United Kingdom
Chipping Sodbury Tunnel - Structural Monitoring CLIENT: AECOM / COLAS RAIL SRSA / NETWORK RAIL How the use of precise wireless triaxial tilt & optical displacement sensor nodes informed decision-making & ensured safety during track lowering & drainage works in a 4 km/2.5 mile tunnel
Challenge Chipping Sodbury Tunnel, near Bristol (UK), was completed in 1902. The twin track masonry structure is 4,000 m long and is the second longest tunnel on the Western Route linking London and South Wales. Network Rail has been implementing an extensive programme of improvements to the route since 2010 which includes installing overhead electrification. In order to accommodate the overhead line equipment (OLE), there was a need to modify many structures, including Chipping Sodbury Tunnel. Works included lowering the track by 150 mm and improving the track drainage. In the Autumn of 2020, AECOM engineers undertook a structural assessment to evaluate the potential impact of the track lowering works on both the tunnel and the central track drainage culvert. This assessment informed the implementation of a monitoring regime which was required to provide the necessary assurance that the works could be undertaken safely.
Devising a monitoring solution for Chipping Sodbury was not straightforward. Factors to consider included: the need to maintain continuous operation and data transfer from the system at distances greater than 2 km from access points; the need to avoid interference with the construction works and to avoid damage to the system arising from the works, and the need to maintain reliable monitoring performance in conditions that were, at times dusty, waterlogged and prone to significant vibration.
The monitoring scheme aimed to verify the findings of the structural assessment and inform decision making, particularly in the event of measurements exceeding predicted values. The monitoring design was guided by principles outlined in the British Tunnelling Society publication, ‘ Monitoring Underground Construction - A Best Practice Guide’ , alongside the Network Rail standard NR/L2/CIV/177 – ‘ Monitoring track over or adjacent to Construction Works ’. Monitoring design and arrangements also drew on experiences gained by AECOM on similar track lowering projects where structural monitoring was also required. This included several locations of the Great Western Railway Modernisation Scheme, namely; Patchway Tunnels, Box Tunnel, Middle Hill Tunnel, Dundas Aqueduct, Sydney Gardens, and Alderton Tunnel.
Key Points:
• AECOM engineers needed a monitoring scheme to verify findings of their structural assessment and ensure safety during construction works • Wireless monitoring solutions ensured a continuous data transfer in challenging site conditions • Sensors on the tunnel lining allowed the anticipated movements to be monitored without interfering with the works
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