High speed electric trains require high precision placing of the overhead conductor. On the £270M Newhaven to Boston electrification project Balfour Beatty Construction, in a joint venture with the Mass Electric Construction Company, has been given the task of individually designing and erecting some 14,000 pylons for client Amtrak. But this was the least of the challenges facing the venture.
At one end of the job the line crosses a major ground freezing operation on the mammoth Boston Artery project. At the other end, the contractor has had to devise ways of allowing historic lifting bascule bridges to be electrified safely.
'We have invented completely new mechanisms for the bridges,' says Balfour Beatty project manager Chris Elliot.
BB's follow-on 'audit, verify and complete' contract was awarded in 1995 after a previous electrification contract ran into difficulties.
The company has based itself in the small town of Old Saybrook, one of Connecticut's favourite holiday spots. In the summer thousands of visitors, mostly New Yorkers, flock to the little town and its neighbours on the scenic Atlantic coast. Lift bridges and swing bridges are up and down all day to allow yachts and boats to pass through the inlets and river mouths.
That is the big problem for Amtrak, the US's biggest rail operator, which has to run one of its main services along the coast from Boston to Newhaven and then on to the Big Apple. Boats have priority over 'new fangled' trains.
It is an even bigger problem for BB. Running catenary across a bridge that lifts or swings open, without causing a dangerous live 25kV tangle, is demanding, especially as trains must keep running.
The project team has invented and tested travelling gantries on the bridges which roll on steel wheels on extra tracks added to the bridge. A rigid cable holder keeps the catenary in position and collapses out of the way when retracted (see box).
The huge gantries for Boston's South Station, which are up to 60m wide, also require complex on-site work. These are the biggest gantries on the job since they support cables for several tracks, and points where tracks interchange. This means much bigger foundations than normal and in a place where the ground is moving.
Heave from ground freezing will go on throughout BB's work because soft ground is being made solid for work on three tunnels underneath the tracks. These will carry interstate highway I93, which is a small part of the billion-dollar Boston Artery project.
Gantry designs must cope with this movement and the physical work on site must cope with conflicting site activity by other contractors.
'Once we have set the levels for overhead line and track, we cannot allow them to sink down again,' says deputy project director Paul Stubbings. Together with Amtrak working on the track itself, the JV will have to ensure that the hole is maintained at the constructed level as the ground reverts to normal after the freezing.
In between these two demanding sectors is a 250km stretch of mainly double track east coast line which Amtrak is upgrading from a 140km/h diesel service to a high speed line. New electrified trains will run at up to 230km/h with a 240km/h maximum, the fastest in the US.
Installing catenary power lines for these is a precise civil and structural engineering exercise. Each cantilever or portal line support has to be tailored exactly to its position on the line, matching precise line tensions, heights and foundation positions.
Static and dynamic precision is needed for high speed locomotives, explains Stubbings. The train's pantograph must stay within a fairly narrow envelope. If there is too much pressure it can collapse. If there is too little, contact is broken, causing flashing, sparking and power surges.
The conductor wire, formed from an almost pure copper and silver mixture, is kept as a fixed straight line above the track, supported from a 'messenger' wire curving from pole to pole. It is clipped to variable length 'droppers' as on a suspension bridge. Clips fit two grooves in the conductor wire.
The messenger allows spans of about 60m, Stubbings says, and provides the wire with a certain 'give'. However, 'hard spots' are difficult to avoid completely and can occur where the wire has to be supported more directly, such as in a tunnel section at one end of the line, where there is no headroom for a messenger wire.
'We have 200 assorted bridges across the tracks,' says Elliot. Some of these allow the messenger to loop underneath but others have clearance only for the catenary, which is attached directly to the soffit. Others need modification to allow enough clearance and in two cases the bridges have had to be demolished and rebuilt.
BB has devised a special hinged insulator bracket for these points to 'soften' them. Hard spots can create problems because of sparking and also accelerated wear effects on the wire, which can need renewal in two years instead of 20 at such points.
There are complex wave dynamics as a train moves along. Hard spots reflect these back, creating standing waves. They are even more difficult when a locomotive has two pantographs, mounted one behind the other.
'All this has to be analysed to get the positions and tensions right in the catenary,' Elliot explains. And, adds Stubbings, an analysis is needed for each type of locomotive.
Additionally, Amtrak wants a particularly robust system for the new line, with higher tensions than normal, up to 2000kg, because of the operating speeds and to use a reduced number of power feed substations.
Robustness will also cope with icing problems from notorious east coast freezing rain. This can quickly coat everything in a
15mm thick ice layer in winter.
Finally, the line has to zig zag from side to side so it does not always contact the same part of the carbon wear bar on the pantograph. This is called stagger and affects support positions and tensions.
To create such precision some 14, 000 line supports are needed, for a total 580 track kilometres, forming marshalling yards, stations and the main line .
'That means 14,000 drawings,' says Stubbings, who explains that the supports are drawn from a repertoire of basic designs worked out in Kirby in Britain.
Three main types are used, a single cantilever, a double cantilever for when the opposite trackside cannot support a pole, and a portal for multiple track sections and sidings.
The appropriate version is customised for its position and then assembled from a battery of components. Without a computerised materials control system, for which BB holds a patent, keeping track of parts could cause a major tangle.
A computerised system produces the drawings for the 14,000 cable supports, Stubbings adds.
That is vital since each support design cannot be finalised until the foundation position is precisely established. Ground conditions are quite variable along the route, ranging from a very hard pink granite to very soft peat.
Developing designs for the lifting bridges and the Boston foundations has pushed up the original $321M projected cost. A small time extension has taken planned completion to October 1999.