Friday 31 January 2014

Rock Mass - Tunnel Support Interaction Analysis

This post presents a spreadsheet which can be used to perform parametric studies of support interaction (based on the book "Support Underground Excavations in Hard Rock", E.Hoek et. al, A.A. Balikema Publishers). 

This calculation considers circular tunnel subjected to hydrostatic stress field in which horizontal and vertical stresses are equal. Kirshch's elastic closed form solution os one of the commonly used analytical solution and is used for this spreadsheet. Although this is never a real case but as the author suggests,"a great deal can be learned by carrying out parametric studies in which different combinations of in situ stress levels, rock mass strengths and support characteristics are evaluated". Also, this solution is considered to be a good tool for a "Sanity check" of the results obtained from numerical analysis[4].

Download support_interaction_analysis here.
Snapshot of the Support Interaction Curve.
The calculation has following support systems available in the spreadsheet for parametric studies.
  • 16mm dia - Very light rock bolts
  • 19mm dia - Light rock bolts
  • 25mm dia - Medium rock bolts
  • 34mm dia - Heavy rock bolts
  • 50mm thick, 1 day old shotcrete
  • 50mm thick, 28 day old shotcrete
  • 300mm thick, 28 day old shotcrete
  • 6I12, Light steel sets @ 1.5m spacing
  • 8I23, Medium steel sets @ 1.5m spacing
  • 12W65, Heavy steel sets @ 1.5m spacing
Maximum pressure and maximum elastic displacement of the above support systems are based on Hoek and Brown [2] and Brady and Brown [3]. Note that, in all cases, the supports are assumed to act over the entire surface of the tunnel walls. In other words, the shotcrete and concrete linings are closed rings; the steel sets are complete circles; and the mechanically anchored rockbolts are installed in a regular pattern which completely surrounds the tunnel.

I hope to post similar spreadsheet soon which can be used to draw support interaction curves for user defined rockbolts, shotcrete, concrete linings and steel sets.

References:
[1] E.Hoek et. al, "Support Underground Excavations in Hard Rock", A.A. Balikema Publishers, The Netherlands
[2] Hoek, E. and Brown, E.T. 1980. Empirical strength criterion for rock masses. J. Geotech. Engng Div., ASCE 106(GT9), 1013-1035
[3] Brady, B.H.G. and Brown, E.T. 1985. Rock mechanics for underground mining. London: Allen and Unwin, USA. This book is available at Mr. Partha Das Sharma's bloghttp://miningandblasting.wordpress.com/
[4] Chapter 6, Technical Manual for Design and Construction of Road Tunnels (FHWA-NHI-10-034), Washington, December 2009.

Monday 27 January 2014

Geological Aspects of Tunnel

Today we had a guest speaker from Geodata spa. Mr. Luca Soldo is a Sr. Engineering Geologist and principal contributor of the book: Mechanized Tunnelling in Urban Areas, Taylor and Francis Publication. Mr. Soldo gave us a Geological point of view for drilling and geophysical testing to be performed for Tunnel alignment and typical examples of Tunnel projects.


Mr. Luca Soldo's Lecture on Geological Aspects of Tunnel on 27th January, 2014

Saturday 25 January 2014

Risk Management in Tunneling



This article is a brief summary of Chapter 2 of Mechanized Tunnelling in Urban Areas by Vittorio Guglielmetti et. al by Taylor and Francis Publications and Guidelines for Tunnelling Risk management proposed by International Tunneling Association (WG No. 2), published in Tunnelling and Underground Space Technology 19 (2004) 217–237 (doi:10.1016/j.tust.2004.01.001).

“No construction project is risk free. Risk can be managed, minimized, shared, transferred, or simply accepted, but cannot be ignored”[1]. Due to inherent uncertainties, including ground and groundwater conditions, there might be significant cost overrun and delay risks and as well as environmental risks and hence Formal Risk Management is becoming more common for underground projects to systematically and continually conduct formal risk management evaluations at all stages of underground projects. Risk Management Plan (RMP) can be broadly divided into the following steps:
  • Step 1: Hazard Identification
  • Step 2: Assigning probability of occurrence (P)
  • Step 3: Assigning consequence/impact of hazard (I)
  • Step 4: Risk Analysis
  • Step 5: Risk response & monitoring
Probability-Impact pair (from step 2 and step 3) defines “Initial risk level”. In cases where “Initial risk level” is above acceptable risk level, step 5 is performed to bring it down to “Residual risk level”. Figure 1 illustrates this concept of initial risk and residual risk.
Figure 1 Risk Level Definition
Following sections brief each of above steps of Risk Management Plan (RMP).
Risk Identification (Step 1, 2 & 3)
Types of hazards depend on the type of project and the method of construction. However, in any tunneling project, the main risk stages would be:
  1. Data collection stage (Geology, Hydrogeology, Geotechnics, Hydraulics, etc.)
  2. Design stage (insufficient experience, difficult solution, lack of design flexibility, etc.)
  3. Construction stage (method, technology, human factors, etc.)
For each of the above design stages, project specific objectives and project tolerances are to be identified and documented in Reference Design Scenario. Once the Reference Design Scenario is defined, workshops with experts, desktop study and engineering judgment based on past experiences are used for identifying associated risks, likelihood, impact etc. and are compiled in Risk Register. Risk register would also include identification of specific strategy to reduce each initial risk and quantification of residual risks.
Risk Analysis (Step 4)
In the early stage of project, qualitative risk analysis is used. The timing of the qualitative risk assessment should be such that major design changes are still possible. Probability (P) and Impact (I) are assigned using qualitative scales that are prepared using engineering judgment, brainstorming, etc. Risk, R is defines as the product of P & I. P and I can be defined on 3 point scale or 5 point scale depending on the requirements. I can be further divided based on specific project requirement. Typical example of a qualitative scale (on 5 point scale) is shown in Figure 2.
Figure 2 Qualitative scale of Risk­­­­

A preliminary estimate of project vulnerability to different types of risks is achieved if qualitative evaluation methods are used while a more reliable estimate can be provided if quantitative methods (probabilistic analysis) are used.
Quantitative risk analysis is performed by substituting qualitative P & I with quantitative estimates of P & I. Probability associated with data regarding the ground characteristics, construction variables and unpredictable events can be treated statistically to identify the most appropriate probability distribution function for each variable. Quantifying the impact of a hazard is mainly done to quantify its consequence in terms of project time and cost at different stages of the project. At the end of Quantitative risk analysis, following key parameters are arrived at:
  1. Normal cost of project                    : Calculated using deterministic design
  2. Variance in project cost                  : Calculated using foreseen variations
  3. Base cost                                        : Normal cost (item no. 1) + Variance cost (item no. 2)
  4. Sum of all individual risk events (calculated using quantitative risk analysis). Assuming that all risk will act together
  5. Range of probable cost                   : Base cost (item no. 3) + Summed risk cost (item no. 4)
The above process of probabilistic estimation of time and cost can be performed using the software system DAT (Decision Aids in Tunnelling). DAT not just calculates the above parameters but also simulates the construction cycle of a tunnel by following a proposed construction sequence along a probabilistic geological profile that stochastically changes for each simulation process for probabilistically significant number of runs. Using the computational effort, it is possible to make a comparative evaluation of the performance of the project alternatives.
Risk Response and Monitoring (Step 5)
The authors opine that for effective use of Risk Management Plan (RMP), the designer can assume two important roles in construction phase:
  • Interact with TBM Manufacturer and contractor in order to contribute new ideas of technological innovations
  • Validate the design hypothesis by observation and monitoring during construction
Elaborating on the monitoring point, the author describes about “Plan for Advance of Tunnel” (PAT). The PAT is a live document that provides a dynamic link between design and construction and facilitates the management of residual risks.  A PAT is updated as the tunnel progresses (say every 200-500m stretch). It summarizer both the design and construction requirement in order to achieve a safe performance and is based on the content of the initial design, construction feedback from previous PAT and on new input data.


[1] Sir Michael Letham, 1994 also reported in Clayton, 2001



Friday 24 January 2014

Hydrogeological Aspects of Tunnels

Today we had a guest speaker from SEA Consulting srl. Dr. Antonio Dematteis is a Senior Hydrogeologist and General Manager of SEA Consulting (subsidiary of Geodata spa). Dr. Dematteis is also the Chairman of Working Group on Sustainable water management in Tunnels (GESTAG).

Today's lecture was useful in understanding:

  • The critical issues related to water inflow into tunnels
  • Legislations concerning water and Environment
  • "WATERMAT" Procedure (Based on Dematteis et. al., 2007)
"WATERMAT" procedure is about creating/establishing a Reference Hydrogeological Model (RHM) and Quantifying the reliability of the RHM using R-Index. Based on the RHM, water inflow into the tunnel is estimated (by empirical / analytical / recharge method / numerical method). Due to water inflow into the tunnel, the impact on aquifers are predicted and Drawdown Hazard Index (DHI) are different water points on the surface to judge the "water hazard" due to Tunnelling. If the DHI is high, appropriate remediation to be planned accordingly. Finally, re-use and replenishment of drained water is planned to compensate the effect of Tunneling on surrounding water resources. 

Examples projects and working methods for the above procedure is explained to give us an overall picture of Water management in a Tunnelling project. 


Dr. Dematteis Lecture of Hydrogeological Aspects of Tunnels on 24th January, 2014

Thursday 23 January 2014

Week 1 Tunnelling & TBM Course: General Aspects of Tunnelling

This 2nd Level Specializing Master is intended to cover the following aspects in detail:
  • Tunnel design and construction methods
  • Rock mass characterization. Geotechnical investigation and risk assessments
  • Tunnel supports
  • Mechanized Tunnelling
  • Hard Rock TBM
  • Soft ground Tunnelling
  • Plants and Microtunnelling
  • Contractual and Legislative aspects
  • Work site management, safety and environmental issues
Since the group is from various background and various experience levels, the initial course content is intended to homogenize the group with quick overview of fundamentals before the area experts are invited for specific lectures.

Week 1 of this course is intended to cover General aspects of Tunnelling and Underground structures design.

Key concepts and basics of underground structures design was covered in this week. As a further reading, following Guidelines and books are reviewed:

  • 2008, Kersten Lecture - Integration of Geotechnical and Structural design in Tunnelling, E. Hoek et al.
  • Support of Underground Excavations in Hard Rock, E. Hoek, A.A.Balkema
  • Mechanical analysis of circular liners with particular reference to composite supports. doi:10.1016/j.tust.2009.02.001
  • Chapter 2 (Risk Management) - Mechanized Tunnelling in Urban Areas, V.Guglielmetti et. al, Taylor and Francis

Additional Reading and References:

  • Sections of the book Practical Rock Engineering
  • Engineering Design Manual 1110-2-2901: Tunnels and Shafts in Rock. US Army Corps of Engineers
  • Tunnel Lining Design Guide - British Tunnelling Society and The institution of Civil Engineer
  • AFTES, Choosing Mechanized Tunnelling Techniques



Monday 20 January 2014

Kickstart: 2nd Level Masters (MAS) in Tunnelling and TBM

Today was the first day of our class. Professor Daniele Peila (Coordinator of this course and Vice President of International Tunnelling Association) gave an overview of the course content and the grading system. Tentative site visit plans to Germany, Austria and Switzerland was discussed. Professor Daniele Peila has decided to club Master thesis defense presentation with an National conference on Tunnelling which is due to be held in Turin, Italy. Personally, I feel, it will be a great opportunity for all the students as this will be an amazing platform to "sell" ourselves in front of the who's who of the industry.

Later, we had an usual round of introduction with all the participants of the course. I am very excited to interact and learn from each of the classmates. There is a very good mix of students from different parts of the world, different backgrounds and different experience levels. Couple of PhD students from Politecnico di Torino are also attending the course as a precursor to their thesis in Tunnelling and Geomechanics.

At this point I also want to thank sponsors of our course: