Application of Measurement While Drilling Technology to Predict Rock Mass Quality and Rock Support for Tunnelling

Håkan Schunnesson,Jeroen van Eldert,David Saiang,Daniel Johansson

doi:10.1007/s00603-019-01979-2

Håkan Schunnesson, Jeroen van Eldert + Show 2 more

Open Access

https://doi.org/10.1007/s00603-019-01979-2

Copy DOI

Abstract

A tunnelling project is normally initiated with a site investigation to determine the in situ rock mass conditions and to generate the basis for the tunnel design and rock support. However, since site investigations often are based on limited information (surface mapping, geophysical profiles, few bore holes, etc.), the estimation of the rock mass conditions may contain inaccuracies, resulting in underestimating the required rock support. The study hypothesised that these inaccuracies could be reduced using Measurement While Drilling (MWD) technology to assist in the decision-making process. A case study of two tunnels in the Stockholm bypass found the rock mass quality was severely overestimated by the site investigation; more than 45% of the investigated sections had a lower rock mass quality than expected. MWD data were recorded in 25 m grout holes and 6 m blast holes. The MWD data were normalised so that the long grout holes with larger hole diameters and the shorter blast holes with smaller hole diameters gave similar results. With normalised MWD data, it was possible to mimic the tunnel contour mapping; results showed good correlation with mapped Q-value and installed rock support. MWD technology can improve the accuracy of forecasting the rock mass ahead of the face. It can bridge the information gap between the early, somewhat uncertain geotechnical site investigation and the geological mapping done after excavation to optimise rock support.

Highlights

Before a tunnelling project starts, a site investigation is performed to determine the in situ rock mass conditions
This paper investigates the quality and usability of Measurement While Drilling (MWD) data in tunnelling and suggests an approach to incorporate the use of MWD technology into the rock support design process in the production phase of a tunnelling project to improve time and cost efficiency
A number of rock mass classification systems have been developed over the years; of these, rock mass rating (RMR) (Bieniawski 1973) and tunnelling quality index or the Q-system (Barton et al 1974) are the most commonly used

Summary

Introduction

Before a tunnelling project starts, a site investigation is performed to determine the in situ rock mass conditions. Site investigations use rock mass classification systems such as those introduced by Barton et al (1974), Bieniawski (1973) and Hoek and Brown (1997). Since these systems often have limited availability of information (surface mapping, geophysical profiles, limited bore holes, etc.), the rock mass classification may be inaccurate, leading to increased excavation time and costs (Wahlström 1964; U.S National Committee on Tunneling Technology 1984; Kjellström 2015). Scandinavian hard rock tunnelling is most commonly performed by conventional drill and blast excavation This method ensures flexibility in different tunnel cross sections and has the possibility to be adapted to changing rock mass conditions. The as-designed support systems are validated against the observations or in some cases the

Methods

Results

Conclusion