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Identifying the underground hazards
20/04/2012 | Channel:
An internal geotechnical investigation of the disused Connaught railway tunnel was commissioned by Crossrail to inform the planning of the regeneration work
At an approximate depth of 17m below ground level, the Connaught Tunnel had, prior to its closure in 2006, allowed the railway to be diverted under the Connaught Passage, a water link which connected the Victoria and Albert Docks.
The primary aim of the site investigation was to establish the strength of the structural elements comprising the single arch tunnel invert at the disused station to determine the structural interaction with the brick lined wall. This data would inform engineers of the geotechnical hazards at the site and aid the design and planning of the regeneration work.
The investigation was considered high risk with regards to the delivery of the programme and its costs due to the unknown working conditions and the anticipated challenges of groundwater, potential ground gas leakages and confined space working within the tunnel.
Due to the challenging working conditions, ESG Soil Mechanics were required to adopt a number of innovative techniques to ensure that the investigation was delivered within budget, on schedule and without incident or injury.
The project itself consisted of two phases:
In phase one of the project, twenty-eight hand and machine dug trial pits were advanced to ascertain the depth of the track ballast and expose the tunnel invert. Twentytwo concrete cores were undertaken (100mm minimum diameter) to prove the thickness and composition of the tunnel invert, and sixteen Cone Penetration Tests (CPTs) to obtain information on pore water pressures and the geology surrounding the tunnel.
For phase two of the project, six rotary cored boreholes were advanced to a depth of between 10.00m and 23.50m below invert level.
During drilling, all exploratory holes were found to be subject to artesian groundwater conditions. As a result, vibrating wire piezometers were installed, rather than conventional standpipe piezometers. A series of combined gas and water sampling valves were installed on the steel headworks upon completion of drilling. This was to enable both water and gas sampling to be undertaken if required.
Each exploratory location was also fitted with a pressure gauge to enable ongoing monitoring and assessment of the water pressure for both design and health and safety considerations during the site works.
To limit these risks and ensure the site was safe for the project team, ESG Soil Mechanics devised an innovative valve assembly and methodology which, in the event of a water or gas leakage, could be closed to allow the safe evacuation of equipment and personnel from the tunnel. The highly original blowout valve was developed by refining existing industrial purpose valve equipment more commonly seen in industrial applications to create a unique solution.
The blowout valve assemblies were installed part-way through the concrete coring process to ensure that protection was in place in advance of penetrating the tunnel invert. Consultation and experimentation with products from a leading supplier of fixing materials to the construction industry allowed ESG Soil Mechanics to create a methodology of grouting in the valve assemblies to ensure a water and gas-tight seal.
This innovative approach to dealing with the unforeseen ground conditions enabled the project to stay on track and ensured that there was no lost time due to accident, injury or incident. Following the fieldwork, the investigation included geotechnical laboratory testing, geoenvironmental laboratory testing, post-fieldwork monitoring, location survey and associated factual reporting. The project was completed under budget and provided 50 per cent more data than originally specified.