realignment of the single road line connecting Pakyong and Siliguri has been avoided.

If weather conditions permit Sikkim will have its first airport by 2013 – an airport that will be known for one the tallest reinforced walls of the world.

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