Bishopsgate – Early one Saturday morning
In May 2016 Camfaud Concrete Pumps Ltd supplied pumps and staff for London Underground’s (LU’s) Liverpool Street Diamond Crossover Renewal. This contract was carried out by Track Partnership, a strategic alliance between LU and Balfour Beatty Rail, responsible for delivering track and drainage renewals across the Tube network.
The Liverpool Street Diamond Crossover is situated approximately 50 metres east of the station, allowing Central line trains to enter and exit the two sidings. At over 60 years old, the track condition was in line for a full renewal. To facilitate the work, over 300 cubic metres of concrete were pumped from street level over 15 metres, down a vent shaft on Bishopsgate Road. Over 400 tonnes of rubble were removed and new track was installed to complete the project, which involved extensive collaboration between a number of stakeholders including Transport for London (TfL), Track Partnership, Broadgate Estates, the City of London Corporation, Hanson Concrete and Camfaud Concrete Pumping.
Pump 1 BSA1409 – the start of a long day
Pump 2 BSA1409 – the roads are getting busy
Three pumps were used for the contract, two working and one on standby. The working pumps were both Putzmeister BSA1409D static models while the standby pump was a Schwing S20 high pressure mobile one. The mobile pump was chosen as the standby because it’s quicker to deploy in the event of one of the statics failing. Fortunately, the backup solution was not required. Both of the static pumps were used for the duration of the pour, the pumps fed separate pipelines to supply concrete to two concreting gangs working in parallel in each of the running tunnels and the cavern.
The pumps were situated in Bishopsgate in the northbound bus lane with traffic lanes subject to a road closure. Track Partnership were extremely careful to protect the environment in this sensitive area with polypropylene sheeting, geotextile fabric, rubber mats, plywood sheets, bubble wrap and drip-trays all being used.
High pressure pipeline – down the shaft and into the tunnel
Working in the cavern
The pipeline route was rather convoluted and required a large number of bends, more than ideal, but was chosen as the best solution in the circumstances. The pipeline exited the pumps and ran along the Bishopsgate pavement turning down into a 15m deep shaft. A 90 degree bend diverted the pipeline along a cross-passage before another 90 degree bend took the pipeline down 6m into the tunnels. A final 90 degree bend, a footed bend to add support, diverted the line along the tunnels to the work area.
The concrete for the project was supplied by Hanson Concrete. The mix used was designed for an earlier contract, the major engineering works on the Victoria line at Walthamstow, carried out in August 2015. It was successfully pumped through 500m of pipeline on that contract and so it was an easy decision to choose the same mix for the Liverpool Street works. The mix contained a CEM 1 / GGBS blend, superplasticiser and retarder admixtures and a small addition of microsilica slurry to ensure that the mix held together in the line and did not bleed as it was placed and finished. Hanson also supplied 4 cubic metres of ready-mixed grout ahead of the concrete to grout the pump and pipelines.
Concrete flowing into the trackbed
The concrete pumping operation was completed in two stages. Firstly, two parallel chambers in the cavern beneath the track had to be filled: each chamber had a volume of approximately 105 cubic metres. These were filled through five ports opened along the length of the chambers. Secondly, the track bed had to be poured, embedding the sleepers into the concrete slab.
Filling the chambers was a relatively simple process. The concrete was introduced through the ports and vibrated to ensure that it was fully compacted and the chambers were completely full. The concrete to fill the chambers was pumped at approximately 50 cubic metres per hour. Consideration was given to designing a separate mix for the chambers but, given the proven quality, flow and ‘pumpability’ of the slab mix, it was decided to use this mix for both sections of the job.
Pouring the slab concrete was a more onerous task due to the obstructions presented by the sleepers and the track, and the complex finishing required. The pumping rate was reduced to around 15 cubic metres per hour and this part of the job was a much more stop / start affair.
Pump washed out – ready for collection
At the end of the pour, the pipes were cleaned out by Camfaud’s staff using compressed air. This is an efficient method of cleaning medium length pipelines but must be carried out by competent, experienced operatives. Using the latest blow- out adapters fitted with pressure gauges and oversized safety vents, the pressure in the line was very closely controlled at 2 bar with the line full, 1.5 bar once the line was half empty and 0.5 bar at the end of the blow-out procedure. Despite having this fine control, all of the concreting gang were required to stand in a safe area behind the pipeline or in the other running tunnel while the concrete was being cleaned from the pipeline.
In total, just over 300 cubic metres of concrete was poured during the concreting works. The pour took 12 hours to complete and this included grouting the pipelines, pumping the concrete into the chambers and track slab, and cleaning out the pipeline and pumps. With good planning and first class teamwork between Track Partnership and Camfaud Concrete Pumps, the concrete pumping operation was completed ahead of schedule allowing Track Partnership more time to complete their works prior to handing the Central line back to Transport for London.
Achieve something remarkable – we did!