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London Underground’s Victoria station is at the heart of London’s transport network. Currently undergoing over £700m of capacity enhancement improvements and with the North ticket hall now open, discover how specialist contractor ITM Monitoring has closely measured every millimetre of construction movement.
Used by over 80 million passengers each year, Victoria Station is one of London's busiest stations. Over the previous five years main contractor Taylor Woodrow BAM Nuttall JV (TWBN) has begun to provide improvements to the station’s capacity, reduce passenger journey times, provide step free access and improve emergency evacuation.
Major works include:
In close proximity to existing London Underground (LU) assets, listed buildings, sewers and an interminable flow of passengers, the Victoria Station upgrade posed a significant challenge to the project team and required a comprehensive movement monitoring system, delivered by ITM.
LU's primary objective in delivering station upgrade schemes is to maximise value across the project from design to delivery. With experience gained on the Channel Tunnel rail link, Green Park step free access and numerous LU projects, ITM was well-placed to ensure LU’s ambitious targets were accomplished using a value-engineered design.
The monitoring instrumentation and monitoring contract was procured and is being delivered in stages. With first-hand experience of managing NEC3 contracts, ITM complied rigorously to the construction, design and management (CDM) regulations required by the project and worked closely with both TWBN and LU.
Stage One saw ITM contribute to the completion of detailed station design. “ITM were employed directly by London Underground to install monitoring equipment and record baseline data to establish how the local infrastructure was reacting to ambient conditions such as seasonal change. This allowed a clear understanding of movement actually induced by the subsequent station upgrade works," comments David Smith, senior construction manager for London Underground.
Substantial traditional surveying and 3D laser scanning works were undertaken of all above and below ground structures providing LU with an accurate 3D representation of the site and adjacent structures to facilitate the creation of a an integrated BIM model. Provision of the model data allowed the project team to design the works from a 3D perspective leading to clash avoidance.
With secant piles forming the walls of the north ticket hall within 3m of a Victorian culvert carrying the River Tyburn; the passenger tunnel linking the north ticket hall to the Victoria line passing under the culvert and skirting over the top of the southbound Victoria line platform tunnel; and an additional escalator squeezing between the twin bores of the Victoria line north and southbound platform tunnels with a mere 300mm clearance, having a spatially accurate and fully co-ordinated 3D model was paramount to construction success.
The early partnership between LU and ITM provided a wealth of background movement monitoring data of the third party structures and LU assets within the zone of influence of the works. As the project progressed through to construction, TWBN were able to use ITM’s monitoring data to address key challenges such as tunnelling in shallow water-bearing ground in the paid area links (PALS) tunnels.
TWBN and designer Mott MacDonald utilised precision jet grouting as the ground treatment method for the PALS tunnel works. This process involved installing over 2,000 columns of jet grout into the ground, forming a wall of weak concrete which could be mined through to create the tunnels.
Close proximity to an urban environment meant the interaction between the jet grouting with existing basements, foundations, utilities and tube tunnels needed a robust monitoring system. The surrounding buildings were closely monitored using Robotic Total Stations (RTS), prisms and back-up manual monitoring.
Below ground saw the installation of precise liquid-levelling cells in a number of building basements, accurately monitoring vertical movement in near real-time during this critical stage of the project.
LU commented: “The initial installation was successfully completed including an innovative rope access approach to high level installation. The work involved installing equipment in and on existing buildings which was managed well, interfacing with stakeholders and avoiding conflict."
Monitoring the vast assets across the project required over 20 different sensor types, installed either above or below ground, to accurately monitor all movement throughout the programme of works.
ITM’s approach supplemented an optical monitoring system with the use of Basset convergence strain gauge rings to overcome a problem with optical refraction caused by temperature gradients in the Victoria line running tunnels. In addition to these, the RTS and prisms, as well as continuous electrolevel beams, monitored 3D tunnel settlement, rotation and ovalisation.
Borehole instrumentation such as biaxial horizontal in-place inclinometers (IPIs) and Multi Point Borehole Extensometers (MPBX) were fitted to monitor changes in subsurface ground movements such as settlement and heave.
Above ground, liquid levelling cells, water level loggers, vibration monitors, piezometers and inclinometers were installed, to name a few. An additional 200 prisms were installed and read by seven RTS to accurately monitor the surrounding buildings, a number of which had listed facades.
The 3D coordinate positions of the prisms were read by networked total stations hourly, with each unit linked to a data logger housing a GPRS/3G modem for remote data transmission and control.
ITM use their in-house Star*Adjust™ software to implement a precise least-squared network adjustment to correct for any movement of the RTS themselves due to construction works.
Data visualisation through Argus, now superseded by ITM’s Calyx OMS software, houses all manual and automatic monitoring data generated across the site, providing a central repository for the project.
The web-based software is designed for assisting with interpretation of large amounts of instrumentation data and is accessible to the project team 24 hours a day. It provides continual monitoring of site conditions, offering status updates for each sensor and information on whether that sensor is currently in alarm.
Using monitoring data to better inform construction decisions provides LU with a value-engineered station upgrade, TWBN with a successful project and passengers with a service throughout the construction process.
The monitoring data gave TWBN the ability to refine a number of construction designs and techniques, resulting in significant savings in time and labour.
To undertake extremely complex civil engineering work safely and without incident, despite a highly congested construction zone with live rail traffic and heavy pedestrian footfall, is no small endeavour.
The team at VSU have been commended for their efforts with the project winning the ‘Tunnelling Project of the Year’ at the 2014 NCE / ITA Tunnelling Awards, ‘Complex Infrastructure’ category at the 2016 ICE London Civil Engineering awards, as well as ‘Specialist Tunnelling Project of the Year’ at the 2016 NCE Tunnelling Awards.
ITM are proud to be a significant part of this project, delivering step free access to Victoria underground station for the first time in its 157 year history.
Find out more: Follow the VSU story and download the case study at: http://www.itmmonitoring.com/projects/victoria-station-upgrade/
Rhiannon Walker, Business Development Manager, ITM Monitoring
m: +44 (0) 7742 298 975
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