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Willmott crams in mixed-use revamp with a twist

In west London contractor Willmott Dixon is working on a tiny site to create a development of epic proportions.

Project: Dudley House, Paddington
Client: Westminster Council
Contract value: £85m
Contract type: JCT Design and build
Region: London
Main contractor: Willmott Dixon
Piling subcontractor: Miller Piling
Frame subcontractor: JRL Group
Start date: December 2016
Completion date: May 2019

Paddington Basin is the quintessential urban canal wharf.

In its heyday it was a busy commercial centre with barges bringing pottery, coal and livestock from the Midlands and the North-west and returning with imported fabric and spices, gin and brandy.

While some of the warehousing remains, today the hustle and bustle of the industrial revolution has been replaced with the clink of glasses in bars, the footfall of commuters on their way to work, and the noise of construction as the area undergoes an overdue revamp.

In a tiny section of land alongside the basin, contractor Willmott Dixon is working on one of those redevelopments: a mixed-use scheme that is a little different from the norm.

Here, instead of the usual commercial offices mixed with retail space or a hotel, the team is working on a project that will provide a new school, accommodation for locally employed medical staff, a church, social housing, community leisure facilities and retail space.

Clever use of space

It is difficult to see how all this is going to fit into a site that once featured a few low-rise blocks of flats.

But by maximising every inch of space available, the team will be able to provide 197 apartments alongside key worker accommodation and brand new facilities for the 840 pupils of Marylebone Boys School.

It is the school that brings the Dudley House scheme into sharp focus for Willmott Dixon project director Mark Chamberlain. “The school has been working from various temporary sites for the last three years,” he says. “That must have an impact on the boys’ education. We are building a permanent new home for that school here. It will make a tremendous difference to the pupils, staff and teachers.”

In fact, two of the boys that will be attending the new school lived in the post-war low-rise flats that were demolished to clear the site. It is fitting that they will be among the first to gain their qualifications at the new facility.

Mixed-use obstacles

Before the team reach the August 2018 deadline to hand over the school and allow pupils to settle into their new home, there are a few obstacles for the project team to overcome.

“We couldn’t have built it without using the lightweight steel frame and the post-tensioning solution, the spans were just too great”

Mark Chamberlain, Willmott Dixon

The building itself is split into three main sections. The first is the school, a seven-storey reinforced concrete-framed building at the south end of the site. The main building is a 22-storey tower housing the rented residential units, which also features a cast in-situ reinforced concrete frame.

Sandwiched between the two is the medical worker accommodation. At six storeys it is the shortest block in the trio and – thanks to the large spans required across the frame – is built using a lightweight steel frame at its upper levels, but with a reinforced concrete basement that will provide a sports hall and retractable bleacher seating for the school.

Grenfell considerations

The tragic events that played out just a few miles away at Grenfell Tower last year brought some of elements specified in the Dudley House design into sharp focus.

“After Grenfell we went back to look at the design and the specification,” Mr Chamberlain says. “We went through a complete and full review, looking at everything we could. There are no combustible materials involved in the cladding of this building and the checking system we have in place is extremely robust.”

The cladding is predominantly a standard terracotta rainscreen system, but it is the installation and inspection regime that differs. Willmott Dixon has brought in a specialist fire safety company to install the cavity barriers and the installation is checked at every turn by a third-party inspection team.

“We want to be sure of everything concerning the safety of this building and felt it was important to carry out that review and demand that sort attention,” he says.

Due the requirements of Sport England, which specifies dimensions for these facilities, the design has necessitated the installation of three 2 m x 1.5 m-deep post-tensioned beams.

“The sports hall needs spans of 20 m,” Mr Chamberlain explains. “This requirement influenced the design of the [medical] worker accommodation above it. We couldn’t have built it without using the lightweight steel frame and the post-tensioning solution, the spans were just too great.”

Duckbill anchored wall

The team moved onto the site during Christmas 2016, clearing the low-rise apartment blocks before beginning the bulk excavation and muckaway.

Site workers removed more than 20,000 cu m of earth during that process, which saw the project team install a series of temporary duckbill ground anchors around the excavation.

There are 163 in total, reaching 12 m into the surrounding earth to tie down the contiguous piled wall installed around the basement dig. These sacrificial anchors provided support to the 257 contiguous piles until the team could cast the capping beam around the wall.

“There is a large brick-lined Victorian sewer that runs down one of the roads alongside the site,” Mr Chamberlain says. “We worked closely with Thames Water on an extensive temporary works design to make sure the ground anchors didn’t clash with it. The site is so constrained we realised we couldn’t work with traditional propping. We couldn’t get the piling rigs in.”

“We physically couldn’t fit the vibrating pokers between the steel bars, there was so much in there”

Mark Chamberlain, Willmott Dixon

Miller Piling installed 662 CFA bearing piles across the project. There are 431 piles of 450 mm diameter down to 28 m, 194 piles of 600 mm diameter to 34 m, and 37 piles of 750 mm diameter to 35 m underpinning the whole building.

The basement ground-floor slab is packed so densely with reinforcement that the team needed to use self-compacting concrete during casting. “We physically couldn’t fit the vibrating pokers between the steel bars, there was so much in there,” Mr Chamberlain says. “Using self-compacting concrete added around £15,000 to the cost, but it was the only way we could have cast the slab to that design.”

On the slipform limit

Cores throughout the three buildings have been slipformed rather than jumpformed, although Mr Chamberlain considers the smaller buildings at the limit of cost-effectiveness for slipforming.

“It was borderline [as to whether] the slipform was the right decision, but by using that method it meant we were able to install the lifts without having the floorplates complete,” he says. “The slipform that we used in the 22-storey building was a bespoke design for this project because of its elaborate layout.”

Table forms and the Peri Skydeck panel slab formwork systems were used to cast the slabs across the structure, although most of the vertical components were precast concrete. It was a tactic agreed with frame subcontractor JRL largely because of the speed of installation and the safety improvements it offers through reduction in time spent working at height.

In common with most city projects, the team working at Dudley House has had to juggle the difficulties of supplying a fast-moving construction project with the materials it needs.

There are many ongoing projects in this part of the capital, with a large office scheme on the site directly alongside the Dudley House project. This has meant close negotiation and liaison over access, with two tower cranes handling materials that are brought in exactly when needed. “There isn’t even enough room for an artic truck here; everything has to come in on the back of a flat-bed,” Mr Chamberlain smiles.

With just a few months left before the team must hand over the school section of the development, the Willmott Dixon team is focused on delivering that facility for its 850 pupils.

But with affordable housing, medical worker accommodation and a new church to be completed by May 2019, this is a project that will offer a real legacy for this corner of west London.

Prefabrication motivation

Constructed to BIM Level 2 throughout, the Willmott Dixon team has focused heavily on the use of prefabricated systems wherever possible.

Other than the use of precast concrete for the vertical members of the structural frame, M&E contractor Kane has helped satisfy that prefabrication desire with its installation of offsite manufactured pipework and flue systems that have been produced in four-storey sections.

“As a company we recognise the benefit that BIM offers, particularly during the design and construction phases, and want to encourage the quality and efficiency of prefabrication,” Mr Chamberlain says. “We are on the journey toward our ultimate goal of defect-free construction. We worked closely with Kane to help develop the prefabricated plant room solution and the four-storey flue sections.”

With the building needing to hit a Very Good BREEAM environmental rating, a dedicated apartment has been set aside with for the sole purpose of checking the acoustic performance and airtightness, before rolling those findings out across the rest of the building.

“We want to make sure that we have checked and double-checked everything,” MrChamberlain adds. “We can flag up any areas that need attention or that may need a slightly different approach before we really get stuck into the fit-out of the apartments.”

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