TBM Tunnels: Unforeseen Geological conditions in Hard Rock

Some colleagues and co-workers agree it seems that  geo-investigation is loosing its importance in most of owners and contractors's views. It is becoming a concern since a number of fatal accidents has been occurred abroad, mostly in Asia and southeast Asia projects.  It is relevant in how contractors and owners should avoid or minimize all risks surrounding the lack of geo-information prior tunnelling. Seems to be estrange but the "TBM machines can do it !" is, unfortunately, the reality in countries where this excavation method is not well-known by investors, owners and contractors.

After years working in overseas projects, it is clear that most of investors and owners are loosing interest in a careful supervision if the excavation require a change, for instance, in support systems, even in the necessary geological updates along the alignment.

This post is just a compilation of discussion that unforeseen geological ground condition ahead the face, such as faults, weakness zones, plays an important rule during tunnelling.

It is a common sense that unexpected events are often caused by geological features because of lack of information from field investigations, data collection, and management matters.

They not only cause increased tunnelling and project cost, but in many cases are of vital importance for the safe completion of a project, and can at times cause substantial delays.

"An understanding of the geological development and the occurrence of such weakness zones features is important in the planning and construction of a project" (Barton, 2002). 

In our views, one of the most important item to be taken into account is the close cooperation between the engineering geologists, the design engineers and the contractor that can often give large savings of costs and time.

So, how to minimize risks prior commencement and during tunnelling? and why it is becoming a normal practice neglect such elementary principals of safety conditions?

In summary, according to international experience and engineering practices, it is suggested to follow elementary engineering procedures such as:

1. Extensive field investigation and good quality descriptions will enable the engineering geologist to predict the behavior of a tunnel more accurately (it cannot eliminate the risk of encountering unexpected features).
2. A good quality characterization of the rock mass will, however, in all cases except for wrong or incorrect interpretations, improve the quality of the geological input data for evaluation and analyses, and hence lead to better design.
3. The methods, effort and costs of collecting geo-data should always be balanced against the probable uncertainties and errors.
4. A detailed Method Statement of excavation, and remediation of possible unforeseen ground conditions ahead the face and the assessment of the risk of the possible altered ground conditions.
5. Before tunnelling commences, the contractor, under owner supervision should ensure that all systems are operational including all temporary and permanent ground support systems.

A good example of these discussions is, for instance, what to do when the tunnel alignment runs towards significant overburden? Probe drills? Geophysics? The answer is clear!!!  However, more and more difficult to convince the ones who insist to neglect these important source of investigation.

Please share your opinions, and experience.

Have an excellent day!