Terraforce®, a longstanding (CMA) Concrete Manufacturer Association member, supplied over 110 000 L12 blocks for the construction of retaining walls on both sides of Terminal Boulevard, a newly constructed 10-lane thoroughfare which feeds directly into India’s Kempegowda International Airport at Bengaluru.
Completed in November 2022, Terminal Boulevard has been widened from an existing 2+2 lanes to 5+5 lanes to cater for the addition of a second airport terminal and an anticipated increase in traffic. Approximately 1.2km of the road has been lined with retaining walls on both sides which cover a total surface area of 9,000m².
Running on an east/west axis, the new road was constructed by cutting a corridor, which at its deepest, is nine metres below existing ground level. This has enabled the new road to pass under the airport forecourt roads to facilitate junction-free traffic flows.
The excavated corridor resulted in embankments which varied in heights of between 0.5m to 9m and in slope angles of 52° to 77° off the horizontal.
After considering various retaining wall options, the Bangalore International Airport Limited (BIAL) project team comprising of Prasenjit Biswas, General Manager Landside Planning, Monnappa BC, General Manger Projects, and Prasannamurti Desai, Vice President Landscape, chose the Terraforce retaining wall system for several reasons, aesthetics having been a major consideration. The blocks also allowed the planting of flowers and other vegetation and the inclusion of hume pipes facilitated the planting of trees. In addition, the Terraforce option required less soil excavation and was more cost-effective than either reinforced concrete or reinforced earthen walls would have been.
The slopes were cut to allow for the insertion of 5m hume pipes to facilitate planting of trees on the vertical slope at every 4m. Placed on concrete foundations, the pipes were filled with soil and the bottom ends of the pipes were kept open to allow for root penetration into the soil below.
Where additional cutting was required, for instance, near bridge abutments or the retaining wall ends, these sections were backfilled with gabion blocks and clad with Terraforce blocks to maintain the required slope angles.
One of the main challenges of this project was the fact that additional cutting into the embankments for the installation of geogrid reinforcement was not possible. This was because the top sections of the excavated embankments were only 2.00m from the upper-level roads and there was no space for the additional cutting. Moreover, most of these sections were operational areas. In addition, the required loading on these roads was up to 45 tonnes for fire-brigade trucks and other heavy vehicles. Therefore, soil nailing was specified as an alternative to geogrid to keep the slope stable.
This involved drilling 115mm diameter holes of up to 9m in length, perpendicular to the slope of the embankment. Nails, 28mm in diameter, were inserted into the holes and grouted with a non-shrinkable cement slurry. Once the nails had been inserted, PVC drainage pipes up to 2.5m long and wrapped in a geotextile membrane, were installed – in pre-drilled holes in the embankment at a slight upward incline to the horizontal plane. The drainage pipes were installed in a grid of 3m by 2m to release entrapped water and to reduce the hydrostatic pressure in the embankment. The slopes were then covered with layers comprising a geodrain membrane, two layers of 8mm steel mesh and 100mm of shotcrete. The geodrain membrane was installed to prevent the leaching of soil and fine material.
Steel plates with metal hooks were bolted to the nails and tied to the 8mm steel mesh layer with 100 mm shotcrete. The hooks were used for tying to a second layer of steel mesh after which a second 100mm layer of shotcrete was applied. The steel plates ensure the nails are affixed firmly to the steel mesh and the shotcrete layers to form a monolithic structure and to prevent the nails from penetrating through the wire mesh and the shotcrete layers.
Reinforced concrete shear-key blocks, which arrest both the vertical and lateral forces acting on the Terraforce blocks, were cast to form the foundations of the Terraforce walls. Steel rods were inserted into the hollow portion of the blocks for interlocking between each vertical layer of blocks and the spaces between the block facades and the shotcrete layers were filled with lean concrete.
The bottom block rows followed the slope of the road, and similarly, the top rows were adjusted to match the profile of road or the ground at the upper level. This was done either by stepping the top row or by adding a concrete layer to match the required road or ground profile.
The sections between the top of the Terraforce walls and the upper-level road kerbing was paved with precast concrete paver blocks to prevent water ingress into the retaining wall structure and the soil behind it. However, any water trapped within the embankment structure will drain through the PVC drainage pipes.
The construction of these embankments required a mock-up section to identify constraints in the installation process and to improve the installation methodology and sequencing during the actual installation.
Some of the other challenges were the varying friction coefficients of the embankment soil which was the deciding factor in determining the length of nails. And managing the levels of the walls when both the top and bottom roads sloped at different angles, was another.
Once the Metro is established in the centre of the two access lanes, this project will shine in full, with the plants fully established and the trees fully matured, providing a visually lush and green entry point to Kempegowda International Airport.
Layout and Concept: Prasenjit Biswas, Prasannamurti Desai, Monnappa BC (BIAL) & Fred Laker (Terraforce)
Landscape Architect: Grant Associates and Design Accord
Engineers: Fred Laker/Terraforce, Maccaferri, Struct Geotech, Smart Minds, STUP India Pvt. Ltd
Quantity Surveyor: BIAL Inhouse Team
Main Contractor (Civil): Balajee Infratech & Constructions Private Limited
Landscape Contractors: Hariyali Landscapes
Terraforce Supplier: Prayosa Buildmat
Prasenjit Biswas, Bangalore International Airport Limited, General Manager – Landside Planning
Prasannamurti Desai, Bangalore International Airport Limited, Vice president Landscape
|Monnappa BC, Bangalore International Airport Limited, General Manager – Projects|