Prof. Kalman Kovári's Lecture on Urban Tunnelling and Case Studies

Yestersay (25th March, 2014), we had a special lecture by Prof. Kalman Kovari, as a part of Second Module of Tunnelling and TBM course and also as a part of  ITACET Training seminar being organized in Politecnico di Torino. During the lecture Prof. Kalman Kovari gave us case studies of Urban Tunnelling with extreme conditions of constraints / exceptionally different geologic conditions. Based on these case studies, Prof. Kovari explained us the thought process during the conception stage and the design stage.

Prof. Kovari also gave us the fundamentals required to understand the upcoming lecture on face stability calculations for Slurry and EPB type TBM drivel.

[Update: Prof. Anagnostou's lecture followed this lecture to cover the details about face stability. I have shared a spreadsheet for face stability estimation based on his lecture at this location (link)]

In particular, Prof. explained us the design and construction of the Ceneri Base Tunnel cavern (24m wide and 17m height) along with the complex problems in executing and monitoring [1].

Prof. Kalman Kovári's Lecture on Urban Tunnelling

We also used this opportunity to clarify some of the concepts explained in his paper on NATM [2]. In my previous blog post (here), there was a discussion about Convergence-Confinement Curve and I had mentioned that the trough-shape response curve (as depicted in all NATM literature) are realistic for shallow tunnels and is due to the material softening. But according to Prof. Kovari, trough-shaped ground response curve is simply not realistic. He emphasized that it does not have any theoretical background and can not be accepted even for shallow tunnels. The trough-shaped response curve could be noticed only in certain special cases (eg. when there is a blocky wedge failure in the tunnel).

References:

[1] Filippini, R., Kovári, K., & Rossi, F. (2013). Ceneri-Basistunnel. In Swiss Tunnel Congress 2013 (pp. 236–249).
Retrieved from: http://www.filippini-ing.ch/documenti/STC_2013-CeneriBasistunnel.pdf

[2] K. Kovári, “Erroneous Concepts behind NATM,” in Rabcewicz-Geomechanical Colloquium, Salzburg, 1993, p. 21. (Available at Swiss Federal Institute of Technology site, here)

Dr. Harald Wagner's Lecture on Urban Tunnelling

Module 2 of Tunnelling and TBM course at Politecnico di Torino (which focuses on Mechanized Tunnelling using TBM in soft ground and Hard Rock condition) started on 24th March with special Lecture by Dr. Harald Wagner. Dr. Wagner gave us an overview of use of TBM for Urban Tunnelling with various examples. He also discussed with us about the design of lining segments and ideal design workflow for any tunnelling project. 

Dr. Harald Wagner's Lecture on Advanced Technologies in Urban Tunnelling

This whole week is power-packed with expert lectures from various countries to kick-start our 2nd Module on Mechanized Tunnelling. Politecnico di Torino is offering this week's of lectures as ITACET Training Seminar, open to all practicing engineers and geologists (details here).

Week 9 Tunnelling & TBM Course: Numerical Modelling (Part 1)

Week 9 (17th March to 21st March) of the Tunnelling and TBM course in Politecnico di Torino was completely dedicated to the Numerical Modelling (basics and practical example in commercially available softwares). We had a chance to simulate Tunnel excavation sequence in Phase² software and compare results with Plaxis Software. Ms. Vincenza Floria from Geodata assisted us in Numerical modelling and clarified our practical difficulties. I intend to blog more about the numerical analysis and software comparisons  (Plaxis with Phase²)in coming weeks.

Eg. Simulation of Tunnel excavation sequence in Urban Environment 

and Volume Loss Calculation

Week 8 Tunnelling & TBM Course: Jet Grouting & Forepoling

Week 8 (10th March to 14th March) of the Tunnelling and TBM course was dedicated to Ground Reinforcement using Jet Grouting and Steel Pipe Umbrella. Prof. Peila explained us the recent trends and the commonly used methods for the design of Steel Pipe Umbrella as Tunnel Supporting structure.

Two main design approaches of Steel Pipe Umbrella were discussed:

1. Equivalent 2D model by considering each Steel Pipe individually deducing bending moment and reaction forces (as shown below):

More on this topic to be discussed in future blogs.

2. 2D Numerical Model, by considering an area of improved Geomechanical properties. This topic is discussed in Hoek's 2000 Terzaghi Lecture (more details here)

3. 3D Model by modelling individual elements in rockmass.

During this week, we also had an opportunity of hands-on experience with the Rock testing, Prof. Peila organized a visit to Rock Mechanics Laboratory in Politecnico di Torino.

Shear Test on Discontinuities

High Pressure Triaxial Apparatus (HPTA)

Triaxial Apparatus for Gravels

Geothermal Model

References:

[1] Oreste, P. P., and D. Peila. "A new theory for steel pipe umbrella design in tunneling." Tunnels and Metropolis. Proceedings of The World Tunnel Congress on Tunnels and Metropolises. Vol. 2. 1998.

[2] Pelizza, Sebastiano, and Daniele Peila. "Soil and rock reinforcements in tunnelling." Tunnelling and underground space technology 8.3 (1993): 357-372.

[3] Peila, D., and S. Pelizza. "Ground reinforcing for tunnelling the example of Steel pipe umbrella." AITES-ITA Training Course Tunnel Engineering. Istanbul, Turkish (2005).

[4] 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)

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)