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Neighbourhood Watch

Construction of a six storey office development is currently underway on St James’ Square in central London, while opposite a new multi-storey building is progressing about six months ahead of the office building. Despite the close proximity of the two schemes, the office project has had to overcome significantly greater ground engineering challenges.

The new office being built by Galliford Try at No.8 St James’ Square will not only have several basement levels but also wraps around and below No.7 St James’ Square - a Grade II listed Edwin Lutyens designed building - and also had to contend with a scour feature in the London Clay. The combination of these issues meant that the raft foundation solution being used on the development at No.5-6 St James’ Square was not an option and a piled design was needed. But that was just the start of the complex issues on the site.

St James Square

The excavation called for some complex temporary design

No.8 St James’ Square will be a steel framed building with two basement levels at the property’s front side facing into the square, deepening to three storeys to the rear due to the rise of ground level over the 80m length of thesite. To meet the demands of the development, the solution involved cased rotary bearing piles, cast in-situ, typically 750mm diameter and up to 35m deep. These piles would take column and core loads up to 3500kN and allow for 1000kN of tension from ground heave.

The footprint of the site is an L-shape in plan, wrapping around No.7 St James’ Square, which has added to Galliford Try’s ground engineering challenge (see box). An excavation 10 to 14m deep was needed to get down to basement level, so demanded a substantial temporary works design.

“Effectively we had to condense six months of piling and temporary works design into a six week period.”

Mark Creighton, Galliford Try

The temporary works designed for this massive excavation involved 10 to 11m long sheet piling between the two sites and trench sheets with close steel framing along the southern end of Galliford Try’s site. Along the north and western sides, the 5m high outer wall of an existing basement was to be left in place, temporarily propped from the basement slab by steel raking props while the inner walls were demolished.

For the lower half of the excavation on its northern and western sides, the temporary works consisted of a contiguous piled wall, of 700 piles with 900mm diameter temporarily cased piles, varying from 16 to 26m in length and at 1050mm centre spacings.

The old 1960s-built No.8 St James’ Square was demolished down to basement level last summer and subcontractor Keltbray Piling had begun installing the contiguous wall along the northern perimeter of the site. At that point everything changed. Instead of piling through a relatively thin layer of river terrace gravels then good London Clay as expected, Keltbray encountered several instances of gravels and high water pressure extending muchdeeper, preventing sealing of the pile casings.

Additional detailed ground investigation by Card Geotechnics (CGL) revealed a series of drift filled hollows. This is a ground formation characterised by hilly contours and deep holes in clay strata, usually filled with alluvial material and widely thought to be caused by the scouring action of swirling glacial meltwater.

The challenge then was to review the whole groundworks design and redesign where necessary in the knowledge that instead of encountering a limited perched water table, the construction team would be dealing with a head of water up to 6m in alluvial material.

Prior to this discovery, the project team had already devised an innovative method of providing lateral support to the northern and western sides. Cross-propping to opposite sides could not be done, so unusually for such a deep excavation, a system using a single layer of raking props was designed.

Key to this was Keltbray Piling’s proposal for a series of ‘king posts’ - plunge columns cast into every second or third male pile of the contiguous wall to support the top 5-6m of excavation above. The king posts would be prestressed by jacking them 35mm away from the basement wall before packing was built against the deflected U-beams and the jacks released.

St James Square

Keltbray’s king post solution helped reduce props to a single raking layer

Completion of that stage would allow construction of a capping beam on the contiguous wall. Steel hydraulic raking props could then be installed once enough of the building’s central pile caps had been built to prop against.Only then, with these stage two props in place, could a substantial soil berm be removed and the excavation completed.

According to Galliford Try chief engineer Mark Creighton, the time needed to get sufficient solid structure built to prop against was the programme’s critical limiting factor, until the reality of the ground conditions came to light.

“Effectively we had to redesign everything, and condense six months of piling and temporary works design into a six week period to mitigate the problems. From an engineering perspective it was good fun, but from a project manager’s view, a nightmare,” Creighton says.

The redesign, by Keltbray’s specialist piling and temporary works consultant Wentworth House Partnership, resulted in the lengthening and further reinforcement of the building’s bearing piles. Along the northern perimeter, the contiguous wall became one of 1000 900mm piles at 1250mm centres, while the western side became a secant piled wall of similar dimensions. Furthermore, with design loads approximately 50% higher, assurance of the adequacy of the propping design became critical.

Galliford Try is now working to pull back lost time in the overall construction programme. Means for doing this have included slipforming the building’s main northern lift and stair core, which reached full height while work on the excavation below was still ongoing. Other structural work has been resequenced where possible including leaving out some non-critical elements of the basement construction.

“Everything that can be done to speed up the programme is being done, such as precasting walls and columns wherever possible and we’re looking at mechanisms for leaving certain slabs out till a later time,” says Creighton. “In that respect we have a very proactive partner in CGL in buying into different ideas and coming up with good ones of its own.”

 

Heritage protection

A project within a project is how Galliford Try chief engineer Mark Creighton describes work beneath No.7 St James’ Square. The overall scheme includes construction of a new two-level basement beneath the Grade II listed building to provide for car parking and future development of the property.

For this aim, No.7 has been underpinned to a depth of 7m from access shafts dug in the building’s basement and partially supported on a platform of steel needles and five braced steel frames. These were constructed top down from the start point of installing 300mm diameter mini piles with king posts using segmental flight auger techniques in about 3m of headroom.

“Wentworth House excelled in the design of this tower arrangement. Not only does it support vertical loads, but also lateral surcharges from the adjacent development, cross-propped to the No.8 St James’ Square ground support,” Creighton says.

The challenge of supporting No.7 was heightened by the discovery of a subterranean slip circle in the London Clay around 4m below the building’s footprint. As the demolition and excavation around the No.7 basement progressed, the listed building started to move more than initially expected, but fortunately for all concerned, the temporary works extended below the influence of the slip plane. “Only when the underpins and mini piles reached this point could we be sure we had the building under control. It was the quirky nature of the ground conditions, a complicated geotechnical problem,” says Creighton.

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