Groundforce Shorco is no stranger to basement box propping, but a project in Wandsworth, London, pushed the company’s products to the limit. Jo Stimpson reports.
The Riverside Quarter residential development by Fraser Property is not particularly unusual in itself, but the propping system being used on the joint basement box for these two blocks of flats is ambitious in both scale and technical capability.
Groundforce Shorco devised a system for main contractor Galliford Try of 12 props, four of which have a remarkably wide span of 47.5m, and a diameter of 1.22m.
These enormous props are just shy of 28t in weight - but they have brains as well as brawn.
Housed inside the props are the electronic load pins, integrated strain gauges and wireless transmitters that make up Groundforce Shorco’s new wireless load monitoring system.
The system allows the prop loads to be monitored at any time, from anywhere, providing instant on-site data that can be accessed via a roaming handheld device within 100m, or via the internet or SMS from greater distances.
The new system allows the fluctuation in load due to temperature changes to be monitored.
A change of around 10-15ºC can cause a load fluctuation of 50-60t, says Groundforce Shorco senior major projects engineer Matthew Green.
The props’ hydraulic element is designed to absorb these changes - but the monitoring system allows engineers to know exactly what is happening and to pick up on emerging trends.
“These are the longest horizontal span props we have ever done”
Trigger points for green, amber and red warning levels can be programmed in to let engineers know when the props are approaching their load capacity.
“Before, you never really knew what the load was,” says Green.
Groundforce Shorco technical sales manager Mark Whitmore says he looks forward to a time when there are years of observational data to inform the monitoring of props such as these. But for now the data system is still young, having been developed over the past 18 months, and the focus is on proving its reliability.
The Riverside Quarter is a stringent test, given the size of its props.
“These are the longest horizontal span props we have ever done,” says Green.
Groundforce Shorco typically supplies props around 36m in length.
For Galliford Try, the reliability of the monitoring system played a key part in showing that the horizontal props could be used with confidence.
“There’s always an element of caution,” says Galliford Try head of design management John Hallums. “As the spans get larger the props get bigger very quickly, and loads go down.”
The benefit of using the system is that the site has a completely clear working space underneath the props, which both Groundforce Shorco and Galliford Try say has contributed to bringing the project in on time.
The props on this project are all MP250s, modular hydraulically adjustable tubular struts with a maximum axial capacity of 250t/2500kN.
The seven largest props feature 1,220mm Super Tube extension sections to maintain axial capacity without intermediate supports, while the five smaller props use standard 610mm diameter tube extensions to reach the correct span.
The props are designed with a pre-camber to avoid sagging over such a large span.
Installing the props was a relatively simple process - but their removal will be the real challenge.
From February onwards, secant piling was used to create a retaining wall 7.5-8m in depth.
A partial excavation 1.5m deep was then created in May, so that the retaining wall could be topped with a capping beam.
The props were then installed at a rate of roughly two per day, with earth mounds built to support them from beneath until they were fully secured.
Basement box excavation could begin in earnest once the props were in place.
When the time comes to remove the props, things could be a little more complicated.
“Going in it’s at ground level, but coming out is different because the props need support,” says Groundforce Shorco’s Whitmore.
The current plan is to use the partially completed upper basement slab, known as B1 (as opposed to B2 which sits below it) to carry out the props’ load-bearing function.
“At the moment we’re looking to use the B1 slab as the prop - but we have to look at the stress from removing the ground floor prop,” says Hallums.
The props would be supported during the removal process with back-propping from the B1 slab.
There is more riding on the success of this removal operation than just the immediate project.
Fraser Property plans to construct up to seven apartment blocks within the Riverside Quarter, if market conditions allow.
“When you find a good subcontractor and you know his ways and he knows yours, it’s a good recipe”
A model result on these first two towers would put all parties in a good position in the talks that are already happening over the second phase of construction, as well as being a testament to the quality of Groundforce Shorco’s wireless monitoring system.
By all accounts, the project looks headed for success so far, and all parties are keen to collaborate again.
“When you find a good subcontractor and you know his ways and he knows yours, it’s a good recipe,” says Hallums.