A new model for TBM performance prediction in blocky rock conditions

•Development of a TBM performance database in blocky rock conditions.•Introduction of the new FPIblocky and establishment of an empirical relation for the prediction of FPIblocky.•Development of equations for the estimation of the penetration per revolution and rate of penetration.•Computation of the machine utilization factor and estimation of the daily advance rate.•Comparison with the QTBM model and discussion of possible solutions to overcome blocky rock conditions.

The term “blocky rock conditions” is generally associated with face instabilities in blocky/jointed rock masses. These events are generally promoted by unfavorable rock mass structural conditions, in terms of joint frequency and orientation, and acting stresses. As a result, rock blocks are formed and then detach from the excavation face which becomes “blocky”, with a markedly irregular and uneven profile. This condition may have a paramount effect on TBM tunneling, leading to a high maintenance frequency and a low TBM advancement rate. Based on the TBM performance data recorded during excavation of tunnels in blocky rock conditions, a TBM performance prediction model has been developed. The model is based on the Field Penetration Index for blocky rock conditions, FPIblocky, which was previously introduced to analyze the TBM performance in blocky grounds at the Lötschberg Base Tunnel. Through a multivariate regression analysis, a new expression has been introduced to predict the FPIblocky based on the volumetric joint count (Jv) and the intact rock uniaxial compressive strength (UCS). An attempt has also been made to quantify the downtimes that may occur in blocky rock conditions and to estimate a reliable value of TBM daily advance.