Victoria Station Upgrade PAL 10. The challenges of mining an escalator barrel within an operational station

Transcription

1 Victoria Station Upgrade PAL 10 The challenges of mining an escalator barrel within an operational station Ryan McCarron Harding Prize Submission Paper 2015

2 CONTENTS Table of Figures... 3 Acknowledgements... 3 Abbreviations... 3 Introduction... 1 Project Organisation... 1 Design Considerations... 2 Requirements... 2 Location... 2 Spatial Constraints... 3 Design... 3 Geometry... 4 Modelling... 4 Ground Conditions... 4 Sprayed Concrete Lining... 5 Construction... 5 Sequence... 5 Ground Treatment... 6 Upper Machine Chamber... 6 Preparatory Works... 7 Upper Barrel... 8 Cross Passage Construction... 9 Lower Barrel and Lower Machine Chamber Conclusion RYAN MCCARRON

3 TABLE OF FIGURES Figure 1: Congestion outside Victoria... 1 Figure 2: Upgraded Scheme... 1 Figure 3: Congestion on platform... 2 Figure 4: Victoria Line and Concourse Tunnel... 2 Figure 5: Existing Assets... 3 Figure 6: Elevation showing ground conditions and existing assets... 3 Figure 7: Proposed geometry... 4 Figure 8: 3D model showing advance sequence... 4 Figure 9: Elevation showing jet grout column layout... 4 Figure 10: Section through existing platforms and new escalator... 5 Figure 11: Sequence of Works... 6 Figure 12: Installation of inclined jet grout columns at the rear of the VPT... 6 Figure 13: Menzi Muck in operation... 7 Figure 14: Upper Machine Chamber under construction... 7 Figure 15: Temporary adit to facilitate preparatory works... 8 Figure 16: Construction of ramp within concourse tunnel... 8 Figure 17: Elephant's Foot Detail... 9 Figure 18: Completed UMC and upper barrel... 9 Figure 19: Concourse support frame Figure 20: Excavation between concourse and platform tunnels Figure 21: Location of IPI Figure 22: Steelwork encountered during existing cross passage breakout Figure 23: SCL Hood Figure 24: SCL Hood Constructed ACKNOWLEDGEMENTS Taylor Woodrow BAM Nuttall Joint Venture London Underground Ian Heath Taylor Woodrow BAM Nuttall JV Anmol Bedi Mott MacDonald ABBREVIATIONS VSU Victoria Station Upgrade TfL Transport for London TWBN Taylor Woodrow BAM Nuttall (JV) PAL Paid Area Link SCL Sprayed Concrete Lining SER Signal Equipment Room VPT Victoria Palace Theatre LU London Underground RIBA Royal Institute of British Architects UMC Upper Machine Chamber LMC Lower Machine Chamber CI Cast Iron IPI In Place Inclinometers RYAN MCCARRON

4 INTRODUCTION The Victoria Station Upgrade (VSU) project is part of Transport for London s (TfL s) multibillion pound Investment Programme to improve and expand the transport network. The Taylor Woodrow BAM Nuttall (TWBN) joint venture were awarded the contract in 2010 to upgrade Victoria Station. This entailed enlarging the Victoria Line ticket hall by fifty per cent, constructing an additional ticket hall leading to the northern end of the platforms, connecting the new and existing ticket halls via approximately four hundred metres of new passenger tunnels and providing step-free access to the station. These improvements will address the congestion problems (Figure 1) in the station, extending its life by approximately 75 years. The redevelopment of Victoria Station (Figure 2) is essential to facilitate the upgrade of the Victoria Line. The two together will result in quicker, easier, and less congested journeys for thousands of Londoners. This paper describes the design and construction of Paid Area Link 10 (PAL 10), a nine metre diameter, thirty metre long sprayed concrete lined (SCL) escalator barrel. The new tunnel descends at 30 degrees through an existing concourse tunnel and between the operational Victoria Line platform tunnels. In its completed form, the barrel will house a three-bank escalator providing the Figure 1: Congestion outside Victoria required access to the northern end of the Victoria Line platforms. The proximity of the escalator to existing assets, the ground conditions through which it is constructed, the complexities of the construction sequence and the client s requirement to maintain an operational station render this endeavour a unique tunnelling achievement. During this time I was the Sub Agent in charge of all the SCL works at Victoria Station Upgrade. PROJECT ORGANISATION Client London Underground Ltd Principal Contractor Taylor Woodrow BAM Nuttall Joint Venture Designer Mott MacDonald Sub-Designer Alan Auld Limited Specialist Sub- Keller Ground Engineering Contractor 2018 Figure 2: Upgraded Scheme RYAN MCCARRON PAGE 1

5 DESIGN CONSIDERATIONS REQUIREMENTS Two banks, each of three escalators currently serve the Victoria Line transporting passengers from street level to the Victoria Line platforms. However both of these provide access to the southern end of the Victoria Line platforms, naturally limiting the length of platform used by passengers. The congestion at the southern end of the platforms frequently results in platform overcrowding (Figure 3). To ensure safety, this requires closure of the whole station at street level causing disruption to many passenger journeys. between the two lines (Figure 4). These cross passages allow redistribution of passengers along the platform. It is this concourse tunnel where the new escalator is required to connect to the Victoria Line. Figure 4: Victoria Line and Concourse Tunnel The platforms tunnels at Victoria converge at the northern end to provide a crossing of the running tunnels just north of the station. Therefore the resulting clearance between the northbound and southbound platform is significantly compromised. Subsequently the concourse tunnel between the platform tunnels reduces in diameter. In reality, to achieve a three bank escalator the entire concourse tunnel would have to be demolished to provide sufficient space causing major disruption to the station. Figure 3: Congestion on platform Extensive passenger modelling of the existing station at Victoria was undertaken and it was identified that a significant proportion of journeys start and end on Victoria Street, at the northern end of the station. Modelling identified the requirement to provide access from street level to the northern end of the Victoria Line platforms to alleviate congestion. Construction of a new ticket hall which would be connected to the Victoria Line through the construction of an escalator barrel and additional cross passages. Further pedestrian modelling confirmed that a bank of three escalators would be required to carry the predicted passenger flow. LOCATION The Victoria Line platform tunnels are currently separated by a concourse tunnel that runs along the platform length. This tunnel is where the current escalators connect into the Victoria Line and forms part of the existing cross passages Further defining the precise location at which the concourse tunnel received the new escalator is one of the Victoria Line signal equipment rooms (SER) controlling a southern section of the Victoria Line. During a signal upgrade of the Victoria Line this was located to the northern end of the concourse and could not be relocated to facilitate the upgrade works. The available footprint within the station, the existing access and operational assets governed the location of the barrel; the passenger modelling governed the size of the barrel. At ground level the conditions were not favourable. Directly above the proposed location lies the Victoria Place Theatre (VPT) a grade II star listed building entertaining Londoners with six performances of Billy Elliot per week. The foundations of this building extend 4m below the ground level. RYAN MCCARRON PAGE 2

6 Figure 5: Existing Assets SPATIAL CONSTRAINTS The geometry of the proposed escalator was heavily influenced by the proximity to existing London Underground (LU) assets (Figure 5). The main requirement was the escalator had to be of sufficient size to house three HD Metro escalators and their associated equipment. The end location within the existing concourse tunnel was dictated by the need to ensure there was sufficient run off between the end of the new escalators and the start of the existing escalators within the concourse tunnel. The detailed constraints in location and in particular the location of the SER (Figure 6) rendered a uniform profile impossible. The design team were required to model a number of significant changes in the geometry of the profile as the escalator descended to accommodate the requirements and fit within the various restrictions. The only way this was possible was to control the excavation profile as it exposed the crown of the concourse tunnel housing the SER and subsequently expose the back of the cast iron platform tunnels. Throughout the majority of the length of the proposed escalator, the clearance to existing structures was negligible. The construction sequence developed by the design and build contractor was integral in managing this risk. Figure 6: Elevation showing ground conditions and existing assets DESIGN The TWBN JV were engaged on a design and build contract at RIBA stage E allowing early involvement in the design. The design development phase commenced a year prior to the enabling works on site where the Designer and Contractor worked together to ensure efficiency in the design and construction. TWBN were committed to ensure the following principles were maintained throughout the design phase: Minimise the impact on the operational railway Simplicity in design and construction Provide programme certainty Provide cost certainty Minimise risk RYAN MCCARRON PAGE 3

7 GEOMETRY Following the development of the initial design the basic geometry of the permanent works was developed. The escalator would be split into three sections (Figure 7): Upper machine chamber - horizontal Barrel - Inclined 30 degrees Lower machine chamber horizontal Figure 7: Proposed geometry MODELLING Due to the complexity of the geometry and the construction sequence it was necessary to develop a sophisticated three dimensional numerical model to analyse the effects of the construction on the operational station and surrounding assets. The model developed a step by step analysis of the proposed advance sequence and a three dimensional representation of the existing assets which was used to finalise the advance sequence (Figure 8). Figure 8: 3D model showing advance sequence Following input of the defined constraints into the model, the advance sequence could be analysed. The initial assessments modelled a one metre heading and bench excavation and predicted unacceptably large deformations in the platform tunnels as the ground was removed between the tunnels. The predicted deformations would be large enough to cause significant damage to the platform tunnels. These would have to be closed during construction and possibly rebuilt. GROUND CONDITIONS The Upper Machine Chamber (UMC) of the escalator was to be constructed through water bearing gravels. Throughout the VSU project, the tunnels have all been constructed through this material and to allow this by pre-treating the ground using a method called jet grouting. Jet grouting is an erosion replacement process whereby the granular soils are mixed at high pressure with grout creating a composite stiff, dry medium. The resulting engineered columns are designed to provide a stable, impermeable medium for construction of tunnels (Figure 9). Figure 9: Elevation showing jet grout column layout The ground treatment was designed to provide a full face of treated material through which to excavate. This would additionally provide a two metre annulus around the excavation perimeter of the tunnel to provide stability to the saturated sands and gravels. It also provides a water cut-off between the excavation and the untreated ground. The works would be undertaken from ground level therefore the positioning of the rig and drill rod would be further constrained. With the Victoria Palace Theatre directly above the jet grout columns would need to be heavily inclined. The proximity of the theatre foundations meant it was necessary to install the jet grout columns through the foundations of the building whilst not disrupting the week s performances or rehearsals. RYAN MCCARRON PAGE 4

8 Jet grouting as a methodology was unproven on this scale. The critical nature of the escalator combined with the unfavourable ground conditions meant this treatment was the only viable solution. SPRAYED CONCRETE LINING The complex geometry associated with PAL 10 called for the tunnel to be constructed using a sprayed concrete lining. This provided flexibility in both lining thickness and profile. This allowed the tunnel to fit between existing assets with the profile varying as required to create sufficient space for the future escalators. The design premise was to provide a sequential excavation and support method, which utilises early strength gain in the sprayed concrete to minimise ground movements. In addition the sprayed concrete is fibre reinforced to provide flexural toughness and mesh can be introduced in areas of the lining under significant stress, which was required through the acute changes in geometry of the lining. CONSTRUCTION SEQUENCE Throughout the development of the design the concerns of the excessive movement predicted in the existing assets was reinforced. It became clear that whilst the design of the tunnel itself worked, it did not work within the strict movement limits on the existing assets (Figure 10). In order to safely construct PAL 10 the Contractor would have to combine the design with a novel construction sequence and the use of a number of different construction methodologies. The only solution was to develop a sequence whereby the existing tunnels could be stiffened prior to sinking the barrel past the axis of the existing platform tunnels. The traditional solution to this was through internal propping of the tunnel. This would not be viable as it would call for the closure of the Victoria Line platforms. The resulting proposal involved constructing a new pair of cross passages between the existing platform tunnels to provide the stiffness prior to the excavation of the lower section of the barrel and the lower machine chamber. The large deformations predicted by the numerical modelling demonstrated the locations at which the deformations would occur in the tunnel. It was suggested that by constructing the permanent cross passages first and therefore increasing the overall stiffness of the platform tunnels, construction of the barrel could be undertaken with manageable movements. In conjunction with this issue there was also a requirement identified to stabilise the concourse tunnel during construction. The sequence was modelled and whilst there were still significant deformations they fell within acceptable limits. The existing concourse tunnel was only accessible through the operational station, however due to the magnitude of preparatory works required a new access into the concourse would need to be formed. Assessing all the sequence issues the design produced adhered to the following sequence shown in Figure 11: A - Construct Upper Machine Chamber B - Drive temporary adit to closed section of concourse tunnel to carry out preparatory works C - Complete upper section of barrel to provide access to existing concourse D - Construct new cross passages from existing concourse tunnel to platform tunnels and backfill E - Construct lower section of barrel and lower machine chamber Figure 10: Section through existing platforms and new escalator RYAN MCCARRON PAGE 5

9 The construction of the new cross passages required to facilitate the closure of the concourse involved the sinking of a shaft between the Victoria Line running tunnels at the northern end and the construction of an access passageway between the Victoria Line platforms using traditional square works techniques to form the new cross passage. This was a significant phase of works not covered in this paper. The closure of the concourse tunnel and removal of all services within the area was also a key milestone for the commencement of the major tunnelling works. GROUND TREATMENT TWBN s specialist sub-contractor Keller installed the jet grout columns. Due to the proximity of the proposed columns to the foundations of the VPT it was necessary to drill an inclined core through the foundations of the VPT (Figure 12). This allowed the installation of the drill string through the core hole to reach the location of the inclined jet grout column. Figure 12: Installation of inclined jet grout columns at the rear of the VPT Figure 11: Sequence of Works To provide the required coverage of jet grout approximately two hundred columns were installed with varying diameters and inclination to provide the cut off of groundwater from the excavation. The installation of these columns was carried out in a very restricted worksite during adjacent piling works and sewer diversion. UPPER MACHINE CHAMBER In order to construct the escalator a series of specialist items of plant were required. RYAN MCCARRON PAGE 6

10 TWBN purchased a Menzi Muck M540 to carry out the excavation of the tunnel, an excavator designed to work on inclines (Figure 13). Additionally TWBN modified a Putzmeister SPM300 spraying robot to work on the slope. The development of this specialised plant was key to the successful and safe construction of the escalator. Figure 13: Menzi Muck in operation The upper section of the escalator is known as the Upper Machine Chamber (UMC). Due to the relative size of the cut and cover structure and the escalator barrel the invert was significantly deeper than the slab from which the drive started. This defined the excavation and support sequence that was employed for the construction of the UMC. Figure 14: Upper Machine Chamber under construction The UMC was driven initially through a series of headings with a temporary invert. This facilitated the construction of approximately 7m of tunnels (Figure 14). At this point the invert was excavated from the face towards the tunnel eye. This allowed the full tunnel profile to be excavated for the UMC despite the relative difference in level from the cut and cover structure to the invert of the tunnel. Throughout the construction of the UMC the excavation was undertaken through the jet grouted sand and gravel. To provide assurance to the client and the construction team a series of Daily Review Meetings were employed to compare the as-built records to the 3D model showing the asbuilt positioning of the columns. This allowed the generation of the theoretical face logs identifying areas where the extent of ground treatment was reduced. Throughout the excavation in the jet grout a series of expected gaps were found within the tunnel profile. The associated ingress of the untreated sand and gravel was successfully managed during the construction phase. The UMC was excavated directly above the operational Signal Equipment Room. The average clearance from the crown the concourse tunnel housing the equipment room and the invert of the excavation was 400mm. The key control measure was to ensure there was no excessive vibration during the construction of the UMC as the signalling equipment within the room was extremely sensitive. TWBN commissioned the installation of real time vibration monitors with trigger levels set in conjunction with the LU signals team allowing the production team to understand the impact of each advance. PREPARATORY WORKS Ordinarily the only access to the concourse tunnel was from within the station. It would be impossible to carry all the required tools and equipment down through the station during engineering hours and carry out the works using this as an access. Further still, concrete construction was required on a large scale which could not be completed during LU Engineering Hours. To this end a temporary adit was driven from the headwall of the UMC (Figure 15). The adit was constructed as a traditional timber box heading large enough to facilitate an access route for labour and materials into the existing tunnel. This provided access from the UMC into the existing concourse tunnel. Once the access was created into the concourse tunnel, the existing cross passages were sealed with a reinforced concrete wall. The location in which the escalator barrel RYAN MCCARRON PAGE 7

11 entered the existing concourse tunnel required the construction of a permanent concrete ramp. Figure 15: Temporary adit to facilitate preparatory works This was to provide suitable support to the invert of the barrel as it descended into the concourse tunnel. Upon completion of the permanent concrete ramp (Figure 16) the remaining section of the concourse tunnel was backfilled with a low strength fill to allow it to be easily excavated. UPPER BARREL Once the preparatory works within the existing station were complete, the inclined barrel construction could commence. The upper barrel was constructed on a 24/7 shift pattern taking a total of 13,000 man hours. The escalator barrel was to be constructed through the remaining jet grout, London Clay and subsequently the existing station. Through these various mediums, no two advances were the same. The variability of materials, skilled labour and plant that was on site to deal with any inevitability was unprecedented. The initial pair of cross passages was encountered, these were constructed in the 1960 s and the excavation of these was undertaken from the top. This involved extensive breakout and removal of existing concrete and steelwork utilising low vibration tools and techniques. As the escalator barrel descended into the exiting station, the profile intersected the last cast iron ring in which the SER was housed. The geometrical requirements for the lining profile called for this to be sprayed within the profile. The proximity of the cross passages and their connections to the existing station made this phase of work extremely demanding. The barrel was to be excavated in a staggered heading and bench sequence. During the construction of the first phase of the barrel the profile descended through the jet grout, and fully into the London Clay, encountering and demolishing the existing station as it progressed. As the excavation profile descended fully into the clay the low clay cover became a primary concern. The crown of the barrel and subsequently the Lower Machine Chamber (LMC) had only two metres of clay cover with the saturated gravels overlying. In addition to this the saturated gravels formed the foundation material for the Victoria Palace Theatre. Figure 16: Construction of ramp within concourse tunnel Due to the critical nature of the clay cover and the relative size of the excavation the duration for which the crown of an advance was open posed a significant challenge. However as the excavation included sensitive demolition and existing of cast RYAN MCCARRON PAGE 8

12 iron (CI) tunnel rings and the breaking out of existing cross passages this required a change in the construction sequence. The design sequence for the barrel called for a staggered heading and bench sequence. This meant that the heading was hanging for the duration of the invert construction. As the time taken to removing the existing cross passages was significant, and the presence of the existing concourse tunnel meant that a temporary invert could not be installed it was decided to develop a Elephants Foot solution where the size of the top heading was increased and was footed onto the existing cross passage s (Figure 17). Figure 17: Elephant's Foot Detail This ensured full support to the vault of the excavation and subsequently reduced any potential impact of the low clay cover. This change in support sequence was developed onsite in conjunction with the designer s representative, the production team and the Client. It proved to be a key decision in the reduction in potential movements to the surrounding assets. Figure 18: Completed UMC and upper barrel The barrel was constructed down through the concourse tunnel to a point where suitable access could be gained for the construction of the cross passages (Figure 18). At this location a full-face headwall was sprayed to allow the commencement of the handworks require to construct the new cross passages. It was envisaged that the headwall and the barrel would remain in this temporary state throughout the cross passage construction. CROSS PASSAGE CONSTRUCTION At the bottom of the proposed escalator a pair of new cross passages had to be constructed. Traditionally a new tunnel is required through which the cross passages could be built, however in this case the concourse tunnel was already present. The cross passage construction required the formation of two new openings on the operational Victoria Line platforms. Historically the formation of a cross passage opening requires significant propping to be installed on the operational side of the asset. This was not feasible in Victoria as the existing platform width was small too small. This quandary led to the development of a propless cross passage construction. It called for a small hoarding to be installed on the platform side of the works (approx. three hundred millimetres into the passenger envelope) and all remaining work would be carried out from the new tunnels. To ensure the constructability, TWBN would need to gain sufficient access to the existing concourse tunnel to construct the new cross passages. The basic design premise of the cross passage construction was to limit deformation of the existing linings during construction. This was achieved through the use of two basic principles. Firstly excavation should be limited to small headings prior to the installation of support members, and secondly the additional support members should provide a significant increase in stiffness of the existing asset allowing further excavation and subsequent deformation to take place. The first activity associated with the cross passage construction was to mine out the remaining RYAN MCCARRON PAGE 9

13 temporary backfill in the concourse tunnel to provide access. At this time due to the extent of breakout required within the existing concourse tunnel to form the new cross passages a series of Y-frames (Figure 19) were installed to support the crown of the concourse tunnel. Figure 19: Concourse support frame The cross passages were excavated in the series of benches installing the permanent works as the excavation progressed. Initially the top bench was excavated, the upper strongback was installed and the permanent concrete for the cross passage was poured. This provided significant strengthening to the existing platform tunnels to prevent excessive deformation. The subsequent benches were excavated with a series of temporary props being installed to support the platform tunnel. This excavation was carried out by hand, and approximately three hundred millimetres away from passengers (Figure 20). The formed a six metre deep excavation between the operational platform tunnel and the concourse tunnel. Following completion of the excavation the lower strongback was installed along with the jamb frames allowing completion of the remaining permanent works concrete. The permanent works concrete was cast to the profile of the future LMC excavation. Due to the relative size of the concourse tunnel to the proposed LMC that would replace it, it was necessary to temporarily backfill the void left. This would allow the LMC chamber to be excavated through a weak backfill in contrast to a void which would not allow the formation of the full primary lining profile. LOWER BARREL AND LOWER MACHINE CHAMBER Following completion of the handworks this released the construction of the lower barrel and the lower machine chamber. Due to the significant durations required for the construction of the upper barrel there were additional concerns over the low clay cover through the LMC. To this end it was decided to install a horizontal In-Place- Inclinometer (IPI) through the crown of the LMC (Figure 20). This was completed prior to the restart of excavation works. The IPI would allow the deformation of the clay vault to be monitored during the excavation and would allow an assessment to be made on whether additional support measures would be required during the construction. This provided a key insight into the performance of the small layer of clay above the crown of the tunnel. Figure 20: Excavation between concourse and platform tunnels Figure 21: Location of IPI RYAN MCCARRON PAGE 10

14 As the excavation recommenced the barrel descended below the axis of the existing Victoria Line platforms. At this point the excavation profile exposed the back of the cast iron for the existing platforms. The barrel then levelled out to form the LMC. This involved the excavation of clay, breakout of cross passage concrete, removal of permanent and temporary steelwork, all with negligible clearance from the back of the cast iron platform tunnel and within three hundred millimetres of passengers. The successful completion of this phase of work hinged heavily on a collaborative relationship between London Underground and TWBN. TWBN identified the requirement to station an operative on the platform throughout all works. This provided eyes and ears on the platform and SER during the works. The face advanced further and encountered the second set of existing cross passages. This set of cross passages was constructed as part of the congestion relief scheme in the 1990 s. When the existing cross passages were exposed the quantity of temporary steelwork encountered was significantly larger than initially expected (Figure 22). This led to significant increase in the time taken to excavate and advance. the clay vault during the removal of the temporary work (Figure 24). Figure 23: SCL Hood Figure 24: SCL Hood Constructed Figure 22: Steelwork encountered during existing cross passage breakout Once again this duration posed a significant problem to the stability of the vault with the low clay cover. To this end the sequence for excavation of the advance was modified. This allowed the top of the vault excavation to be excavated over the existing concourse tunnel to form a SCL hood (Figure 23). The hood rested on the existing CI and provided temporary support to As the excavation of the lower machine chamber progressed through the existing cross passages and encountered the now redundant propping installed for the construction of the new cross passages, the excavation and dismantling of the propping was an intricate sequence to provide continuous support to the clay vault and existing platforms. The temporary fill installed following the cross passage construction was removed and the SCL lining was sprayed up against the permanent concrete installed during the construction of the new cross passages. The primary lining and permanent cross passages forming PAL 10 was completed in January 2015 with minimal disruption to the operational station. RYAN MCCARRON PAGE 11

15 CONCLUSION The construction of PAL 10 escalator was a critical element in the congestion relief scheme at Victoria Station Upgrade. Constraints above the ground and below the ground meant PAL 10 was challenging from the design phase right through to construction. The collaborative approach between TWBN and MM delivered a design solution in a very complex environment. The follow on construction phase demanded a robust approach to client assurance. The development of the propless solution for the cross passage construction resulted in a limited impact on the operational side of the platform satisfying both client and contractor requirements. An innovative design coupled with an adaptive construction sequence provided a solution to a seemingly insurmountable problem. The strong working relationship developed between contractor and client provided the client with the confidence that the risk associated with working in such close proximity to their asset was understood. Effective communication between the contractor and the operational staff on the station was key in ensuring the works could progress effectively and ultimately the successful delivery of PAL 10. RYAN MCCARRON PAGE 12