Prof. Georgios Anagnostou's Lecture on Face Stability

50th Lecture day of the 2nd Level Specializing Masters in Tunnelling course and the Last day of  ITACET Training Seminar (28th March '14)  was presented by Prof. Georgios Anagnostou from ETH Zurich, Switzerland. Prof. Anagnostou's lecture was focused on Tunnel face instability in soil and the potential hazards related to it. Professor explained the idealized failure mechanism (wedge in front of face + prismatic body extending upto the surface based on Horm 1961) of face instability using model tests and the derivation of limit equilibrium equations for calculation of support pressure. 

Prof. Anagnostou's Lecture on Face Stability

Professor also gave examples of face stability calculation using Limit Equilibrium equations and based on monograms.

Examples of Support Face Pressure Calculation using LEM Approach

References:

[1] J. Messerli, E. Pimentel, and G. Anagnostou, “Experimental study into tunnel face collapse in sand,” Phys. Model. Geotech., vol. 1, pp. 575–580, 2010.

[2] G. Anagnostou and K. Serafeimidis, “The dimensioning of tunnel face reinforcement,” in World Tunnel Congress 2007, May.

[3] R. Schuerch and G. Anagnostou, “Analysis of the stand-up time of the tunnel face,” in World Tunnel Conference 2013 Geneva, 2013, pp. 709–714.

[4] G. Anagnostou, “Urban tunnelling in water bearing ground – Common problems and soil-mechanical analysis methods,” in 2nd International Conference on Soil Structure Interacton in Urban Civil Engineering, 2002, pp. 233–240.

[5] K. Serafeimidis, M. Ramoni, and G. Anagnostou, “Analysing the stability of reinforced tunnel faces,” in 14th European Conference on Soil Mechanics and Geotechnical Engineering, 2007, pp. 1079–1084.

[6] G. Anagnostou, “Some remarks concerning EPB and slurry shields,” in Development of Urban Areas and Geotechnical Engineering, 2008.

[7] G. Anagnostou and K. Kovari, “Face stability in slurry and EPB shield tunnelling,” in Geotechnical Aspects of Underground Construction in Soft Ground, 1996, pp. 453–458.

[8] P. Perazzelli and G. Anagnostou, “Comparing the limit equilibrium method and the numerical stress analysis method of tunnel face stability assessment,” 7th Int. Symp. „Geotechnical Asp. Undergr. Constr. Soft Gr. “. Rome, 2011.

[9] G. Anagnostou and K. Kovári, “The face stability of slurry-shield-driven tunnels,” Tunn. Undergr. Sp. Technol., vol. 9, no. 2, pp. 165–174, Apr. 1994.

[10] G. Anagnostou and K. Kovári, “Face stability conditions with earth-pressure-balanced shields,” Tunn. Undergr. Sp. Technol., vol. 11, no. 2, pp. 165–173, Apr. 1996.

[11] L. Cantieni and G. Anagnostou, “The interaction between yielding supports and squeezing ground,” Tunn. Undergr. Sp. Technol., vol. 24, no. 3, pp. 309–322, May 2009.

[12] G. Anagnostou and L. Cantieni, “Design and analysis of yielding support in squeezing ground,” in 11th ISRM Congress, 2007, p. 4.

Prof. Nuh Bilgin's Lecture on Cutting Tools and TBM Performance Assessment

Prof. Nuh Bilgin's Lecture of Cutting Tools and TBM Performance assessment

On 26th and 27th March '14, we had a special lecture by Prof. Nuh Bilgin 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. Prof. Bilgin explained us the mechanics of Rock cutting and different types of Rock cutters used in Roadheaders and TBM. With the help of example case studies, cutting forces (FC & FN) were calculated for different types of cutters. 

Different methods for assessing the performance of mechanical excavators (Roadheaders & High energy impact hammer) were discussed using numerical examples. A good example for the calculation and comparison between excavation rate of Roadheaders, High energy impact hammers and Drill & Blast method can be studied in his paper on Istanbul Kadikoy–Kartal metro tunnels [1]. Types of tools used in TBM and principles of Disc cutters were discussed in detail with numerical examples for V type disc cutters and CSS disk cutters. Case Study of Tuzla-Dragos Sewerage Tunnel was considered for explanation. Prof. Bilgin’s paper on performance prediction of TBM gives further insights and details of the above exercise [2], [3]. The above paper presents prediction model based on Earth Mechanics Institute (ESM) of Colorado School of Mines (CSM). This method is based on full-scale laboratory cutting tests. Prof. Bilgin also introduced another prediction model developed by University of Torenthiem and Norwegian Institute of Technology, which is an empirical model based on database accumulated over time. 

Another study covering 262 different types of TBM and statistically analyzed data presents relationship between TBM diameter, TBM thrust, cutterhead torque, TBM weight, number of disc cutters and maximum rotational speed of cutterhead [4]. 

Numerical examples were taken from the book: Mechanical Excavation in Mining & Civil Industries (link here).

References:

[1] I. Ocak and N. Bilgin, “Comparative studies on the performance of a roadheader, impact hammer and drilling and blasting method in the excavation of metro station tunnels in Istanbul,” Tunn. Undergr. Sp. Technol., vol. 25, no. 2, pp. 181–187, Mar. 2010. 

[2] N. Bilgin, C. Feridunoglu, D. Tumac, CINAR, M, and L. ÖZYOL, “TBM cutting performance in Istanbul,” T T Int., no. FEV, pp. 17–19, 2006. 

[3] N. Bilgin, C. Balci, H. Tuncdemir, S. Eskikaya, M. Akgul, and M. Algan, “The performance prediction of a TBM in difficult ground condition,” AFTES, Journees d’Etudes Int. Paris, pp. 25–28, 1999. 

[4] U. Ates, N. Bilgin, and H. Copur, “Estimating torque, thrust and other design parameters of different type TBMs with some criticism to TBMs used in Turkish tunneling projects,” Tunn. Undergr. Sp. Technol., vol. 40, pp. 46–63, Feb. 2014. 

Some of the research papers of Prof. Bilgin could be accessed here.

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)

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)

Week 6 Tunnelling & TBM Course: Shotcrete and Lining Design

Week 6 (24th Feb '14 to 28th Feb '14) of 2nd Level specializing course in Tunnelling and TBM was on Shotcrete and Design of lining, including the use of support interaction capacity curves. Majority of this week's time was spent on understanding the evolution of design philosophy for tunnel liners using shotcrete and steel-sets. 

Initial Support & Lining System Used in Current Practice [1]

It is understood from the above figure that the application of Shotcrete + Steelsets is used for wide range of ground conditions and is one of the most widely used support system. 

Concepts of three main design methods: Distributed ground pressure approach, subgrade reaction approach and Ground-support interaction approach were discussed. The paper on composite supports (Steel-sets + Shotcrete) by Hoek's research group (Carranza-Torres et al) [2] was helpful in understanding the concepts of Capacity diagrams.

Other paper which was reviewed during this week was Oreste (2007) which is on Hyperstatic reaction (numerical method) to simulate interaction between the support and rock surroundings using springs. Based on several calculation using Hyperstatic Reaction Method (HRM), design tables are presented for dimensioning support system (with Steel-sets + Shotcrete).

For complete understanding of shotcrete usage and admixtures used, we had invited lectures by  Mr. Enroco Dal Negro, Carlo Pistolesi and Cristiano Maltese (from Maipei) on practical aspects of shotcrete use in a site and on Concrete Technology.

Also during this week,  we had a special lecture by Mr. Juha Kukkonen (from Sandvik) on Drilling Jumbos and tools. This is in continuation with the Drill and Blast lecture series which was covered during Week 4 of this course.

Mr. Juha Kukkonen and Mr. Giovanni De Mattia from Sandik

References:

[1] Transportation Research Board (2006). "Making Transportation Tunnels Safe and Secure", NCHRP Report 525, Vol. 12.

[2] Carranza-Torres, C., and M. Diederichs. "Mechanical analysis of circular liners with particular reference to composite supports. For example, liners consisting of shotcrete and steel sets." Tunnelling and Underground Space Technology 24.5 (2009): 506-532.

[3] Oreste, P. P. "A numerical approach to the hyperstatic reaction method for the dimensioning of tunnel supports." Tunnelling and underground space technology 22.2 (2007): 185-205.

Guidelines and Precautions for Continuum Analysis

Continuum analysis is gaining importance nowadays in all the design offices. Complex multi stage models can be easily created and quickly analyzed as the computing power is increasing day by day. But the irony is, sometimes, the users are not powerful enough, with little understanding (of both - the problem at large and the physics behind the code). In retrospect, during past 5 years of my experience as a design engineer, I have run umpteen number of numerical analysis but hardly any of them was perfect (from my current standpoint). However, it has been improving continuously with experience. It is not an area which could be mastered overnight, anyway. 

During my current course in Politecnico di Torino, I am able to interact with different experts and learn from different practicing engineers about the efficient and practical way of Tunnel continuum analysis. I want to summarize my understanding and the guidelines given in different books regarding the Continuum Analysis of Tunnels, to share with other aspiring Tunnelling engineers (I intend to update this post as the course progresses).

General procedure and precautions to be taken in numerical modelling:

• Step 1: Assess the requirement and purpose of continuum analysis. Precaution in selection of selecting FoS. Safety factors and load factors commonly used in conventional methods should not be used in Numerical analysis.
• Step 2: Choose suitable material model, anaysis dimension (2D/3D) and computing technique (FEM/DEM/BEM) / software based on the purpose of the analysis. Summary of different numerical modeling programs and their applications are summarized in Table 6-12 of FHWA-NHI-10-034 [2]
• Step 3: Stress state: Effective stress is more appropriate for saturated rock masses and for sedimentary rocks (Warpinski and Teufel 1993, Berge, Wang and Bonner 1993, Bellwald 1992)
• Step 4: Mesh Size and Boundary Condition: Depends on the hydrologic conditions. Approx. boundaries at 5 to 10 times the size of the opening from centerline. 
• Step 5: Construction sequence: Main sequence to be modelled in numerical simulations are: excavation/ material removal (in steps, if required), liner/support installation (in steps). To simulate the lag time in shotcrete strength gain, one way of modelling is to sequence installation when shotcrete develops its full strength.
• Step 6: Support installation near the face is a 3D problem. However, to simulate this in 2D model, the rock is allowed to deform a percentage of its otherwise free deformation prior to “installation” of the support. This percentage ranges between 50 and 90 percent (Schwartz, Azzouz, and Einstein 1980) depending on how far the supports are installed behind the tunnel face and can be judged using convergence-confinement method. Discussed in detail in my blog post, here)
• Step 7: In a 2D analysis, the properties of bolts and lattice girders are "smeared" along the length of the tunnel. Equivalent properties (per unit length of tunnel) to be used in analysis.
• Step 8: Analysis approach: Unless required, the number of details in performing the analysis should be maintained minimum and Accuracy of results should be read with the accuracy of input data in mind.
• Step 9: Interpretation: Key aspects to be investigated

1. Is the deformation according to the expected trend
2. Compatibility between support system deformations and rock deformations
3. Compatibility between stress state and failure criterion
4. Numerical Convergence
5. Evaluation of support system performance and redesign (if required)
6. Prepare capacity diagrams
7. Parametric studies to develop design charts for other opening size
8. Check yielded and overstressed rockmasses. Check for spalling etc.
9. Pore-pressure distribution and check direction of water flow

Also refer my previous post on 2008 Kresten Lecture for specific learnings from this paper.

References:
[1] Chapter 8, Tunnels and Shafts in Rock, US Army Corps of Engineers (EM 1110 2 2901), Washington, May 1997.
[2] Chapter 6, Technical Manual for Design and Construction of Road Tunnels (FHWA-NHI-10-034), Washington, December 2009.
[3] Hoek, Evert, et al. "The 2008 Kersten Lecture Integration of geotechnical and structural design in tunneling." 56th Annual Geotechnical Engineering Conference. 2008 (available for educational purpose at: Hoek's Corner)