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Spreading the load

Substructure requirements for the redevelopment at 5 Broadgate, in the heart of the City of London, are all about retaining a deeper basement and a substantial load-bearing raft. Is this the shape of things to come on previously developed sites?

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The emphasis has been on retaining the perimeter to construct the deeper basement

You’ll never guess who I had in the back of my cab” is not something you would expect to hear from a piling rig operator - but that, no doubt, is exactly what Cementation Skanska’s Mick Sixsmith was telling his friends after chancellor George Osborne visited the 5 Broadgate site in London. In fact, Osborne even took to the rig’s controls to break ground at the prestigious new London headquarters for investment bank UBS.

At a superficial level, the foundation work is demanding, but not exceptional for a major development in London.

It incorporates a complicated two-level basement and there is a substantial piled perimeter retaining wall. Add in the usual demands of managing deliveries to and from site in the heart of London, a single restricted site access point, lack of storage and the need to operate major plant in a tight environment during a phased handover from the demolition contractor, and it becomes a project that only a handful of UK foundation contractors could manage.

The project is, however, unusual on a number of counts. First, for some of Cementation’s long-serving staff there was a strong sense of déjà vu because the company previously piled the site in 1986 as part of the original Broadgate development. As such, the project is one of the first redevelopments of a major 1980s commercial property development. Significantly the original project marked the point at which buildings in London got a lot taller and basements became commonplace.

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George Osborne visited the site to break ground on the scheme

As a consequence, from a buried obstruction point of view, the site is peppered with heavily reinforced underreamed piles. No doubt this was a significant factor in explaining why structural engineer Buro Happold designed the scheme with a beefy and heavily reinforced 2m-thick raft to support the 14-storey development.

Essentially piling work on this latest phase in the site’s evolution is all about retaining the perimeter to enable construction of a deeper basement and installing the loadbearing raft. There are no loadbearing piles. Just possibly, this may well become the new norm for the next phase of commercial real estate development in central London.

It creates some novel challenges in terms of the secant piled wall, particularly around the detailing of the couplers in the pile reinforcement cages. There are four sets to tie into the raft and a further two for the level one basement slab. These had to be positioned with very tight vertical tolerances of just 50mm.

Cementation used Romtec’s safe splice system, in which following cage sections are connected with bull-dogs, and the operation does not require operatives to work with their hands through the suspended cages. Not only is this safer, should the cage move, but it is also more accurate and was a major factor in helping the contractor achieve the demanding tolerances, says Cementation design engineer Zoe Baldwin.

Ironically, one of Cementation’s biggest challenges has been dealing with the existing piles, installed back in 1986 using Watson & Hillhouse rigs. Coring through the existing - and it has to be said extremely well-made - piles proved a very demanding and noisy process, often requiring night working to ensure the surrounding Broadgate occupiers where not disrupted.

Nevertheless, not all obstructions could be removed, and in places the wall has to kink around these, further complicating the arrangement of the couplers in the reinforcing cages. Two sewer connections, which cut through the wall, create the need for local stitching, adding to the complexity.

Secant piling is, in fact, only needed on the west and north boundaries of the approximately rectangular site. On the east and south boundaries, lower retaining requirements mean sheet piling, installed under a separate contract with Sheet Piling UK, is sufficient. A resin injected at the junctions between the sheet pile and secant pile walls controls groundwater ingress. Cementation’s contract includes the capping beam for the whole site perimeter wall; installed under a subcontract by Byrne Brothers.

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Building 5 Broadgate required the demolition of 4 and 6 Broadgate, which are the first buildings to be redeveloped within Arup Associates’ iconic 1980s-built Broadgate Circle Development, adjacent to Liverpool Street Rail Station in London.

The new building, designed by architect Make, is set to become the UK headquarters for the investment bank UBS, which currently occupies a number of adjacent properties.

At more than 110,000m2 and 14 storeys high, the building will include four trading floors each capable of accommodating 750 traders, and an overall building capacity of around 6,000 people.

Developers British Land and Blackstone Group appointed Mace to deliver the project under a construction management agreement.

The rotary bored secant wall totals 162.5m, not allowing for the kinks to circumnavigate the immoveable obstructions, and is made up of 243, 900mm diameter piles. These vary in depth between 19.5m and 21.5m with three extending to 24.5m.

This was needed where the new wall runs very close to the sheet piled basement of the neighbouring Gaucho restaurant. Cementation designed and constructed some temporary props as a contingency measure, should wall movement during pile installation exceed the trigger level - 70% of the allowable 40mm movement. In the event, movements were just 3mm.

Another element of the project is that at the tender stage Cementation was asked to bid for the supply of six huge hand dug underreamed piles, a construction method that has all but disappeared in the UK, principally on safety grounds.

This was needed as two vehicle access ramps cut down at the edge of the site. One is Network Rail’s main delivery access point to Liverpool Street Station and the other is used continuously by Broadgate Estates. Both had to remain fully operational and it was not logistically possible to get a large rig in to install the necessary structural reinforcement to protect these potentially very sensitive structures.

Structural engineer Buro Happold designed 20m deep hand dug piles, each with an 8m diameter underream and Cementation won the bid on this basis. While preparing method statements on how this complex work could be achieved safely in conjunction with the Health & Safety Executive, Cementation in parallel brought in GCG to develop an alternative micropiled solution.

GCG came up with an innovative approach, based around the group effect of a rectangle of closely spaced 300mm diameter micropiles, in which the total capacity is greater than the sum of the parts. Former Cementation employee Jim Martin, who now runs a micropiling specialist firm, carried out the installation, which although to exacting tolerances, proved straightforward - particularly when compared to what might have been.

 

Green Ambitions

The design of 5 Broadgate will make it the “greenest” building in the Square Mile and it is on track for a BREEAM “excellent” rating. The building will have the largest number of solar photovoltaic panels on a commercial building in London, a green roof, rainwater harvesting and heat reclaim driven underfloor heating.

Buro Happold engineer Edward Sauven carried out a detailed carbon footprint analysis of the development, which he presented at the British Land embodied carbon seminar in October 2011.

This looked at the carbon embodied in the building’s fabric over its 60-year expected lifetime. He compared this to the projected carbon that will be used in the building’s operation over the same period.

This found an almost equal split between the two when decarbonisation of the UK power grid was considered. This means that heating, cooling and lighting the building over its lifetime will create the same amount of carbon as building and maintaining it.

Sauven attributed around 5% of the total carbon to the sub-structure; 18% to the superstructure, 5% to the façade and 19% to the fit out.

He also looked at potential carbon mitigation measures during construction and identified over 3,500 tonnes of cost effective carbon savings, representing a relatively modest 4% of the total embodied carbon in the build stage. The lion’s share of this - around 70% - was achieved using low-carbon concrete in the superstructure.

Changes in the foundation design produced the second largest carbon savings of 600 tonnes. Sauven estimated that switching from a load-bearing piled solution to a raft reduced the volume of concrete and produced an 8% saving in embodied carbon.

It’s important to note that this compares a raft to a new piled solution, and the analysis did not consider the potential of reusing the original piles installed in 1986. Nevertheless, the analysis highlights the importance designers and developers are placing on carbon; which looks set to become a major driver in developing foundation solutions that maximise the structural effectiveness of the concrete they employ.

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