Use of geopolymer ground improvement techniques have helped ready workshops in Ilford for the arrival of Crossrail.
Crossrail’s tunnels have been headline grabbing but other parts of the network have been undergoing strategic upgrades to meet the demands of the new rail route. In Ilford this has called for a £50M investment being undertaken by Volker Fitzpatrick to upgrade existing facilities to create 10 new sidings for Crossrail trains.
Volker is now working on the final stages of the refurbishment at the Ilford Train Yard ahead of the planned 2016 completion date but reaching this stage has not been straightforward. The project team has had to contend with poorer ground conditions to upgrade an existing structure as part of the refurbishment.
“Crossrail will bring significant benefits to Ilford with reduced journey times and new routes,” says Crossrail project manager Jonathan Hunter. “As part of the Crossrail project, we are upgrading the existing depot at Ilford Yard, which will have an ongoing strategic role in the delivery of Crossrail as the yard will take delivery of the new Crossrail trains from 2017 and will serve as a base for drivers and trains.”
Chris Evans, managing director of Volker Fitzpatrick’s civils division says: “Our role as principal contractor has involved detailed design, enabling works, construction, as well as installation, testing and commissioning of the new facilities.
“Our work on site is taking place in a live rail environment and this provides a number of challenges such as ensuring our staff remain safe at work.”
Geopolymer injected system
Techniques such as minipiling would have been the conventional solution to overcome the problems but once the time and disruption, as well as the ownership issues, were factored in, it was not considered a viable solution.
Volker’s consultant RPS proposed working with Uretek to use a geopolymer injected system to overcome the challenges in dealing with the steel framed workshop building.
“Ground investigations undertaken by ESG showed that ground conditions were inadequate for the proposed redevelopment with around 5m of made ground over sands and clays,” says Uretek project engineer Liam Bromley. “It was believed that the existing foundation design did not have the capacity that was required and would not support the increased loads of 350kN/m2 that the refurbishment would place on the foundations.”
According to Uretek engineering manager Dan Hadfield, despite involving Uretek in the scheme, the geotechnical specialist working for RPS had some serious concerns about the use of geopolymer injection at the site, particularly the post-treatment testing. “Having the close working relationship meant that we were able to work with the team to revise the testing regime from using dynamic cone penetration testing and develop a plate load testing approach that could be carried out before and aft er treatment which would replicate the new loads,” he says.
Initial calculations ahead of both the testing and treatment suggested that total settlement of the pad foundations, which are 9m apart, would be 5mm. The analysis also showed that differential settlement between columns would be less than 10mm. The plate load tests showed that the result would actually be 2.5mm – well within the design criteria of less that 25mm demanded by RPS’s specification.
The analyses were put through their paces with a scaled down on-site trial carried out ahead of the main work. Work took 10 days to complete with four teams from Uretek working on the site simultaneously.
The proposal for the site was drawn up on the basis of having full unrestricted access to the site but the reality was that Uretek had to work around other trades during the treatment. “The Uretek system meant that we were able to be flexible with the on-site logistics and adapt the phasing of the treatment to fi t around the other work being undertaken at the same time,” says Hadfield.
The treatment itself involved four 52mm diameter, 5m deep injection points per 3m by 3m pad foundation – two were drilled vertically and two were inclined through the foundation itself. The inclined injection points were designed to treat areas on the boundary of the site without the need to access parts of the workshop that were under different ownership.
The injection was carried out continuously as the injection tube was extracted at a rate that was based on calculations determined by the fl ow rate of the pumps and the results of the plate load tests. Hadfield is keen to underline that the geopolymer material used by Uretek is very different from cementitious grouts. “The geopolymer is injected at low pressure and the ground improvement comes as a result of the chemical reaction that causes the geopolymer to expand horizontally and vertically and from the strength of the hardened polymer material itself,” he says.
Hadfield adds that during the injection process the material expands five to 10 times its liquid volume and exerts a considerable expansive force. “As the material expands, it fi lls any voids, compacting the underlying ground until thrust is applied to the underside of the foundation,” he says. “Despite the expansion in volume, the movement of the ground can be controlled to within 0.5mm.”
“One of the main benefits of the system meant that no excavation was needed and work could be carried out where there was minimal access”
Liam Bromley, Uretek
Injection work at Ilford was monitored in real time using a series of sensors and laser levels. In total there were 19 pad foundations to be treated and each 5m injection was completed in just 10 minutes.
For the strip footings, the whole 168m length of the building under Crossrail ownership was treated by injecting at 1.5m centres along the walling.
“One of the main benefits of the system meant that no excavation was needed and work could be carried out where there was minimal access,” says Bromley. “Geopolymer injection can often solve problems that would be considered untreatable by conventional methods due to the scale of disruption, time and cost involved.”
Bromley adds that the standard of the work delivered at the Ilford site has influenced some Uretek processes. “The highly regulated nature of the rail sector meant that the quality assurance demand at Ilford was scrutinised in greater detail than other projects of a similar scale and we introduced additional quality assurance processes for the work,” he says. “In the past we have been involved late in the process but working with clients through the proposal stage of the project enabled us to identify areas where additional time and cost could be minimised and offer a more engineered solution.”
Evans says: “Uretek’s approach and input at pre-contract stage was of great benefit to the project as it was vital to demonstrate to Crossrail that the solution was technically sound. Its proactive approach was key to this.”
With the ground improvement now complete, work on site has progressed well and Volker expects to hand the site back to Crossrail on schedule.
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