Due to its hilly terrain and road meandering along river Teesta, connectivity to Sikkim’s petite-sized capital Gangtok is uncertain, especially in rainy season.
With a view to improve connectivity, the government decided to construct a new airport. Site selection for an airport in a hilly region is a tedious task, as the airport’s runway and apron requires flat land due to operational considerations. Pakyong, which is situated at approximately 30 km from Gangtok, was finally found to be a suitable location.
The new airport will be able to handle ATR-72 class of aircraft. Its runway 02/20 is 1,700x30 m and apron is able to park two ATR-72 aircrafts. The terminal building has a capacity of handling 100 passengers at a time. All other facilities and features required for a civil airport, ie, ATC tower, fire station, communication and navigational systems, runway lighting, meteorological aids and systems, are included in the project.
The biggest challenge while taking up construction of runway, parking apron, etc, was cutting hills and filling up valleys and ravines to achieve a flat terrain. Other challenges included:
• Retaining wall/structures upto 75 m height• Disposal of extra earth• Source for borrow earth material for filling• Stability of slopes• Impact on environment due to cutting and filling
The challenges were overcome by:
• Adopting a design with optimum level of runway at which volume of cutting matched with filling requirement• Choosing an innovative composite soil reinforcement system for hill slope retaining structure
Composite soil reinforcement system
This innovative system is created by the combination of high strength geogrids, as primary soil reinforcement and heavily galvanised and PVC coated mechanically woven double twisted (DT) wire mesh panels, as secondary reinforcement.Depending on the availability of space, a vegetated slope face is provided for the soil reinforcement system by installation of Green Terramesh (GTM) units as fascia. Wherever the space is limited, Terramesh (TM) units have been used as the fascia to construct a wall.
Final selection of the system was done based on following parameters:
Seismic consideration: Sikkim airport is located in a seismic region, and flexibility of the structure is very important to absorb seismic shocks. Rigid RCC walls are not suitable. Reinforced soil wall with panels, segmental walls, wraparound system walls and composite soil reinforcement system walls are flexible and have an ability to withstand and absorb seismic shocks, making them more suitable.
Foundation considerations: Normally RCC walls require greater depth of foundation needing increased excavation. Reinforced soil walls require lesser excavation effort as foundation depth requirement is smaller compared to RCC walls. Speed of construction: RCC wall construction consumes more time as it involves erection of formwork, placement of reinforcement bars, curing, removal of formwork, etc. Composite soil reinforcement system walls are less labour intensive to construct, and by utilising soil, the construction is speedy. They have modular blocks for ready use and dont require any special form works.
Use of local material: As mentioned earlier, one of the stringent requirements of the project is to use cutting material like soil and stones for filling on valley side. Composite soil reinforcement system uses locally availabe stones.
Cost of construction: RCC wall construction requires greater excavation for foundation, and costly materials such as cement and steel, scaffolding materials. Composite soil reinforced walls are 50-70 per cent cheaper than RCC walls.
Based on the above criteria, composite soil reinforcement system was the best possible solution as retaining structure.
Thus an environment friendly solution on cutting side has been adopted by minimising the use of any synthetic material. The hillside is cut to slope with an angle not exceeding 360. This slope is protected by erosion control blanket made of coir in order to prevent erosion, allowing vegetation to grow. The erosion control blankets are suitably anchored to the finished slope, using “U” pins. Specific seeds are to be planted on cut slopes for greenery.
Generally, for retaining structure use, an RCC retaining wall is an obvious choice. However, there is a big impact on the environment whenever an RCC structure is constructed. For construction of RCC retaining structure for the Sikkim airport project requiring 53,600 sq m of retaining structure with height ranging from 20-75 m, over 150,000 cu m of concrete of 18,000 mt of steel would have been required. To produce 150,000 cu m of concrete, approximately following quantities of material would be required:
The amount of CO2 emissions connected to the electricity required to produce steel at the steel plant has been calculated to be 0.65 tonnes of CO2 per tonne of steel.
Cement production result in CO2 emissions, accounting for 1.8 gt CO2 in 2005. The average CO2 intensity ranges from 0.65-0.92 tonne of CO2 per tonne of cement across countries with a weighted average 0.83 t CO2 per tonne of cement. For coarse aggregate production the CO2 emission is of the order of 4-4.5 kg/mt of aggregate. Similarly for fine aggregate CO2 emission is expected to be of the order of 0.5 kg/mt.
Thus, overall CO2 emissions for producing concrete and steel for constructing the RCC wall would have been about 65,000 mt.
For the proposed solution, only 608 mt of steel will be used for the facing. Secondly, reinforcing element, ie, high strength geogrid used for reinforcing the soil are made up of polyester and LLDPE which are a byproduct of petroleum purification. Therefore, the reinforcing element made up of polyester and LLDPE has very limited impact on the environment.
Thus, using composite system is cost-effective and very environmental friendly.
In addition to the above the gabion facia, ie, Terramesh unit uses locally available stones and sloped facia, ie, Green Terramesh promotes growth of vegetation at the facing.
Thus, apart from substantial reduction of CO2 emissions, there is an increase in the oxygen level in the environment due to the vegetation on the facing.
Channelling surface water and sub-surface water
The site at Pakyong has a rugged and mountainous terrain where contour levels vary from 1,330-1,520 m. A huge catchment area exists above the airport land and there are about 11 water streams flowing across the location. In addition, there are a number of sub-surface water streams, water springs across the airport site, which serve the water needs of the surrounding population. Channelling these surface streams and sub-surface streams is very important, not only for stability of filled up earth embankments, but also to meet the water needs of the people around the airport site. The sub-surface water springs act as a lifeline to the villages.
AAI designed a stormwater drainage system keeping in mind the environmental aspects taking care to avoid any environmental damage by following the topography of the land. The design takes care of both surface and sub-subsurface water to keep it intact. The design also takes care to use the water to upgrade the groundwater table.
All the water streams are channelled through box culverts with a network of intercepting drains. Box culverts pass below the runway and follow natural topography to the extent possible. Steps are provided in box culverts to keep velocities within limits.
Sub-surface water is channelised by constructing chimney drains using non-woven geo-synthetic and local stones from site alongwith a network of perforated pipes covered with non-woven geo-synthetic to stop the entry of soil in pipes.
The water from the jhoras and sub-surface streams will be collected for supply to the villages. A water supply scheme to villages, Dikling and Lossing, situated on the periphery of the airport site, has been executed and operationalised. The scheme is presently serving a population of more than 660 persons and is designed to serve 2,400 persons when fully operationalised.
Sikkim is a green valley with lot of flora and fauna. An RCC structure would have had a major impact on environment. The proposed structure with local stone and green fascia shall blend very well with the surroundings causing minimum adverse affects on environment. A system designed to tap natural water and making it available through a well designed network of non-woven geo-synthetic pipes, is slated to be hugely beneficial to local villages. Thus, AAI has been able to weave sustainability in such a way that it is beneficial to all local inhabitants, both environment as well as project construction requirements.
With sustainability goals, AAI now pays special attention at the planning and design stage for:
Site• Conservation of top soil• Prevention and erosion during construction• Avoiding noise, water, solid wastes and air pollution• Protection of existing trees and plantation of trees in and around building after construction
Building form• Shape, orientation and envelope of building, depending upon solar movement• Maximising daylight and heat gain • Wind direction• Use of insulated glass
Solar energy• Solar power PV panels and cells • Solar water heaters• Use of solar energy for street lights
Energy conservation• Adherence to ECBC norms • Use of all electrical appliances that conform to BEE 3 Star or above • Occupancy sensors, timers to shut off lights automatically
Water and waste management• Use of faucets/taps with sensors• Use of cisterns with low water consumption• Recycling of water through Sewage Treatment Plant (STP) • Use of recycled water for HVAC, gardening and flushing• Rainwater harvesting through recharging groundwater by borewells
The author is VP Agarwal, Chairman, Airports Authority of India (AAI).