Convergence-Confinement Curve based on Depth

Last week, we had an interesting discussion among classmates regarding Convergence-Confinement Curve. This interesting discussion led to further reading on this topic and helped us to understand the influence of depth on convergence-confinement curve. As it gave me a good insight on the behavior of the system with respect to depth and reference to few landmark research articles, I decided to blog on this topic.

From the beginning of this course, our understanding of convergence-confinement curve is as shown in figure below [1]. This concept was described and a spreadsheet for parametric analysis was shared in my previous blog here.

Convergence Confinement Curve

But during the Lecture by Prof. Galler on 3^rd March (details), he gave us a different behavioral concept, as shown in figure below [1]:

NATM - Convergence Confinement Curve

Second behavior as explained by Prof. Galler was proposed by Pacher [2], [3], with a trough-shaped ground response curve. This concept became a main argument point of NATM technique – In order to keep the load upon lining as low as possible, the support-reaction line should intersect the ground reaction line at point minimum point (Point B).

This concept was however questioned by Kovari [4] and was criticized by saying that the minimization of the lining resistance is not possible at all, because its prerequisite of trough-shaped ground response curve could not be explained theoretically.

Several authors [1], [5], [6] defended Pacher’s [2] trough-shaped response curve using numerical calculations, justifying that the variation could be because of the depth of the considered tunnel. It was demonstrated that the trough-shape response curve (as depicted in all NATM literature) are realistic for shallow tunnels and is due to the material softening [5], [6]. Softening is related with loosening and is responsible for increase of the ground pressure relative to the increase of convergence. The deeper the tunnel, the smaller is the softening behaviour and thus trough-shape behavior is not observed in numerical models of deeper tunnels [7].

Softening Behaviour

Further, it is to be noted that softening occurs both for friction angle and the cohesion. Cohesion softening is more dangerous than friction softening as it can lead to sudden loss of stability (as cohesion is destroyed completely after small deformations) [6], [8].

Significance:

For shallow tunnels, cohesion softening is proven by researchers [8] and hence this has to be considered in the numerical modeling. For an excavation with Shield TBM, the excavation face is supported by the TBM itself. Whereas in NATM (shallow tunnel), cohesion softening would play a vital role in stability of the tunnel. Vermeer [9] has also suggested to use 3D tunnel-heading stability and 3D settlement analysis for shallow analysis, if possible.

References:

[1]      D. Kolymbas, Tunnelling and tunnel mechanics: A rational approach to tunnelling. Springer, 2005.

[2]  Pacher, “Deformationsmessungen im Versuchsstollen als Mittel zur Erforschung des Gebirgsverhaltens und zur Bemessung des Ausbaues,” in Grundfragen auf dem Gebiete der Geomechanik/Principles in the Field of Geomechanics (in German), Springer, 1964, pp. 149–161.

[3]      L. Müller and E. Fecker, “Grundgedanken und Grunds{ä}tze der‘ Neuen {Ö}sterreichischen Tunnelbauweise’, Felsmechanik Kolloquium Karlsruhe” (in German), Trans Tech Publ., Claustal, 1978.

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

[5]    G.-M. Vavrovsky, “Development of groundpressure, deformation and tunnel design (in German),” Felsbau, pp. 312–329, 1994.

[6]      P. A. Vermeer, T. Marcher, and N. Ruse, “On the Ground Response Curve,” Felsbau, vol. 20, no. 6, pp. 1–8, 2002.

[7]      C. Bliem and W. Fellin, “Die ansteigende Gebirgskennlinie,” Bautechnik, vol. 78, no. 4, pp. 296–305, 2001.

[8]      S. C. Moller, P. A. Vermeer, and T. Marcher, “NATM-tunnelling in softening stiff clays and weak rocks,” Numer. Model. Geomech., p. 407, 2004 (available at Stuttgart Univ. site, here)

[9]      P. A. Vermeer, S. C. Moller, and N. Ruse, “On the application of numerical analysis in tunnelling,” Post Proceeding 12th Asian Reg. Conf. soil Mech. Geotech. Eng. (12 ARC), Singapore, pp. 1539–1549, 2003 (available at Stuttgart Univ. site, here)

PS: Citations managed using Mendeley. Started trying Mendeley for research papers and reports and found it amazing :)

Week 7 Tunnelling & TBM Course: NATM, Ground Reinforcement & Ground Monitoring

Week 7 (3rd Mar '14 to 7th Mar '14) of 2nd Level specializing course in Tunnelling and TBM started with a lecture by Prof. Galler on NATM technique (details here). Followed by lectures on Geotechnical Monitoring, Ground Reinforcement and Lining Segments.

Prof. C. Oggeri explained us about the concepts and importance of Ground monitoring with case studies. Ground response monitoring forms a key aspect of Observational Method / NATM Technique.

Observational Design Cycle

For further reading on Ground Monitoring, we referred to the book by Dunnicliff [1]. In the latter part of the week, we were given the basic concepts of Ground Reinforcements and Ground Improvement for Tunnelling applications, detailed discussion to follow in Week 8. 

References:

[1]   Dunnicliff, John. "Geotechnical instrumentation for monitoring field performance." (1993).

Prof. Robert Galler's Lecture on NATM & Case studies

Prof. Robert Galler at Politecnico di Torino, Italy

Today (March 3rd, 2014) we had a special lecture by Prof. Robert Galler (official page) from University of Leoben, Austria. Prof. Galler gave us an overview of the NATM Technique with few examples. Basically, the lecture was a brief of the contents presented in his book "NATM - The Austrian Practice of Conventional Tunnelling" published by Austrian Society of Geomechanics (review of this book was covered in my blog, here) and his paper on New NATM Guidelines[1]. Prof. Galler explained us about the Design stage workflow and construction stage followed in NATM with emphasis on monitoring and role of a Geotechnical engineers during construction.

Prof. Galler also explained in brief, about the Austrian Practice of "Works Contracts" for Underground works based on Austrian Standard ÖNORM B2203-1. This standard addresses the need for flexibility in contracts in order to take advantage of NATM's strength. ÖNORM B2203-1 proposes costing guidelines to prepare Advancement (Round Length) vs Support Factor Matrix for deciding different support classes.

Example of Costing matrix proposed in B2203-1 [2]

Also we received a personalized signed copy of the book "The Austrian Art of Tunnelling in Construction Consulting and Research" (link) from Prof. Galler.

The Austrian Art of Tunnelling - ISBN 978-3-433-02924-4

References:

[1] Galler, R., et al. "The New Guideline NATM–The Austrian Practice of Conventional Tunnelling." BHM Berg-und Hüttenmännische Monatshefte 154.10 (2009): 441-449.

[2] The Austrian Practice of NATM Tunnelling Contracts, Austrian Society of Geomechanics, ÖGG Salzburg, 2011 (available at ÖGG's website)

Terzaghi's Rock Load Theory

[Update: This post is by Mr. Ramadoss Muthukuar, details here. He will also be contributing to this blog hereafter]

Karl von Terzaghi (October 2, 1883 – October 25, 1963) was an Czech civil engineer and geologist known as the "father of soil mechanics".

 

In 1900, Terzaghi entered the Technical University in Graz to study mechanical engineering, where he also developed an interest in theoretical mechanics. He was nearly expelled at one point but ended up graduating with honors in 1904.Terzaghi translated and greatly expanded a popular English geology field manual into German. He returned to the university for one year and combined the study of geology with courses on subjects such as highway and railway engineering. Shortly thereafter he published his first academic paper on the geology of terraces in southern Styria.

The Rock load classification method is one of the first methodologies for rock mass classification for engineering. Karl von Terzaghi developed the methodology for tunnels supported by steel sets in the 1940s. By many regarded as obsolete as ideas about rock and rock mass mechanical behavior have since further developed and the methodology is not suitable for modern tunneling methods using shotcrete and rock bolts.

But still the Rock load theory have been in use for shallow tunnel design of tunnel width smaller than 6m.Later the rock load theory was modified by Deere et al to arrive an Quantitative approach to design the support system.

{update} Terzaghi's Rock load theory is covered in the book referred below 

 “Tunnelling in Weak Rocks” by Bhawani Singh and. R.K. Goel.

Book Review: NATM - The Austrian Practice of Conventional Tunnelling

NATM book by Austrian Society of Geomechanics,
ISBN: 978-3-200-01989-8

NATM - The Austrian Practice of Conventional Tunnelling is the book released by the Working Group "Conventional Tunnelling" of The Austrian Society of Geomechanics in the year 2010. I had a chance to go through this book and want to share my impression through this blog post.

As described by Professor Robert Galler, during the release of this book, this book contains all the topics concerning NATM with the necessary references. Although the topics are not covered in-depth, this book acts as a primer to all the disciplines of engineers/stakeholders to start understanding about conventional tunnelling / NATM.

First four chapters covers the general definitions, applicability of the method and explains the typical stages in design phase. Chapter 3 on "Application of NATM" briefly explains the construction sequence clearly with the help of illustrations and site photographs. I found it very useful in relating with site conditions and visualization of the construction sequences. For the soft ground conditions, three examples of Tunnels (Road tunnel with shallow overburden, Metro tunnel in urban area with shallow over burden and Railway tunnel with shallow overburden) are explained with typical excavation sequence & tunnel cross sections.

Chapter 5, 6 and 8 covers the details about Geotechnical investigation, geotechnical design and geotechnical monitoring along with flowchart for design process at different stages of project.

The remaining chapters gives a general overview about safety management, contracts and risk management. Since the target audience for this book is varied (which includes engineers, geologists, owners, decision makers, etc.) specific topics like Rock mechanics, rock classifications, numerical modelling, design of supports etc, are not discussed in this book. Very good book to start understanding about Tunnelling and allied aspects.

-Senthil Nath G T