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Mon August 02 2021

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Giant props shore up Bayswater basement excavation

18 Jun Constructing the basement of a £500m residential development on London’s Hyde Park has required some of the strongest propping equipment on the market.

The excavation on Bayswater Road is 65m by 25m by 15m
The excavation on Bayswater Road is 65m by 25m by 15m

Park Modern is a 190,000 sqft  development of 57 apartments varying in size up to six bedrooms, as well as 30,000 sqft of commercial space.  It is being built by Turkish contractor Ant Yapi under a £100m contract for developer Fenton Whelan on Bayswater Road and Queensway in central London.

Modebest has the groundworks and concrete frame package, which  includes construction of an enormous three-level basement – alongside busy roads and adjoining buildings. To further complicate things, London Underground’s Central Line passes under the Bayswater Road, parallel to the excavation’s southern retaining wall.

Support for the excavation, which measures 65 metres x 25 metres x 15 metres deep, is provided by more than 30 of Groundforce Shorco’s modular hydraulic props. The props, which include some of the company’s heaviest MP750 units, are braced against a waling beam comprising Super Mega Brace units to support the secant piled retaining wall.

Construction of the basement started in late summer 2020 with installation of a guide wall followed by construction of a secant-piled retaining wall comprising 1,000mm diameter piles at 1,200mm centres. Temporary kingposts were installed in certain locations to support the capping beam, which was then constructed.

Excavation then began and in September the first props for Level 1 were installed. 

The propping solution is planned in three phases to accommodate the three levels of support required.

Level 1, which began in September 2020, will continue until the last props are removed in July 2021. Props for Level 2 started going to site in during October 2020 with the last ones off-hired in May. The Level 3 props were installed in November 2020 and removed following construction of the basement slab in April 2021.

In total, 33 Groundforce props will have been used to support the excavation. Level 1 uses fourteen MP250 units, each with a rated load capacity of 250 tonnes. Five of these props, spanning the central section of the basement, range in length from about 17m to 23m. To ensure sufficient stiffness over such long spans, these units employ 1,220mm diameter Super Tubes instead of the standard 610mm diameter extension tubes.

Level 2 also employed fourteen props: three MP250s (of which one features Super Tube extensions), seven MP375s (375 tonne capacity) of which four had Super Tubes; and four of the largest MP750 props, each with a load capacity of 750 tonnes and each featuring Super Tubes.

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Level 3, at the bottom of the excavation, was supported by three MP250s and two MP750s and all props used Super Tubes.

Groundforce’s Super Mega Brace was used around the perimeter at all three levels: Levels 1 & 2 each used 150 metres of Super Mega Brace, while Level 3 used 50 metres. One large section of bespoke steel waler beam was also incorporated at all three levels.

Groundforce says that the project is notable not only for the scale of the temporary support equipment deployed, but also for its exacting technical requirements.

“The job involves high loads and has very onerous deflection criteria,” said Mark Whitmore, general manager (major projects) with Groundforce. “To keep prop count to a minimum the highest capacity props on the market were needed.”

Because of the close proximity of occupied buildings and a live underground tunnel, ground movement has had to be limited to the absolute minimum and constantly monitored.

The props themselves, besides being given a visual inspection on a weekly basis, have Groundforce’s own remote load monitoring system to detect any movement or change in loadings. Two props at each level were fitted with the system, which monitors the situation and sends an alert via SMS text or email if set criteria are exceeded.

On this project, the design criteria required that a movement in the secant piled wall of as little as 25mm would trigger an ‘amber’ alert to require investigation and possible remedial action. A ‘red’ alert would be triggered by 30mm of deflection – resulting in an immediate halt to all work pending investigation.

Modebest managing director Michael Brennan said: “The success of the project to date is a testament to the collaborative, open minded approach we adopt to problem solving at Modebest and Groundforce have been a key partner in this journey.“

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