Monitoring a tunnel during construction with safety, quality and reliability in mind…
In August 2019, Central Rail Systems Alliance began excavating 11,000 tonnes of material from the closed Eastern bore of the Gasworks Tunnel, a tunnel which is less than 500 metres from Kings Cross Station. The tunnel has two operational bores which carry the East Coast Mainline. The excavations are planned works which facilitate the station remodelling that will see track, signalling, overhead line and geotechnical improvements to the infrastructure, and the eastern bore being reopened to traffic. The remodelling works will ease pressure on the movements in and out of the station, increasing accessibility by 50 per cent. Once the material is removed, and following a short period of contingency, slab track will be laid through the 490 metres long tunnel bore, connecting to the north of the tunnel into the existing fast lines. It is predicted that removal of the material may result in some minor uplift of the tunnel invert and could cause some deformation of the tunnel structure. The tunnel is brick lined at the invert, sidewalls and crown except at the areas where the tunnel passes underneath the Regent’s Canal. Here, the crown is constructed from cast iron girders and supported by the U-shaped brickwork forming the invert and sidewalls.
The Victorian Railway infrastructure is ageing, degrading in condition yet being stretched to cope with more use. Construction work to upgrade the network takes its toll, and less is known about how some of the older assets will react to this construction work than would be liked. Understanding how much a tunnel moves during a twelve-month long construction project is fraught with challenges, including:
• the lack of understanding of how much tunnels naturally move
• the limited understanding of what external factors can cause it to move further
• uncertainties around structural integrity of assets which were built over 150 years ago
• how to measure a tunnel to an appropriate level of accuracy to satisfy risk models
• how to transmit this data out of the tunnel to have constant insight into the behaviour
• how to classify thresholds for movement, and decide what escalation procedures are needed
• how to avoid having any of our people inside the tunnel for the duration of the work.
Add to the above a description of the site conditions: dusty, noisy, low visibility, heaving with operational plant, no phone signal and no natural light; it’s apparent there are difficulties. Now comes the really interesting part: a third-party construction site exists above the tunnel, with active piling and excavations directly above the Eastern bore. This ‘perfect storm’ of a monitoring situation now presents further challenges not just around has the tunnel moved, but also who has made the tunnel move.
Amey Consulting’s solution
Amey Consulting began by undertaking 3D Finite Element Analysis (FEA) to assess the influence of the removal of spoil material and subsequent installation of a slab track system on the stability and displacements of East Gasworks Tunnel. The potential displacements are dependent on the geotechnical stiffness parameters of the tunnel structure and the surrounding ground included in the 3D Finite Element Model. The results of the FEA revealed that the maximum and minimum values of expected heave after removal of all spoil material from the tunnel invert along the entire length of the tunnel were 7.65mm and 4mm respectively. It was therefore decided that trigger levels should be set for movement of the tunnel in accordance with the results of the FEA:
• green: Less than 4mm
• amber: Between 4mm and 7mm
• red: Above 7mm.
Options were depleting as more was learnt about the challenges and constraints. Ultimately, what was needed was a solution that could record movement data in absolute form, at regular intervals, day and night, with no requirement for people to visit site. Similarly, the data needed to be stored in a secure, accessible location so that teams could react to any movements as they happened. In a situation like this it can be easy to lose track of the priorities: work should stop immediately if the tunnel moves, so that the infrastructure, the travelling public and people at work inside the tunnel can be protected from adverse risk.
Amey Consulting’s technical monitoring solution comprised:
• five high-accuracy Trimble S5 Total Stations
• over 500 metres of steel toughened multicore fibre optic cabling, connecting the total stations to a 4G gateway at the south portal
• provision of isolated 110v power to the tunnel
• five environmentally toughened cabinets, each housing the data communications and uninterruptable power units
• over 150 toughened glass prisms, installed in arrays throughout the tunnel
• a network of reference prisms to coordinate all movement data to control points.
The solution was designed so that it could work in adverse conditions, record reliable and quantifiable data and be trusted to conform to the project specification. The result:
• accuracy – millimetre accuracy was achieved
• regularity – data was recorded every 15 minutes 24/7
• safety – people did not enter the tunnel during construction work as the system was totally automatic.
All monitoring data was hosted on Amey Consulting’s secure Azure cloud, processed and presented graphically using Trimble T4D software. This has enabled multiple stakeholders to gain access to historic and live data in real time, understand trends, correlation with temperature and weather conditions, but more importantly see the effect of the construction activity on the tunnel.
Back-analysis consists of obtaining more realistic parameters of the soil by using the displacements measured by monitoring sensors. The sensors are represented and linked to a Finite Element Model. Installation of monitoring sensors at the invert of the tunnel, where maximum displacements were expected, was not feasible. Therefore, back-analysis technology (Daarwin) was implemented to calibrate the geotechnical stiffness parameters using readings taken in areas of the tunnel cross section other than the invert. This allowed for a more precise estimation of the displacements at the tunnel invert.
Throughout the project, 3D laser scans were undertaken to accurately build an archive of the geometry through time, with a particular focus on when the tunnel is totally stripped of all 11,000 tonnes of material. The results of the point clouds have allowed us to produce change detection ‘Heatmaps’, again translating technical movement data into intuitive graphical results.
The monitoring system recorded lower displacements than those estimated using FEA during the excavation activities at the East Gasworks Tunnel. This indicated that the stiffness parameter values used in the previous 3D Finite Elements Analysis were lower than real. The back-analysis revealed that the real stiffness parameters of the London Clay were greater than those utilized in the previous Finite Elements Analysis.
The overall system design proved to be up to the task of measuring accurately, reliably and robustly. This meant that reassurance was provided for the site personnel that the tunnel was reacting in a manner that was within safe engineering tolerances. It also provided reassurance to the asset owner (Network Rail) that the tunnel as a whole (eastern, central and western bores) was structurally integral, and safe for the travelling public.
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