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Tunnels and tunnelling

Effects of Tunnelling on an Existing Tunnel in Clay

Constructing long open cavities in soft ground is a challenging task in its own nature but even more so in an urban environment where it is often constructed next to existing structures; both above and below ground. Advances in tunnelling construction technology such as those of tunnel boring machines have allowed tunnelling works to be carried out efficiently and more importantly, it can be done safely, even in poor soil conditions. 

Fig 1 Tunnelling
Figure 1: Bond Street Station Upgrade passenger tunnel constructed directly below Royal Mail tunnel.

Attention in recent years has thus shifted to investigate the impacts of the tunnel construction on existing structures. This is very common in major cities in the past, present and more likely in the future. Major underground lines are currently being constructed in London in close proximity to existing lines, historic and prestigious buildings, and utility tunnels that are centuries old. This research focuses on the former where a new tunnel is constructed directly beneath an existing tunnel in clay. 

Despite the frequency of the problem and the severity of its implications in the event of a failure, little is understood of its deformation mechanisms. This forces design engineers to impose a higher degree of conservatism in damage assessments, commonly carried out through resource demanding complex numerical models. There is a pressing need to improve efficiency of these predictions; however, this cannot be achieved without a good understanding of the mechanisms involved.

The main objectives of the research are as follow;

  1. To investigate the stresses and strains imposed on the lining of an existing tunnel when a new tunnel is excavated underneath it, both in parallel and perpendicular undercrossing.
  2. To study the effect of the clear distance between the two tunnels to the response and behaviour of the existing tunnel.
  3. To serve as a database for tunnelling in clay for future parametric studies with numerical models.

A 10m beam geotechnical centrifuge at the Schofield Centre will be used to conduct this research. In complex problems such as these, the geotechnical centrifuge becomes an invaluable tool of research as it allows the response of the soil and tunnel at model scale to be investigated at realistic stress levels by increasing the centripetal acceleration to account for the non-linear behaviour and plasticity of soils.

Fig 2 Tunnelling
Figure 2: Staged volume loss tunnel model.

Tests were conducted at 100g, effectively scaling up both of the tunnels to 6m diameter tunnels at prototype scale. Two scenarios were investigated; parallel tunnelling (also known as piggy-back tunnelling) and perpendicular undercrossing at different clear distances between the two tunnels with a novel staged volume loss tunnel design. This novel tunnel modelling method allowed simulations of the forward progression of the tunnel construction in 5 stages, covering a distance of 60m at prototype scale at various volume losses. Longitudinal behaviour and thus be measured and recorded as a significant marked improvement from the previous plane strain 2D models of tunnelling. 

 As part of this project, a new centrifuge package has been commissioned. It was designed and built from scratch with the three dimensional nature of the experiment in mind, while making improvements based on lessons learnt from existing packages to keep soil disturbance to a minimum and to ensure a high efficiency of model making. Constructed out of grade 6082-t6 aluminium allow, the package has an internal dimension of 750mm(L) x 600mm(W) x 440mm(H) designed to accommodate preconsolidation pressures of up to 1000kPa. Fully loaded, the package weighs just over three quarters of a ton and will be spinning at a rate of approximately 151 RPM.

Fig 3 Tunnelling
Figure 3: Fully loaded centrifuge package.

Complementing centrifuge test results, field monitoring of a live existing tunnel; Royal Mail tunnel in London, with similar tunnelling scenarios were carried out through a range of state-of-the-art instrumentations including distributed fibre optic strain sensing (DFOS), wireless sensors and photogrammetry. This group field work through Centre for Smart Infrastructure Cambridge (CSIC) has resulted in numerous publications and was recognised with the International Tunnelling and Underground Space Award for the category of Ground Investigation and Monitoring 2014.

Combining results from both centrifuge tests and field monitoring have allowed much of the data and knowledge gaps to be filled, allowing for a much more detailed picture of the mechanism to be observed and studied. This research is currently on going and approaching its final phase. The research was made possible by the generous funding from Laing O’Rourke Construction PLC and technical support from Royal Mail Group Ltd, CH2M Hill and ARUP. 

 - Mr C.Y. Gue

See the news article on winning the prestigious Ground Investigation and Monitoring Award for The Smart Tunnel at the International Tunnelling and Underground Space Awards.