This article explores some of the technical challenges in implementing runway underpass and the way forward with regard to Mysuru airport runway expansion.
Mysuru airport at Mandakalli is all set for expansion and the euphoria around it is only increasing with every passing day. Mysuru is currently the only tier 2 city of the state devoid of a full-fledged airport. Expansion plans were mooted multiple times but all were kept in cold storage forcing Mysoreans to use Bengaluru airport for a faster connectivity to the outside world.
But now we are close to realising the dream of a full-fledged airport in our own backyard. As per Member of Parliament Shri. Pratap Simha, a portion of the total fund for Land acquisition(LA) will be made available by the state government soon, and the LA process shall commence early next year.
Air tarmac over a Highway
The peculiarity of Mysuru airport is that it is sandwiched between Mysuru-Chamrajanagar railway line and the Mysuru-Nanjangud NH 766. So, when viewed in plan, the centre line of runway is aligned perpendicular to both rail and highway. Diverting or realigning either of them proved to be cost prohibitive. Hence, expanding the airport in the current configuration was challenging and required a very unconventional solution.
The solution was to expand the runway across the National highway by grade separating them with the construction of an underpass along NH 766. Such concepts have been successfully adopted by various airports around the world from as early as the 1900s. But there are engineering challenges in implementing them. Varanasi and New Delhi are among other Indian cities which are exploring similar options for their runway expansion.
A Look at the various aircraft bridges around the world
Couple of examples of the Taxiway/Runway
Sydney Airport, Australia
Stapleton Airport runway tunnel in east Denver, US (Non-operational now)
Leipzig Airport, Germany
Crash Impact and bomb proof underpass: Operating the runway over a live highway underpass is fraught with safety challenges. The design of such underpasses must account for extreme events like traffic crashes or a terror act involving explosives that are detrimental to its structural integrity. Such events impose large transient dynamic loads on the structural elements. Hence, it is necessary to provide adequate structural capacity to meet the loading demands. In simple words, the structure must dissipate the energy of ultimate imposed load without collapsing. And post event, the structure may be designed such that it is serviceable with repairs.
With regard to the extreme event of explosion, it is noteworthy to mention that the underpass in lieu of its confined configuration will cause reflection of blast waves within and further amplify the loads. So, Well-designed Structural joints, damping elements and optimum reinforcement ductile detailing will play a major role here.
To reduce the magnitude of this risk, certain interventions can be adopted as a first layer of defence-
- Regulating Underpass traffic movement during an event of take-off or landing. Either through speed reduction or complete closure of traffic.
- Continuous surveillance of the buffer zone along the underpass for detecting suspicious activities.
- Installation of crash barriers around structural elements.
Robust design of Underpass roof slab and foundation: The roof of the underpass would serve as a foundation for the runway across the highway, so It is very essential to ensure that there is absolutely no differential settlement between the runway over underpass and the at grade runway pavement at any given time. In simple words, the underpass roof slab should maintain its level at all times without deforming. If a differential settlement were to occur it would then produce a big jerk to the aircraft in motion, not only proving dangerous to the aircraft frame but also rendering the runway non-functional.
Such settlements may happen due to long term effects like- ground settlement beneath the foundation and creep induced deformation of structural elements or due to seismic activity. (With sustained loading the roof slab may deform with time, a phenomenon called creep.)
Aircraft weight: In the design of runway underpass the major parameter that governs is the weight of the aircraft and the impact factor. The roof and lateral frame of the underpass are subjected to significant braking forces during an aircraft landing. Usually it is assumed to be 70% of the total weight of the aircraft.
For example, Boeing 747-8 Freighter has a maximum take-off weight (MTOW) of 447.7 tons and a maximum landing weight(MLW) of 346.091 tons. So Boeing 747-8 F generates 242 tons (70% of MLW) of braking load in lateral direction which the substructure of the aircraft bridge needs to resist safely.
- So if Mysuru airport is aiming to go international it must have its airport underpass designed for the maximum weight of the aircraft (passenger/cargo/military) it envisages to cater for. Ideally the underpass must be designed to support heaviest plane like Airbus 380 and Antonov M225 (600,000 KG)
Speed and ease of construction: During the construction of the runway underpass highway traffic needs to be diverted or at times it may warrant closure of airport operations if safety is an issue. Traffic diversions increase commuters travel time and cost. Hence, the process of construction of the underpass needs to be expedited. This is where RCC precast, RCC-Steel composite and prestressing technology helps.
Precast construction involves casting of structural elements in a factory under controlled environment and then shipping them to the site for their installation and integration. This modular technology is century old and proven. Once the necessary land is cleared for underpass construction, various substructure and superstructure precast elements can be brought to the site and stitched in-situ in record time while still ensuring quality. This modular construction is very advantageous unlike the RCC in-situ staged construction which is time consuming and resource intensive.
The above points shed some light on the structural design engineering challenges such aircraft bridges bring on, which only calls for robust fool proof designs of underpass structural elements.
The Way Forward
Mysuru airport expansion project involves construction of a new passenger terminal, underpass, additional apron and taxiways. From here it is necessary to plan in such a way that the time taken to get the project completed is reduced. For which the owner AAI needs to be open minded to take up the works in phases as the entire land parcel might not be available in one go. So start the works wherever land parcels are available. This requires phased planning of construction activities.
To start with, the AAI can initiate construction of Underpass in Phase 1 and subsequently plan for other construction works as and when the land parcels are made available. Underpass construction would require minimum land requirements as it is along the National highway 766. All that is needed to be done is temporarily divert the NH traffic to nearby existing road networks. Once the underpass works are done, the road traffic can be reinstated back on the original NH 766 route.
As mentioned earlier the underpass needs to be designed to support operations of heavy duty cargo planes like Anotnov M225 (6,00,000 KG) and the width of the underpass be sufficiently wide enough to accommodate increase in highway traffic in lieu of future industrialisation.
The design and construction of runway underpass is critical and is fraught with challenges, hence, it is in the best interest of the project that phased construction be explored immediately with open mind.
Get started with the Mysuru Airport Runway underpass construction.