Thermometers – the large half metre long industrial type – are not exactly standard kit for site engineers. But piling crews, forming a large secant wall basement in the centre of Bath, would not dare be without one instantly available.
Any groundwater disturbed by their rig's auger must be immediately sampled. If the water is hot – around 250C – concrete wagons, always on standby just a few metres away, are quickly activated to plug the auger hole. The pile must then be redrilled 24 hours later.
Geotechnical contractor Expanded Piling is currently sinking the last of its 784 secant piles and so far this scenario has remained theoretical.
The concern has been to not disturb the city's invaluable and protected supplies of hot artesian spa water, flowing deep beneath the site, which is located just 200m from the city's famous Roman Baths.
The maximum 17m long piles stop well short of this 10,000 year old 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.
For Expanded this means a copy of the all powerful local parliamentary Act safeguarding the spa water is always on hand in its site office.
"Section 33 of the 1982 County of Avon Act has been a major factor in our risk profile of this job" explains Expanded's regional business manager Chris Thomas. "Protecting the spa water is paramount and we always need two concrete wagons on site before we can start any pile bore."
A further potentially disruptive factor to this already congested work area has been the discovery of archaeological remains. Slap bang in the middle of the site is Bath's "Big Dig" – one of the region's most extensive archaeological investigations aimed at revealing the city's history stretching back to 8,000 BC.
Cordoned off from the bustle of construction, a team of Museum of London archaeologists is exploring some 25% of the total 5ha site and is painstakingly unearthing evidence of earlier site occupation ranging from Stone Age to post-medieval.
Expanded arrived to start its 20 week, £3.2M piling contact last autumn. Included in the £360M mixed retail, office and housing SouthGate development will be a 142m long rectangular basement car park for 860 vehicles.
Expanded's task for main contractor Sir Robert McAlpine is to form the basement walls with hard-firm (male-female) secant piles. The basement will later be excavated to 10m depth, exposing over half the bore of the 17m long male piles.
Watertightness is essential, to ensure a dry basement beneath the 4m deep water table, and to minimise disturbance of the spa water reservoir lower down.
"The demand for a 1 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," claims Expanded senior project manager Rob Howarth.
Adding to the challenge of achieving this accuracy, the contractor knew that the clay ground contains several 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, and Expanded brought in two of its high-torque rigs capable of sinking a single full length casing for each pile.
Headed up by a brand new £800,000 Bauer BG28 rig, the decision to go for single cased piles – a technique that Expanded itself introduced into the UK from mainland Europe two years ago – rather than more conventional segmental casings, is proving extremely cost effective.
"This powerful rig is the key to the whole job's progress and bores through the hard limestone bands effortlessly," Thomas reflects. "We can achieve up to 14 single cased piles a day – double the production, and bored more accurately, than is possible with a segmentally cased operation."
The 28 tonne metres 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 piling start last year.
When Expanded arrived in Bath last October it was faced with an already busy site, rather than the empty work area
favoured by piling contractors.
Competing for space were earthworks lorries, concrete delivery wagons and several other geotechnical contractors sinking bearing piles and forming a grout curtain beneath the line of the secant wall. All equipment and supplies had to enter through just two site access points, and the available piling platform which marks the outline 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.
Negotiating with the main contractor for a 50% wider piling platform, an additional site access plus Saturday working has all helped Expanded recover its programme. But key to the speed up 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's the best rig I have ever driven," says Mathew 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 onto the piling programme's critical path.
The site's now numerous concrete consuming operations – piling, capping beams, casting superstructure columns - demand some 60 ready mix deliveries every day.
Expanded needs up to 40 of these; but a shortage of Bath concrete suppliers means the contractor must rely on batching plants up to 30km away. Given the congested city centre location, that translates into a two hour round trip.
And with piling crews ever conscious of the Avon Act, those two concrete wagons must already be on site before any pile is started.
Just 200m north of the site lie the city's famous Roman Baths boasting the UK's only active hot springs.
Sections of the artesian reservoir, feeding the baths with water at up to 46 degrees C, flow just 100m beneath the site.
The city centre site consists of stiff Lias clay, with possible joints and faults, underlain by more competent rock. Any excavation deeper than 5m is automatically controlled by the 1982 County of Avon Act.
Discovery of any "hot" spring water during piling or deeper grout curtain wall operations, triggers immediate sealing measures to ensure the
continued integrity of the
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 irrefutably 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" 880mm diameter male piles. It has proved so efficient that a second, unused CSP rig on site, an older modified Soilmec CM120, has been kept only as a standby.
Compared to boring standard segmental cased piles - an operation which at Bath would have needed six, 3m long casings
for each bore - the Bauer has proved twice as quick and, claims Expanded, significantly cheaper overall.
Secant walls normally demand the male pile being completed no longer than 10 days after the softer, unreinforced females either side, into which it must cut at least 225mm. But the Bauer rig's impressive 28tonne metre torque and full casing allows the flexibility of an up to two week gap.