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Observational method supports Hammersmith car park conversion

Excavation of a major basement project in Hammersmith has been eased through use of the observational method to remove props ahead of the ground floor slab casting.

Load monitoring on hydraulic props spanning the 50m excavation for the transformation of the Kings Mall car park site in Glenthorne Road for a luxury residential development allowed the observational approach to be used.

The basement excavation is being delivered by Joseph Gallagher with excavation support from Groundforce Shorco for developer St George West London for its Sovereign Court project.

“The construction sequence dictated that the ground floor slab had to be cast prior to the props being removed,” said Groundforce Shorco major projects design manager Ajay Nagah. “The concrete pour was therefore done in sections, with the props removed a few at a time.”

The roughly triangular site will incorporate a two-storey basement and 17 of Groundforce’s modular hydraulic props have been used to provide support during the excavation phase. Some sections of the work called for props to span 50m with no intermediate support.

“We had strict performance requirements that had to be satisfied,” said Nagah. “There were stringent deflection criteria due to the presence of a London Underground line against the southern boundary of the site. We had to provide a clear space beneath the props for Gallagher to excavate the basement.

“The large size and irregular shape of the excavation added to the complexity,”

The irregular shape of the excavation called for special brackets to be developed to allow the props to brace against the concrete capping beam constructed at the top of the secant pile wall.

Most of the props were 250t capacity units but two 500t props were also used and, to cope with the loads, they had to be placed closer than normal at 7.5m centres.

Use of continuous on site deflection monitoring, Gallagher was able to remove props ahead of schedule using the observational method to ease the excavation and slab casting work, as well as reduce the programme time.

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