Urban Tunnel - Sink Holes & Face stability

Metro Tunnel (Source)

Lately, after a recent event (details: link 1, link 2, link 3), there has been much attention in media about the sinkholes created by underground tunnel construction in an urban setting. So, in this post I would like to discuss the theoretical basis behind the stability calculation, which is one of the engineering parameters used to avoid sink holes. 

During an urban bored tunnel drive, instability of the face is one of the prime concern for any tunnel manager. While the workers in TBM may be protected with the closed-face machine, the instability could cause over-excavation and thus excessive settlements & at the worst case, a sink hole on the surface.

Usually, based on the geology, overburden, loads, water condition etc, the type of mechanised tunnelling is chosen for the construction (more on selection of TBM is discussed here). Regardless of the type of TBM (unless its open face rock TBM), during the TBM drive, the Tunnel engineer constantly monitors the applied TBM face pressure with respect to the Target face pressure estimated for the anticipated geotechnical properties. The forces/factors contributing to stability and instability of the tunnel face are:

Factors affecting the stability

Since the cohesion of the soil depends on the pore pressure dissipation, the methods can be broadly divided into:

1. Undrained Condition (widely used method in practice - Kimura and Mair, 1981)

2. Drained Condition (widely used method in practice - Anagnostou and Kovari, 1996)

The face support could be exerted using (a) The Pore pressure in the TBM chamber and (b) The effective support pressure excerted by the TBM. Usually in EPB, the pressure is measured by load cells in the excavation chamber which measures the total stress, ie (a)+(b). The following plot clearly indicates that the total pressure required for the case with maximum delta H is always less than the case in which pressure gradient is the least. However, it is still preferred to have the pore pressure in excavation chamber that is equal to the in situ pore pressure in the ground. This is clearly explained in Dr. Benoît Jones' article in Tunnelling Journal [2]. It can also observed that, as the cohesion increases (stabilizing factor), the effective pressure required decreases (and hence the total pressure).

Comparison of Face Pressure - Above plot is prepared for a 6.6m dia Tunnel with 10m overburden and 20kPa surcharge. Ground water assumed at ground level

In Slurry TBM, the pore pressure in the TBM chamber can be increased by increasing the slurry pressure. It can be set even higher than the water pressure in the ground. Whereas in EPB, the pore pressure in the TBM chamber is maintained by soil plug (formed in the screw conveyor) and can not be set higher than the fluid pressure in the ground.

References:

[1] Anagnostou, G. & Kovári, K. (1996) Face stability conditions with earth-pressure-balanced shields. Tunnelling and underground space technology. Vol. 11, No. 2, pp. 165-173.

[2] Benoît Jones, A Bluffer's Guide to Stability (Part 1 to 3), Tunnelling Journal Magazine (Feb to Jun '14).

[3] Davis, E. H., Gunn, M. J., Mair, R. J. & Seneviratne, H. N. (1980) The stability of shallow tunnels

and underground openings in cohesive material. Géotechnique. Vol. 30, No. 4, pp. 397-416.

[4] Kimura, T. & Mair, R. J. (1981) Centrifuge testing of model tunnels in soft clay. Proceedings of the 12th Int. Conf. of Soil Mechanics and Foundation Engineering, Stockholm. Vol. 2, pp. 319-332.