Between two of the world's busiest airport runways, an excavation is taking place to allow construction of an underground railway junction at Heathrow. David Hayward reports.
The notice greeting travellers on London's Piccadilly Line is clear: 'Heathrow Terminal 4 underground station closed from January 2005 until September 2006 - replacement bus service from Hatton Cross.' This disruption for the thousands of airline passengers and staff who use the railway every day to reach Britain's biggest airport is to allow the Piccadilly Line to be linked to an extension to the new Terminal 5.
The 20-month closure could have lasted much longer, were it not for the fast-tracking of groundworks and diaphragm walling. This is under way on a site in the middle of the busy aircraft taxiways and stands between Heathrow's two main runways.
Both the excavation and station closure are needed to allow the already tunnelled extension to be connected to the main Piccadilly Line.
Directly above the junction point and just metres from the operational airfield, Bachy Soletanche brought in a hydraulically operated diaphragm wall grab to ensure that it could form, in seven weeks, the perimeter of a 41m long rectangular cofferdam box.
This will soon be excavated into a large, 18m deep access shaft, to allow temporarily closed Piccadilly Line running tunnels beneath to be broken into and the junction formed.
'We completed up to five, 25m deep diaphragm wall panels a week, ' says Bachy Soletanche contracts manager Barrie Arkwright.
'That is 50% quicker than is possible with a conventional rope operated grab.' The 1.6km long twin-tunnel Tube extension, now being fitted out, starts from the new Underground station at T5. It is routed deep below aircraft stands between the runways and at present stops at the diaphragm wall site, a few metres short of the Piccadilly Line tunnels.
These tunnels, at the western end of the line, are part of a 5km long loop from Hatton Cross, which curves round beneath the airport before rejoining the main route back to central London. The loop connects two Underground stations, one serving Terminals 1 to 3 and the other Terminal 4.
It is part of this loop line, including T4's station, that is closed to allow a 40m length of the twin 4.3m diameter tunnels to be broken out to create the T5 link.
To form the diaphragm walled box, Bachy brought in a KS3000 hydraulic grab which offered both To ensure accurate control of the KS3000 hydraulic grab, its alignment is guided by a kelly bar fixed to the top of the grab unit.
As the grab is raised, this vertical bar slots into a box ensuring the whole frame remains central and vertical for the next bite. Before it takes that bite, the frame rotates horizontally through 180¦.
The grab's interlocking jaws have six teeth on one side but only five on the other. This would encourage successive bites to veer slightly sideways, so instead the automatic rotation ensures the cut remains straight.
the required cutting speed and 1 in 200 verticality - much greater than that achievable with rope grabs.
The wall's 25 panels, designed by airport operator BAA's consultant Mott MacDonald, vary in both length and depth. At 7.1m the longest panels were excavated in three bites of the grab's 2.8m wide jaws and most extend 25m deep, well below tunnel inverts.
But several 1.2m thick panels, lying directly above the two tunnels as they pass across the box, were excavated only 12.5m deep and stop only 800mm above the crown of each Tube tunnel.
To ensure bentonite could not escape into a tunnel from the base of these shorter panels, a 6m length of each tunnel directly beneath the wall was plugged in advance with a lean mix concrete.
The original plan for forming these four concrete plugs was to access the tunnels from a ventilation shaft a 900m walk from the site. In the event the construction team formed a new temporary access manhole in the centre of the compound.
Ventilation holes into the old tunnels were needed before excavation began. The contractor was to drill a series of 200mm diameter bores but instead mobilised a Casagrande HT55 piling rig to enlarge one of the planned holes into a 2.1m diameter lined, bored pile tube large enough for both ventilation and access.
Short cross adits were hand excavated from the pile tube bottom across into each tunnel. This allowed concrete to be pumped down from the surface to form the tunnel plugs only 20m back from the manhole.
The 4000m 3 of diaphragm walling finished in mid-July - an operation that at any one time demanded up to 700m 3 of recirculated bentonite to hold open the excavated panels - engineers proudly highlight a site compound free from support fluid.
'We have strict controls preventing and containing any spills, ' says Bachy Soletanche operations manager Barry Osborn.
The site is surrounded by a lined 1m high precast concrete bund, while the compound fl oor is covered with a 500mm thick lean mix working platform. This protects the airport concrete beneath and is laid to drain off any bentonite spills into settlement tanks.
Valuable site space is given over to temporary spoil bunkers where excavated clay from the panels is allowed to drain for 48 hours before being taken off site in covered trucks. A bespoke drainage system routes drained mud back for recycling in the desanders, desilters and hydrocyclones of the 700m 3 capacity onsite bentonite plant.
An early threat to the construction programme occurred as the first wall panel was being excavated. An old backfilled adit, used during construction of the original Piccadilly Line tunnels, crossed the line of the wall, and panel design had accommodated this obstruction.
But when Bachy's initial panel dig unexpectedly hit the adit, and skewed 3m off its planned alignment, the prospect of delay was very real. Fast redesign, involving changes to wall panel sequencing, minimised the setback and an overlap of construction operations means works are back on target.
Strict security The project's tight construction programme is compounded by its 'airside' location. Everyone and everything travelling to the site must negotiate the airport's strict security regime.
Personnel, materials and vehicles are all thoroughly searched every time they go airside. Delivery and site vehicles must pass the airport's own MoT test, while drivers need an airside 'driving licence'.
Specific controls are required on aircraft manoeuvres from the closest stands and, as an added precaution, temporary concrete screens surround the site to ensure construction workers are protected from airport operations.
Access shaft sequence After completion of the diaphragm wall Morgan Vinci will excavate inside the box, creating an 18m deep access shaft to the Piccadilly Line tunnels.
The first task is to form a 1.3m thick ground level slab to double as the box roof, allowing a cover and cut excavation beneath, through a large access hole left in the concrete.
The slab will also mean this part of the site can go back into use as an aircraft stand.
With planes moving across the slab, the tube tunnels beneath will be unearthed, broken into and removed.
From one end of the box, short adits will be dug to locate the ends of the two completed extension tunnels to T5. Tracks from these tunnels will be connected to the Piccadilly Line route and an intermediary roof slab cast half way up the box to create a covered junction area.