The farnworth tunnel project is part of the UK-wide GBP 1bn (USD 1.32bn) Railway Upgrade Plan to provide improvements throughout the network which includes the Manchester to Preston route. Electrification of the heavily trafficked route by Network Rail required the reconstruction of the Victorian-era Farnworth Tunnels to accommodate 21st century high speed trains. The existing Grade II Listed Heritage tunnels were both too small for OLE equipment, were passed their original design life and in a near life expired condition in places. The upgrade works were required to provide sufficient clearance for OLE, increase the route speed and to upgrade the tunnel(s) to modern standards. The chosen scheme involved enlarging the original “Up” Tunnel to accommodate twin track, enabled rail service to be maintained through the Down Tunnel during the works to the Up Tunnel it posed significant engineering challenges. The close proximity of the masonry tunnels ensured that any works to one would result in a complex interaction between the two. The risks presented by the low ground cover to the busy A666, which crosses perpendicular to the tunnel, needed to be managed in addition to the risks associated with working next to a live railway line. In order to successfully achieve the project goals Network Rail, J. Murphy & Sons (JMS) and OTB Engineering collaborated closely to implement proven and innovative tunnel techniques to overcome the challenges posed by the project.
FARNWORTH TUNNELS
The existing Farnworth Tunnels were Grade II Listed Heritage Structures that ran parallel to each other at approximately 1m apart and were 270m in length. On either side of the tunnels the railway is located in a steep sided cutting with listed masonry portal structures at the tunnel mouths. The A666, a strategically important dual carriageway road for Greater Manchester, crosses over the tunnel alignment at approximately mid length. Ground cover above the existing tunnels varied from approximately 2m at each portal to 8m beneath the A666.
The original Farnworth tunnel (now the Up Tunnel) was constructed between 1835 and 1838 by John Hawkshaw. The tunnel was originally a double track bore, with a large elliptical air shaft in the centre. The introduction of Pullman coaches gave insufficient clearance and a decision was made to bore a new single track tunnel for the down line, leaving the original double line tunnel for the up line. Works began in early 1880 on the Down Tunnel with the new bore in use by late 1880. The Down Tunnel and Up Tunnel are connected by 13 cross passages. Extensive strengthening works to the Up Tunnel were carried out in 1881, which involved relining parts of the tunnel.
The Up Tunnel has a 3.4m radius semi-circular arch with 2.5m high curved sidewalls and an invert. The tunnel was originally constructed from sandstone masonry blocks with a lining thickness of approximately 400mm thick.
Approximately 50 per cent of the tunnel is now relined with masonry brickwork. The Down Tunnel has a horseshoe shaped (radius 2.3m), brick-lined profile with an invert. The lining was constructed from seven courses (800mm) of masonry brickwork. Based on previous cores through the lining the inner ring consisted of blue Staffordshire engineering brick under-ringing six rings of red masonry brick.
SCHEME DEVELOPMENT
Network Rail’s goal was to increase the passenger capacity on the route and to raise the line speed from 50mph to 100mph. The rehabilitated tunnel(s) were, therefore, required to provide sufficient clearance to new OLE equipment and aspirational rolling stock. The modern standards requirements for fire protection and the effect of transient air pressure were defined in SRT-TSI. Network Rail’s project specific waterproofing requirements defined a “tanked” tunnel lining.
A number of options were reviewed by Network Rail at GRIP stages 2 and 3 (feasibility and option selection) to achieve compliance through the tunnels for OLE installation, gauge clearance for stock and aspirational line speed. The options reviewed are summarized below:
¦ Track lower/invert replacement.
¦ Tunnel enlargement (Down and/or Up).
¦ New online cut and cover tunnel.
¦ Re-bored Down tunnel.
¦ New offline bored tunnel.
¦ New offline cut and cover tunnel.
The options were assessed against the amount of modification to the existing heritage assets, amount of rail disruption caused, feasibility, risk and whole life cost. The enlargement of the Up Tunnel to accommodate a twin track with increased line speed and subsequent decommissioning of the Down Tunnel was selected as the preferred solution. While achieving the required project goals the number of residual risks remained which needed to be managed by the delivery team:
¦ Obtaining listed building consent approval from English Heritage
¦ Managing the geotechnical risks as a result of low ground cover and limited site investigation
¦ Sequencing and executing major upgrade works next to a live railway.
¦ Ensuring stability of the already deformed Down Tunnel during the enlargement works.
CONSTRAINTS ON PROJECT DELIVERY/ CONSTRUCTION
In October 2014 JMS and OTB were awarded the contract to complete the upgrade works from GRIP 4 to 8 (single option development, construction and hand-back) under a design and build contract. The chosen solution was developed in conjunction with Network Rail allowing for the following constraints:
¦ The enlargement and handback of the Up Tunnel needed to be completed in a 23 week blockade of the Up line.
¦ The blockade start date of the 28th June 2015 could not be moved.
¦ A blockade of the Down Tunnel was not possible during or in advance of the Up Tunnel, with any strengthening works to the Down Tunnel to be done in engineering hours.
¦ Additional ground investigation and surveys needed to be specified, procured and interpreted to permit the construction and design of the works.
¦ The long lead in times in certain items provided a limited timeframe to procure and design parts of the works.
INVESTIGATION WORKS AND SURVEYS
Following contract award a detailed condition survey and assessment of the Down Tunnel was undertaken to identity defects in the tunnel lining, current stability of the lining and to evaluate the impact of the Up Tunnel enlargement on tunnel stability.
Visual, tactile and intrusive surveys were undertaken as a part of the tunnel investigation works under the supervision of JMS and OTB. The suit of tunnel survey works included, photographic recording, tactile hammer tapping, laser sweeping of the tunnel profile, masonry sampling and testing and an endoscopic survey of the lining. The survey of the tunnel recorded significant deterioration and deformation of the tunnel lining as well as extensive defects including a number of major defects. The observed major defects in the tunnel are summarized below:
¦ Water ingress was observed in the majority of the tunnel.
¦ Up to 100m of the tunnel crown had experienced significant flattening of up to 350mm.
¦ Over a length of 20m two large longitudinal cracks in the tunnel crown were present.
¦ Delamination of the brickwork lining and or voids behind the tunnel lining was present in extensive areas of the tunnel crown, particular between the two tunnels.
A number of traditional masonry tunnel repair techniques had previously been carried on the masonry lining, including patch work lining replacement, pinning of the lining and drilling of weep holes. These repairs appear to have had limited beneficial effect of the lining
Ground investigation
The British Geological Survey geological map shows the site is underlain by superficial deposits mainly consisting of Glacial Till (Boulder Clay). The superficial deposits are in turn underlain by Carboniferous age (Coal Measures), dominantly shales and mudstones with interbedded sandstones.
Site investigation work in the vicinity of Farnworth Tunnel undertaken prior to the design and build contract consisted of 11 No. light cable percussion boreholes and indicated the presence of Made Ground, especially behind the Farnworth and Kearsley portals, adjacent to the A666 and between the existing tunnels. In addition, within the Glacial Till deposits there are horizons of glacial sands and occasional gravels. The thickness and lateral continuity of these granular units was not established by these previous investigations. No bedrock was encountered during the initial investigations.
The additional ground investigation included 21 No. rotary and window sample boreholes which confirmed the succession of soil that underlies the Farnworth Tunnel site area: Made Ground, overlying Glacial Till deposits with interbedded sand and silt lenses. The Glacial Till deposits consist predominately of slightly silty, slightly sandy clay with gravel and occasional cobbles with some bands of laminated silty clay. No boulders were encountered in the ground investigations.
Small perched water bodies associated with some of the sand lenses within the Glacial Till are present above and around the existing tunnels and it was anticipated that any water intersected during the tunnel excavation would be at low pressure. Below the tunnel invert level there are discontinuous sand horizons and lenses that contain water and have a moderate head of water pressure. However most of the permanent works excavations were above these deeper sand bodies.
Potential risks to the Up Tunnel enlargement included low strength clay with short stand up time, requiring face support, small pockets and lenses of loose sand and or silt, water ingress from small perched bodies in the crown and shoulders of the tunnel excavation and intersection of large cobbles or boulders within the Glacial Till.
DOWN TUNNEL STRENGTHENING WORKS
A soil structure interaction structural assessment of the existing tunnel was carried out to determine its current stress state and to assess the effect of the Up Tunnel enlargement on it. The analysis included, for the existing deformation in the Down Tunnel lining, properties of the masonry determined from laboratory masonry tests and geotechnical information obtained for additional ground investigation works. The assessment concluded that the existing masonry lining was highly stressed in its current condition and that enlargement of the neighbouring Up Tunnel would result in excessive deformation and stresses in the masonry lining. Strengthening of the tunnel was, therefore, required in advance of the Up Tunnel enlargement.
The performance criteria for these strengthening works were to ensure stability of the Down Tunnel during the Up Tunnel enlargement, form the permanent lining to resist long term ground loads negating the impact of long term tunnel instability on the Up Tunnel and to ensure minimum gauge requirements were maintained through the tunnel.
Various options were reviewed against the performance requirements and programme constraints, with the installation of an inner (secondary) steel reinforced sprayed concrete lining (SCL) considered the most practical and robust solution to strengthen the tunnel. The works were programmed to be undertaken in a series of nominal 54 hour track possessions in advance of the Up Tunnel blockade with a start date of March 2015. The SCL strengthening provided:
¦ A rapid method of installing a secondary lining.
¦ Earlier strength gain compared to cast concrete enabling increased production.
¦ Flexibility on the application thickness and profile to enable gauge clearances to be maintained.
¦ Provision of steel mesh in the SCL provided enhanced lining capacity.
¦ Ability to return the down line to service after a weekend possession.
Existing gauge clearances to rolling stock were marginal in the Down Tunnel crown, requiring the thickness of the SCL to be minimized, while ensuring still a sufficiently thick lining to withstand the ground and construction forces acting on it. To provide the required gauge clearance and sufficient capacity of resist the anticipated stress acting on the lining a 200mm deep SCL was adopted this was locally thinned at discrete pinch points in the tunnel.
Down Tunnel reconstruction and enlargement
Over a 15m length the direct application of SCL to the tunnel intrados would foul gauging requirements due to deformation of the existing brickwork lining in the tunnel crown. Longitudinal cracks were also present in this area. An alternative solution was, therefore, required in this section of the Down Tunnel.
External support to the existing lining was not possible due to the poor ground conditions and lining condition, therefore, local removal of brickwork from the tunnel crown to enable application of a SCL was considered the most appropriate solution. To achieve the required clearance up to three courses of the seven ring lining required removal from the tunnel crown. As a consequence of the observed deformation and large cracks in the crown of the tunnel in this area temporary pre-support to the existing tunnel lining was considered essential before removal of any brickwork and installation of the permanent SCL. The chosen temporary pre-support consisted of rib and post steel UC sections with an invert strut positioned between the existing railway sleepers.
These works would also be carried out in the 54 hour possessions, however, the installation of the temporary ribs in the tunnel would foul gauging requirements requiring the design to incorporate safe hold points to enable partially enlargement of the lining before handback of the tunnel for train operations. The temporary ribs were installed at two sleeper’s width apart and were braced in the longitudinal direction and timbered out between ribs to provide support to any loose brickwork. The installation of a two temporary ribs enabled a 400mm width slot to be cut into the tunnel arch to a depth required for clearance. A larger permanent rib was then installed within this slot enabling removal of the temporary rib closest to the previously construction SCL lining and removal of the remaining brickwork in the arch in this area before construction of the SCL. Once the SCL had gained sufficient strength the remaining temporary rib could be removed to enable handback of the tunnel for train operations or the sequence for the next rib started in time permitted.
These works were carefully programmed and sequence between OTB, JMS and NR to ensure sufficient time to enable construction of at least one permanent rib in a possession beforehand back of tunnel for train operations. A total of 11 No. permanent ribs needed to be installed as part of the sequence. The rib weight was economized as much as practical while still ensuring a robust design to aid installation of the ribs. The total weight of each rib was 0.5 tonnes and was each fabricated in six sections and bolted together on site.
UP TUNNEL ENLARGEMENT
In order to achieve the client defined project requirements within the allowable blockade period a precast segmentally lined tunnel, excavated and constructed using an open face tunnelling shield, was adopted. Before enlargement of the tunnel a number a significant temporary works elements were required to the tunnel and the portals.
Advanced works to the Up Tunnel
Voids detected between the shoulders of the Down and Up Tunnel during the Down Tunnel survey possessed a risk of excessive local ground movement and face stability, resulting in a hazard to the SCL strengthened Down Tunnel, the New Tunnel construction and the A666. Mitigation of this risk in advance of Up Tunnel enlargement was therefore required. The chosen solution involved the systematic filling of voids using a cementitious grout. The grouting works required careful planning with defined control criteria to ensure the integrity of both existing tunnel linings were not compromised by excessive grouting pressure, while ensuring the voids were adequately filled
Instability and collapse of the existing tunnel lining ahead of the excavation was also a hazard to the works. A number of options were considered to mitigate this risk, however, given the project constraints the infilling of the tunnel and cross passages with a low strength foam concrete was considered the most appropriate solution. The infilling of the tunnel in conjunction with the grouting works removed the hazard that instability of the existing Up Tunnel might pose to the Down Tunnel and construction of the enlarged tunnel. The existing air shaft was also infilled with foam concrete as it was no longer required.
Temporary works at the portals
To enable launch and reception of the tunnelling shield at the required level 3m deep pits were required in front of the existing masonry portals and next to the operational railway. The excavation at its closest was 2.0m from the nearest rail. The launch pit was 10x20m in size to enable shield and back up to sit within it, the reception was 10x10m. In order to protect the live track from movement associated with the pit excavation a cantilever hard firm secant piled embedded retaining wall was installed. The piled wall consisted of 600mm diameter piles at 450mm centre to centre. In addition to limiting track movement the cess side piled wall at the reception pit was required to provide toe support to the recently constructed soil nailed slope. Intrusive works to the gravity masonry portal retaining walls showed the foundation for both portals would be exposed by excavation of the pits. To avoid undermining the foundations and cracking of the portal structure, traditional mass underpinning of the portal walls was undertaken in a hit and miss sequence.
In addition to influencing the vertical stability of the wall, the excavation of the pits could effect the horizontal stability of the walls by temporarily increasing the retained height. A 500mm deep reinforced concrete headwall was installed locally in front of each portal to ensure stability of the portal during the works. The headwall was dowelled into the existing portal structure.
The relatively shallow existing ground cover behind both portals to the extrados of the New Tunnel required a pipe arch to be installed through both existing portals into the surrounding soil behind. The pipe arch consisted of 8m-long, 32m-diameter steel pipes at 300mm centres. The pipe arch was also connected into the concrete headwall. The pipe arch has been installed to protect the existing portal and control ground movements. Very limited cover was present behind one of the portals relative to the enlarged tunnel requiring the installation a reinforced concrete roof slab in this area.
A condition survey of both parapets undertaken in advance of the enlargement works showed survey established that one of the parapets was in poor condition and the decision was taken to temporarily remove this structure in advance of the works on safety concerns.
A specially designed and built steel shove frame was installed within the launch pit to enable initial launch of the tunnelling shield. As far as practical the frame was prefabricated off site to reduce the risks associated with site assembly.
Permanent tunnel lining
In order to achieve the required space proofing an 8m i.d. tunnel was required. Each tunnel ring consists of 10 steel cage and steel fibre reinforced, trapezoidal, precast concrete segments, each 0.3m thick and 1.4m long. As the minimum radius of curvature on the alignment is 2,650m tapered rings were not required. The segments were provided with a primary 3.5 bar EPDM compression gasket towards the outer edge of the segment and a secondary hydrophilic seal towards the inner face of the segment. In addition a caulking groove was provided on the internal face of the segment. The circumferential joints were connected with three plastic compression dowels per segment and the radial joints were fitted with spear bolts. Each segment contained two lifting sockets/grout holes.
Tunnel shield
To enable the construction of the 8m i.d. tunnel a new 8.9m o.d. open face tunnelling shield was built for the project. The shield is believed to be the largest tunnelling machine in the UK for 40 years. The machine was 8.9m in length, and including the back up equipment it had an overall length of 22m and a weight of 350 tonnes. The shield needed the capability to excavate through a variety of materials including weak foamed concrete, brickwork, masonry, variable glacial deposits. The machine’s 3.3m-long main body housed two telescopic cutting booms, controlled from the operator stations behind them. The booms were mounted one above the other to enable the upper and lower halves of the working face to be excavated, supported and loaded out simultaneously. The 1.8m-long hood section in front of the main body was fitted with hydraulic fore-poling rams and face support rams. The shield was advanced and steered using 20 main propulsion rams pushing against the last segmental ring built. Each ram had a capacity of 100 tonnes with a total thrust of 2,000 tonnes being available. The shield was fitted with a heavy duty primary chain conveyor feeding a secondary rubber belt conveyor. The machine was equipped with a PLC controlled two-component grout system, mechanical segment erector and segment handling system in the backup area.
MONITORING
Prior to the commencement of construction works on site an extensive, robust and predominantly automated monitoring system was established in order to monitor ground/structure movements and to trigger a response in the event of the movement exceeding a predefined trigger.
The safety critical assets potentially affected by the works included the Down Tunnel, down line track, the existing heritage listed portals and the A666. Impact assessments on each structure as a result of ground movements associated with the works were carried out, with a hierarchical level of trigger values defined for each structure. The adopted monitoring system involved the installation of six automatic total stations and more 500 targets.
Optical targets were installed at 3m centres on the down line track throughout the tunnel and along the length of the launch and reception pits to enable measurement of cant and twist. Five targets were installed along the profile of the Down Tunnel arch at 10m centres to record any deformation of SCL due to enlargement of the Up Tunnel and or grouting works behind the existing Up Tunnel lining. Targets were also installed either side of the A666 at 5m centres and on both portal structures. Four inclinometers were installed in boreholes undertaken as part of the additional ground investigation to determine ground movements at depth. Access to results of all instruments and targets was provided by a web portal by the surveying contractor.
CONSTRUCTION
SCL strengthening of the Down Tunnel along with the local enlargement works commenced on the weekend of 13th March 2015. The existing brickwork lining was air blown and jet washed, removing dirt and calcite deposits from the lining. The steel mesh was fixed from a bespoke scaffold platform and secured to the lining using masonry dowels. The bespoke scaffold enabled up to 30m of tunnel to be meshed in possession. Three 30 tonne silos were used to store the dry mix sprayed concrete material. The concrete was pumped from the site compound above the tunnels down the Up Tunnel airshaft into the Down Tunnel, where rail mounted robots were used to spray the concrete.
In the enlarged section the inverts of all steel ribs were installed over three weekend possessions before work started on enlarging the tunnel crown. A swivel arm lifting device mounted on a rail road vehicle was used removing the safety risks associated with manual handling. Both the SCL works and enlargement of the tunnel were complete within the 54 hour possessions as planned.
The grouting and piling works commenced simultaneously on site with over 180 m3 of grout pumped behind the existing Up Tunnel lining to fill preexisting voids. The grout take was nominal in most areas however significant grout takes of up to 6m3/m were recorded locally particularly around the cross passages.
Filling the Up Tunnel with concrete involved the pumping 7,500m3 of concrete into the tunnel. The concrete needed to be strong enough to support the existing lining yet weak enough to enable the tunnel machine to mine. The filling of the tunnel was undertaken in approximately 1m lifts and was completed in 15 days and nights from a plant established on site.
The construction and excavation of the launch pit piled wall and construction of the pipe arch and headwall were completed in less than eight weeks.
Construction of the tunnelling shield commenced in mid-December 2014 and the shield was delivered to site on the 19th July 2015 following a partial commission of the machine on site. The construction of the 8m i.d. tunnel commenced on the 1st August 2015 but unexpectedly challenging ground conditions resulted in slow progress initially. During the initial parts of the excavation it became apparent that the original Up Tunnel lining had under gone significant distress and distortion and had at some point previously collapsed in the crown and shoulder. This situation was completed masked by the present of the inner brick lining. Progress was further slowed by the almost rock like state of the Accrington brick inner lining in places during the initial parts of the excavation. Initial progress was slowed to one ring per day instead of the intended four, however by the completion of the tunnel drive, advances of up to 8.4m over a 24 hour period were achieved.
The shield successfully passed under the busy A666 in mid-October. The passing of the shield beneath the A666 coincided with the planned upgrade works to the highway allowing individual lanes to be closed as the tunnel passed underneath. On the 1st November 2015 the tunnelling shield broke through at the reception portal. Normal rail services were restored through the new twin track 8m id tunnel on the 14th December 2015.
CONCLUSION
By working collaboratively and with careful planning J. Murphy & Sons, OTB and Network Rail ensured the successful completion of this complex and challenging project. While these were all proven techniques, they were combined in a way which was unique in the UK. The SCL strengthening of the Down Tunnel was the first time that Network Rail had completed such intrusive work in possessions rather than a blockade. The design, procurement and commissioning of the bespoke 8.9m id tunnel shield was completed in only seven months. Overall the project was taken from feasibility stage to handback in under 14 months. During the construction works there were no accidents to the workforce or environmental incidents.
The newly reopened Farnworth Tunnels have turned assets created in the 19th century into ones fit for the 21st century and will ensure the security of rail services on the route for the next 150 years
