Systematic errors in the characterization of rock mass quality for tunnels : a comparative analysis between core and tunnel mapping

Abstract: This thesis analyzes the potential systematic errorin the characterization of the rock mass quality in borehole and tunnel mapping. The difference when assessing the rock mass quality refers to the fact that the characterization performed on drilled rock cores are commonly done on-meter length, while the tunnel section can be up to 20-25 m wide. At the same time, previous studies indicate that the engineering geologist tends to characterize the rock mass quality during tunnel excavation with a conservative estimation of the parameters defining the rock mass quality to ensure a sufficient rock support. In order to estimate this possible systematic error produced by the size difference when assessing the rock mass quality, a simulation was performed within a geological domain, representative of Stockholm city. In the simulation, each meter of the tunnel section was given a separate value of the rock mass quality, randomly chosen from a normal distribution representative for the studied geological domain. The minimum value was set to represent the characterized rock mass quality for that tunnel section. The results from the simulation produced a systematic error due to the difference between the geological domain, reproducing the borehole mapping, and the simulated values, representing the tunnel mapping. The results showed a systematic error in the RMR basic index around 15 points in average, which compared to the difference of 5-7 points obtained in Norrström and the Norrmalm tunnels in the Stockholm Citylink project recently constructed, are found to be excessive. However, in the simulation, it was assumed that (1) the results obtained were the same in the bore hole mapping and in the tunnel mapping, (2) with the only difference of the engineer geologist assigning to the tunnel section the lowest RMR basic value, and (3) that there was no spatial correlation between the quality RMR basic index. After analyzing the three assumptions the simulation was based upon, the absence of spatial correlation was found to be the most significative, which indicate that spatial correlation in rock mass quality needs to be included if a more correct value should be obtained.