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Construction risks - The problem with Tunnelling

In this article Iftikhar Ali considers the various issues which can arise on projects involving the construction of concrete lined pressure tunnels and how these issues can be managed from a legal and/or insurance policy perspective.

Concrete has an inherent and natural tendency to crack.  When it is poured it undergoes a thermal dehydration process as it cures, which results in the inevitable cracking of concrete.  Now, cracking in itself is not a problem, but it may well become an issue for the operational performance of a particular structure.

Introduction

It is unrealistic for a design to seek to eliminate all cracking in concrete – instead the more realistic approach is to accept that cracking will occur and then seek to control it to ensure the structure is fit for purpose and can function under the intended operational conditions.  Cracking of concrete can be controlled in a number of ways – for example, with the use of aggregates, steel reinforcement and even by restricting the amount of movement and thus stresses that might be induced into the structure.  By way of example, the inclusion of ash as an aggregate contributes to a stronger, more durable and more chemically resistant concrete mix and reduces shrinkage during curing. The addition of steel reinforcement also reduces shrinkage and can be used to control the extent and nature of the cracking. 

Cracking is an inherent vice for concrete much in the same way as iron rusts and wood rots – concrete cracks. Therefore, designers will try to cater for a particular design crack tolerance within the structure.  Distinguishing between cracking that is an inherent issue such as early thermal cracks from other cracks that may well be structural and constitute damage can be a challenge.  From an insurance policy perspective, project and property policies contain physical damage triggers not only as part of the insuring clause but also for those provisions that write back cover following physical damage – under clauses such as the London Engineering Group and Design Exclusion clauses. Therefore, distinguishing what is fortuitous cracking and cracking which is intended by design or an inherent component is key to determining whether coverage is afforded and, if so, to what extent.

Concrete lined pressure tunnels

It is useful to look at what particular issues can arise in respect of the construction of concrete lined pressure tunnels. First, the development of cracks in the concrete lining of a pressure tunnel cannot be prevented by reinforcing the concrete. Therefore, the main purpose of the steel reinforcement is to increase the number of cracks and to limit their width.  The purpose is to reduce the permeability of the lining and thus the water losses from the tunnel.  Reinforcement therefore prevents uncontrolled cracking of the lining and high water losses, which can cause the washing out of joint fillings and instability of the tunnel – that could lead to cracking or collapse.

Second, the design of underground concrete lined pressure tunnels relies heavily on the geotechnical survey conducted as part of the front end design package. In this front end design, the purpose of the geotechnical survey is to communicate the site conditions and the design and construction recommendations to the site design, building design and construction personnel. The survey is supposed to provide specific representative information on the subsurface soil, rock and water conditions. This data may be presented to underwriters to communicate the viability of the project and to obtain insurance cover. 

Third, in reinforced concrete structures, international codes typically seek to limit cracking to a maximum of 0.3mm widths – however for structures that are to convey liquids 0.2mm is considered the appropriate design standard. 

Fourth, deformation of the steel reinforcement within the concrete lining is typically designed into the tunnel lining structure and is intended. The designer's intention is to calculate the range of the elastic deformation of the steel reinforcement (including a factor a safety).    

Conclusion

The geotechnical survey conducted at the front end of the project should be thorough and moreover representative of the project site soil, rock and water conditions.  In any event, as the ITCOP Code prescribes, this basic design should be monitored, updated and enhanced during/after the excavation process for the tunnel based on an ongoing sampling exercise of the geological conditions.  What this ultimately means is that the design for such structures is amended on a progressive basis through the construction sequence. This does not, in the author's view, fall into the category of being an experimental design – as the computation method for reinforced concrete-lined pressure tunnels is well known – and further, classifying everything that arises during the construction sequence as a design defect does not seem the appropriate conclusion.  The particular event or issue needs to be evaluated on its own merits.

In respect cracking, it seems fairly clear that cracks within design tolerances are not fortuitous physical damage and do not engage the insuring or write back exclusions in standard policy wordings. In respect of steel deformation, this is an issue that is in part intended and expected as part of the design at least.

Contact

For more information please contact Iftikhar Ali, Partner  T+44 20 7280 8908 M+ 020 7280 8908 or email Iftikhar.Ali@dwf.law 

This information is intended as a general discussion surrounding the topics covered and is for guidance purposes only. It does not constitute legal advice and should not be regarded as a substitute for taking legal advice. DWF is not responsible for any activity undertaken based on this information.

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