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Power Base

Fragile ground made conventional piling risky at a mill conversion in Macclesfield, so a more gentle solution was required. GE reports.

The current fashion for converting former industrial premises into apartments often requires retaining as much of the existing building as possible, while keeping disruption and costs to a minimum.

In Macclesfield, an old silk mill is being converted into flats, and a new structural steel frame is being constructed inside the existing perimeter masonry walls.

Site investigations showed the ground beneath the existing foundations to have insufficient load bearing capacity to support the new frame.

“We’re not constructing new foundations here, we’re going to transfer the steel frame loads through the existing masonry at basement level, which is strong enough. It’s just that the ground isn’t good enough,” explains consultant engineer Bingham Davis managing director Robert Seymour.

Piling was initially considered as the best way to create additional bearing capacity, but, given visible signs of movement in the walls, it was thought to be too risky.

“With the fragile nature of the building, piling - and especially driven piles - might have caused some distress on the building,” says Seymour. “There would be too much vibration during the piling and underpinning works. Also, the basement has difficult access, which posed a problem for the piling rigs.”

Bingham Davis needed a more gentle solution that would also be suitable for the limited access.

Polymer injection specialist Uretek devised a solution that involved deep injection of polymer resins to improve the ground beneath the load-bearing walls.

Where ground was particularly weak, and where there were party wall constraints, the company installed PowerPiles at close centres to a depth of 2.5m.

“The PowerPile is tightly wrapped for its insertion in the ground, and expands within the soil,”

The PowerPile system was a logical development from the firm’s deep injection system, according to Uretek UK’s geotechnical manager Stephen Cooper.

“In occasional situations we were finding that resins migrated through - rather than consolidated - very soft soils,” he says. “We wanted to contain that migration, which we set out to do with the original PowerPile system.”

The concept is simple: expanding resins are injected into a pre-installed geotextile-wrapped tube, constraining the migration of the resins to a column and allowing polymer injection to be used in very weak ground.

The deep injection method is still the preferred treatment for most cases, says Cooper.

If a foundation was originally designed to bear upon a soil stratum, then a deep injection solution consolidates the ground as a mass under the footing, and can provide a uniform support for the foundation.

But if the material can escape into softer soils beneath the foundation, such as peat or very soft clays, PowerPiles can be useful, offering two different techniques for supporting the structural loads.

The first (the original UPP100 version) is via a confined consolidation of the soil, and the second (the new UPP400) is effectively the creation of a load transfer system in which loads are physically transferred from the structure beyond the soft soil into the firm strata at depth.

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“The PowerPile is tightly wrapped for its insertion in the ground, and expands within the soil,” explains Cooper. “There’s benefit to the adjacent soil, and therefore an increase of skin friction onto the PowerPile itself, and there’s also a major advantage of the large diameter of the PowerPile once expanded, which gives a greater basal resistance to loading.”

The pile expands from around 35mm to 330mm, and with the latest version of the system loads can be over 200kN per pile, which, at close centres, is more than adequate for a four- or five-storey building.

During its research Uretek found that, in some instances, the pile’s geotextile bag would tear along the seam with the use of more aggressively expanding resins, and
also that necking or bulging might occur.

“We now use a seamless bag in UPP400 piles, which are designed for load transfer. This prevents tearing during the expansion process and means we can pack more resin in and have a very dense pile capable of high loads,” says Cooper.

The company has also invested in a bespoke system to assess and monitor resin consumption as the PowerPiles are being filled from the base upwards, a method which has eliminated bulging and necking and provides better installation control.

On the Macclesfield site it took only three weeks for 76 linear metres of deep injection and PowerPiles to be installed.

Dynamic probing was carried out before and after to confirm the required strength had been reached.

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