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”.
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:
- Data
collection stage (Geology, Hydrogeology, Geotechnics, Hydraulics, etc.)
- Design
stage (insufficient experience, difficult solution, lack of design
flexibility, etc.)
- 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:
- Normal
cost of project : Calculated using deterministic design
- Variance
in project cost : Calculated using foreseen variations
- Base
cost : Normal cost (item no. 1) + Variance cost (item no. 2)
- Sum
of all individual risk events (calculated using quantitative risk
analysis). Assuming that all risk will act together
- 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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