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Geotechnical | Foundations for Cardiff’s biggest property development

Geotechnical | Major Cardiff development

Cardiff aeriel

If you have not been to Cardiff lately, you might not recognise it at first glance. Because a few steps from the central train station is one of the largest construction sites the Welsh capital city has ever seen.

About 47,500m³ of material – about 18 Olympic swimming pools – has been removed for what is set to be the new BBC Wales building.

It is part of a larger, five-phase eight-building development for Cardiff Central Square – including offices, a bus station, retail space and public amenities.

Cropped cs4

Cropped cs4

Excavation site, end-2016

Fitting it into eight blocks in the city centre sounds cramped, but it is actually opening up the space. It is a completely new look for a city experiencing residential growth not seen since the coal industry’s boom.

Last year, Cardiff Central station recorded 12M passenger entries and exits – up 100% on 1998 (including an estimated 500,000 visitors heading to Rugby World Cup matches at the Principality Stadium). And the growth is expected to accelerate. Network Rail, in its March 2016 Welsh Route Study, forecast that the number of passenger journeys through Cardiff Central would be upwards of 23M by 2023 and 32M by 2043.

“The increase in use of trains is quite significant. The numbers in terms of how many million per year is very drastic growth,” says  ISG project director Kevin McElroy.

Cropped3d view 1

Cropped3d view 1

The design, as viewed from the north

International construction firm ISG is building what will greet all these passengers, which is known as  “the centrepiece” – the £80M  BBC Wales Broadcasting House. It is also preparing the basement for the adjacent Two Central Square development, which will include two office buildings.

Architects Foster & Partners and Lawray, as well as consultant Arup worked on the design, including the 4,645m² BBC building set over four floors. Inside will be offices, studio and production space, providing for more than 1,200 staff. The BBC building’s floorplan is a rough 90m square, but when you add in the public space at ground level, and the triangular Two Central plot – in total it is about 2.5 football pitches.

New Civil Engineer visited in week 44 (mid-October 2016) of the 122-week construction programme, which began September 2015 and which is due to finish spring 2018.

This massive excavation has unearthed a changing city. Excavators uncovered what was originally the River Taff at the bottom of the excavation, unearthing the river pebbles and alluvial river sands.

Diggers uncovered what was originally the River Taff at the bottom of the excavation, unearthing the river pebbles and alluvial river sands

Filled in and built upon, the next layer up contained the remains of Temperance Town, a working class suburb established in the late 1850s. The suburb’s name came from owner and teetotaller Colonel Edward Wood who told developers that under no circumstances could alcohol be sold on the site.

Cardiff Central station arrived in 1930, bringing more business and the local authorities sought to shift Temperance’s growing and visible poverty and overcrowding.

“They knocked the first and second floors into the ground floors, mass filled it, and when we dug down, we got down to what were fully intact ground floors of the former houses,” says McElroy. “So the guys cleaned off the old floors, old fireplaces, took photographic evidence and we carried on.”

Cropped over view 2

Cropped over view 2

The development is between Cardiff Central Station and the Millennium Stadium

Then, from 1954 to August 2015, the plot humbly served as Cardiff’s central bus station, with visitors to the city greeted by a pretty ordinary view of concrete slab and 34 bus shelters. The bus station’s future home is not far away, just one plot east, where a car park is currently being demolished. But new plans will bring in an 11-storey building with a glass skyway connecting the bus and train terminals.

All said and done, ISG’s basement dig extends 6m down. Piles (600mm and 750mm – varying with the basement structure) went up to 13m down.

Secant piles

A total of 925 secant piles went around the perimeter to seal the site, and 442 load bearing continuous flight augured piles were inserted  inside the building’s footprint.

To dig, seal and prepare such a gargantuan hole, 5,588 vehicle movements were required over 10 weeks, and about 100 HGVs were needed for every 10-hour day.

“And we’ve had very little feedback from anybody,” says McElroy.

To avoid complaints from the public or other stakeholders, the team uses a “back entrance”, avoiding the city centre. There’s also an app in use, to direct and inform drivers who attend site.

Deliveries app

“When placing an order, we issue them with the app, which helps the drivers before they get here. They can get out their phone and see: access through town, on site, holding area locations, requirements on site, who to report to, not just for people on site, but those visiting. It informs everything. If anything changes to the job it gets updated automatically.”

More technology was drawn in at the design stage, using a Revit Model created by each consultant Foster & Partners, Arup and Lawray. It is a coordinated model updated monthly.

“It’s not Level 2 BIM – BBC weren’t mandating that. Level 2 is more coordination and assets, measuring and monitoring,” says McElroy.

Peak site activity has passed, with the forward plan featuring less concrete and more steel. “They [steel contractors] can only do two deliveries a day. But for concrete you might have 10 to 14. So compared to… excavation, it’s easy,” says McElroy. 

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