It is nearly 10.30pm on a freezing winter night at Glasgow International Airport. Dozens of yellow-clad construction workers and their machines stand in line at the airport gates, poised like paratroopers about to jump. Suddenly, on some hidden signal, the machines roar into life. The gates open and slowly the queue shuffles forward. In just a few minutes the yellow procession has disappeared into the darkness beyond.
Main joint venture contractor Amec/Tarmac began this routine in November 1999 and will repeat it nearly every night through to March 2000. It is central to the airport's plan to replace the entire length of its 2.4km long, 60m wide main runway. Every one of the 88 machines and 150 workers that make up the mobile runway production line has to go through the gate in exactly the right order as soon as the site gate is opened.
Such detailed programming is needed because the work has to be carried out between 10.30pm and 5.40am every night so that a fully operational runway can be handed back each morning to client BAA.
Planning actually began nearly four years ago. In summer 1996 the airport's main runway, last resurfaced in 1985, was approaching the end of its design life. In that time flights in and out of the airport had doubled and BAA was unsure how many more harsh Scottish winters the runway could take.
Original construction was a 20mm bituminous wearing course on top of a 100mm Marshall Asphalt base course - a specialist asphalt developed in America for use on runways that uses a higher aggregate content to provide a stronger mix - on a 228mm concrete sub base. The relative thinness of the top layer meant any movement in the concrete joints below could result in potentially hazardous cracking in the surface. Runway lighting cables were also becoming brittle and some of the lights were going out of adjustment.
BAA carried out a detailed survey of the runway, including taking core samples over its length to establish the integrity of various structural layers. By April 1998 three elements of upgrade work were identified.
First was to strengthen the central section of the runway to meet the demands of larger and more frequent aircraft for the coming 15 years.
Second, replacing the airfield ground lighting system would bring the runway up to the highest Category Three standard. This included recabling the entire system, building new cable ducting and installing 920 new runway lights.
The final element was replacement of the existing 20mm friction course with a 50mm Marshall Asphalt wearing course with 4mm wide, 5mm deep grooves at 25mm centres running across the runway to remove surface water.
The major challenge for the team planning the job was the short window in which the work could be done. The resurfacing work in 1985 was carried out in the summer but this is no longer possible because of the large number of holiday charter flights. By working at night, through the winter, disruption was kept to a minimum.
Work was scheduled to be carried out over two years. The first phase, from November 1998 to March 1999, saw 90 structural joints replaced on the runway and cable ducting and new electrical pits installed around the runway.
A second phase, planned to run for five months from November 1999, involved removing the old and installing the new lighting system, planing off the old 20mm friction course and laying 144,000m2 of new asphalt surface.
In order to get the work completed in the six month window the tasks had to be planned to run concurrently. This meant dividing the runway into 200m to 400m segments so that planing, surfacing and grooving is carried out in different sections at the same time.
To allow all this to happen and to maximise possession time, engineers from consultant Babtie, the Amec/Tarmac JV and BAA spent a year assessing issues of sequencing and logistics. They had to look at the full sequence of works, including everything from replacing the existing lighting to clearing the runway of debris after each shift.
'Risk management is absolutely key,' explains Babtie project supervisor Keith Waywell. 'We had to design out the risk. We have been meeting fortnightly throughout the year to plan this operation.'
Failing to meet handover times was identified as the key risk. And central to minimising this risk is an innovative system of interlocking security gates that allows men and equipment to pass through the Airport's strict security system throughout the day into a site compound to get ready, although it prevents them from passing through the final gate into the live airport until handover at 10.30 pm.
During the day vehicles are lined up in the compound in order of works. So when the gate opens the mobile lighting units go in first, followed by each trade in a carefully planned sequence.
To date the project is ahead of programme and Waywell says this is entirely down to planning. He adds 'It's funny, the more you plan the luckier you seem to get.'
Why read this
Possession management is the key to runway resurfacing
Innovative lighting speeds
More than 140,000m2 of high
performance surfacing laid to strict timetable.