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.

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