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Comparison of the drill -and- blast and the TMB techniques for the excavation of tunnels

Souvatzi Georgia

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Summary

Τwo possible alternative tunnel driving (or boring) techniques are the sequential excavation method with drilling and blasting (D+B) with temporary support (sometimes called New Austrian Tunnelling Method, NATM) or the mechanical method with a Tunnel Boring Machine (TBM) with simultaneous load-transport of excavated rock and installation of the support. The choice of the most effective technique for the rock excavation during driving of a drift or adit or a tunnel depends on many factors like the length and cross-sectional area of the tunnel or drive, rock mass conditions and water, the cost, the required time of completion of the drive, risk of delays etc. In the present thesis these two techniques are compared in an actual case study of a planned adit of a length of 6.7 km and diameter of 5.7m of connecting the two underground mines of Olympias and Madem Lakkos in Chalkidiki (Northern Greece). Considering rock mass conditions an open-type hard rock TBM was first selected and its cutting-head equipped with disc-cutters was properly designed for the case study under consideration. The D+B technique was also designed based on rock mass quality and tunnel dimensions and shape with a drilling pattern having a cylinder cut with parallel drillholes around an emplty or dummy drillhole. Based on the rock mass and tunnel design parameters of the two alternative methods we performed a cost and performance analysis and we predicted the time of completion of the tunnel. All calculations were performed in Excel™ spreadsheets to facilitate transparent and quick computations of relevant quantities. From this comparative analysis it turned out that the D+B technique is the most time and cost effective and is characterized by the lowest risk of time delays due to its flexibility.

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