Weston Sewage Treatment Works
Shaft Construction and Tunnelling works
Principle Contractor: Dean & Dyball
Client: Wessex Water
ATL Scheme Ref: 317
The scheme was required as part of the improvement works to Bleadon STW (which deals with both storm & foul water for the Weston Super Mare area) for Wessex Water.
ATL’s part of the scheme involved the construction of a 3no. segmental shafts varying in size from 6.5m dia at 14m deep, a 7.5m dia at 23m deep & a 10.5m dia at 21m deep.
Further to this was a 1.8m dia link tunnel between the 7.5m to the 10.5m shaft, a 3.3m link tunnel between the7.5m & an existing wet well, along with RC baffle walls & finishing works within the shafts.
Prior to this works taking place, ATL also had to supply all temporary works designs such as entry /exit portals, thrust walls & guide collars.
SPECIALIST EQUIPMENT/ WORKS INVOLVED
ATL installed all 3no. segmental shafts using caisson sinking method, with the aid of ATL’s own vertical jacking rams. However, rams were not required initially, exactly the opposite, due to the nature of the very soft clays encountered. ATL came up with an anti drag channel fixed to the lowest ring to prevent the shaft from sinking too quickly under its own weight.
ATL also used their own excavators & grabs which excavated the bulk of the shafts, through the soft clays & running sand into the soft limestone, down to the mudstone, approx 20m deep. The hard mudstone, was then broken out via excavator & breaker, lowered into the shafts via 50ton crawler crane. ATL also organized specialist dewatering prior to the works starting to aid in the works, mainly to deal with the water in the running sands.
On completion of the shaft sinking ATL constructed the RC bases, the thickest being 3.5m deep, heavily reinforced with 360m3 of concrete, placed via concrete pump.
1800mm dia x 37.5m long tunnel
A 37.5m long, 1800mm dia link tunnel was required between the 10.5m & 7.5m shafts, which was carried out using ATL’s own full faced TBM & pipe jacking rams. The ground conditions for the drive were very onerous, with the soft clays being at the top of pipe, running sands in the middle & a harder limestone to the bottom. However, this was no problem for the TBM, which finished bang on bulls eye, exiting the portal perfectly.
More updates to come
Published 24th April 2012
We are pleased to Announce the succesful completion of the 3.3 m dia tunnel
3.3m dia tunnel
To enable a 2m dia outfall pipe to be installed between the 7.5m shaft & the existing outfall wet well, a 3.3m dia steel sleeve had to be installed 1st. The steel sleeve, used as a tunneling shield had to be designed by ATL, not only as a shield, but to house a lifting beam capable of lifting a 10ton load. The beam was required for a specialist coring contractor to remove a 1.8m dia concrete core from the wet well tie-in. ATL also installed face rams to aid with the stabilizing the weaker spoil at the head of the excavation.
Prior to the tunneling being able to progress, ATL came up with a scheme to deal with the weaker material above the shield, to prevent ground collapse. This came in the form of a lance grouted arch above the shield & horizontal dewatering direct into the running sand band to draw the water out via vacuum pump.
Please click here to view a feature on WSM Sewage Treatment Works scheme