In the last contribution in this forum, we made reference to a paper presented in the World Tunnel Congress last year which identified that design errors and engineering misjudgment are the primary failure factors in underground works. These failures were caused mainly by tunnel face and excavation support collapses and the projects were disrupted with sometimes significant and even un-insurable financial loss and project schedule overruns.
Financial loss and schedule overruns (where DSU/ALOP coverage is procured or even when an organization’s reputation is on the table) can be argued to be a core element of the ‘Total Cost of Risk’ which could be vaguely defined as the total cost of the insurance policy premiums, costs below the deductibles/uninsured losses, hazard prevention and risk control costs, costs of procuring professional services post a failure event and reduced productivity. An important consideration is the realization that the components of the total cost of risk are controllable and can be monitored and properly managed.
In Underground works in particular, which entail a higher level of risk (primarily attributed to unknowns and variability of the surrounding ground) insurance coverage is typically provided at the early project stages, when still many of these uncertainties exist; then the risk management documents and procedures are nourished with only conceptual qualitative information. We will take the ‘risk’ to ‘introduce, here the notion of design risk engineering and provide a different insight on the value that risk engineering can add to the construction and insurance industry and to the early assessment of the risk profile and exposure of a project. Through the use of qualitative and quantitative tools and methods, proper application of design risk engineering can reduce components of the total cost of risk and thus through addressing the issue of limited project information at the early stages of a project, assist also in structured and informed decision making.
Let’s focus here on the tunnel face instability, which is considered to be responsible for the higher number of tunnel failure cases. In the early project stages, only a very limited piece of information is available to assess the required face support pressure and confinement profile to ensure safe tunneling excavation conditions. Through the use of probabilistic methods (i.e. by considering a wide range of applicable controlling parameters), the project stakeholders can assess the probability of failure and agree in advance on a proper framework for risk allocation, an approach similar to the Geotechnical Baseline Reports (GBRs). The Tunnel Stability Factor (TSF) could be used in this regard in the early project stages. In a further step, the risk of face collapse could be properly defined in a structured risk register which could also be part of the project documents, including also financial contingency release points. Such an approach can reduce the insurance premium and deductibles, since the project stakeholders could demonstrate to the insurers thorough risk assessments and due diligence from the early project stages, an element which is also included in the schedule of deliverables of the Code of Practice for Risk Management of Tunnels and Underground Works. In addition, such an approach would increase the hazard prevention and risk control measures and hence their associated costs. However it would reduce by default the number of tunnel failure cases and any reinstatement costs below the deductibles (including of course the high costs of specialist and legal services). Last but not least, any project disruption can very likely lead to reduced SPI’s and productivity losses, not to mention also the non-tangible element of project and stakeholder’s reputation. Can someone argue that through such an approach the ‘Total Cost of Risk’ could be reduced? We would definitely argue that is does indeed.
Now, let’s touch upon the excavation support design and failures, which is considered the second most important factor of tunnel failures and collapses. There are cases, such as in fast track projects for example, where the design of tunnel excavation support and lining and even the construction method, is based on assumptions which should be verified and validated from the geotechnical investigations campaign. Of course, amending the construction methodology post a geotechnical campaign is costly and time consuming, particularly with the logistics and general set up. Among the various tools available to the industry, the CULT-I (Capacity Utilization of Lining in Tunnels-Index) is a practical probabilistic approach that can be applied to different construction methodologies in an environment of uncertainty for both risk mitigation and optimization. To cut the long story short, the abovementioned advantages with regards to the reduction of the ‘Total Cost of Risk’ are also perfectly valid herein.
Design risk engineering or in other words approaching a project through continuous risk assessments to minimize the uncertainties to the extent possible or better said to mitigate the risk ALARP! And the reduction of the ‘Total Cost of Risk’ will inevitably follow!
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