Tunnel Face Stability - VB Module for Quick Estimation

The stability of the tunnel face is one of the fundamental factors in selecting the method for excavating a tunnel in soft ground and in urban areas. When using TBMs, evaluation of the face-support pressure is a critical component in both the design and the construction phases. However, specific recommendations or technical norms are not available as common guidance for the design. In current practice, different approaches are often employed, both to evaluate the stability condition of the face and to assess the required face-support pressure [6].

This blog post presents a VB module built on Microsoft Excel which is set to calculate the face support pressure in Tunnel. This post is based on Prof. Anagnostous' lecture, ITACET training seminars and related practice exercises. This code is set to calculate the Tunnel face pressure to maintain a stable face when there are no seepage forces and for closed EPB drive (with seepage forces). Although these results can not be used for detailed analysis/design but this could help in a quick check on pressure magnitudes and for rough parametric studies.

Screenshots from the VB program - Input and Output

Click here to download. 
[Update: One of the reader reported that the program seem to show some errors in Mac OS. I will update it soon. However, it is working perfectly fine in Windows 8 & Windows 7]

Details about the program are briefly explained below.

Part 1: Support pressure at Tunnel face (without Seepage Forces)

This part calculates face support pressure based on Horn (1961), wedge failure mechanism. As of now, this program calculates face support pressure for Cohesive Soils or for short term condition in low permeable soils. This could be further developed for all types of soils.

Part 2: Support pressure in case of Closed EPB Drive (with Seepage Forces)

The construction methods used in soft ground tunnelling beneath the water table must ensure control of the ground at the tunnel heading and additionally prevent seepage flow towards the working face. In an EPB drive, the face is stabilized by direct support of the pressurized muck and by the reduction of seepage forces. Hence, higher the head difference, the higher the effective support pressure. Higher effective support pressure will cause excessive cutter wear and will require higher torque to operate. The above program calculates effective support pressure using normalized diagrams. For detailed analysis case specific FEM coupled analysis shall be performed.

In some cases, the program shows "No pressure required". This is possible when the compensation of water pressure (along with cohesion in ground) suffices for face stability.

References:

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

[2] 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.

[3] 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.

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

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

PS: Please let me know if I have missed any error handling scenario or any other bugs. Thank you.

Week 19 Tunnelling & TBM Course: Segmental Lining Design (Part 2)

[News: I would like to thank all subscribers of my blog. Last month I reached total page views of more than 10,000! That is a lot more than I anticipated. Thanks for encouraging, following and sharing with fellow tunnelling engineers.]

Week 19 (26th May to 29th May) covered the details about the Segmental Lining design. This is the third week of classes on Segmental lining and related issues (previous posts: Week 18 and Week 16).  Mr. Michele Mangione from Arup, UK took us through the Design Methodologies and case studies on Segmental opening support systems.

Mr. Michele Mangione's Lecture on Segmental Lining Design

The general approach of segmental lining design could be briefly summarized as:

1. Determine the level of loading

2. Design Model 

• Analytical Model (Continuum model - Muir Wood / Curtis or Bedded beam model - Duddeck and Erdmann) 
• FEM analysis

3. Compute Normal forces, Shear Forces, Bending Moment and Deflection

4. Design for above member forces

5. Additional design and checks for following conditions: 

• Thrust jacking loads 
• Secondary grouting loads, 
• Storage and lifting loads 
• Birds-mouthing of radial joints 

Flowchart for Tunnel Lining Design

One of the problems often faced by young Tunnel designers is the lack of a single reference (a Textbook/ Guideline) for all design approaches required during segmental lining design. I have made an attempt to review the following guidelines/recommendations and summarize the concepts covered in each reference.

• DAUB recommendations [5] summarizes the design principles very briefly (including fire loads and steel fibers) and serves to be a handy reference. It does not include any example design calculations.

• ITA-WG No. 2's guidelines on Tunnel lining design [6] is another very useful reference for Tunnel designers. The guideline presents the basic concepts yet in detailed way with examples and references. However, it does not cover the concepts like Fire loads and steel fiber segmental linings. 

• British Tunneling Society's Tunnel lining design guide [12] is a comprehensive document for general design philosophy of tunnels. Chapter 5 and 6 summarize the design aspects related to Segmental Lining design. The guideline does not include the details of structural design and does not have examples but has a detailed list of references for further study. The guideline however presents a case history on Channel Tunnel and Great Belt Lining design. 

• Singapore, Land Transport Authority - Guideline for Tunnel Lining Design (Part 1) [9] covers the design guidelines for Segmental Lining. This brief document clearly summarizes the loads, load combinations, typical K values for different soils and numerical models to be considered for the lining design. It also includes a detailed set of example design.

• AFTES (French recommendation) [3] for segmental lining design covers full range of the areas affecting the design and construction of precast concrete segments. This guideline discusses in detail about composition of rings in different types of rings (rectangular, trapezoidal and parallelogrammic segments), contact joints and gaskets. Section 4 of the guideline presents various load combination for design of segments in Serviceable and Ultimate limit states. The unique feature of this guideline is, it presents a table indicating sensitivity/importance of various parameters for different stages of project and for different methods of analysis. 

In a future post, I intend to discuss the different design models and share a spreadsheet to quickly estimate (for pre-dimensioning purposes) the member forces using different design models.

Post class hangout with Mr. Michele Mangione, Arup UK

References:

[1] T. R. Kuesel, E. H. King, and J. O. Bickel, Tunnel Engineering Handbook. London: Springer, 2011, p. 528.

[2] FHWA (US), and Parsons, Technical Manual for Design and Construction of Road Tunnels--civil Elements. AASHTO, 2010.

[3] AFTES, “The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM),” 2005.

[4] A. M. Wood, Tunnelling: management by design. CRC Press, 2002.

[5] DAUB, “Recommendations for the Design , Production and Installation of Segmental Rings,” pp. 1–56, 2013.

[6] ITA working group on general approaches to the design of tunnels, “Guidelines for the Design of Shield Tunnel Lining,” Tunn. Undergr. Sp. Technol., vol. 15, no. 3, pp. 303–331, 2000.

[7] ITA working group on general approaches to the design of tunnels and F. Report, “Guidelines for the design of tunnels☆,” Tunn. Undergr. Sp. Technol., vol. 3, no. 3, pp. 237–249, 1988.

[8] H. Duddeck, “Future trends in the structural design of tunnels,” Tunn. Undergr. Sp. Technol., vol. 2, no. 2, pp. 137–141, 1987.

[9] J. Poh and G. K. Hun, “Guidelines for Tunnel Lining Design,” Singapore, 2006.

[10] H. Duddeck and J. Erdmann, “Structural design models for tunnels: Tunnelling 82, proceedings of the 3rd international symposium, Brighton, 7--11 June 1982, P83--91. Publ London: IMM, 1982,” Int. J. Rock Mech. Min. Sci. Geomech. Abstr., vol. 20, no. 1, p. A15, 1983.

[11] A. M Wood, “The circular tunnel in elastic ground,” Geotechnique, vol. 25, no. 1, pp. 115–127, 1975.

[12] The British Tunnelling Society, “Tunnel lining design guide,” Thomas Telford, London, 2004.

Week 18 Tunnelling & TBM Course: Ground Settlements

Last lecture of Week 17 and most of the lectures during Week 18 (16th May to 23rd May) were focused on Ground Settlements, influence of TBM drive on surface buildings and on Segmental lining. 

Prof. Stefano Invernizzi, explained the different available models for predictions of greenfield ground movements. Pickhaver [1] has briefly summarized all the semi-emperical, empirical methods and numerical methods in his PhD thesis. Detailed version can be studied in Geodata's book on Mechanized Tunnelling [2]. (related post which I came across recently: Interesting article about empirical estimation and numerical estimation).

Prof. Fritz Gruebl (University of Stuttgart, Germany) and Dr. Davorin Kolic (ITA-Croatia President)  gave us an account TBM drive overview (including logistics, ventilation and emergency systems) with an emphasis on Tunnel Induced settlements and Universal Segmental Lining.

Models to explain the possibilities of different curve radius using Universal Segments

Prof. Fritz Gruebl's Lecture on TBM Drive & Segmental Lining

On 22nd May, we had detailed case study analysis of Ceneri Tunnel with the designers from Pini Swiss Engineers, Switzerland. Mr. Davide Merlini, Mr. Francesco Rossi and Mr. Stefano Morandi gave us an overview of the Ceneri Base Tunnel from a designer perspective (from the project conception stage to the construction stage). 

Case Study on Ceneri Base Tunnel by Pini Swiss Engineers

On 23rd May, (as explained in an earlier post) we has another case study analysis from a Contractor's perspective. 

References:
[1] J. A. Pickhaver, “Numerical Modelling of Building Response to Tunnelling,” University of Oxford, 2006.

[2] V. Guglielmetti, P. Grasso, A. Mahtab, and S. Xu, Mechanized tunnelling in urban areas: design methodology and construction control. CRC Press, 2008.

Meeting with ITACET Board Members

Today we had an opportunity to meet the Board members of ITACET Foundation (details about ITACET is in this link) as they were here for an ITACET Board meeting at Politecnico di Torino, Italy. His Excellency A. AL-Mogbel (ITACET President) wished all the participants for successful completion. 

Board members Mr. Piergiorgio Grasso, President of GEODATA S.p.A (first from left); Mr. Claude Berenguier Executive Director, ITA (third from left); Mr. Felix Amberg, President of the Amberg Group (fourth from left); Prof. Daniele Peila (third from right) and Mr. Søren Degn. Eskesen, ITA-AITES President (first from right) can be seen in the photo below.

ITACET Board Members at Politecnico di Torino

Today, we also had an invited guest from Ghella S.p.A construction company. Mr. Giovanni Giacomin (TBM & Tunnelling Department Director) presented us two case study projects. First case study was on Legacy Way motorway Tunnel being executed by Ghella S.p.A for Brisbane City council. This award winning Tunnelling project (ITA Tunnel Project of the Year - over $500 million category) executed by Ghella demonstrated the perfect combination of innovative solutions and perfect execution. Second case study was on Maldonado project in Bonous Aires, Argentina.

Mr. Giovanni Giacomin from Ghella S.p.A, Italy

What Happened to Big Bertha?

Anyone who is involved in Tunnelling industry would be aware of the name "Big Bertha" as it is the largest Tunnelling Boring Machine (TBM) built so far [1] (although there were plans to build even bigger TBM [2], they are held up due to various reasons [3]). There is so much of anticipation and excitement about its progress that, I wanted to share my two-cents on its current situation. This is just an attempt to summarise the prognosis that would have been carried out by a TBM engineer.

Workers walking into the Big Bertha Tunnel [WSDOT Flickr Page]

A little introduction about Big Bertha (for those who are not aware):

At 17.5m (57.5 feet) in diameter and around 100m (~300 feet) in length, Big Bertha is a Tunnel Boring Machine specially built for The Alaskan Way Viaduct replacement tunnel in Seattle, Washington, USA to replace a 60 year old Viaduct which is vulnerable to seismic events in the future. Big Bertha is named after the first female mayor of a major U.S city (Ms. Bertha Knight Landes). This $80 million machine was manufactured by Japanese company and the TBM started digging in July 2013 and as of January 29, 2014, Bertha had tunnelled around 312m (~11% of total tunnelling length - 2.8 km)[4]

What happened then?

In December 2013, the TBM was stopped due to an unexpected blockage with an 8 inch diameter steel pipe [4, 5]. After more than 158 hours of "Hyperbaric" interventions, the Tunnelling work resumed in January 2014, just to stop after few meters of Tunnelling because of clogged cutter head, high temperature readings and damaged seals!

Steel Pipe which caused the blockage in December 2014 [4]

Is this Unusual?

No & Yes! It's not uncommon to see damaged seals or bearings on TBMs. Contractor even had a budget of $5 million to have bearing on hand in case of a problem with the original. But fixing them isn't so simple and such huge repairs/replacements are never anticipated in the beginning stage of a TBM drive. Moreover, the cause and extent of damage to the seals remains a mystery (at least till they have opened the main bearing).

Bertha's Broken Seals [8]

Was the Choice of TBM Correct?

TBM selection is often the most crucial decision for any tunnelling job and probably the first aspect to be suspected in instances like this (my previous post discusses about various international guidelines and how to Select TBM). For Big Bertha, this aspect became controversial right from the beginning of the project. 

Geological Profile and Pressure Levels (maximum design face pressure is 7 bar) [6]

The tunnel has to pass through a heterogeneous mix of clays, silts, sands, gravels, cobbles and boulders along the geotechnical profile. EPB or Slurry Type could be used in such circumstances. The presence of sands, gravels, cobbles and boulders would have made, Slurry Type TBM as the ideal choice but EPB machine could also be tricked to suit such situation. The client (WSDOT) left the design choice with the contractor and the winning bid chose to use EPB Machine (which Big Bertha is). Some argue that it is impossible to use EPB machine for such soils and for such face pressure values. Conventional wisdom would predict that Slurry Type TBM only is suited for face pressure more than 6 bars.

Approximate suitability of TBM based on Grain Size

Along with many innovations applied for the first time to the EPB system, the manufacturer included the ability to change cutters at atmospheric pressure from within the arms of the cutterhead. This enables to replace disc cutters at atmospheric pressure itself.

Okay, its EPB Machine! So what? - Problems

• When an EPB Machine is being used in a geotechnical strata with cobbles and boulders, the disc cutters are expected to cut the rock pieces in to smaller fragments. However, in a TBM with openings as big as in the Big Bertha (minimum cutter head opening of 37%), it is expected to have a Secondary Crusher Mechanism [6, 7] which is missing in Big Bertha and this could lead to clog. Although ribbon screw type is envisaged to handle the presence of boulders during the excavation, the absence of crusher mechanism is being questioned by experts.

37% minimum opening ratio in Big Bertha [WSDOT Flickr Page]

• Handling high face pressure using EPB is questioned by experts. When the face pressure is more than 6 bars, achieving pressure distribution in the screw conveyor could be tricky and would depend on the consistency of the material churned.
• The EPB churns the soil which is relatively in a paste state and for a TBM of this size this would mean requirement of high torque to advance the TBM. This could result in a lot of heat generation and especially when the TBM is resuming its drive after a maintenance period, the torque required and heat generated will be very high. If not properly managed, this could damage the seals.
• Provisions of accessible cutter head made the cutterhead thick and heavy (~630 Tons). This means difficulty in handling during repair. Special cranes and special crane footings need to be designed to lift such cutter head in single piece.

Way Ahead

• It has been planned to create a ~40m deep and ~25m dia shaft (TBM is at ~20m below ground level), lower the ground water and excavate 20m below ground level and drive the TBM into the shaft so that the repair work can be carried out at atmospheric pressure. Piles shown on either side of the Tunnel alignment were installed before the Tunnelling started - to limit ground movement.

Access Shaft to Repair the TBM [4]

Following illustrations are taken from WSDOT website [4] which conceptually shows the proposed stages involved in the repair works.

Current situation above the ground [4]

Current situation below ground [4]

Proposed injection between the piles. Piles were installed before tunnelling started to limit settlements [4]

Identify and Divert the Utilities [4]

Proposed grouting behind the Machine [4]

Proposed construction of access pit (40m deep and 25m dia) [4]

Proposed installation of crane and dewatering system before digging the soil in access pit [4]

Proposed system for dismantling the cutter head and repairing the seals in main bearing [4]

References:

[1] Foley, Amanda (3 April 2013). "Big Bertha arrives in Seattle". Tunneling Journal. Retrieved 21 December 2013.

[2] World's largest tunnel boring machine (September 2011), World Highways. Retrieved 13 May 2014.

[3] World's largest TBM on Hold as Orlovsky tunnel cancelled (June 2012), Tunnels and Tunnelling International, Page no. 5 (link). Retrieved on 13 May 2014.

[4] Tracking Bertha, the SR 99 tunneling machine, Washington State Department of Transportation. Retrieved on 15 May 2014.

[5] What's blocking Bertha, The Seattle Times. Retrieved on 16 May 2014.

[6] Technical parameters of Seattle's mega EPBM, Tunnel Talk (Dec 2012). Retrieved on 16 May 2014.

[7] Why is Bertha Stuck, Crosscut Seattle (January 2014). Retrieved on 16 May 2014.

[8] Bertha’s big troubles started in Japan, The Seattle Times, (February 2014). Retrieved on 16 May 2014.

Week 16 Tunnelling & TBM Course: Segmental Lining Design (Part 1)

This is a weekly update after a really long time. Week 14 & 15 was off because of Easter Vacations. Along with lot of catching up with the course work, I also had a chance to visit some places in Italy :) (some of the photos are shared here)

Week 16 (May 5th to 9th) of the Tunnelling & TBM course was dedicated for understanding the concepts and design steps related to Segmental Lining design for Tunnels excavated using TBM. With the background on Lining design for SEM/NATM construction (Week 6 of this course), this week, Mr. Moreno Pescara (Technical Director from Geodata) gave us an overview of Universal rings, gaskets, design steps and precautions to be taken as a Tunnel designer. We also had a Guest Lecturer from Turin public transport authority who gave us a design case study on Torino Metro.

This was Part 1 of lecture on Segmental Lining, more details on the segmental lining and case studies are expected to be covered in the future weeks (Week 18-19). Keep looking for Segmental Lining Design (Part 2) for summary of the lessons, references and spreadsheets.