In the first use of its kind, the Tottenham Court Road Station Upgrade (TCRSU) project has designed and constructed two sprayed concrete lined overbridges passing directly over the operational Central Line platform tunnels. As trains and passengers pass underneath the feet of the SCL miners, and they walk over the back of the existing platform tunnels, they have achieved a first on the London Underground (LU) system. Moving away from the traditional three-stage, hand mined solution; the tunnelling operations have created two caverns above the existing platform tunnels into which the permanent linings can be installed. Significant work was completed in order to ensure adequate ground support was maintained and to minimise ground movements and damage to the platform tunnel linings.
PROJECT OVERVIEW
The major upgrade to the LU Tottenham Court Road Station has been developed to relieve congestion, achieve step-free access and modernise the station. The major civil engineering elements of the LU Station upgrade are:
A. An enlarged sub-surface ticket hall beneath Charing Cross Road, with three new entrances;
B. A combined fire fighting and emergency escape shaft with emergency access tunnels to both Central and Northern Lines;
C. Three new escalators to the Northern Line from the new ticket hall;
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By GlobalDataD. An additional concourse tunnel adjacent, and parallel to, the Northern Line platforms, leading to three new staircases to access the platforms;
E. An additional concourse tunnel adjacent, and parallel to, the Central Line platforms, with two new overbridges leading to new stair and lift to access the platforms.
Halcrow worked as the multidisciplinary design consultant for the client, LU to develop the design between 2007 and 2009. The specialist sub-consultant for the sprayed concrete tunnelling works was Dr. Sauer & Partners (DSP). The tender was awarded to the Taylor-Woodrow-BAM-Nuttall (TWBN) joint venture in January 2010 and then Halcrow was subsequently appointed to provide design and site supervision services.
The primary objective of all tunnelling works at TCRSU was to minimise ground movements. To this extent reducing excavated area, speed to tunnel ring closure and controlled tunnel advances were critical.
The primary ground support for the main tunnels is steel fibre reinforced sprayed concrete with a spray applied waterproof membrane located at the boundary between the primary and secondary linings. Secondary linings consist of both traditionally reinforced cast-in-place concrete and steel fibre reinforced sprayed concrete linings.
CENTRAL LINE OVERBRIDGE DESIGN
Overbridges
The new SCL Central Line interchange tunnel extends from the existing TCR station interchange level and runs approximately 110m parallel to the existing platform tunnels. The new overbridges extend from this tunnel, over the westbound Central Line platform tunnel and terminate in an enlarged upper lobby.
From here, traditional hand mining methods are to be adopted to descend between, and break into, the existing platform tunnels to form the new stair and lift structures. The level and alignment of the overbridges was significantly constrained by a number of existing structures. The tunnels need to tie into the existing station interchange level, minimum headroom requirements must be met on the platform and, to add to this complexity, an existing Victorian brick lined mid level sewer is only 5m clear of the platform tunnel (i.e. lying less than 1.5m above the upper lobby).
New stair and lift structures descend between the platform tunnels from each overbridge and provide additional access to the Central Lines from the existing station interchange level.
The original tendered design was for the overbridges to be constructed by traditional hand mining methods involving significant use of timber and steel heading. In producing the CDS for this extensive temporary works, TWBN’s temporary works designer, OTB, proposed an alternative to this traditional solution. On Contract award, TWBN pursued this SCL option.
EXISTING ASSETS
As noted above, the site was particularly constrained, the overbridges:
A. Pass over and break into the west bound Central Line platform tunnel. This is 21ft 2½ inch (6,464mm), 12 segment cast iron lined tunnel constructed in 1908.
B. Pass within 1.5m under the existing Victorian brick lined mid level sewer which was constructed circa 1880 and consists 6½ft (1,981mm), three ring (350mm) thick engineering bricks.
C. Have only 3m lateral clearance to a branch of the Mail Rail Tunnel. This is a 9ft (2,743mm) ID cast iron, six segment tunnel constructed in 1915.
Early 20th century cast iron used in the platform tunnels is known to be extremely variable in quality and the asset assessments during the design phase were based on strength properties of old grade 10 cast iron as per LU standards and in accordance with industry practice. During initial construction phases, samples were taken and tested to show that the quality was marginally better than assumed.
Surveys were carried out prior to construction to ensure the lining was in adequate condition to resist the expected deformation and additional loading. The existing concrete cladding was broken back, and a detailed inspection was carried out of the condition of the cast iron lining. An "asfound" deflected profile was determined from the exposed sections of the platform rings.
Prior to work on the overbridges, significant assessment and protection works were developed and installed for the overlying Thames Water owned mid level sewer. The final sewer protection scheme required some 90 350mm diameter hollow steel tubes to be cored out from the adjacent cast iron lined Mail Rail tunnel. The tubes were installed into the ground between the sewer and the overbridge temporary works and fully grouted, thereby providing protective support to the sewer. The works in the Mail Rail Tunnel required the approval of the owner – the Royal Mail Group.
Close collaboration and engagement amongst TWBN, LU, Thames Water and RMG ensured that the proposed construction methodology for the overbridge was both challenged and robustly developed.
Permanent works
The permanent works are entirely encompassed within the primary SCL. Fabricated steel transfer beams have been installed onto the extrados of the platform tunnels where the crown of the tunnels have been removed, these pick up and transfer the hoop thrust around the opening and into reinforced concrete side saddles. The overlying soil load is resisted by a sprayed concrete roof that sits on the concrete side saddles.
The upper lobbies consist of traditional concrete encased steel square-works. The installation of the steel frames was made significantly easier because they were carried out within the temporary SCL shell. Removal of the crown segments of the platform tunnels required a full line closure and each was carried out during a 52 hour weekend possession. To ensure speed of installation, steel I-beams were half cast in concrete so they could be lowered into place and act as permanent formwork for the overbridge floor slab.
Temporary works
The sprayed concrete linings of both overbridges were designed to accommodate all aspects of the permanent works including concrete saddles; two steel transfer beams on each side of the WB platform tunnel; and the steelwork of the upper lobby structures. Of the two overbridges, overbridge two comprised the most onerous constraints since clearances and space were more restricted with the distance between Central Line platform tunnels only being 1.2m. Total overbridge length is approximately 17m with the largest cross section of 50m2 occurring at the upper lift lobby (approximately 7.8m in diameter). Two transitions in geometry were adopted to optimise the design, the first at the location of the southern transfer beam and start of cast-in-place concrete saddles and the second one at the location of northern transfer beam and start of upper lift lobby. The use of SCL allowed for such diametric changes with minimal construction implications and provided flexibility with regard to connections to existing underground structures. The end profile comprised a domed headwall at its upper part and the lower part formed around the presence of the Central Line eastbound platform tunnel.
Both SCL overbridges are located entirely within homogenous London Clay with a total soil overburden of 16.3m, equivalent to approximately two tunnel diameters. The typical geology across the site is 2.5m of made ground overlying approximately 4m of Terrace Gravels overlying approximately 30m of London Clay with the Lambeth Group below this.
The SCL had a typical design thickness of 300mm and was steel fibre reinforced. The thickness increased in the crown over the length where the tunnel advanced underneath the Thames sewer protection pipes. This was to remove the risk of any clay wedges falling off the under side of the pipes and ensured a safer working environment for the tunnelling personnel. At locations where the SCL was supported on the Central Line platform tunnels, a local elephant foot type thickening was adopted with a minimum width of 825mm sitting on the annulus grout of the tunnel. This reduced the stresses being transferred from the new structures to the existing cast iron lining and maintained levels within acceptable limits and factors of safety.
The design was based on concrete compressive cube strengths of minimum 35MPa after 28 days and an early compressive strength development following the J2 curve [1]. Residual flexural strengths of minimum class two [2] were specified.
The SCL structures were designed as temporary works only without longterm load carrying requirements. The permanent concrete works were designed and detailed to carry all loads for the structures’ 120 year design life.
Analytical Assessment
Due to the criticality of ensuring adequate ground support, minimising ground movements and impacts to the platform tunnel linings, a threedimensional finite element model (FEM) was set up.
The model provided loads and stresses for the SCL design and for the assessment of the interaction between new SCL structures and existing LU structures. Predicted deformation values for the existing structures related to individual construction stages could be retrieved from the model. This allowed comparison between predicted deformations and those actually encountered on site to be carried out. This increased confidence for all involved parties during construction.
ABAQUS v6.10 EF, a general purpose finite element software package, was used to perform the numerical analysis.
LU’s Central Line platform tunnels, the Royal Mail Mail Rail Tunnel and the Thames Water mid-level sewer were modelled. From the new works at TCRSU the pipes supporting the sewer were modelled as well as all new SCL tunnel works in the vicinity of the SCL overbridge, i.e. Central Line SCL interchange tunnel and the new overbridge structures.
For the impact assessment on existing Central Line assets, section forces and deflections were extracted from the FEM and plotted onto a moment – thrust envelope taking due account of existing condition and additional effects due to the "as-found" deflected profile of the tunnel.
Local bearing stresses were checked where the foot of the SCL shell sat on the platform tunnels; this resulted in increasing the foot to ensure adequate load spread. Further checks against local stress increases due to lining deflection were carried out at bolt-hole locations (i.e. flange cracking and deformation of the radial joint seats). Circumferential and radial bolts were loosened to ensure localised damage and bolt head stripping would not occur.
LU engineers were concerned how the short term heave was resisted when the overburden above the platform tunnel was removed. Calculations showed that the load could be safely transmitted along the length of the tunnel via the existing wrought iron circumferential bolts to a point where the soil overburden act to resist it again.
Additional assessments were carried out on the overlying bricklined mid level sewer to ensure deflections and associated strains were within those stipulated by Thames Water. Similar assessments were carried out to safeguard the Royal Mail Group owned Mail Rail tunnel.
Central line Overbridge Construction
Construction sequence
The primary driver for the sequencing of construction was to provide adequate ground support as quickly as possible, and thus minimise associated movement to the surrounding 3rd party assets. All aspects of the design focussed on achieving ground support quickly, and ensuring asset and personnel protection.
For the tunnel parts south of the Central Line westbound tunnel, a top heading and combined bench/invert excavation and support sequence was chosen. Typical standard advance lengths of 1m for top headings and 2m for combined bench/ invert advances were used.
Full face (top heading only) 1m advances were carried out until the crown of the platform tunnels was passed. Following this the face was split into left and right hand side advances. Following completion of the remaining top headings, a partial SCL headwall was sprayed and a shaft type excavation was dropped between the platform tunnels in 1m advance steps.
Excavation was performed using a 2.5t excavator until space was too tight to allow the machine to enter. From there onwards a 1.5t rubber track excavator was used to complete the top heading excavation.
The 1m slices excavated downwards between the platform tunnels were hand mined.
Concrete spraying operations were performed using a mobile spraying robot until limited space restricted access and the Contractor switched to hand spraying.
Both SCL overbridge chambers were constructed between February 2012 and March 2012, each one taking three weeks.
Mitigation and Contingency
LU approval of the scheme was dependant on TWBN putting in place a number of preventative measures to safeguard the platform tunnels, the Mid-Level Sewer, the Mail Rail Tunnel and buildings at street level. These measures included:
A. Maintaining a 500mm exclusion zone over the crown of the platform tunnels within which hand excavation was mandatory;
B. Continuous depth probing was carried out during construction to determine the exact location of the platform tunnels and to confirm the ground conditions;
C. Limiting the maximum size of plant and providing timber sleepers on the temporary backfill over the crown of the platform tunnel to spread the plant loading. The minimum protection measures on the outside of the exposed platform tunnel comprised 250mm backfill or unexcavated soil together with an appropriate cover;
D. Removal of the soil lying between theoretical crown extrados and horizontal pipes (minimum four steel pipes exposed). This avoided areas of loose ground and mitigated the risk of soil blocks falling;
E. Enlarged SCL footings where the temporary lining bears on the existing platform tunnels; F. Bolt loosening in the platform tunnels local to the works to minimise risk of thread stripping and localised stress concentrations in the flanges of the tunnel segments;
G. The installation of wire mesh to the intrados of the platform tunnels to protect the public from the risk of loose render falling onto the platforms;
H. Daily RESS (required excavation and support) meetings attended by the contractor, the designer and LU where a review was undertaken of the automatic monitoring of the platform tunnels, the SCL convergence monitoring and deformations of the Mid-Level Sewer and Mail Rail Tunnel;
I. 24/7 attendance at the face by SCL inspectors employed by Dr. Sauer & Partners;
J. Continuous attendance of a watchman on the platforms during the works to check for distress in the tunnels during traffic hours;
K. Detailed visual inspections undertaken by TWBN during engineering hours to check the tunnel lining for signs of distress such as cracking, movement or tightening of bolts.
Monitoring
As part of the TCRSU construction works, a comprehensive monitoring system was set up on site and in the local vicinity. This included street and building monitoring prisms throughout the anticipated zone of influence and levelling studs over most of the LU infrastructure. A system of five point monitoring arrays was installed at 5m centres within the Central and Northern Line platform and running tunnels. Due to the criticality of the existing assets and the innovative nature of the structure being designed, additional monitoring was proposed in and around each overbridge location, this included: A. One five point prism array on the cast iron platform ring directly under each SCL footing B. One five point prism array on the platform tunnel cast iron at the centre line of the overbridge C. One five point array at the centre line of the overbridge on the east bound platform.
One of the main agenda items on the daily RESS meetings was the review of monitoring data. The deformation values of the Central Line platform tunnels were reviewed and compared to what the three-dimensional finite element model predicted for the corresponding construction progress. The encountered movement (direction and magnitude) compared well with the predicted ones.
Crack surveys of the cast iron segments were performed each night in order to assess potential weak points in the platform tunnels.
5-point prism arrays and longitudinal electrolevels were also installed throughout the impacted zone of the RMG Mail Rail Tunnel. Horizontal in-plane inclinometers were also placed into sub horizontal pipes that had been installed in the ground between the mid level sewer and primary lining to monitor the movement of the sewer. The Mail Rail Tunnel monitoring allowed these to be tied together and located in space.
Conclusion
Both the temporary and permanent works for both overbridges were completed early 2012 with minimal disturbance to the Central Line platform tunnels.
Close collaboration between the JV Contractor (Taylor- Woodrow-BAM-Nuttall) and eventual client (LU) engineering directorate resulted in the successful design and construction of the scheme achieving estimated project saving of approximately GBP 400,000 (USD 620,000) and seven weeks in construction programme.
As a testament to the engineering, the predicted movements were found to be within 15 per cent of those measured. Additional benefits gained from this exercise were:
A. A reduction in Health and Safety risks to personnel due to:
¦ Minimising exposure to Hand Arm Vibration syndrome by adopting mechanised excavation;
¦ Minimising exposure of personnel to unsupported ground due to remote excavation and spraying;
¦ Reduced work in confined spaces during excavation (i.e. no timbering, installing walling);
¦ Constructing the permanent works in a relatively open excavation reducing risks associated with handling heavy steel and SGI items;
B. The increased speed of construction associated with the SCL option significantly de-risked the construction programme. The removal of the crown segments of the platform tunnel had to be carried out in pre-arranged line closures that are booked up to four years in advance. Missing these closures would have had significant ramifications to the overall construction programme. Adopting the SCL methodology gave sufficient benefit that one of the closures could be used for other construction activities.
C. The adoption of the sprayed concrete lined system means that a waterproofing system can be installed between the primary and secondary linings. This is not something that is achievable when adopting traditional multi-face hand mined excavations. The inclusion of this waterproofing increases the design life, durability and positively influences whole life costs of the structure through reduction in maintenance cost and requirements. The adoption of a spray applied waterproof membrane installed in the SCL tunnels was another first on the LU system for the TCRSU project.
