Why were sophisticated grout control techniques needed to stabilise a simple rail embankment in North East England?
David Hayward has the answer.
It appeared to be little more than a solitary drilling crew grouting into the side of an anonymous rail embankment in a remote County Durham field.
But far from insignificant, this crucial embankment on the East Coast Main Line had a complex history of instability, and the drilling team was armed with the most advanced grout control software yet available.
The speed restriction on Browney Curve, imposed for safety reasons since last spring, was needed to prevent embankment slippage - an event which could close the East Coast Main Line indefinitely.
A background of unstable ground had combined with the collapse of a drainage culvert, resulting in water ingress through the embankment.
Network Rail, committed to providing a safe and reliable railway, had to carry out immediate repairs.
'We believed there was a real risk that the deformations we were experiencing would become unmanageable and result in us being unable to maintain the safe operation of the railway, ' says Andrew Hinton, Network Rail's regional engineer.
The site team overseeing embankment strengthening - Network Rail's partnering contractor May Gurney Rail, consultant Donaldson Associates and geotechnical specialist Bachy Soletanche - faced tight timescales in meeting the client's requirement for total safety and minimum disruption to the high-speed trains that monopolise its most prestigious Intercity route.
So it was little wonder that the job called for such sophisticated grout control. Instead of relying on traditional engineering judgement to largely determine grouting rates, Bachy Soletanche's new software automatically selects and controls optimum grouting pressures, volume and flow rates. This offers engineers a real time computer screen 'picture' of exactly how the grout is performing deep within the embankment.
The software minimised both the risk element of the grout's effectiveness in sealing the collapsed culvert running through the embankment, and the likelihood of surface heave - a common occurrence with such grouting operations.
' We expected some heave but recorded a very pleasing zero, ' claims Bachy Soletanche project manager Richard Piggin as he watched his grouting team leave site last month. Days later, the seven month old, 96km/h speed restriction was removed.
With intercity trains once more crossing the field at their normal 145km/h, yet another chapter in Browney Curve's complicated history was over.
It is a history that has plagued railway operators for over 130 years. From the outset in 1870, when engineers began building the curved embankment across the sloping field, instability of the underlying land was suspected. The whole area is an overstep glacial valley running down to the River Browney. The 7m high ash-topped, clay core embankment had been founded on a raft of sandstone blocks underlying timber beams.
Movement of the hillside had led to minor embankment instability, first recorded in the 1970s. Design schemes a decade later included deep drainage to divert groundwater away from the embankment.
But by 1996 this movement had migrated up to track level and the sudden appearance of a 2.5m deep hole alerted engineers to the weakness. The hole was infilled from the surface with ballast, but a similar failure in August last year suggested the problem had not been solved.
Hillside movements had deformed an original 30m long, 600mm square brick and stone culvert, built to re-route surface and field drains under the embankment.
In 2002 a major collapse of the stone slabs blocked the culvert.
To prevent total embankment failure, the culvert was grouted and water flows redirected to 1980s drains.
'We really had little idea what condition the old culvert was in, nor the exact route of water paths through the embankment, ' recalls Michael Purkis, Donaldson Associates design engineer.
Network Rail suspected the old culvert was the cause, but emergency investigations were needed to establish the precise length affected. Two dozen boreholes were sunk into the field around the embankment to check overall slope stability;
but only shallow pit 'window sampling' was allowed in the structure itself for fear of causing further disturbance.
The simple solution would have been to dig out the entire section of embankment and start again. But since the track carried on the embankment is the East Coast Main Line used daily by more than 300 trains, this was not an option.
'Three years at least' is Steve Richardson's estimate of how long in advance such a 'big bang' closure operation would have taken to plan - especially with no easy diversionary route available.
As May Gurney Rail's construction manager, it was Richardson's task to project manage investigations and then to implement the project team's solutions immediately. Speed was critical, especially as rail geometry in the canted and curved track was in danger of becoming compromised.
The technical challenge was not eased by a client that demanded not only zero risk to train passenger safety, but also a solution which retained use of both rail tracks and caused minimal disruption to train movements.
Conventional piecemeal grout injection into the embankment carried with it the strong likelihood of surface heave, which would lead to track misalignment. Early ideas also involved grout tubes drilled vertically down from track level, an operation that would have been slow and expensive as well as requiring repeated weekend track possessions.
Bachy Soletanche's £150,000 solution was to use tubes Ó manchette - to Donaldson's design - with grout injected from the embankment sides. This not only allowed uninterrupted train running, but also incorporated an unprecedented grout control regime where engineers could 'see', rather than just interpret, the success of their stabilisation measures.
Based on software developed by the contractor's French parent company, Soletanche Bachy, the SystÞme de Production en Injection et de Contrôl Embarqué (SPICE) is a program which automatically monitors and adjusts the grout's precise flow, volume and pressure rates reacting instantly to conditions found underground.
The collapsed culvert ran largely through the embankment's clay foundation. This, plus localised seepage flow paths around it, could be sealed and strengthened without over-pressurising or weakening other areas of the embankment outside the immediate grouting regime.
Constant production of flow charts and graphs created a real time, three dimensional computer image of the grouted area. This allowed engineers from Bachy Soletanche, Donaldson and Network Rail to analyse the success of the operation daily and, as a joint team, to confirm the grouting strategy.
All the area's drainage needs were diverted to a new 1.2m diameter pipe, thrust-bored through the embankment at a point 20m distant from the unstable zone. Meanwhile, Bachy Soletanche planned its network of 32 grout holes, drilled in five rows and each running up to 17m into the embankment to blitz the entire collapsed culvert area.
Grout injection using tubes Ó manchette followed. Given the possibility of heave at rail level, the operation was considered so sensitive that the first few holes were drilled and injected during track possessions.
Only when round the clock measurement of track alignment recorded no change was the contractor allowed to continue grouting under the live loading of passing trains.
Network Rail was also taking no chances. Specialist contractor WA Developments was called in to provide a permanent way gang constantly on site and charged with repairing any track movement. But the fiveman gang had little reactive track work to do throughout the six week grouting operation.
'Despite a track movement tolerance of just 3mm, we were pleasantly surprised that the grouting caused no measurable settlement or heave, ' confirms Richardson. 'This was a very sensitive job, yet the grouting technique proved both effective and low risk to the railway.'
Piggin estimates that the total 30m 3of grout injected into the embankment was roughly double the initial estimates, the clay having proved softer and the ash looser than expected. 'This is a measure of the system's flexibility, ' he says, 'And I anticipate the technique's further use in similar situations where control and minimal settlement are important.'