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In deep with the basement jacks


Three floors underground, in a congested central London basement, mini-piling rigs are building the new foundations for a city office retrofit.

David Hayward reports

JUST A stone's throw away from the Bank of England, in the heart of the City of London, number 107 Cheapside resembles any other typical 1960s office block which is about to undergo a significant retrofit.

Cocooned within scaffolding and plastic sheeting, its eight floors of denuded, temporarily abandoned office space await a major £35 million upgrade, including the addition of two storeys to be built on top.

In contrast to the dusty silence above ground, the congested and confined three-level basement deep beneath is currently a maze of construction activity.

Here site teams are busy with concrete coring, demolition and excavation, while a fleet of mini-piling rigs compete for basement working space beneath two and a half metres of headroom and between closecentred concrete columns. 'Managed congestion' sees small dumptrucks weaving around temporary ventilation ducts and grout supply lines while negotiating the pipes and pumps of an extensive dewatering system.

It is impressive, then, that the outcome of all this act ivity will be a network of over 250 up to 20 m-long minipiles, bored from three levels and designed to strengthen existing foundations to carry two new off ice floors soon to be built high above.

It is praiseworthy that this innovative, contractorsuggested piling redesign has removed the need for extensive propping and large access holes to be cut in basement slabs, temporary works that would have been essential with earlier piling options.

The pre-tender large-diameter piling option has been changed to an extensive mini-piling system and the client has been offered a time saving of more than a month on original programme estimates.

'Large-diameter piling would have proved impractical and could have undermined the temporary stability of the basements, ' claims Anthony Lucas, project manager for ground engineering specialist Cementation Foundations Skanska. 'Extensive vertical, and even lateral, propping of basement floors and walls would have been needed for earlier schemes.

With such large holes broken out of the slabs to allow rigs to operate beneath, sometimes only the f loor beams would have remained intact.'

Competent London Clay, overlain by silts and gravels, allows the exist ing building to be founded entirely on spread footings, doubling as 550 mm-thick basement f loor slabs. Conveniently these three slabs shorten in width on each side as the building deepens from ground level to the lowest basement, which lies some 9 m below the street.

This facilitated early thoughts of a foundation strengthening scheme, based on piles of varying length positioned close to the sides of each basement slab, bored directly into virgin soil beneath. Further piles could be bored from ground level, all still located within the building's footprint and all drilled immediately into undisturbed soil.

Original ideas centred on a network of 120 large, 600 mm-diameter piles. They were to be bored either directly from each basement or just from ground level.

But at tender stage CFS put forward practical and economic challenges to both these options.

The average 2.5 m basement headroom, falling to just 2 m below integral floor beams, imposed severe restrictions on any CFS rig capable of installing 600 mm piles. The company's most practical mini-rig, a Klemm KR709, weighs 12 tonnes and has a 5m mast, attributes that would have needed extensive temporary works to allow it to operate in the basements.

A forest of slab support props, congest ing the work area, would have been essential, argues Mr Lucas, and a series of access holes would have been required in overhead slabs to increase headroom for the mast.

In some locations, such large areas of removed slab could have reduced the structural integrity of the basement box so that side walls could need temporary lateral propping.

Pile positions, which were just 300 mm from the columns and walls, would also have triggered drilling problems.

CFS did consider an obvious alternative, a confinedspace piling rig, only to dismiss it on safety grounds.

Tripod piles - offering low headroom and 600 mmdiameter drilling capability - could, several years ago, have offered a practical solution without the need for major temporary works.

Th is manually operated, three-legged tripod rig, with a rope-driven dropshoe, is no longer part of the company's fleet. As a safeguard to the workforce, health and safety rules now ensure that the required 75 kg segmental augers are posit ioned by winch or forklift truck - there is no longer any manual handling.

Winches and trucks in Cheapside's already crowded basement environment were simply not an option.

The other design option outlined at tender stage - all 122 large-diameter piles being bored from a rig at ground level - was also dismissed by CFS.

Installing up to 10m of temporary casing, for piles initially 30m long, which then needed to be cut off at the lowest basement level, was just about feasible, claims Mr Lucas. But the 70-tonne rig needed to install them, plus its support crane, would still have demanded extensive propping of basement slabs and could have resulted in a slow, uneconomical solution.

Instead CFS, which tendered to main enabling works contractor Keltbray, put forward its own scheme. The contractor proposed slimming down all but a dozen of the 600 mm-diameter piles into 300 mm versions, in addition to doubling the number of piles needed and installing them floor-by-floor using mini-piling rigs with low headroom.

Willingly accepted by demolit ion specialist Keltbray, the £560,000 bid offered several advantages.

The adoption of an underground, three-strong hydraulic piling fleet (two light 1.6-tonne Twin Tech TD308s and a more powerful higher-torque Hutte HBR202 weighing in at 6 tonnes) has removed the need for propping any slabs. Their 2.4 m-high masts can operate beneath the slabs - just - in order to avoid temporary cutouts.

The six 600 mm piles that are still needed at ground level, plus a similar number at upper basement level, 3.3 m beneath ground, are all being installed from the surface. Here, the 12-tonne Klemm KR709 rig also needs no slab propping and installs the upper basement piles through short, temporary sleeving.

Crucial to this design's economy was minimising the average length of the 252 minipiles. This would, using the normal safety factor of th ree, have been 24 m.

CFS staged a £25,000 preliminary pile load test, using specially developed lightweight equipment for easy handling in the confined basements. This convinced Cameron Taylor, consultant to project manager Second London Wall, that the safety factor could be reduced to two, allowing a 30 per cent reduction in pile length. The new design was also validated by tests at a nearby site.

More important to Keltbray, however, was the overall saving in project time that was offered by the CFS design. The 11-week piling programme is no quicker than other options. But, unlike alternatives, the adopted scheme's lack of temporary works allows Keltbray to make an immediate start on the building's partial demolition works required for the retrofit.

'Earlier schemes would definitely have resulted in us doing no demolition work at all during the piling phase, ' explains Keltbray project manager Marvin Dyke. 'Even though it has still needed several weeks of detailed planning to ensure close teamwork between the piling crews and our operations in such a confined space, the CFS design is proving to be the best economic solution overall.

'It offers significant time savings by allowing us to start basement work alongside the piling operations.'

Additionally, he argues, the overall risk to the ultimate client, property developer Carlisle Group, is also reduced. 'We take responsibility for both the piling and demolition work, coordinating interfaces ourselves, ' explains Mr Dyke.

He points out that the retrofitting of aging, yet expensive, city cent re off ice blocks (a technique known as 'cut and carve') is a niche market. Such projects currently account for 40 per cent of Keltbray's £120 million turnover - a percentage that Mr Dyke expects to r ise to about 70 per cent within f ive years.

'Our joint scheme with CFS has worked well. And there is a lot of potential out there for similar projects, ' he concludes.

An underground obstacle course

'THIS is one of the most confined, congested and awkward site areas that we have ever worked in, ' claims CFS project manager Anthony Lucas as he reviews the challenges of operating and servicing his basement piling f leet while surrounded by a myriad of 'obstacles'.

All three underground rigs have been operating at their practical limit. There is just a few millimetres of mast clearance to overhead slabs and piles have been installed within 300 mm of concrete side walls.

On four separate occasions significant structural analysis was needed to gain approval to nibble out the underside of beams running through overhead slabs.

This allowed for a further reduction in clearance to masts butting up to the beams.

Surrounding congestion had to be both organised and safe. With the water table just 6 m below ground level, a major dewatering network ensures piling crews remain dry but still leaves them negotiating a network of pipes at the lowest basement level.

For every piling position, separate coring crews are needed to cut through the 550 mm-thick reinforced concrete slabs.

Pile spoil is temporarily stockpiled alongside, pending its reuse as backfill where a large basement area is being infilled as part of the retrofit.

Small dumptrucks compete with ventilation ducts and grout supply lines.

And, even when the rig teams focus on their own machines, further underground obstacles have been overcome.

More than a third of pile bores have had to be drilled through old bricks and concrete debris, which is encountered up to 5 m below piling platforms.

Rigs at every level are serviced entirely through large, single-access holes, cut at the same point into the basement and groundlevel slabs, so as to provide a direct vertical route to the surface. Through these 8 m by 4 m openings, known as moling holes, must pass all equipment, materials, services and grout supply lines to each rig.

Pile grout is site-batched with preweighed bags to ensure quality and consistency. A pumpable 40 N/sq mm mix is needed, with equal proportions of sand and Ordinary Portland Cement, plus a 0.45 water/cement ratio. A high, shear colloidal mixer is used to ensure a homogeneous low-bleed grout.

Only the dozen 600 mm-diameter ground and upper basement level piles, which are all installed from ground level by the Klemm KR709, need ready-mixed concrete delivered by truck. This minimises the need for concrete delivery wagons to travel through central London streets.

Despite the site congestion, programmed piling rates of 11 piles per rig every five day week are being maintained; with good weeks achieving up to 14 piles per rig.