ROCK TUNNELS | SQUEEZING GROUND
TRANSFERABILITY OF EXPERIENCE
PART B:
1 INTRODUCTION In this paper, the comparison between creep and consolidation investigates whether experiences gained from previously constructed tunnels, about required thrust force, can be transferred to tunnels of different diameter or to adjacent tunnels of the same diameter built under the same geotechnical conditions. The investigations into transferability of experience
are valuable in practical situations where, for example, a smaller-diameter pilot tunnel is constructed prior to the main tunnel for exploration, advance drainage or ground improvement; or, the opposite (e.g., upgrading of a road tunnel by later construction of a safety tunnel with a smaller diameter); and, cases of sequentially excavated twin tunnels.
The paper addresses: (i), the effect of the tunnel
diameter on the risk of shield jamming (‘scale effect’); and, (ii), the effect of a tunnel on the required thrust force in an adjacent, later excavated tunnel (‘interaction’). While there is a significant volume of literature on
creep and consolidation in tunnelling in general, and on shield jamming in mechanised tunnelling through squeezing ground, however the ‘interaction’ effect has not been studied so far. The ‘scale effect’ has only been investigated by Ramoni and Anagnostou (2011) for the case of consolidation.
1.1 Scale Effect Besides the question of transferability, investigating scale effect is also motivated by a theoretical consideration— that consolidation time increases with the second power of the drainage path length, e.g., the thickness of a low- permeability layer. Based on this, squeezing would develop slower
for a larger-diameter tunnel, as the drainage paths are longer in relation to a smaller diameter tube. In turn, this should also affect the rock pressure acting on the advancing shield and thus the force required to overcome shield skin friction and the risk of shield jamming. There may be significant differences in scale effects
between consolidation and creep. The effect of the tunnel diameter on the risk of shield
Right, figure 5:
Plane-strain scale problem: (a) geotechnical situation for tunnelling through a horizontal aquitard; and, (b) simplified rotationally symmetric computational model
jamming is more complex to assess, for three reasons: ● Depending on diameter, there are technical limitations for certain TBM parameters.
● Diameter poses limitations on certain operational parameters.
● A larger cross section has a higher potential of encountering adverse conditions.
Considering the limitations, Ramoni and Anagnostou (2011) showed that a smaller diameter is consistently less prone to shield jamming in the case of consolidation; however, the differences between them decrease in weaker ground. Two questions arise: Is the scale effect the same in the case of creep? Is a larger diameter more favourable in weaker ground?
1.2 Interaction Tunnelling through heavily squeezing ground can cause stress redistribution and deformations in an
16 | December 2025
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