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

doi:10.1016/j.tust.2014.07.009

Tunnelling and Underground Space Technology

Volume 44, September 2014, Pages 108-120

https://doi.org/10.1016/j.tust.2014.07.009 Get rights and content

Highlights

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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.
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The present paper validates the influence of rock mass fracturing on net penetration rates for hard rock TBMs.
•
The rock mass fracturing factor (k_s) is shown to be a good indicator of rock mass boreability in hard rock tunnelling.

Abstract

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 (k_s) 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 (k_s) have been identified for a variety of k_s values.

Introduction

The prediction of construction time and costs is important in the selection of excavation method and the planning and risk management of tunnelling projects. Good prediction facilitates the control of risk and enables delays and budget overruns to be avoided.

In the case of hard rock TBM projects, net penetration rate (m/h) is an important factor used to estimate advance rates, cutter life and excavation costs. The main influences on net penetration rate are intact rock properties, rock mass properties, and machine parameters.

Many studies have examined the influence of geological parameters on tunnel boring machine (TBM) performances (Wanner and Aeberli, 1979, Howarth, 1981, Lindqvist and Lai, 1983, Sanio, 1985, Zhao et al., 2007, Bruland, 1998, Barton, 2000 Ribacchi and Lembo-Fazio, 2005, Gong et al., 2005, Gong et al., 2006, Bieniawski von Preinl et al., 2006 Yagiz, 2009, Yagiz et al., 2010, Hassanpour et al., 2009, Hassanpour et al., 2010, Hassanpour et al., 2011, Bejari and Khademi, 2012, Farrokh et al., 2012.

Wanner and Aeberli (1979) considered in their studies the joint frequency which is determined by the total joint area per unit volume of excavated rock. It was found by field observation that only joints produced by shear stresses influence significantly the specific TBM penetration. Howarth (1981) performed an experimental work concluding that moderately fractured rock can improve the TBM performances. Lindqvist and Lai (1983) indicated by laboratory experiments the influence of the intact rock properties. Sanio et al. (1985) considered and studied the importance of the anisotropy in addition to the discontinuities. Zhao et al. (2007) developed an ensemble neural network to establish a relationship between the specific rock mass boreability index (SRMBI) and four influential rock mass properties: rock compressive strength, rock brittleness, joint spacing and joint orientation. Ribacchi and Lembo-Fazio (2005) performed an analysis of excavation data from a TBM project in a gneiss formation. It was found out the influence of rock quality on TBM performances. It is indicated that for a given type of rock, fracture spacing has a predominant influence. Gong et al., 2005, Gong et al., 2006 studied numerical modelling of joint spacing and orientation getting good agreement with field investigation in the results.

The influence of both, intact rock properties and rock mass parameters, have a great importance on the TBM prediction models developed by Bruland, 1998, Barton, 2000, Bieniawski von Preinl et al., 2006 and Delisio et al. (2013). Hassanpour et al., 2010, Hassanpour et al., 2011 and Farrokh et al. (2012) analysed some TBM prediction models in several TBM projects considering the rock geological influence on the performance predictions.

The main purpose of the present research is to find an appropriate tool to enable an assessment of the influence of the rock mass conditions on the penetration rate of hard rock TBMs.

As part of the NTNU prediction model for hard rock TBMs, the rock mass fracturing factor (k_s) has been developed as a means of representing the simultaneous effect of the degree of fracturing (fracture type and spacing) and the angle between the tunnel axis and the rock mass’ planes of weakness using a single coefficient. The rock mass fracturing factor is the most important rock mass parameter currently used in tunnel boring (Bruland, 1998).

The rock mass fracturing factor (k_s) represents rock mass boreability. Higher values imply greater boreability during hard rock TBM excavation. Throughout this paper, the term rock mass boreability refers to rock mass resistance to boring, while the term drillability, or intact rock boreability, refers to intact rock resistance to boring (as tested at the laboratory).

In order to assess rock mass fracturing (k_s) as an appropriate tool for the empirical evaluation of the net penetration rate of hard rock TBMs, field work involving geological engineering back-mapping and TBM log data analysis, recorded along selected sections amounting to a total length of 1200 m, was carried out. The EIDI2 Tunnel project in Faroe Islands was chosen for this study, since it provides satisfactory research conditions.

The present research validates the influence of rock mass fracturing, as expressed by the fracturing factor (k_s), on penetration rates for hard rock TBMs. The fracturing factor (k_s) is shown to be an ideal tool for evaluating the influence of rock mass fracturing, and is thus a good indicator of rock mass boreability during hard rock TBM tunnelling.

Section snippets

Rock mass boreability

Boreability can be defined as the resistance (in terms of ease or difficulty) encountered by a TBM as it penetrates a rock mass composed of intact rock and planes of weakness (Bruland, 1998).

The penetration rate is influenced by intact rock and the properties of the rock mass. Intact rock properties are typically defined in terms of rock petrography, abrasivity, porosity, schistosity and strength.

The NTNU/SINTEF laboratory method is used extensively in major international TBM projects to

A practical approach to geological back-mapping for hard rock TBMs

The engineering geological back-mapping of a TBM-bored tunnel consists of the following steps:

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Determination of rock type.
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Identification of the strike and dip of Marked Single Joints.
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Notes on other singular phenomena such as intrusions, mixed face, water, and rock support.
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Determination of the number of fracture systems and type of fracturing (joints or fissures) for each system.
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Measurement of the strike (α_s) and dip (α_f) of the fracture system(s).
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Measurement of the strike (azimuth) of the tunnel

Field data

A major part of the research presented in this paper is TBM performance data analysis, together with geological back-mapping. The geological back-mapping was carried out along 240 sections of the Eidi II Tunnel. The total length of tunnel from which data were obtained is 1200 m.

TBM performance studies have many purposes. These include the improvement of on-site tunnel boring operations, feedback to the planning process and, in some cases, knowledge and expertise enhancement promoting the

Analysis and discussion of the field data

The present study shows a clear relationship between the fracture factor (k_s) and net penetration rate to the extent that high k_s values indicate a higher penetration rate. It is important here to distinguish the section where mixed face (sill and basalt) conditions were encountered, i.e., along the previously described first 270 m of the tunnel length.

The relationship between the fracture factor (k_s) and net penetration rate is shown in Fig. 17. High fracture factor (k_s) values indicate easier

Conclusions

This present research has analysed the influence of rock mass fracturing on net penetration rate for hard rock TBMs. Higher degrees of fracturing result in higher net penetration rates.

According to the NTNU prediction model for hard rock TBMs, the fracturing factor (k_s) is shown to be an adequate expression of rock mass fracturing and rock mass boreability. The field penetration index (FPI) also exhibits good correlations with the fracturing factor.

Geological mapping along short sections fails

Acknowledgements

The authors would like to thank the research project “Future Advanced Steel Technology for Tunnelling” (FAST-Tunn). This project is managed by SINTEF/NTNU and funded by the Research Council of Norway, the Robbins Company, BASF Construction Chemicals, the Norwegian Railroad Authorities, Scana Steel Stavanger and BMS steel. We also extend our thanks to Jacob Clemmensen at MT Höjgaard in Denmark, and former M.Sc. student Anders Palm for assisting in arranging the field work and sharing data.

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

Highlights

Abstract

Introduction

Section snippets

Rock mass boreability

A practical approach to geological back-mapping for hard rock TBMs

Field data

Analysis and discussion of the field data

Conclusions

Acknowledgements

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

TBM Tunnelling in Jointed and Fault Rock

Simultaneous effects of joints spacing and orientation on TBM cutting efficiency in jointed rock masses

Rock Mech. Rock Eng.