Tideway Tunnels

Chambers Wharf was one of Tideway’s main drive sites for the launch of tunnel boring machine (TBM) ‘Selina’, which travelled underground 5km eastwards to Abbey Mills Pumping Station. Chambers Wharf also received Ursula the TBM from the central site.

The Greenwich Pumping Station site was used to drive a connection tunnel to Chambers Wharf using the TBM ‘Annie’. The Greenwich Pumping Station site (GREPS) lies within the grounds of a fully operational Grade II Listed pumping station. Shotcrete constructed ‘Annie’s’ launch adit in the most challenging tunnelling conditions imaginable.

GREPS Adit

Resourcing multiple specialist roles within a challenging environment became even more constrained when the CoVID-19 pandemic struck halfway through the project.

Added to the logistical challenges of working underground, under the river Thames, in central London, several technical challenges had to be overcome to ensure the smooth and safe progress of this critical path project:

• Restricted workspace at the pit bottom of 70m deep shaft
• Constant overhead lifting operations, muck out, materials in
• Saturated chalk strata affecting mucking operation, shotcrete application, tunnel electrics and surveying equipment
• Rapid water ingress and occasional flooding due to ungrouted boreholes

It became evident that when the diaphragm wall was stitched drilled, and tunnelling commenced that this was not the case. Large inflows of water were immediately encountered resulting in a rapid re- think on the complete pit bottom set up and tunnelling methodology.

This included:

• Installation of 180nr concrete blocks placed on the pit bottom to lift the tunnelling turn out area above the water covered in geotextile and sub-base.
• Cut off drains at the tunnel entrance to ensure water flowed out of the tunnel invert and through the blocks to the sump.
• Upgraded shaft pumps to allow for chalk slurry.
• Increased excavated profile to allow ‘high accelerator sealing layer’ to be applied prior to structural lining.
• Pressure relief holes were required to be drilled into the SCL lining at 1.0m centres using an excavator mounted drilling rig.
• Specialist spraying head for tunnel excavator manufactured as original tunnelling equipment was not suitable.
• Invert ‘letter box’ to control invert water during construction.

CHAWF Adits

Following the experiences learnt from GREPS It was decided to assume the chalk would be of similar condition and the pit bottom set up and tunnelling methodology was revised:

• 340nr concrete blocks placed on the pit bottom to lift the tunnelling turn out area above the water covered in geotextile and sub-base
• Concrete pit bottom slab cast
• Cut off drains at the tunnel entrance to ensure water flowed out of the tunnel invert and through the blocks to the sump
• Upgraded shaft pumps to allow for pumping chalk slurry 80m vertically to the surface treatment plant
• Increased excavated profile to allow ‘high accelerator sealing layer’ to be applied prior to structural lining
• Invert ‘letter box’ devised at GREPS to control invert water during construction average flow of 4 ltrs/second
• Pressure relief holes were required to be drilled into the SCL lining at 1.0m centres using an excavator mounted drilling rig
• J2 Modified strength curve with 28 day strength of 40Mpa.
• Two tunnels constructed concurrently, with mucking and spraying gangs alternating between drives.
• Use of onsite batching plant to produce all shotcrete 24/7
• Shotcrete was delivered to the spraying unit using a combination of all terrain mixers and concrete skip to transfer the material to pit bottom.
• The Project utilised a 4 gang shift rota to ensure working time directive were met.

Remarkably, despite the multitude of challenges faced the Shotcrete SCL team completed the construction of the 7.5m diameter GREPS adit successfully and without incident.

Despite the multitude of challenges in the middle of a first wave of a pandemic, the Tideway Tunnels project was handed over snag free two weeks ahead of schedule and without incident.

This Project team proved that the depth of experience and technical knowledge in overcoming these extreme conditions was of true benefit to our Client. Lessons learned was captured during a collaborative workshop and shared widely with other Contractors on the Tideway project.