Tunnelling in soft ground - The Mechanics of Soils and Foundations - page 407

settlements S a . Notice that ground loss and ground movements will decrease as the
tunnel support pressure is raised. This is opposite to a foundation which will settle
more as the bearing pressure is raised.
It is convenient to define a load factor as
L f = σ

− σ

t
σ z − σ tc

z

(25.8)

so L f σ t is increased the load

factor L f and ground movements decrease. This definition of load factor is consistent
with the definition of load factor for a foundation given in Sec. 22.5: in both cases
settlements decrease as L f reduces. Relationships between settlement and tunnel load
factor like that shown in Fig. 25.10(b), based on observations made in centrifuge tests
on model tunnels in soft clay, are given by Taylor (1984). From these data a load factor
of 1/3 would give volume losses less than 1%.

= 1 when σ t = σ tc and as the tunnel support pressure

25.8 Summary
1. Tunnels in soft ground have linings which are relatively strong and stiff so they
are stable once the lining has been built. They require substantial support during
construction.
2. Tunnelling in fine-grained soils is undrained and pore pressures near the heading
are reduced. Dissipation of excess pore pressures decreases the effective stresses
near the heading and the factor of safety against collapse will reduce with time.
3. There are simple analyses for undrained and drained stability of tunnels and
headings which determine the collapse tunnel pressure

tc .
4. Groundwater has a major influence on lining loads and heading stability.
5.

σ

Surface settlements due to tunnelling take the shape of a normal probability curve
and can be determined from the ground loss at the tunnel during construction.

6. Surface settlements are related to a tunnel load factor which is similar to the load

factor used to limit settlement of a foundation.

References

Atkinson, J. H. and R. J. Mair (1981) Soil Mechanics Aspects of Soft Ground Tunnelling,
Ground Engineering , July, pp. 20-26.
Taylor, R. N. (1984) Ground movements associated with tunnels and trenches. Ph.D Thesis,
Cambridge University, Cambridge.

Further reading
Burland, J. B. R. J Mair, and J. R. Standing (2004) Ground performance and building response
due to tunneling, Proc. The Skempton Conference , London, 2004. Ed. Jardine, R.J.,
D.M. Potts and K.G. Higgins, Thomas Telford, London, Vol. 1, pp. 291-342.
Mair, R. J. (2007) 46th Rankine Lecture. Tunnelling and geotechnics - new horizons,
Geotechnique (to be published).
Mair, R. J. and R. N. Taylor (1997) Theme lecture: Bored tunnelling in the urban environment,
Proc. 14 th Int. Conf. Soil Mech and Foundation Eng , Hamburg, pp. 2353-2385.