Paper Review: Big Tunnels in Bad Rock

This paper was year 2000’s Terzhagi lecture, presented by Evert Hoek in ASCE Civil Engineering Conference held in Seattle. This was later published in ASCE Journal of Geotechnical and Geo-environmental Engineering, Sep 2001 (link to doi) [1].

Prof. Peila introduced this paper to us during initial lectures on Tunnel design concepts and most recently; it was discussed again during a lecture on pipe umbrella design.

As the title suggests, in this paper, Hoek presents case-studies of big tunnels (10 to 16m dia) in poor quality rock mass, which results in squeezing condition and discusses the practical options for pre-reinforcing the face and the support. The discussion starts with explanations on development of support design concept from classical Terzaghi’s ground arch concept (1946) to “Plastic zone” concept as conceived in Convergence-Confinement curve.

Following aspects regarding the spatial distribution of the support pressure in excavated tunnel are highlighted:

• At a distance of approximately 1D ahead of tunnel face, the rock mass is not influenced by the excavation
• About 2D behind the face, the support pressure provided by the face is zero (and hence radial convergence reaches its final value).

It is further pointed out that the understanding and controlling the behavior of the “core” ahead of face is important in assessing the stability of tunnel. Based on the previous research work, the paper justifies that, when the ratio Rock Mass strength to In situ stress falls below 0.2, there is an onset of severe instability (squeezing) and without adequate support, the tunnel wall and face would collapse. The paper also presents a curve to give a first estimate of tunnel squeezing problems based on the ratio: Rock mass strength / in situ stress.

Strain vs (Strength/Stress) ratio

Comparison of strains in different projects

Following are the highlights from the case history studies:

• Sakurai (1983) and Chern et al (1998) suggested that tunnel strain levels of more than 1% might indicate the onset of tunnel instability.
• Frequent design error is the use of excessively large forepoles, which tend to overload the steel sets behind the face (although, they provide good support for the rock mass ahead of the face).
• In extremely poor quality ground with clay minerals, self-drilling rockbolts may be ineffective
• Overstressing in the shotcrete at the corners of the invert is due to sudden change of curvature in numerical model and could be neglected
• Design of face reinforcement could be done using axi-symmetric analysis but the pattern is usually chosen based on rock bolt pattern

Examples of the support design presented in the paper give a clear design basis, justification of each step and a clear construction sequence. I found this very helpful in understanding the perspective required for a Tunnel design engineer.

This paper addresses the lack for an established design procedure for pipe umbrella and presents a crude approach of simulating in a 2D model. As concluding remarks, the paper gives general guidelines on handling water in tunnel construction and possibility of using TBM in squeezing conditions (which advances using shuffle-shoe process).

Typical Layers for Waterproofing

Refereces:

[1]      E. Hoek, “Big Tunnels in Bad Rock - 2000 Terzaghi Lecture,” ASCE J. Geotech. Geoenvironmental Eng., vol. 127, no. 9, pp. 726–740, 2001. (PDF is shared in Hoek's corner, here)