Convergence confinement method (2)

This blog continues from this previous blog and elaborates on the Convergence-Confinement method in tunnelling.

In-situ stress of ground matters

As the previous blog explained, the in-situ stress of the ground will be shared between the ground itself and the supporting structure once the tunnel is done. During this process, convergence continues to happen gradually. When the in-situ stress of the ground is low compared to the undrained shear capacity of the ground, the behaviour of the ground is relatively more elastic, and the convergence of the ground is smaller and happens slower. In other words, the relaxation of the ground will be higher for a ground with higher in-situ stress. This is easy to understand, since a ground with high in-situ stress is ‘bursting’ to relax and therefore will release its stress much quicker than a low in-situ stress ground.

Stiffness matters

As the previous blog explained, the ground continues to converge until installation of the structural support. Delayed installation up to certain point, leads to a reduction in stress in the structure.

At certain point, when the structural support is installed, convergence is intercepted by the structure. The ground and the structure converges together. An increased support stiffness, as a result of the addition of the structural support, limits convergence to a smaller value than without the support.

Convergence is smaller, and stress in support is greater, when:

• Support is installed earlier

• Support is stiffer

Limitations of the Convergence-Confinement method:

As mentioned in the previous blog, the Convergence-Confinement method is only capable of dealing with (overly) simplified situations which include:

1. The tunnel geometry is assumed to be perfectly circular

2. A single and consistent ground condition

3. Full open face tunnel excavation

4. K0 = 1

If any of the above is not true in the practical situation, which is the case most of the time, a numerical model is a better option for analysis.

Nowadays, design directly based on the Convergence-Confinement method is mostly obsolete and replaced by numerical analysis. However, the Convergence-Confinement method still has great value from an educational perspective, and more importantly, still can provide guidance in terms of the share of the stress by the structure. This will be discussed in the next blog post.