Building information modelling (BIM) and load monitoring are making their debut on a large basement propping scheme in Oxford.
Westgate Oxford Alliance – a joint venture between the Crown Estate and Land Securities – has embarked on a £500M project to redevelop the Westgate Centre in Oxford. The plan involves demolishing part of the existing Westgate shopping centre and refurbishing the remainder to create new homes, 82,000m2 of shops, 27,000m2 of cafés and restaurants and a five screen cinema.
Work started on site in early 2015, with Laing O’Rourke winning the contract to complete the new development in time for Christmas 2017.
Beneath the footprint of the development will be a basement measuring around 85m by 300m. It will accommodate car parks, loading areas and plant rooms and has been excavated to a depth of up to 8.5m within 16m deep sheet piled walls, supported by a proprietary system of waling beams and raking props supplied by Mabey Hire.
The excavation was split into two phases, allowing the props to be “leapfrogged” from one section to the next, keeping the total number to a minimum.
“This was one of the benefits of using a proprietary system,” explains Mabey major projects manager Matthew Green. “We could supply equipment for the phase 1 works which could then be re-utilised as the excavation progressed into phase 2 – typically this involved 220 linear metres of waling beam, 44 upper level props and 70 lower level props.”
Subcontractor Expanded has been using a staged method to form the basements. Once the sheet piles were in place, the earth was excavated from the central area, leaving a berm to support the piles. At this stage, Mabey installed its waling beams, and Expanded started to cast a 1.2m thick heavily reinforced raft slab at the base of the excavation. Once the slab concrete had achieved the required strength, Mabey installed the upper level of raking props, spanning up to 13m between the waling beam at the top and the edge of the raft slab, which was acting as the thrust restraint.
With the props in place, Expanded could remove the berm material, leaving the piles supported by the temporary support system. It could also start casting a concrete lining wall up against the piles.
Mabey’s second – lower – level of raking props spans around 4m from this wall to the slab; once they were in place, the upper level of props was removed and taken to the next phase of the excavation, while Expanded worked within the excavation to build walls, columns and lift pits.
The design of the lower level of props is quite complex – particularly in the large Phase 2 excavation, so Mabey chose to design the propping solution using Laing O’Rourke’s BIM model to identify where clashes could occur.
“We can provide 3D representations for clients to see what the scheme looks like, but mainly that is for visual effect,” explains Green.
revit model north east corner westgate a
“What we have done with Westgate is used their model to design the temporary works, making use of the fact that all the structural elements were already in there.”
Mabey has developed a “family” of its products within Revit software, so they can be plugged directly into a BIM model.
“We knew that it would be good as a visual aid, but we wanted it to actually become a design tool,” says Green.
“Temporary works are often seen as a necessary evil, but if you don’t pay attention they can cause a lot of problems. For us and Laing O’Rourke, embracing the BIM model for temporary works has saved a lot of time.”
The design uses two different sizes of Mabey’s proprietary Super Bracing Struts – the 400mm square section SBS-400 and the 600mm square SBS-600 – combined with Super Shaft Brace and Super Shaft Plus waling beams. The raking props were fixed to the raft slab via “birdsmouth” end plates.
The props were prestressed using a hydraulic jack, and then locked off mechanically, removing the hydraulic element completely.
“This allows the axial stiffness of the prop to be dictated by the section size and not the hydraulic component,” explains Green. “There was a stiffness requirement on the project, and with this design we could achieve both the required preload and the stiffness.”
One of the design criteria was deflection, which was limited on most of the site to a maximum of 20mm at the mid span of the waling beams.
Mabey also identified the Westgate project as an opportunity to trial its new LIVEpin monitoring system. “It wasn’t a requirement on the job,” says Green, “but because we had been developing the LIVEpin, and were at the point where we could put it into an excavation, we felt there were areas on the Phase 2 basement where it is close to the highways and there are quite stringent deflection criteria, where we could put two pins in and use it as a test.”
The pins have been fitted to the two props that take the most load, and provides real time data of the load going through the prop at any time (see box).
The props will be in position until this summer, when the basement works are set to be completed.
“What we are trying to do is to ascertain the load in the strut by the most accurate means we can,” explains Mabey Hire technical director Dave Holland, discussing the development of the company’s new LIVEpin.
“We thought of a number of different options to get the load in the strut by different means, but the best idea seemed to be to use a shear pin at the end of a strut and to modify the strut itself to accommodate that. The actual accuracy of the pin is 1.5%, which is something we can really shout about.”
“Once we had developed the initial prototype, we did tests that were all about the accuracy and repeatability of the data: concentric and eccentric loads; high and low loads; different temperatures,” he adds.
“The next stage was to figure out a way of getting that information from the pin to our dedicated information website. Our preferred option was for it to be wireless; and we also wanted to make it as small and powerful as possible, while keeping it autonomous to a reasonable period.”
The third part of the process was to develop a system and protocol that would enable the data to be collected and sent as quickly as possible while giving confidence that it is reliable and as accurate as possible. This has been achieved by designing repeaters that are strategically placed around the site that amplify the signals to send load and temperature information every 30 minutes, but to switch automatically to once a minute if the load goes above a pre set threshold and then return to the lesser frequency of readings following a latency of returning below the threshold, thus saving on readings and power, helping the autonomy of the system.
The information from the pin is collected by a logging device on the site, which sends it via 3G to the website. As with the rest of the system, the logger is built with redundancy to ensure that data is always captured.
In association with Mabey Hire