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News: Thinking Outside The Box

Very often, we get asked to deliver awkward projects that our more mainstream competitors either can’t get their heads around, or aren’t in a position to deliver because of their company delivery structure & inflexibility. Here at HB Tunnelling, we’re proud to say that one of our strengths is our ability to think outside of the box, and deliver results that meet or exceed all the clients’ requirements. 

HBT was contacted by Forkers Ltd to look at feasibility of flood relieve project known as Mushroom Green Embankment in the West Midlands.

Upon site visit it was explained that there is existing culvert which is in poor condition and requires upsizing or tunnel a new culvert offline to avoid serious flooding of the upstream catchment explained Forkers.

HBT looked at the possibility of tunnelling offline, now comes the challenging part, the size of tunnel that would be require is 4.8m x 3.6m & located in the embankment is 2 number strategic critical foul sewers at 675mm and 750mm diameter.

Following on from the meeting HBT looked at what & how such a task could be undertaken. HBT worked with all parties to come with a workable solution to allow Mott Macdonald to put together the bones of a scheme which on its completion is expected to take 42 properties off the flood register. The scheme was put out to tender and a £1.5M valued, 36-week contract was duly awarded to West Bromwich-based Forkers Ltd, who were very aware of the task set out in front of them.

Mott Macdonald designed the scheme as a 45m-long section of 4.8m x 3.6m culvert, which on its completion would give the local authority 470m3 more capacity than in the existing culvert. Hugh Boyle said “Jacking a culvert can bring with it its own problems, not at least the fact that jacking a large rectangular structure is totally unlike jacking a circular structure, in that a high degree of friction needs to be overcome with high jacking forces, which can in turn put the integrity of the precast structure at risk”.

First things first HBT had to commission a purpose built box jack tunnelling shield for the works, as this would take 12 weeks to fabricate & manufacture HBT awarded this to Oldham-based Tunnel Engineering Services.



Before any works could proceed HBT was instructed to undertake settlement calculations as such a large structure & coupled with the 2 number strategic critical foul sewers at 675mm and 750mm diameter located within the embankment it was critical to see what the settlement would be incurred so a plan could be set up to support the embankment & the 2 number foul sewers.

Stabilisation works was required to the embankment, Forkers are one of the UK’s premier ground stabilisation and mine consolidation contractors and duly put forward a proposal to stabilise the embankment using a cementitious grout. Forkers set out and horizontally drilled 26 Nr. holes at between 18-22m in length and injected around 40T of PFA grout over a period of 2.5 weeks.

As further enabling works, Forkers provided additional support to the two sewers by installing a series of hydraulic jacks beneath the joints, which if any settlement occurred they could be extended to keep the pipelines to the required gradient. As each culvert weighs a staggering 17.9T, meaning that the overall 45m-stretch of sewer comes in at 805T, it was decided that only 24m would be constructed in conventional ‘bored’ tunnel, with the remainder being constructed either side of the embankment in standard cut and cover.



To enable the construction of the tunnelled culvert, a sheet-piled 9m x 7.8m drive and reception pit, at depths of 7m and 4.5m respectively. HBT excavate both shafts using 406 Mega Shore bracing supplied by MGF to support the structure and to construct the tunnel. As a further part of the enabling works, a 1m-thick, 36m3 RC thrust wall was cast and two 29m-long by 457mm dia. guided auger bores were installed with the steel casing acting as a pair sacrificial guide rails for the culvert to slide on – which on their installation were infilled with grout. A series of eight-110T jacks – three up either side and two in the invert were bespoke for this project & set up in the pit bottom, along with a custom-manufactured shield with internal dimensions of 4.8m x 3.6m.



To excavate the tunnel, a 1.5T Kubota excavator and Bobcat skid-steer loader were utilised, with the latter mucking out into a 6.11m3 (8 yard) skip.



The ground conditions were interesting in that they comprise made ground/fill and fly ash overlying clay, with absolutely no water ingress, Explained Hugh “Predominantly the area is underlain by Carboniferous rocks which contain a number of mudstones and sandstones with coal measures inter-bedded,” explained Hugh. “On this site, however, we have pulled out rail sleepers and what appeared to be some brick arch and possibly some form of tunnel or abutment, which leads us to believe that at one time this was a rail embankment.”



To further counteract the friction, HB Tunnelling are using a polymer bentonite as lubricant along with a standard anti-drag system. The allowed settlement on the scheme is -/+83mm, with the contractor half way through the installation of the tunnel. On completion of the overall 45m-long structure, the contractor are intending to divert the flows through it, and abandon the existing 98m3 structure using either PFA grout or foam concrete.



For more information please contact Hugh or Andy on - 0845 388 3325.

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