Analysis of jacking forces during microtunnelling in limestone

This paper is intended to describe an Italian case study of a microtunnelling project where the boring machine got stuck during jacking of a 760 mm pipe in a limestone formation. Reference is made to rock mass characterization, including site investigations and laboratory tests. The machine's performance is compared to prediction. To allow for a better understanding of the conditions which led to the unexpectedly high jacking forces, continuum and discontinuum numerical analyses have been used. These analyses are shown to be essential at the design stage, when dealing with microtunnelling in a rock mass, in order to obtain a good prediction of the jacking forces.