Concepts for handling changing muck properties caused by TBM excavation

It is a well known practice – most of the claims on tunnelling are due to unexpected geological conditions whereas in many cases where TBM have been used it should be clear that ground properties as described in the Geotechnical Baseline Report (GBR) are very likely to be changed during the excavation by the TBM. So a contractual line has to be drawn between the term unexpected ground conditions and expected permanent changes of muck properties caused by the TBM. From a contractual point of view it is very important for both sides – owner and contractor – to have the best assessment on the ground properties in the GBR which have been gained from traditionally quasi-static exploration tests in the laboratory. However, the excavation and mixing of the ground in the working chamber of a TBM is far away from being quasi-static but are highly dynamic to chaotic why traditional soil mechanics have to be judged critical in this case. Quasi-static ground properties change under dynamic conditions due to the complex 3-phases system of the ground (solids, water, air). The economic prediction of the TBM performance and efficiency of a tunnel project is mainly depending on these ground properties changes because they determine geotechnical phenomena like clogging, abrasion and face stability which are most important for the contractor’s assessment. Furthermore he has to predict the penetration of the cutting tools into the face which again is a highly dynamic process.