Influence of rock mass fracturing on the net penetration rates of hard rock TBMs

• For hard rock TBM projects, the net penetration rate is an important factor to estimate advance rates and excavation costs.
• Field work involving geological back-mapping and TBM log data analysis was carried out.
• The present paper validates the influence of rock mass fracturing on net penetration rates for hard rock TBMs.
• The rock mass fracturing factor (ks) is shown to be a good indicator of rock mass boreability in hard rock tunnelling.

Penetration rates during excavation using hard rock tunnel boring machines (TBMs) are significantly influenced by the degree of fracturing of the rock mass. In the NTNU prediction model for hard rock TBM performance and costs, the rock mass fracturing factor (ks) is used to include the influence of rock mass fractures. The rock mass fracturing factor depends on the degree of fracturing, fracture type, fracture spacing, and the angle between fracture systems and the tunnel axis. In order to validate the relationship between the degree of fracturing and the net penetration rate of hard rock TBMs, field work has been carried out, consisting of geological back-mapping and analysis of performance data from a TBM tunnel. The rock mass influence on hard rock TBM performance prediction is taken into account in the NTNU model. Different correlations between net penetration rate and the fracturing factor (ks) have been identified for a variety of ks values.