A large amount of armourstone is needed for the construction of breakwaters that protect harbors. Sharp tender price and optimized design require an accurate prediction of the blast yield of quarries selected for the production of armourstone. The Blasted Block Size can only be smaller than the In-Situ Block Size. In practice, the IBS is estimated based on mean discontinuity orientation and spacing values derived from scanlines recorded manually. Instead, we propose a novel approach: the voxel method. The voxel approach uses all discontinuities visible in the rock face. The discontinuities are extracted from laser scanning of the quarry face. Blast induced fractures are discarded and discontinuities that have a similar orientation and a close spacing are merged assuming that they belong to the same discontinuity plane and were wrongly split during the segmentation process. The potential of our method is demonstrated for a quarry in Benin.

1. Introduction

Alarge amount of armourstone weighting from 500 kg to dozens of tons is needed for the construction of coastal structures that protect harbors and seawalls from waves or trap beach sand at holiday resorts. Quarries have to be selected for the production of heavy grading armourstone. By matching the expected quarry yield curve (i.e, tonnage of armourstone versus block size after blasting) to the demand for rock materials, the design of coastal structures can be optimized in order to reduce construction costs (Vrijling and Nooy van der Kolff, 1990).

Agood estimation of the distribution of large blocks in blastpiles requires a model that uses as input parameter the In-Situ (or naturally occurring) Block Size distribution (IBSD) (Latham et al., 2006). Obviously, the Blasted Block Size cannot be greater than the In-Situ Block Size (IBS).

Simple methods have been developed to estimate the IBS distribution from the statistical analysis of scan line data recorded on3Dsurface exposures(Wang et al., 1990) or 1D borehole logging data (Palmström, 2001 and Latham et al., 2006). These methods produce a rough estimation of the IBSD as they oversimply the rock mass by averaging spacing and orientation of discontinuities and assuming not more than three discontinuity sets or systematic jointing. The dissection method (Wang et al., 1991) reproduces the blocky rock mass without having to simulate the statistics of the discontinuity sets. It works with the intercept and orientation of discontinuities measured along directions that sample properly the rock mass structure. Thanks to its deterministic nature, the dissection method performs better than the Wang's equation or the Palmström and Latham's method. In this paper, we present a novel approach to estimate the IBS distribution that shares with the dissection method this advantage. Contrary to the dissection method, our method uses all discontinuities visible in the rock face.

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