Project Description
The construction of a new airport at Pakyong in the Himalayan state of Sikkim in North East India will provide direct access to tourists and trade to this remote, yet beautiful region of the world. The region is home to diverse flora and fauna, a major draw to the tourists to the area, yet is currently only accessible by road and the nearest airport, Bagdogra, is 120km away, with the state capital, Gangtok, being 35km away.
The Rup2560 M (£G31M) project features a 550m wide, 1.7km long corridor was required in which the runway and airport buildings are constructed for client, Airports Authority of India (AAI). This was to be carved from the mountainous topography using massive geotechnical “cut and fill” engineering works. Traditional retaining structures of the height required would be prohibitively expensive and may not adequately cope with the seismic conditions. Conversely, shallow earthwork embankments would consume too much area. Therefore, extensive use of geogrid soil reinforcement and slope stabilisation was used to maximize the development area.
Main designer Mott Macdonald India and Dehli based main contractor, Punj Lloyd, worked with Maccaferri‘s Indian subsidiary, Maccaferri Environmental Solutions Pvt Ltd to carry out the soil reinforcement and slope stabilisation works. Beneath the runway, one of the tallest reinforced soil structures in the world is currently being constructed. When completed it will stand at 74m high and features 800kN/m Long Term Tensile Strength (LTDS) geogrids in conjunction with Terramesh (hard) and Green Terramesh (soft) face finishes, to minimizing the visual impact on the surroundings. Designed with specific geogrids at certain spacings within the slope, the structures accurately balance the cut and fill volumes. This eliminated the need for import or export of construction fill materials and polluting truck movements in this region. The cut slopes of between 4 and 111m high above the runway would provide the fill material for the reinforced soil structures required below the runway of between 4 and 74m high. Paralink™ geogrids, used as primary reinforcement, were placed at 1.6–2m centres within the soil slope, significantly greater than the more usual 600-800mm centres in other reinforced soil structures. This innovative use reduced the installation time, the amount of polymer geogrid consumed in the project and the number of deliveries to this remote site, reducing environmental impact.
Design of the slope used BS8006 for static design and FHWA (AASHTO) methodologies for the seismic design. The Paralink reinforced structure was subject to a 6.8 magnitude earthquake in September 2011 without sustaining damage; an event which destroyed other infrastructure in the area. AAI, as part of its corporate and social responsibility also established a paper recycling unit, education facilities within the villages and the Pakyong health centre, This facility caters for more than 100 small hamlets in a 25 km area around Pakyong town, itself having a population of 14,000. In addition to this, the AAI set up a rural water supply scheme for Lossing and Dikling villages.
Concept
The runway strip is planned in a N-S orientation along the natural slope with the uphill (cut zone) to the West and the downhill (fill zone) to the East.
Since a plane surface is required for the construction of the runway and airport infrastructure, cut material from the uphill portion should provide the fill material for the downhill portion. This constraint was mandatory to reduce environmental impact. Analysing the relationship between the volumes of cut, fill and the height and length of any retaining structures, optimised the commercial and technical requirements of the project.
The topography and volumes of cut material available necessitated the requirement of tall retaining structures along the east side. Among the many solutions considered, rigid reinforced concrete structures and numerous varieties of flexible reinforced soil structures, Maccaferri’s Composite Soil Reinforcement System, ParaMesh® was selected due to its flexibility, seismic performance, cost & speed of construction and suitability for use with the local materials. ParaMesh features a combination of primary reinforcement geogrids provided by Paralink™, and secondary reinforcement and fascia elements provided by Terramesh™ units. Paralink offers the highest LTDS (long term design strength) to UTS (ultimate tensile strength) ratio of any other geogrid. This enabled an increased vertical spacing between grids, contributing to the cost effectiveness of the solution by reducing the quantity of geogrids and increasing the speed of filling and compaction. These advantages become clearer still on these structures which reached a height of 74m, some of the tallest reinforced soil structures in the world.
With these reinforced soil structures, drainage is an important consideration in the design; Sikkim receives some of the highest annual rainfalls in the world. Furthermore, 9 natural streams (jhoras) used by local people for their daily water supply and agricultural irrigation crossed the airport site. This was achieved by designing extensive “chimney drains” behind the ParaMesh® structures, to collect the water by means of non-woven filtration/separation geotextiles & semi-perforated pipes. These were diverted below the runway through concrete culverts before finally navigating and exiting through the soil reinforcement of the retaining structure. The flexibility of the ParaMesh® system helped in achieving this. The permeable fascia of Terramesh also enables the drainage of water within the structural backfill, relieving the pore-water pressure developed.
As generating tourism was one of the major drivers to the planning of the airport, the proposed solution had to minimize environmental and aesthetic impacts and enable maximum revegetation and forestation following construction. The approximate estimate of reforestation achievable on the ParaMesh® solution through natural growth and artificial planting of local grass (e.g. Pennisetam Purpurium, Cynnobon Daetylon, Cyprus Rotendus etc), live stakes (e.g. Thysumulaenamexima) and creepers (like Haidra Halis), was 0.25 million m2. This was 50% of the total estimated reforestation for entire project. Added to this, the estimated reduction of carbon foot print by using a ParaMeshR structure (total CO2 footprint = 4,200 MT) was 93000 MT compared to the use of any RCC retaining structure (total CO2 footprint = 97,000 MT).
Design/Planning
Due to the mountainous topography of Sikkim, the required planar landing strip would be created by cutting the West side of the Dikling Hill and then using this as fill material to create the platform. A project constraint was to minimize the land-take (footprint) of the site. After many iterations, the final designed runway level was determined after balancing cut & fill quantity. Land for natural slope for retaining filled up earth was not available due to presence of Pakyong Bazaar area adjoining airport site requiring construction of retaining structures of height up to 74 m height for a length 1480 m. Project lies in seismic zone IV. Thus structures have to be highly resilient to withstand the seismic forces.
The design of the ParaMesh® structures was carried out by main consultant Mott Macdonald Indi
a following British Standard (BS: 8006) for the static analysis and AASHTO Standards (using FHWA guidelines) for the seismic analysis. Seismic Coefficients were calculated from the guidelines of the Indian Roads Congress (IRC-6). Due to the variations in topography & boundary conditions, the structure height and slopes had to be varied from wall chainage to chainage:
- Sufficient space – Embankment slopes not exceeding the angle of repose of the reworked fill material, were designed.
- Insufficient space – Sloped ParaMesh® structures, with revegetating Green Terramesh™ facia, were designed
- Highly limited space – Near-vertical ParaMesh® structures, with stone-faced Terramesh™ facia were designed
For intermediate situations, a combination of slope above (Green TerrameshTM) and wall below (Terramesh™) was designed. Wherever sufficient space was available at the top of the reinforced structures, a natural unreinforced slope was designed which were protected from erosion by using BioMac; coir erosion control blankets. Providing these sloped structures, Green Terramesh™ or natural slopes, wherever possible resulted in cost savings compared to walled structures.
Detailed stability analyses were carried out on representative sections of the Paramesh® structures, depending upon the height other key parameters.
All stability checks were performed using MacSTARS, Maccaferri’s slope stability software. Client Airports Authority of India, requirements are that, the design of these complex structures also had to be verified and approved by pioneer institutions such as the Indian Institute of Technology (IIT). The ParaMesh® structure schemes & designs were approved by IIT Mumbai, who was engaged by Mott MacDonald.
The robustness of the design was proven when the structure was exposed to not only extreme rainfall which has disrupted progress, but critically a seismic event of magnitude 6.8 in September 2011. This earthquake significantly damaged other infrastructure in the area, but the ParameshR reinforced soil structure was unaffected.
Construction
Construction of the entire project was awarded to the Delhi based contactor M/s. Punj Lloyd Ltd (PL) by the client AAI. Punj Lloyd engaged Maccaferri India for design vetting, material supply and construction supervision of the composite soil reinforced structures. Once designs were approved, construction of the full ParameshR structures commenced.
The 800kN/m UTS ParalinkTM primary reinforcement geogrid is laid out horizontally on the prepared ground. The fascia element (Terramesh™ or Green Terramesh™) is installed and fill material placed and compacted in layers. Once backfill reaches the top of the fascia element, subsequent fascia units are placed on top of it. The primary Paralink™ reinforcement was placed at 1.6m vertical centres between geogrids. This not only enabled rapid construction times, as the contractor did not have to stop so often to install geogrid, but also reduced the quantity of geogrid required.
Terramesh™ fascia units are filled with graded rock forming a “dry-stone” wall appearance. Green Terramesh™ units are designed to revegetate. A biodegradable coir matting is factory fitted behind the front galvanised woven wire mesh face of the unit. Topsoil is then placed behind this coir and in front of the compacted backfill to provide the substrate for vegetation establishment on the face of the reinforced slope.
The construction has been particularly challenging for various reasons:
- Progress is limited by heavy rainfall, particularly during the spring and summer monsoon period.
- Availability of skilled labour, especially to operate construction plant
- Plant had to be dismantled during transportation to enable it to pass over bridges and other infrastructure with vehicle weight limits
Construction of embankments of these heights requires significant skill and a critical component is the placement and compaction of the fill, in this case, in 300mm layers. Moisture content must be within a specified range, and the fill material often had to be turned to dry it to a suitable condition.
The contractor has an onsite laboratory to carry out a range of soil testing, including ensuring compaction of the fill material achieved 98% of density at optimum moisture content. The quality plan developed by Maccaferri India for the Paramesh® construction encompassed Maccaferri products; backfill, fascia stones, filter media as well as construction activities; foundation preparation, installation of facia units, soil reinforcement, compaction & placement of structural fill, installation of drainage filter media etc.
Construction progresses around the clock with cutting and movement of fill performed under floodlights at night. Placement and compaction of the fill layers is carried out in the morning. A substantial local workforce was used for much of the gabion Terramesh™ filling and topsoil and seed placement within the Green Terramesh™. The local workforce, are predominantly terrace farmers, operating in 40 day cycle after which they need to return home to work their fields.
As of January 2012, construction of the Paramesh® reinforced soil structures is beyond the half-way point. Exceptionally heavy rains throughout the construction period in 2010 and 2011 will delay completion of the works until December 2012.
See a press release about the award here
