UK POWER NETWORKS BATTERSEA CABLE TUNNEL: POWERING A RENAISSANCE AT BATTERSEA S ICONIC POWER STATION

Transcription

1 UK POWER NETWORKS BATTERSEA CABLE TUNNEL: POWERING A RENAISSANCE AT BATTERSEA S ICONIC POWER STATION Gerard Quigg CEng MICE Senior Civil Engineer COWI UK Ltd.

2 Table of Contents 1 Introduction Ground Movement & Third Party Assets Access Shaft, SCL Launch Tunnel & Bored Tunnel Junction Chamber Review of Monitoring and Conclusion... 5 Figure 1 Tunnel Route and Third Party Assets (Indicative only)... Figure 2 - Proposed Junction Chamber... Figure 3 - Installation of the Steel Ribs and Mesh within the 1600mm diameter sewer... 2 Figure 4 - Access Shaft and Gantry Crane Temporary Works... Figure 5 - Students (front row) on site at the arrival of TBM 'Maggie'... Figure 6 Operational Cables (Left), Cable Protection, Propping & Monitoring Points (Right)... 3 Figure 7-3D visualisation of excavation around existing tunnel... Figure 8 - Junction Chamber Excavation Sequence... Figure 9 - Original sequence with discrete headings (left) / new sequence (right)... 5 Figure 10 - Shaft Settlement - Predicted v Measured... 5 Figure 11 - SCL Tunnel Settlement - Predicted v Measured... 6 Figure 12 - Junction Chamber Settlement - Measured on SAA... 6 Page limit 6 pages; Word Count words; This paper was reviewed and approved for submission by; Name: James Belcher Position: Senior Project Manager Name: Martin McGovern Position: Associate Director Name: Position: John O'Dwyer Director Name: Position: Damian McGirr Director

3 BATTERSEA CABLE TUNNEL 1 1 Introduction A new electricity substation is being built to distribute power to the Nine Elms Strategic development area, including the Battersea Power Station development, which is forecast to have a 110MVA demand over the next twenty years. Led by UK Power Networks, and jointly funded with the power station developer, the project was undertaken in partnership with alliance contractor Clancy Docwra, tunnelling specialists Joseph Gallagher Ltd. and consultant COWI. COWI were commissioned as Joseph Gallagher's permanent and temporary works designer, also providing design assurance by fulfilling the role of sprayed concrete lining (SCL) and Monitoring Engineer during tunnelling works my role encompassed each of these elements. The new substation was constructed at a site in Battersea. The tunnel route follows public road networks where possible and passes under a number of third party assets comprising railway viaducts and cuttings, a highway bridge and utilities mains (Figure 1). An access shaft and SCL Launch tunnel were constructed on the substation site, an open-face shield was used to construct the new 300m long cable tunnel and the two tunnels were connected by hand excavating a 'Junction Chamber' concrete wraparound. The Junction Chamber is a 9m x 6m x 6m concrete structure constructed by timber Figure 1 Tunnel Route and Third Party Assets (Indicative only) heading excavation (Figure 2). The key risks with the chamber were; hand mined connection to a 2.950m OD precast concrete wedgeblock tunnel; containing live 132KV cables; located approximately 20m beneath a busy public road; 12m below live 1600mm & 300mm diameter sewers; 10m from a live railway. Figure 2 - Proposed Junction Chamber 2 Ground Movement & Third Party Assets The tunnel passed under a number of third party assets and I led the approvals process for rail, highway, gas and local borough council assets. Damage assessments were undertaken on the utilities - four clean water mains (3", 4", 9" & 12") and two sewers (300mm & 1600mm). The clean water mains were located close to the surface and were replaced. The 1600mm sewer was located 12m above the junction chamber and was lined with temporary steel ribs and mesh. The ribs and mesh were detailed in small lengths that could be lowered into the sewer through a manhole and bolted together during a series of night-time diversions (Figure 3). The ribs and mesh were designed to support an area of loose brickwork, not total structure movement. A longitudinal Shape Accel Array (SAA) was installed along the sewer crown to monitor movement during Junction Chamber excavation.

4 BATTERSEA CABLE TUNNEL 2 Figure 3 - Installation of the Steel Ribs and Mesh within the 1600mm diameter sewer It was proposed to install a cured in place pipe (CIPP) lining within the 300mm sewer. However, following completion of a CCTV survey a change in level was identified - meaning the CIPP lining could not be installed. A risk reduction meeting was held, during which a number of options such as overpumping or replacement of the 300mm sewer were discussed. I undertook a review of the ground movement and determined the value of settlement at which point the sewer failed, this was used to set trigger levels and allow excavation to continue. The ground movement due to shaft construction was based upon the methodology by New and Bowers, The ground movement due to the tunnel and Junction Chamber was based upon the methodology by O Reilly & New, A volume loss of 2% was chosen as an acceptable and conservative value to satisfy the relevant third parties. The predicted surface settlement was; due to shaft excavation - 11mm. due to the open face tunnelling - 5mm. due to the junction chamber timber headings - 24mm. Each timber heading was given an equivalent diameter and the total settlement value was derived by superimposing the results from each timber heading. Using this approach, a sub-surface settlement of 42mm was predicted at the 1600mm brick sewer level. 3 Access Shaft, SCL Launch Tunnel & Bored Tunnel To minimise disruption to the public, the works were undertaken within an existing warehouse on the site of the new substation (Figure 4). I undertook a survey to ensure the shaft and gantry crane temporary works were located inside the building footprint, designed the gantry crane support frame and confirmed the existing frame could be used during the installation of the gantry crane. Figure 4 - Access Shaft and Gantry Crane Temporary Works The 30m deep, 7.5m diameter, shaft lining was formed from a combination of precast concrete (PCC) segments and SCL. The shaft was constructed by caisson sinking through the upper deposits and underpinning within the clay. The shaft was underpinned using SCL and PCC segments as a secondary lining. A sprayed concrete soft eye was constructed to facilitate future 'breakout' for the SCL launch tunnel. The 30m long/ 3.8mOD SCL launch was required for assembly of the TBM and construction of the 2.44m ID segmentally lined tunnel. I designed the shaft and tunnel structural elements and, working with the contractor, developed a launch methodology and designed a launch frame and cradle for the shield. The shield was launched by building temporary rings and a reaction frame within the tunnel. I

5 BATTERSEA CABLE TUNNEL 3 also fulfilled the role of SCL and Monitoring Engineer, chairing the daily review meeting and issuing the RESS to the contractor for shaft, SCL and TBM tunnel works. Through my role as the 2017 BTSYM Schools and Universities Chair, I arranged to hold a TBM naming competition at a local school. I also arranged for the project team to visit the school and give a short presentation on tunnelling (Figure 5). 4 Junction Chamber The junction chamber was the key project risk. It is located approximately 20m BGL and connects the new tunnel to the existing wedgeblock cable tunnel. Upon completion of the tunnelling works, the TBM was stopped 4m away from the existing tunnel. The machine was disassembled and removed through the tunnel and shaft the shield remained in place. A Figure 5 - Students (front row) on site at the arrival of TBM 'Maggie' connection was then formed by hand mining and constructing a reinforced concrete cast in situ chamber around the existing tunnel and removing selected rings. The existing wedgeblock tunnel was surveyed internally to identify the location and orientation of segment joints. To reduce the risk of the segments becoming unstable, the existing tunnel was internally supported in the temporary case by internal propping. The propping remained in place until the permanent works were complete. I modelled the effect of unloading the existing wedgeblock tunnel and designed the temporary steel props (Figure 6). The propping was designed to support each segment so that when it was unloaded externally the ring did not lose shape. The methodology to support the rings was similar to that adopted during lining replacement on the Jubilee line between Baker Street and Bond Street. A remote total station was installed in the exiting tunnel to monitor movement during excavation and unloading. Live Cables in Wedgeblock Tunnel Figure 6 Operational Cables (Left), Cable Protection, Propping & Monitoring Points (Right) To undertake the excavation, a sequence of timber headings were adopted. The timber headings were used to support the excavated ground for a short duration only and were were filled with mass concrete to encapsulate the wedgeblock tunnel prior to removal of segments. The excavation was detailed based on the location of the segment joints, so that no segment was ever fully exposed over its width or left in 'free air'. This method was chosen over SCL.

6 BATTERSEA CABLE TUNNEL 4 The junction chamber comprises a roof slab, base slab and sidewalls to support the existing tunnel. The excavation sequence adopted (Figure 7) was to burn a hole in the TBM crown, chimney up vertically, tunnel laterally over the TBM and then tunnel forward over the crown of the existing tunnel. Subsequently, two gangs worked to excavate each sidewall down and under the wedgeblock tunnel. The central section, comprising temporary curved I-beams propped off the sidewalls, permanent beams and concrete to the rear of the existing tunnel, was then excavated. The final operation was to tunnel under the existing tunnel to complete the concrete surround. Once complete, the existing segments were removed exposing the steel ribs in the Figure 7-3D visualisation of excavation around existing tunnel existing tunnel (Figure 8). Heading Over Wedgeblock Tunnel Existing Wedgeblock Tunnel Fully Exposed and Spanning Through Excavation Heading around Shield Concrete surround completed and removal of segments from the existing wedgeblock tunnel Figure 8 - Junction Chamber Excavation Sequence

7 BATTERSEA CABLE TUNNEL 5 I undertook the role of monitoring engineer during these works, holding a daily review meeting, reviewing the monitoring installed in the sewer, the UK Power Networks tunnel and at surface level. During construction, the monitoring indicated lower settlement than expected and as confidence grew in the excavation technique, the contractor proposed a change in sequence. The original sequence exposed the minimum amount of ground so that all joints were supported with timbers and the void was filled with mass concrete prior to commencing the next heading. This resulted in approximately 1.2m wide/ tall headings and a number of concreting operations. To excavate under the tunnel, the contractor proposed opening up more ground to allow the miners more working space and to aid programme. I reviewed the monitoring and, along with the contractor, developed an alternative method of supporting the existing tunnel segments with sacrificial steel legs whilst excavating under it allowing a larger excavation and reducing the number of concrete pours in a confined space (Figure 9). Figure 9 - Original sequence with discrete headings (left) / new sequence (right) 5 Review of Monitoring and Conclusion Shaft construction was monitored by precise levelling along the adjacent road and carriageway. The measured values show that the method proposed by New and Bowers [1] provides a reasonable and conservative approach for predicting surface settlements due to shaft excavation (Figure 10). Settlement (mm) m deep Caisson, 6m deep Underpin & SCL Tunnel m deep - New and Bowers (1994) 23m - New & Bowers (1994) Completion of 17m Caisson Completion of 6m SCL Underpin -16 Distance from Shaft Wall (m) Completion of SCL Tunnel Figure 10 - Shaft Settlement - Predicted v Measured Following the load factor approach [2], and using a typical undrained shear strength for London Clay, the theoretical volume loss is approximately ~ % for the open face SCL and TBM. However, a volume loss of 2% was used for open face tunnelling. The clay layer was wetter than expected and contained a number of 'greasybacks'. On review, the chosen volume loss was reasonable when surrounded by sensitive assets. Taking cognisance of the ground conditions, and

8 BATTERSEA CABLE TUNNEL 6 using a lower undrained shear strength results in a volume loss of 0.7-2% [3]. The volume loss was back calculated as ~2% using the trapezoidal area method (Figure 11). Settlement (mm) Distance from tunnel centreline (m) Predicted - Volume Loss (2%) Array 7 14 Figure 11 - SCL Tunnel Settlement - Predicted v Measured During excavation around the existing tunnel, the remote total station within the tunnel showed minimal movement of between 1 3mm. Surface monitoring by precise levelling showed a total settlement of 5mm and the SAA within the 1600mm sewer recorded 8mm vertical settlement (Figure 12). These values were both approximately 20% of the predicted cumulative settlement and resulted in no damage to the assets. When back analysed, a volume loss of ~1% was achieved for the junction chamber. Vertical Settlement (mm) Junction Chamber - Settlement recorded on 1600mm Sewer SAA LT/ RT - Top and Bench 1 Sidewall - Top Bench Sidewall - Middle Bench Sidewalls complete -8 Central Section Figure 12 - Junction Chamber Settlement - Measured on SAA In summary, the methodology adopted for construction of the Junction Chamber was successful to mitigate risk, ground movement and potential impact to the surrounding third party assets. By diligently reviewing the monitoring, confidence was gained in the excavation technique allowing the contractor and designer to work closely and introduce efficiencies to increase working space and aid programme where practical. The monitoring results for shaft construction show agreement with current practice, whereas it seems prudent to consider higher than theoretically predicted for open face tunnelling. References: [1] New B. M. and Bowers K. H. (1994) Ground movement model validation at the Heathrow Express Trial Tunnel. Proc. Tunnelling 94. Institute Mining and Metallurgy, Chapman and Hall, London [2] Dimmock, P. S. & Mair, R. J. (2007) Estimating volume loss for open-face tunnels in London Clay. Proc. ICE Geotechnical Engineering 160, GE1, [3] Macklin S. R. (1999) The predication of volume loss due to tunnelling in overconsolidated clay based on heading geometry and stability number. Ground Engineering, 32, No. 4, BTS Harding Prize 2018