Verticality tolerances are unusually tight during piling work for the secant basement wall of a major commercial development in the centre of Bath. And the prime reason for such accuracy is equally unusual.
Deep beneath this World Heritage Site lies a reservoir of artesian water, formed during the Jurassic period, that feeds the city's famous Roman Baths just 200m away. Any possible disturbance to this ancient water course is contractually outlawed.
"Protecting the spa water is a significant factor in our risk profile of this job and we are acutely aware of it every time we install a pile," says contractor Expanded Piling regional business manager Chris Thomas. "The resulting piling verticality needed is extremely challenging."
A large industrial thermometer is on hand to monitor the temperature of any groundwater disturbed during piling. If it is hot – about 25˚C – piling stops immediately and ready-mixed concrete is brought in to plug the auger hole. The pile is then redrilled 24 hours later.
The up to 17m long piles stop at least 100m short of the spa water reservoir. But there remains a small risk of disturbance through potential fault lines and unpredictable bands of hard limestone in the predominant dark blue Lias clay.
With Expanded sinking the last of its 784 secant piles, this scenario has so far remained theoretical. But an all-powerful local parliamentary act safeguarding the spa water remains on hand in the contractor's site office.
Section 33 of the 1982 County of Avon Act dictates the procedure needed during any excavation or piling deeper than 5m in the city centre. And it demands that sufficient concrete supplies are always available to instantly stem any unusually hot groundwater flow.
The £360M mixed-retail, office and housing Southgate development, being built for joint venture client Multi Development/Morley, includes a 142m long rectangular basement car park for 860 vehicles.
Expanded's 20-week, £3.2M piling contract, for main contractor Sir Robert McAlpine, is to form the basement walls with hard-firm (male-female) secant piles. This basement will later be excavated 10m deep, exposing over half the bore of the 17m long male piles.
Watertightness is essential, both to minimise possible disturbance of the spa water reservoir and to ensure a dry basement beneath the 4m deep water table.
"The demand for a one-in-200 pile verticality, plus a 225mm overlap cut by male secant piles into the females either side at 12m depth, is one of the most onerous waterproofing conditions we have ever attempted to meet," says Expanded senior project manager Rob Howarth.
Adding to this challenge, the contractor knew that the clay ground contained up to 100mm thick bands of very hard limestone – rock that could easily deflect the bore.
Cased piling offered the best option to achieve the required verticality. Expanded brought in two of its high-torque rigs capable of sinking a single full-length casing for each pile, headed by a brand new £800,000 Bauer BG28 rig. The decision to go for single-cased piles – a technique that Expanded introduced into the UK from mainland Europe two years ago – rather than more conventional segmental casings, is proving cost-effective.
"This powerful rig is the key to the whole job's progress and bores through the hard limestone bands effortlessly," says Thomas. "We can achieve up to 14 single-cased piles a day – double the production, and bored more accurately, than a segmentally cased operation."
The 28tm torque Bauer – the most powerful rig in Expanded's 26-strong fleet – incorporates two rotary heads attached to the rig mast. The lead head holds the single 18m long casing, while the second, 1m higher, supports the follow-on auger.
Spoil is air-flushed to reduce friction with the casing. As arisings emerge from the top of the casing during the bore, the spoil is channelled sideways into an enclosed vertical muck chute hanging alongside. This multi-section chute acts like a collapsible concertina routing the spoil down to ground level where it feeds directly into a dump truck for removal off site.
The rig's ability to complete an 880mm diameter male secant pile in just 30 minutes has proved crucial, following a slow start last year.
When the contractor arrived in Bath in October it found not the empty work area favoured by piling contractors, but an already busy site.
Slap bang in the middle of the complex was Bath's Big Dig – one of the region's most extensive archaeological investigations aimed at revealing the city's history stretching back to 8000BC.
Cordoned off from the bustle of construction, a team of Museum of London archaeologists is exploring 25% of the total 5ha site and is painstakingly unearthing earlier site occupation; ranging from early Stone Age to post-medieval.
Competing for space from the onset were earthwork lorries, concrete delivery wagons and several other geotechnical contractors sinking bearing piles, and forming a grout curtain beneath the line of the future secant wall.
All equipment and supplies had to enter through just two site access points, and the available piling platform, marking the line of Expanded's basement, was initially only 10m wide.
"We underestimated the time it would take to assemble two of our largest rigs within this already crowded site," Thomas admits.
After negotiating with the main contractor for a 50% wider piling platform, an additional site access plus Saturday working has helped Expanded recover its programme. But key has been the performance of the new Bauer rig. It has averaged eight piles a day – double the programmed rate of four per rig – and has peaked at 14 a day.
"It is the best rig I have ever driven," says Matthew Sharlotte, a rig operator with over 20 years' piling experience. "And we could achieve even higher rates if we had better concrete delivery."
The challenges with concrete supply have pushed its delivery firmly on to the piling programme's critical path. The site's numerous concrete-consuming operations – piling, capping beams and casting superstructure columns – demand 60 ready-mixed deliveries every day.
Work to install the secant wall piles was due to be complete in April.SUPER RIG
Speed, accuracy, power and economy. These are the qualities claimed by Expanded Piling for the pride of its 26-strong rig fleet – the new computerised Bauer BG28, designed specifically to bore fully-cased secant piles (CSPs). The rig has won its spurs on the Bath contract by averaging double its programmed four, 17m long piles a day, and boasting a daily maximum of 14 piles.
The machine is being used to bore all the secant wall's 394 hard, male 880mm diameter male piles. It has proved so efficient that a further, unused CSP rig – an older modified Solmec CM120 – has been kept on site only as a standby. A continuous flight auger Casagrande B300 is installing the firm, female piles.
Compared with boring standard segmental cased piles – an operation that at Bath would have needed six, 3m long casings for each bore – the Bauer has proved a third faster and as a result is significantly cheaper.
Secant walls normally demand the male pile being completed no longer than 10 days after the softer, unreinforced females either side, and into which it must cut at least 225mm. But the Bauer's 28tm torque and full casing allows the flexibility of an up to two-week gap.