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  Right choice of the tunnel construction method: a key factor to reduce technical and financial risks

  Doubtless, tunnel design involves much higher uncertainties than any other civil engineering work.In recent decades there has been an extensive growth in the world of underground constructions that are the most appropriate response to the lack of available surface space and for the needs in terms of environmental and social integration of infrastructures; on the other hand, tunnels involve high capital investments and take a long time to build therefore constitute a major financial challenge for owners.Looking at statistical and cases history, we can see that tunnels construction involve much higher hazards and risks than any other civil engineering construction and a lot of tunnel projects reported cost overruns and contractors claims in percentage much greater than other civil construction works. One of the main reason of this is that tunnels are linear features requiring linear construction sequencing, consequently small changes in production produce large variations in construction schedule and costs.If we look at cost drivers, we must underline that, even if in modern road tunnels advanced technology is used mainly for safety reasons, the final construction cost is still governed by the cost of the tunnel excavation that is obviously governed by “geo” factors (geology, geotechnics and geomechanics); in first approximation, we can say that excavation cost represents a quote from 60% to 80% of the total construction cost.Another important thing that we have to remember about costs is that the possibility to influence cost is greatest at the early phases of the project and for this reason the early phases are the most critical ones during which the fundamental design choices must be made, among which the choice of the excavation method to be used.The primary choice is between mechanised or conventional tunnelling.Today there is a trend towards mechanized tunnelling sometimes considered a “panacea” for all possible situation.

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