They will demand thorough consideration of the effects of the tunnelling works and effective mitigation of the perceived risks. When the situation is reversed and a new building is to be constructed near an existing tunnel, there is often less awareness of the need to protect the underground.
This might be because the tunnel infrastructure is usually much less evident, "out of sight and thus out of mind".
Whatever its public profile, protecting an existing tunnel is usually vital because of its essential function and the cost of repairs, or worse, replacement.
It is worth noting that it would be unusual for a new building structure, which does not physically clash, to generate loads that would be high enough to lead to structural collapse of a tunnel. However, the levels of loading or unloading that could lead to serviceability problems, such as ground water leakage, are much lower and much more likely to be within the range created, for example, by a typical CBD tower with basements.
Owners and developers need to understand the potential restrictions or costs that may result from measures to limit undesirable effects on the tunnels.
If risks are fully understood at the preparation of the concept, the designer can develop layout and structural forms that accommodate the presence of the tunnels more economically than modifying schemes or applying additional measures later in the design development.
Determining the acceptability of a development adjacent to of tunnels can create a quandary for all parties involved if clear guidelines have not already been established. Unfortunately, this is often the case for many of our older tunnels, which are the very structures potentially more at risk from disturbance.
Even current tunnelling projects include simple loading allowances and physical separation requirements that might not always achieve the level of protection intended. Expert review will always be required, but there are approaches and process that can be implemented to improve certainty in determining the measures that will be required.
While the following discussion refers to tunnels, as their existence is less obvious at the surface, the same principles apply to other underground structures such as shafts, station boxes and caverns.
A tunnel designed for the existing conditions with no reserves of capacity for change will constrain later development.
At the same time, the immediate cost implications of accommodating ill-defined future requirements that might not eventuate is a deterrent to the owner of the tunnel from making what might be perceived as overly generous allowances.
It is an issue where cost and benefit accrue to society, but not equally to the particular parties involved. These matters are often dealt with through legislation or requirements of government authorities.
Nowadays, specifications for tunnels typically include allowance for either a known project or a general loading for possible future developments.
When the details, or even the form, of the proposed development are not available, these allowances are often in the form of a limit on the ground pressure that can be created by the future development at a given level over the tunnel (and possibly on the walls of a station).
Such requirements can be supplemented with defined “no go” zones around the tunnel, which create a physical separation between the new works and the tunnel that we are protecting.
When faced with assessing the acceptability of a development near a tunnel, the obvious first step is to check for the existence of specified, deemed to be acceptable criteria for the proposed earthworks, building loadings and position of new works.
In cases where not such allowances were considered in the original design work, opportunities for proceeding with a development can exist if:
As noted near the start of this piece, the critical issues for tunnel structure are most likely to be centred on cracking, with the associated aspects of leakage and reduced durability. It will therefore be important to decide what acceptable risk levels are, and how these are to be gauged from the results of analyses.
Finally, it is worth understanding that if a development could cause damage in an existing tunnel, there are practical examples of works that can be incorporated into a development to reduce its detrimental effects.
Excavations can be staged with the building works to reduce the maximum unloading effects. Buildings can bridge over a tunnel. Building loads can be transferred past the tunnel stratum using piles sleeved to eliminate transfer of load into the ground until the socket at depth.
While our experience indicates that there is still much to be done for general understanding of this subject, we are also aware of the move towards future proofing of tunnel designs for additional loading, and a case book of examples for dealing with older tunnels.
At the same time it is worth re-iterating that it is important to develop concepts for new buildings that minimise the need for mitigation, and its associated costs.
A good understanding of the response of the tunnel is vital to get to the solution that is right for all the interested parties.