Tough conditions and temperature fluctuations have been resolved by real time monitoring on basement props in Oslo
Contractors working on Norway’s most ambitious building project - the Fjord City redevelopment - have successfully completed a large, complex basement excavation with help from modular hydraulic supports fitted with real time load monitoring.
The project, which is known locally as the “Barcode”, comprises a series of high rise buildings on former dock and industrial land in central Oslo. It has a huge common basement, shared by 15 of the housing units, that is up to three levels deep and covers an area of 35,000m2. The basement has been built in soft Oslo clay and made ground, and so it needed strong, rigid support.
There’s no way we could have responded to the increase without using real time load monitoring
A vital requirement was that nearby structures - including a major road bridge that passes just 7m from the excavation - had to be protected from any damage caused by ground movement. This was especially challenging as the site was already affected by ongoing natural settlement calculated at around 30mm per year.
Basement contractor Kynningsrud Foundations opted for a modular system for the propping, and went to Groundforce to supply it, having previously used the firm’s props on a project in Trondheim.
Groundforce supplied 22 of its MP250 props, the biggest standard hydraulic struts it supplies, each with a nominal capacity of 250t. They were installed in three levels to support the basement’s sheet piled retaining walls while Kynningsrud excavated to full depth.
The excavation was carried out in sections, with the support system dismantled for re-use and “leap-frogged” into position as each new section commenced.
“We could have done the propping with traditional steelwork, but this entails a lot of work and doesn’t lend itself to re-use,” explains Arne Tveit Eigeland, geotechnical engineer at the project’s consulting engineer Multiconsult.
Kynningsrud quickly mastered the removal, repositioning and re-installation sequence and, despite the fact that the largest props were 28m long and weighed more than 18t each, was soon relocating each one in less than two hours.
Continuous real time load monitoring was a fundamental requirement of the operation, according to Tveit Eigeland.
“We needed a system of integral load monitoring to measure real-time loads and monitor load changes due to excavation,” he says.
On the contract, which began in July 2013, temperature fluctuations created increases in the load from 260t to 340t as the props expanded during the day. Kynningsrud installed water sprinklers to regulate prop temperatures, and managed to achieve a constant load of 270t.
“There’s no way we could have responded to the increase without using real time load monitoring,” says Kynningsrud project engineer Vidar Flatekval.