Supply Chain - Site Preparation - Dewatering
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(left) Excavation at the shaft base and (right) 2.5m ID tunnel excavator machine (TEM) being lowered into drive shaft - Courtesy of Barhale[/caption]
(left) Barn Elms shaft under construction and (right) the 2.5m ID tunnel excavator machine (TEM) being lowered into drive shaft - Courtesy of Barhale[/caption]
The TEM roll represents the machine’s rotation about its central axis and there was a continuous tendency to clockwise or anti-clockwise spin. To overcome the issue, inclinometers and laser targets were installed within the machine and used for base readings for roll calculation.
Installing the bespoke guidance equipment, and in-detail mapping of every stage of the drive, ensured that the TEM was able to operate effectively and carry out the pipe jack successfully.
A further challenge was the safe handling and maintenance of the 14 tonne, 3m diameter precast concrete jacking pipes. To address the issue, Barhale purchased a 20 tonne SWL welding roller to be used as a base stand so that the pipes could be rotated 360°. This solution enabled the team to inspect each pipe thoroughly without the need for access equipment and working at height.
It also brought a huge benefit in the preparation of the inter-jack stations. Using this system, the inter-jack hydraulic cylinders could be installed around the circumference of the jacking pipes by the fitters from standing position.
Overall, the use of the welding rollers saved the Barhale team time, eliminated the risk of working at heights, and reduced the number of lifting operations. This idea was nominated for a Tideway RightWay award.
Shaft cover slab under construction - Courtesy of Barhale[/caption]
(left) The probe drill in use, forward probing of the tunnel face before full excavation as part of the WPP and (right) temporary support from within the sacrificial outer skin of the TEM once the tunnel drive was completed - Courtesy of Barhale[/caption]
Once each concrete section had been broken out, the Barhale team could move in and pull the concrete from the invert back to the pit bottom for removal. Positioning the Brokk in advance of the shield meant that the removal of the material was not impeded.
Proceeding section by section, it took two and a half weeks to reach the TEM. Removal of the damaged lead pipe revealed that, under the pressure of sitting stationary in the ground for five months while the recovery strategy was devised and implemented, the tail skin of the machine had deformed more than anticipated.
By restarting and moving the TEM followed by the recovery shield forward into virgin ground, the Barhale team was able to relieve the pressure on the tail skin. This permitted the removal of the hood and the docking of the steel lead pipe after which the drive could be restarted.
Replacing the damaged reinforced concrete pipes created quite a challenge, but with the support of the project team, a pioneering solution to change the cracked pipes was developed. The recovery shield that Barhale developed not only enabled the damaged tunnel to be replaced but also enabled access to the TEM face where repairs and enhancements had to be conducted. Additionally, the purpose made tunnel shield was designed to allow complete dismantling upon completion of the tunnel drive, allowing the final connection to the main tunnel.
Secondary lining shutter in place within the sacrificial TEM outer skin at the junction of the main Tideway central tunnel - Courtesy of Barhale[/caption]
This consisted of a specialist automated tunnel formwork system that would ensure the tunnel secondary lining was constructed efficiently by pouring each section in one pass. This improved quality by reducing the number of construction joints compared to more traditional methods. As the equipment is fully automated, the hydraulic stop ends ensure a robust seal when pumping concrete. A mechanical travel system enabled quick set-up for each subsequent pour with minimal resources and it removed the need to manually strike and reconstruct formwork for each section of the lining.
Shaft cover slab under construction - Courtesy of Barhale[/caption]
Peter Leyton, Construction Director for the BAM Nuttall Morgan Sindall Balfour Beatty JV, praised the Barhale team for its hard work and effort in getting the TEM to the main tunnel connection:
Site establishment - Courtesy of Whitehouse Studios[/caption]
Work on Braidholm Road - Courtesy of George Leslie Ltd[/caption]
Site reinstatement - Courtesy of Whitehouse Studios[/caption]
(top) Collapsed left bank and masonry weir, (left) left hand cofferdam dewatered, exposing the extent of the undermining to the weir foundations, and (right) sewer pipes diverted through a temporary bridge (PortaDam and cofferdam installed on the left hand side of the river) - Courtesy of Ward & Burke[/caption]
(top left) Left bank foundation RC slab and wall built, excavator grab positioning reclaimed stone wall, (top right) left half of weir stabilised, shutter for right hand cofferdam installed, river bed graded ready for flooding, (bottom left) with the weir stabilisation complete the existing fish pass and part of the weir are demolished to make room for the new fish pass, and (bottom right) sewer pipes reconnected through the fish pass walls - Courtesy of Ward & Burke[/caption]
During construction, large stones from the collapsed downstream weir and left bank were recovered from the river. The design was altered to reuse these stones and incorporate them into the structure, avoiding embodied carbon associated with the original proposed concrete structures and imported aggregate. The stones were used to rebuild the left bank, installed as scour protection on the right bank, and used to landscape canoe landings. The stones helped the structure blend harmoniously with the surrounding environment.
Before flooding the left bank downstream cofferdam, a steel shutter was designed and installed on the new concrete slab to seal the proposed right bank cofferdam to the weir face. This enabled part of the right bank cofferdam to be constructed within the left bank cofferdam, ensuring a better seal and reducing risks associated with piling in the river.
Once the left bank was complete and the left half of the weir stable, the cofferdams were flooded and removed.
A similar process was followed on the right bank to stabilise the weir. Sheet pile cofferdams were installed upstream and downstream of the weir. Temporary pumps kept the cofferdams dewatered.
(top left) Propped cofferdam for upstream section of fish pass, preparing for blinding, (bottom left) fish pass wall rebar, upstream section, and (right) upstream baffles, penstock, and access metalwork installed in fish pass - Courtesy of Ward & Burke[/caption]
Midland Valves supplied a penstock upstream and stop logs downstream to isolate the fish pass for maintenance. The penstock and stop log frames were cast into the fish pass walls, flush with the base of the fish pass. This avoids grit being caught in the frame, preventing a tight seal.
The fish pass was constructed in two halves; upstream and downstream of the weir. The two sewer pipes were cast into the walls of the downstream section. The downstream section was constructed first so that the sewer pipes could be re-energised and the temporary pumping station decommissioned before the end of the whole project.
To construct the downstream section of the fish pass, a section of the weir and sewer pipes were removed. Then the ground was formed and blinded with a 8.53° slope. The base steel was fixed and the base poured. Next the wall steel was tied, shutters erected and the walls poured. The walls were constructed with a tolerance of +-5mm to ensure the baffles could fit and there wasn’t excessive space for debris to get caught. For the construction of the walls, the formwork panels had to be stepped to tie in with the run of the slope. The on-site joiners made relevant angled shutter bases to integrate with the main wall shutters so each wall could be poured in one go.
To construct the upstream section of the fish pass, a new cofferdam was installed. The cofferdam was initially propped with MGF frames. The blinding was designed strong enough so that the MGF frames could be removed once it reached strength. It was important that the MGF props were removed because they would have clashed with the fish pass wall, and limited lifting operations in the cofferdam. The construction process was repeated and the fish pass base and walls were poured.
Surveys and inspections of the fish pass by Yorkshire Water and the Environment Agency confirmed that the overall design and construction of the structure was satisfactory and within the design specification. On confirmation of this, the dewatering pumps were decommissioned and the sheet piles removed from the river to allow flows through the new pass.
A 160t crane was employed to remove the sheet piles upstream of the weir. The excavator that had driven them in could not reach over the new fish pass.
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(left) right bank cofferdam, looking at the rebuilt left bank masonry wall (and right) 160 ton crane and vibro hammer removing the upstream cofferdam - Courtesy of Ward & Burke[/caption]
(left) The preferred option and (right) flooding on St Augustines Road - Courtesy of Galliford Try[/caption]
Outfall under construction - Courtesy of Galliford Try[/caption]
Poor ground and underfoot conditions adjacent to the River Rother proved too risky and problematic for traditional mobile crane lifts, so all lifts of the precast sections were to be undertaken with a tracked excavator. This further constrained the design and restricted the maximum mass of each section to 13 tonnes. Consequently, in its final iteration the precast headwall was designed in two sections and was combined with an in situ toe and infill slab/wall which completed the structural design.
(left) Installation of drive pit on St Augustines Road, (middle) guided auger bore rig, and (right) the transition between 600mm and culverts - Courtesy of Galliford Try[/caption]
Settlement calculations were undertaken using the “Guide to best practice for the installation of pipe jacks and microtunnels technique” and a predicted settlement of <3mm was deemed to be acceptable. The pre-condition surveys highlighted only minor cosmetic issues and were used as reference to the state of repair of the properties prior to construction work commencing.
The majority of the civil design was undertaken in house. This element included a design coordination role combining and processing the information received from the consultants and stakeholders. An example of the numerous stakeholders which were consulted is that of the Coal Authority. Initial searches indicated the presence of historical coal workings. More detail was sought and records indicated the presence of an adit within 20m of the proposed pipeline. A coal permit was sought from the Coal Authority and the design progressed whilst this was processed.
Service detection - Courtesy of Galliford Try[/caption]
Cofferdam and silt curtain - Courtesy of Galliford Try[/caption]
Cofferdam
A cofferdam was required to retain the River Rother and this was designed and supplied by National Trench Safety UK and installed by the primary subcontractor Arthur Civil Engineering. The cofferdam was unique in design in that it utilised Larsson piles in a trapezium shape (plan) to minimise over-dig yet still accommodate the headwall and first culvert.
The cofferdam was installed by Ivor King Piling using a 35t excavator with a Movax side grip pile driver attachment. The cofferdam was effective but 4” end suction pumps supplied by Selwood Ltd were still required to maintain the cofferdam and keep water levels low.
Headwall installation
Once the cofferdam was watertight, the headwall toe was cast in situ, after which the precast headwall arrived in two sections as planned. It was lifted into position by Arthur Civil Engineering using a 50t tracked excavator. An administrative issue led to the need to negotiate a 2-week FRAP extension with the EA and the headwall was completed and the cofferdam removed by 14 October in accordance with the conditions of the revised FRAP.
Pipeline works
Construction works on the remainder of the project continued in March 2022. A temporary one-way system was implemented to provide a safe working area to install the remainder of the pipeline with open cut works commencing on 14 March. The open cut works progressed steadily with over 100 services safely crossed. The later spring and summer months provided excellent conditions for online sewer relay with drought-like conditions, culminating in hosepipe bans for the majority of the UK.
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Open cut pipelaying - Courtesy of Galliford Try[/caption]
Prior to the tunnelling works commencing, concern was raised over the proximity of the tunnel as it crossed under a 225mm foul water sewer in Derby Road. With a clearance of just 300mm, the existing foul sewer was inspected by CPS Drainage Solutions using CCTV and a precautionary 5m cured-in-place patch liner was installed to strengthen the sewer prior to the guided auger bore operation. No-dig works commenced in early June and progressed well without incident and were completed towards the end of the summer. No settlement was detected during the installation and no visual deformations were reported from the nearby structures.
Open cut works continued late into the summer with flooding benefits delivered by 5 October. Significant rainfall events have since occurred and the feedback from residents previously affected by the flooding has been positive.
900mm Ridgistorm pipe from Polypipe Civils ready for installation - Courtesy of Galliford Try[/caption]
(left) Works within the cofferdam in the Thames and (right) works ongoing within the cofferdam - Courtesy of Barhale[/caption]
Secondary lining works - Courtesy of Barhale[/caption]
(left) Commencement of shaft works, and (right) diversion works within Victoria Embankment -Courtesy of Barhale[/caption]