Jet grouting crews forming a novel cofferdam inside a listed pump station faced the additional challenge of protecting live sewage pumps only metres from the rig. David Hayward reports.
Ten intense days of jet grouting inside a Victorian pump station in Reading have accomplished the risky business of lowering the building's floor level through waterlogged soil.
Bachy Soletanche has completed 67 jet grouted columns which form the rectangular cofferdam and its 1.2m thick base that surround and underlie the station's concrete floor slab.
This has created a dry environment for contractor John Murphy & Sons to break out the old slab, excavate the ground beneath and build a new floor 1.4m lower.
The challenge was to form a temporary works cofferdam through the waterbearing ground and inside the congested building with limited headroom. Options such as dewatering, ground freezing or secant piling risked either heave or settlement damage to the listed structure, or a less than 100% water seal.
An even greater concern was ensuring the safety of a bank of live pumps only metres away, through which much of Reading town centre's combined sewage and drainage flow has to pass uninterrupted.
'It was very important not to damage or flood these live pumps, ' says Kieran Burke, contracts manager for Murphy, working in alliance with Thames Water Utilities. 'Their security was paramount and no other solution could have given us the required margin of safety.'
The deeper sump level in the 120-year-old Blakes Lock pump station on the banks of the Thames will allow five new pumps to feed town centre flows to a new treatment works along Reading's refurbished sewerage network. The greater head of water will enhance the new pumps' efficiency and the risk of flooding central streets will be further reduced.
'The pumps were approaching the end of their working life, ' explains Thames Water project manager Jamie Shotter. 'The age of the building and its proximity to the river created challenges, but we had to replace the pumps now to avoid future disruption to residential and business developments due to be complete this year.'
Faced with a near ground level water table and the 7m long sump area bordered tightly by the station's external walls, the client's easiest option was to demolish the building and start again. But Blakes is one of the oldest pump stations of its type that is still operational.
'We were not allowed to disturb or damage the building so had to come up with a solution that ensured the integrity of everything around it, especially the nearby live pumps, ' explains Burke. 'So crucial is this pump station's role that half the pumps had to remain operational while the other half were being replaced.'
To simply break out the reinforced concrete sump floor, without any water seal around it, risked immediate groundwater and river ingress through the surrounding gravels and alluvial clays. Murphy called in Bachy Soletanche to suggest waterproofing schemes, and rapid dismissal of most options led to the jetgrouted solution.
'Dewatering or ground freezing would have worked out at roughly the same cost but have taken twice as long and created an unacceptable risk to the building and its live pumps, ' explains Bachy Soletanche engineering manager Simon Lebon. 'Low headroom, internal congestion and concern over a watertight seal also ruled out sheet piling and mini secant piles.'
Jet grouting to create a cofferdam of secanting grout columns carried none of these risks and, with a contract price of £106,000, worked out best value overall. It also offered a useful bonus.
As well as providing a perimeter box of up to 3m deep vertical and inclined concrete columns, Bachy Soletanche's Klemm 704-1-E drill rig has formed - 3m beneath the insitu old floor slab - a base of shorter overlapping jet grouted columns.
This interwoven network of 26 foreshortened columns sealed the cofferdam base and provided a solid plug on which Murphy could concrete the new 400mm thick sump floor.
Location of the sump is so tight against the pump station walls that the track-mounted electrohydraulic drill rig installed the maximum 1.6m diameter perimeter columns vertically and also inclined at up to 27infinity where the cofferdam had to extend outside and beneath the building's shallow foundations.
Here the required installation accuracy of 0.5infinity at the base of 3m long inclined columns called for split-second co-ordination between the rig operator and real time readouts of numerous machine and material functions.
Although jet grouting has been around for 30 years, few of today's geotechnical contractors offer it as a mainstream option. And Bachy Soletanche's double-jet system, using a combination of grout and air, still relies heavily on operator skill (see box).
With the cofferdam walls complete, and the old heavily reinforced concrete sump precored by diamond drilling, the rig operator used these 150mm diameter holes to drill beneath the old slab to form the cofferdam base.
This plug is formed by jet grouting only the lowest 1.2m length of the overlapping column network starting at a depth of 3m underground. Grout flow is then turned off and the drill withdrawn back up through the original slab's precored holes.
Given a watertight cofferdam, Murphy could then break out the old sump floor, excavate down 1.8m to the cofferdam base and pour a new slab at this lower level.
The pump station is now refurbished and ready to fully reopen this spring.