Abstract

Boreability can be defined as the ability of a bore to penetrate a rock mass. Understanding the factors influencing boreability is critical for enhanced project planning and reduce geotechnical risk in an offshore shaft boring environment. Large diameter drills are used for offshore shaft boring, which can be up to 7 m in diameter, and therefore more akin to tunnel boring machines due to the scale of the excavation and extent of ground interaction. With increases in bore diameter, there is a need to properly define and evaluate the effect of the degree of rock mass fracturing on machine performance for improved estimates of boreability. Discrete Fracture Network (DFN) simulation has been used as an innovative approach for stochastic realisation of rock mass fracturing by determination of the P32 volumetric fracture intensity in the context of boreability. P32 shows positive trend to specific penetration (SP), with maximum SP being achieved at moderate to high fracturing levels (20 – 25m−1). However, in this case, P32 shows a similar positive trend to P10, but with peak SP appearing at higher intensity levels. Increased RQD values result in reduced SP, with peak SP reached at moderate fracturing levels, similar to P10.

1. Introduction

Boreability is defined as the ability of a machine to penetrate a rock mass [1], and is commonly associated with Tunnel Boring Machines (TBM). The development of offshore shafts, Large Diameter Drills (LDD) are utilized, which feature similar tools or technology to that of oil well drilling with reverse circulation system for muck removal. The size and scale of ground interaction can be up to seven meters, making these type of LDD's more akin to tunnel boring machines due to their scale. Little research has been undertaken on the performance or boreability of LDD's when compared to evaluation of TBM behavior. Given that the underlying excavation process and ground interaction tools differ between LDD and TBM machines, TBM performance can only therefore be used as a proxy rather than direct comparison.

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