In 2017, Shannon airport carried out the resurfacing of 75 per cent of its main runway, within extremely tight night-time possession windows.
The €14 million investment ensures the integrity of the runway for the next 20 years. Eoin Colgan, who managed the project on behalf of RPS Consulting Engineers, outlines the issues with the runway, discusses the technical solutions available, and describes how the project was successfully and safely delivered.
Shannon airport is located adjacent to the Shannon estuary. Its main runway – 06-24 – at 3,199m in length, is the longest runway in Ireland and is capable of handling all aircraft types. In 2017, the airport handled 1.74 million passengers. The airport is open 24 hours a day, seven days a week, with no curfews, slots or noise restrictions. Commercially, Shannon airport is part of Shannon Group plc. Itis operated by the Shannon Airport Authority (SAA). In 2012, SAA split from Dublin and Cork airports and operates as a stand-alone company. In order to finance the vital runway rehabilitation works, Shannon airport arranged a loan facility from the Ireland Strategic Investment Fund (ISIF).
The first phase of the runway rehabilitation process was the completion of a pavement evaluation survey. This was carried out in 2015. The survey concluded that the runway was structurally acceptable, taking into account future design traffic. RPS Group arranged for a surveyor to carry out a detailed topographical survey. As well as scheduled flights, which operate between 6am and 11.30pm, the airport facilitates a number of transit flights which stop over to refuel throughout the night. Consequently, the availability of access to the runway for a detailed survey was extremely limited. Given the tight window of availability, RPS chose to carry out a point cloud survey of the runway, over five nights, in early 2016.
Runway 06-24 was originally built in 1961. It was constructed using pavement quality concrete (PQC) poured in 50m wide bays. The nominal depth of these concrete bays was 340mm. The runway was subject to a significant overlay in 1981/2, which consisted of a 200mm thick layer of asphalt laid on top of the concrete runway. A 20mm porous friction course (PFC) was laid as the top wearing course.
The PFC and underlying asphalt material, which extend to approximately 75 per cent of the runway length, were in excess of 30 years in age. Both these material layers had already exceeded their intended design life. The PFC material was unravelling and generating an ongoing foreign object debris (FOD) issue. FOD is loose, unbound material laying on a runway which can cause damage to aircraft, particularly if it is sucked into jet engines.
A significant source of FOD is reflective cracking, working its way up through the runway from the original construction joints. The joints in the concrete pavement were located at approximately 60m centres. The propagation of cracking through the runway was increasing maintenance costs. The photo (right) shows the repair dressing that Shannon airport had to apply to the reflective cracking.
The essential requirements for the project were to:
• Produce a runway with adequate pavement bearing capacity for a lifespan of 20 years;
• Create a runway surface with a uniform surface and good wet friction characteristics for landing aircraft;
• Provide good crack resistance so as to minimise any future FOD risk, and;
• Provide new energy-efficient LED aeronautical ground lighting (AGL) for the CAT I edge lighting and the CATII lighting.
RPS examined in detail four possible paving solutions:
1.) Béton Bitumineux pour Chaussées Aéronautiques (BBA). This is a relatively new product, which has been developed and used in France. On investigation, it was found that this option would have significant risks, insofar as it has not been used in Ireland to date and the equipment to produce and test it would have to be specially imported.
2.) Stone Mastic Asphalt (SMA). The southerly 06 end turn pad of the runway was surfaced in SMA in 2014. However, to date, SMA is not commonly used for main runway surfacing.
3.) Porous Friction Course (PFC). The very short night-time possession windows available to the project would mean lots of construction joints in the new PFC. These joints would in turn increase the FOD risk in future years.
4.) Grooved Marshall Asphalt (GMA). Its use on runways is tried and trusted. GMA is a robust and enduring material. However, as a surface course it has to be grooved after it is laid, in order to achieve the requisite wet friction characteristics.
Grooved Marshall Asphalt (GMA)
The preferred technical solution was GMA. However, the constraints of the tight night-time possession window between 11.30pm to 6am during the construction phase meant that it was critical to reduce the volume of old asphalt material been milled out and to minimise the volume of new asphalt being laid. To address this issue, a Polymer Modified Binder (PMB) was introduced into the lower layers of the GMA. This resulted in a reduction in the depth of new asphalt to be laid and also increased the pavement’s ability to resist cracking. The selected design called for 70mm to be milled off the top of the existing runway and a new depth of 150mm of GMA to be laid. On average, the runway was raised by 80mm in height. Localised deepened strips of PMB were laid directly over the transverse joints, so as to combat future reflective cracking.
Other key features of the design included laying new cabling and installing new lighting pots for the replacement LED AGL. The drainage channel and gullies along the edge of the runway had to be reformed and rebuilt. In order to meet European Aviation Safety Authority (EASA) standards, all the chambers and manholes adjacent to the runway in the grassed areas had to be ‘de-leathilised’, meaning hard material had to be placed in slopes around the four sides of the chambers, so that an aircraft could safely ride up and over the chambers if it strayed off the runway into the grassed area.
Runway 06-24 is a critical piece of national infrastructure. It was vital that a well-managed and resourced contractor, with extensive experience of runway rehabilitation, be awarded the contract. Shannon airport could not afford to let a contractor ‘learn on the job’. A collaborative approach between the client, the designer and the contractor was essential to deliver a successful outcome. The competition used the negotiated procedure in accordance with the European Communities (Award of Contracts by Utility Undertakings) Regulations 2007. A pre-qualification process allowed four contractors forward to the award stage. Preferred bidder status was awarded on the basis of a 70 per cent quality, 30 per cent price basis. This reflected the importance of appointing a contractor thatwas best qualified for the project, as opposed to going for the lowest cost tender. Following negotiations, a ‘best and final offer’ was put forward by the preferred tenderer. The offer was accepted by Shannon airport and Lagan Asphalt Ltd was appointed in Q1 2017. The Government Contracts Committee for Construction (GCCC) contracts result in unbalanced risk allocation and are not sufficiently flexible. Therefore it was decided to use the NEC 3 –Option B contract. A key advantage of the NEC 3 contract is that risks are identified early and dealt with by both parties as part of a risk reduction process. There were extensive discussions between client, designer and contractor on all the design and on-site issues in Q1 2017 before mobilisation in April 2017. During the construction period, very few surprises or contentious issues arose.
Work on the runway commenced before the end of April 2017. The contractor had, on average, 90 workers and 70 vehicles at work every night. The available work window was between 11.30pm and 4.30am, five nights a week, from Tuesday to Saturday. Work had to cease at 4.30am every morning so as to allow for a FOD safety sweep of the runway. It was inspected and then formally handed back to air traffic control prior to the landing of the first transatlantic flight from New York at 6am. The pavement overlay was laid in three layers: base, binder and wearing course. The contractor laid only one type of course per night, so as to avoid any confusion with respect to material types. In accordance with EASA standards, a 1:100 ramp had to be formed every night to allow for aircraft to ramp up and down from newly laid layers. Likewise temporary runway markings had to be reinstated at the end of every shift. CAT II AGL lighting was temporarily lost in the first week of construction when the centre CAT II lights had to be switched off and decommissioned. This meant that the airport could not operate in CAT II conditions (low visibility due to mist and fog) while the work was being carried out. In total, just one day’s traffic was diverted to Cork airport near the end of September 2017. On average, the contractor laid the equivalent of a new football pitch of asphalt every night. In total, nearly 60,000 tonnes of asphalt were laid. The project benefited from relatively dry weather and only a few shifts were lost. The upgraded runway was completed by the end of September 2017, ahead of schedule and under budget.
A time lapse video of a typical night shift can be seen on YouTube. Enter ‘Shannon airport runway upgrade’ into the search function of YouTube or copy and use this link: https://www.youtube.com/watch?v=dNRfwvIC5Ts
Author: Eoin Colgan is a senior associate with RPS Consulting Engineers. He has more than 24 years’ experience in the construction industry working for contractors, local authorities and RPS. He specialises in project management, employer’s representative roles and contract administration. During his career he has worked on projects with a cumulative value of more than €2 billion in Las Vegas, New York and in Ireland with companies such as Sisk, John Paul Construction, Bovis Lend Lease and John Mowlem & sons. He is currently working on the High Speed 2 rail project in the UK.