In the centre of London, Crossrail project teams are engaged in a major compensation grouting programme. Paul Thompson reports.
The main headlines coming out of the work on Crossrail so far have centred around the launch of the first few of the eight tunnel boring machines (TBMs) that will drive the 21km twin bored tunnels below London.
But away from the actual tunnel portals, lots of preparation work is underway to ensure the TBMs’ passage below the streets of London doesn’t hit the headlines for the wrong reasons.
As well as negotiating their way around existing underground infrastructure, the TBMs driving the twin tunnels will also pass under a number of listed and historic structures.
To mitigate the impact of the tunnelling, widespread compensation grouting is being undertaken with some of the major elements being carried out by the KBR joint venture between Keller Geotechnique and Bam Ritchies.
The JV is working on the grouting for BFK JV’s delivery partners Bam Nuttall, Ferrovial, and Kier, which have three significant contracts across the project.
BFK’s work includes Crossrail contract C300, known as the western running tunnels.
These, which run east from Royal Oak, under London’s West End and on to Farringdon just outside the City of London. The JV’s other projects include contract C410, which is for the construction of the station caverns at Bond Street and Tottenham Court Road and contract C435 for the station cavern at Farringdon.
Contracts C300 and C410 form the main focus of the KBR’s grouting work. In a £40M deal KBR will provide compensation grouting at three separate locations along the route of the cross-capital link to limit levels of ground settlement caused by the tunnelling work.
Contracts were awarded in January 2011 but the KBR team did not start on site until October last year.
Before that, plenty of hours were put into working out the drilling and grouting regime that could see as much as 20,000m3 of grout injected beneath London from three separate sites at Bond Street Station, Tottenham Court Road Station and Fisher Street Shaft.
“The amount of settlement is generally linked to the amount of time you leave an excavation unsupported”
The work is being carried out in anticipation that there will be a degree of settlement caused by the three different excavation methods expected during the tunnelling work. Settlement is expected to result from tunnel boring, from spray concrete lined excavation and from that caused by the bulk excavation of the ticket hall boxes.
KBR senior technical engineer Owen Francis is leading the JV’s work to make sure these settlement levels are within agreed guidelines. Levels of potential settlement vary according to the different tunnel and excavation methods used, according to Francis.
“The amount of settlement is generally linked to the amount of time you leave an excavation unsupported. A TBM’s advance rate is generally quite quick, so settlement is likely to be smaller. Sprayed concrete excavations tend to settle more as they are open a little longer, and wherever you dig a hole in the ground the sides want to collapse inwards. If you want a 1m3 hole you always have to take out more,” he explains.
Under the Crossrail contracts, measures to mitigate settlement at the stations must be implemented when settlement is likely to be 10mm.
The client has provided the JV team with settlement contours for the entire length of the project. These plot the likely settlement along the scheme’s length and highlight where there may be structures at particular risk, such as heritage or listed buildings.
“The maps of the route highlight every heritage structure or listed building along its length and the degrees of risk of settlement associated with the work. Obviously the project involves tunnelling beneath London so there are plenty of potential issues,” says Francis.
At the Bond Street site, five grout shafts will allow the KBR teams to work at the correct level beneath the ground to radiate the grouting holes through which the specially designed grout mix will be fed under pressure.
Across the project, a total of 13, 4.5m diameter grout shafts are scheduled to be excavated to depths of between 15m and 20m. Seven will be dug at Tottenham Court Road and one at the Fisher Street Shaft, plus the five at Bond Street.
From within the tight confines of the grout shafts, drilling teams will bore almost 50km of grout holes, 940 in total, at lengths of more than 90m.
“The maps of the route highlight every heritage structure or listed building along its length and the degrees of risk of settlement associated with the work”
These holes radiate horizontally from each of the grouting shafts and will allow the teams to pump the 2N to 3N strength mix between the layers of London Clay which make up the bulk of the ground that is being tunnelled through.
“We don’t actually need a high strength grout,” says Francis. “It doesn’t need to be any stronger than the London Clay. What we wanted was a mix with a low ‘bleed’ so we trialled various water/cement ratio grout mixes and ended up with a recipe that also features a 5% bentonite content. Its weakness also allows us to go back and re-inject each sleeve a number of times should we need to.”
The team is using a cased auger drilling system after early trials with a water flushed drag bit proved difficult.
The 127mm outside diameter temporary casing has been installed at distances as far as 65m, and the 110mm diameter boreholes are grouted through steel tubes à manchette with injection sleeves at 500mm intervals.
“We’ve used a cased auger drilling system as well as a water flushed drag bit, but we are probably going to stick with the auger from now on,” Francis explains.
The team is using the latest, most advanced grout modules to carry out the work. These feature six 100bar computer controlled grout pumps to compensate for settlement indicated by the monitoring carried out by sub-contractor KGI - a joint venture between Keller Geotechnique and ITMSoil.
Grouting and drilling work is generally carried out 24 hours a day, seven days a week and a three, eight hour shift pattern.
London’s injection compensation
Grout holes radiate from each of the shafts around the sites so that separation of the bores at the edge of each site is approximately 3m. The problem for the site team is then fitting those grout holes within the shaft so that the minimum permissible spacing of 320mm between each hole is not breached.
Within each of the shafts, boreholes are placed in layers which are 400mm to 600mm apart and spread over a 2m deep band, normally between 3m and 5m above the base of the shaft and within the London Clay.
Normally there are two or four different layers but in some complicated areas as many as five. In particularly congested areas, the team may need to incline the boreholes too.
“There are tight tolerances for deviation of the boreholes,” explains Francis, adding, “We need to maintain a close separation in plan, we can’t get too close to the crown of the tunnel - between 4.5 and 5m - and we must be 2m below the top of the London Clay.”
Scheme: Crossrail Contracts C300 and C410 compensation grouting
Client: BFK JV
Contract value: £40M
Contractor: KBR JV
Ground monitoring subcontractor: KGI JV