Experiences of utility microtunnelling in Irish limestone, mudstone and sandstone rock

• Measured microtunnelling data in Irish limestone, mudstone and sandstone rocks is presented.
• Variations in penetration rates with jacked distance corresponded well to vertical deviations.
• Measured values of the utilisation factor ranged between 48% and 78%.
• Influence of various rock characteristics on TBM performance examined.
• Existing TBM performance prediction models were appraised against the present data.

There is currently no published guidance on the excavatability of Irish rock for microtunnelling applications. In this paper, new data and experiences of microtunnelling through rock (using a Herrenknecht AVN slurry shield machine with a rock head) at five Irish sites are presented and interpreted. The rock type is limestone at three of the sites, with mudstone/conglomerate and sandstone at the other two sites. The jacking forces are separated into face and friction (between the concrete pipe and rock) components. Useful relationships have been established between the excavatability index and the uniaxial compressive strength and brittleness index of rock. Cutter head wear is discussed in the context of rotational distance travelled and rock strength. In addition, the suitability of a number of prediction models documented in the literature for the prediction of microtunnel boring machine performance in Irish rock is examined.