ACTIVE TUNNELLING CONTRACT SUMMARY
SCHEME REF No. :- 294
SCHEME TITLE :- Stoke-25m dia shaft
CLIENT:- North Midland Construction
PROGRAMME DURATION:- 34 WEEKS
The scheme was required as part of the flood relief for the area on behalf of Severn Trent Water. ATL’s part of the scheme involved the construction of a 25m dia shaft, approx 13m deep including finishing works.
SPECIALIST EQUIPMENT/ WORKS INVOLVED
Prior to ATL arrival, a Larsen sheet piles cofferdam had been installed by a piling contractor. This was because there was a very high water table, saturating the top 10m worth of ground, above a mudstone lower strata & dewatering of this cofferdam was necessary prior to starting.
The initial dig down to the mudstone (approx 10m) was carried out using a 13ton & 8ton excavator within the shaft & a 30ton long reach excavator accompanied by a 70ton crawler crane working at the surface. ATL were excavating approx 70 wagon loads a day for this operation. To ensure the sheet piles couldn’t deflect 2no. reinforced concrete ring beams were constructed as the dig progressed, having to reach a specific strength gain prior to dig continuing.
As soon as the dig came into the mudstone the segmental shaft could start to be constructed by underpinning technique, beginning with a substantial concrete hanging collar (111m3 in one pour, via concrete pump). On completion of the underpinning down to approx 13m, ATL had to allow ground anchors to be installed, after which we returned to continue the works.
The next item to progress was the 1m thick reinforced concrete base, which had to be trimmed & blinded, along with ground anchors being cut down prior to starting. Approx 40tons of re-bar & 550m3 of concrete was then used in the construction of the RC base, with the aid of a 32m boom concrete pump for placing. The largest pour being 230m3 in one go, which required power-floating to give a smooth, sealed finish.
Once the base had been cast, the remaining segmental rings could be built upwards to the top, being backfilled with compacted suitable fill as it progressed. A central column consisting of 3.66m segmental rings & 2.7m manhole rings were constructed 12m high on top of the structural base, to support the large roof slab beams.
Incoming service holes & 1.5m dia incoming/ outgoing sewers were then installed, along with making good & caulking/pointing to the shaft to make it complete & watertight prior to roof slab placement.
A 120ton mobile crane aided in placing the main supporting roof slab beams, followed by over 80 roof slab planks, finished with a heavily reinforced structural topping to complete the roof & ATL’s work to the storage shaft.
Both NMC & STW were extremely happy, not only with the project being delivered to a high safety & quality standard, but also finishing some 3weeks ahead of programme.