The project is part of the enabling works for HS2. Severn Trent Water need to upgrade a 230m section of 1640mm Ø brick sewer which crosses under the HS2 embankment where the high-speed rail line will be built. This will ensure the asset is future proofed as it will be situated within the HS2 boundary which introduces new complexities but also it must be able to take the loading of the new embankment.
The contract was awarded to specialist tunnelling contractor Terra Solutions. Their remit comprises the construction two large diameter launch and reception structures which are sunk as segmental shafts, twin 180m-long by 1800mm Ø tunnels running parallel with each other, plus conversion of the launch and reception shafts into permanent structures used to access this section of sewer. The structures will ultimately house an intricate benching arrangement with penstocks to control flows between the twin tunnels, these flow controls will facilitate future operation and maintenance allowing one leg to be fully isolated.
Downstream of this this the Green Lane Sewer will be upgraded to include new sections of 1800mm ID and 1200mm ID sewer which will be installed using a combination of open cut and tunnelling methodology, on land spanning across and adjacent to Gilson Road/Gilson Drive in the Coleshill area of east Birmingham.
The construction works on the twin 1800mm ID tunnels are constrained by the presence of the existing 1640mm brick sewer. Significant design development including ground movement assessments have been developed to ensure no impact to the existing structure and it can remain operational until flows are ready to be switched through the new asset.
Terra installed the drive and reception shafts as 12.5m Ø jacked caissons using a series of eight 80T capacity jacks with tilt function to facilitate ring building. The ground conditions on both shafts comprised a glacial till – consisting of a firm and very stiff, slightly sandy, slightly gravelly clay. Perched groundwater at collar formation was encountered that wasn’t expected, a sump was created within the centre of the shaft to control groundwater whilst blinding was poured, first rings built, and steel in the collar tied. The steel for the collar was prefabricated in eight sections and dropped into place which sped up this element, avoiding the ground being open for longer than required.
The steel cutting edge was made from 25mm steel plate and was custom rolled to suit both the segments and tie bar arrangement. Two crawler cranes serviced the shaft construction element, a 135T Kobelco at the launch shaft and a 100T Kobelco at the reception shaft. The sizes of the cranes were governed by the heaviest repetitive lift, which during shaft construction was the 13te CAT excavator digging within the shaft and during tunnelling an eight-tonne muck skip, six tonne pipes along with the TBM lift. A larger crane will be required on site to install the cover slab beams once the design has been finalised as they are expected to weigh around 20te.
The construction of both RC base plug reinforcement was undertaken on the surface to minimise working within the shafts. Steel was only required at the under ream under the last ring, in the form of U bars and radial bars. Steel was tied in sections on the surface and then lifted it into position which reduced the amount of time operatives spent working in a confined space.
Around 340m3 of concrete forms each base plug, this was poured via a concrete pump over a series of lifts to minimise the risk of shrinkage. The tunnel is to be driven on a very flat gradient of 1:1229, with similar ground anticipated as with the sinking of both shafts.
A key focus for Terra was to drive a positive Corporate Social Responsibility legacy, in addition to working with many local suppliers, reducing our carbon footprint and creating employment opportunities, we ensured our apprentices got to experience the works on site. Feedback from one our newest apprentices was great to hear:
“On site at the Green Lane Embankment is quite the experience, seeing the depth of the shafts and the sheer size of the machinery in real life was eye-opening, the amount of work being put into HS2 project is immense – I am very glad and proud to have been able to continue working on such a prestigious job. The work being carried out at Green Lane is being delivered to such a high standard with regards to quality processes including scrutiny of material conformance, the skills of operatives employed and the Safety standards.”
Terra Solutions have worked collaboratively with the client from commencement to develop the overall scheme design and ensure the once the project started on site it was a success. This was managed through a series of design and constructability meetings/workshops to take the project through the HS2 certification process. The close client contractor partnership fostered from the outset has been the key to project achievements made so far.