Gloucester Flood Alleviation Scheme

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ATL are contracted to carry out 3nr 1200mm diameter pipejacks parallel to one another underneath a Dual Carriageway.

The permanent 1200 pipe hs to withstand a 1.5 bar of pressure to allow the water to be pumped from a 20m diameter shaft, through the 1200 pipes, into culverts and discharge to the River Severn. Each tunnel drive is 135m long and is driven from an 8m x 6m x 6m deep Drive pit.

ATL are using a Full Faced EPBM machine that at the time of writing has successfully completed 2 of the drives and is progressing well with the third drive.

ATL are able to remove the muck from the face with a conveyor belt system or with a closed screw auger system, depending on the ground condition.

The Ground that ATL are encountering is a "Soft becoming firm light brown and greyish brown locally silty CLAY (Alluvium)", which is part of the river bed deposits from the River Severn which runs in close proximity to the tunnelling. The SPT "N" readings vary from 7 down to 4 with some water seepage. (Appendix 1 gives a typical borehole example).

Whilst carrying out the first drive ATL encountered an existing sewer crossing the drive location. ATL stopped work whilst the sewer was diverted, but during this period a large bank of fibre optic cables (BT) were identified approximately 10m ahead of the EPBM. ATL were able to steer the machine under the cables and raise the levels over the final 10m to hit the receiving pit at the correct level. (Appendix 2 provides the wriggle survey).

ATL have been able to achieve an average 4 pipes / shift during the tunnelling, with a turnaround and setup time of 3 days.

SCOPE OF WORK - 1800 EPBM

ATL are contracted to carry out an 1800mm pipejack 230m long, along the length of Worcester Street in Gloucester. Traffic has to be maintained in one direction only whilst the tunnelling work proceeds.

ATL are carrying out the work with our 1800 EPBM machine, with a full face and a screw auger attachment, that passes the muck from the face through the auger, that sits within it own sleeve and has a shut off valve to allow the sleeve to close off the face and hold the muck within the auger. ATL also have an injection system, that allows us to inject polymers directly into the face to either assist in breaking down the ground (i.e. stiff clay) or to solidify the ground (Alluvium, sand etc).

The ground condition that ATL are tunnelling within, is a clay layer at the bottom, with sand and gravel running above. As the drive progresses ATL anticipate that the face will become a full face of sand and gravel, before going back into clay. There is water running through the gravel layer, which ATL are currently reducing with some wellpoints. (Appendix 3 Ground Condition Profile)

At the time of writing the report, ATL had progressed 30m into the drive achieving 2 pipes per shift and continuing to make good progress.


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