Elsevier

Engineering Geology

Volume 84, Issues 3–4, 16 May 2006, Pages 220-228
Engineering Geology

Fuzzy expert system for economic zonation of an ornamental slate deposit

https://doi.org/10.1016/j.enggeo.2006.02.002Get rights and content

Abstract

Investigation of deposits for traditional extraction activities (metals and coal) has generally been based on determining grade, or content, of the required material. In order to apply the grade concept to an ornamental rock such as slate, it is first necessary to define the variables that determine both the geotechnical recovery rate for the rock mass — which conditions the size of the extracted blocks – and the aesthetic features of the slate — which define the quality of the slabs as potential roofing material.

For this research, geotechnical and aesthetic data for a slate deposit were collected from 16 continuous core borehole samples. A fuzzy expert system was then developed using this data, defining the rock mass recovery rate and slab quality in accordance with the criteria of a slate expert, producing as a final output a zonation of the deposit in terms of top quality slate, medium quality slate or waste.

A mathematical model based on fuzzy logic was chosen due to the fact that the boundaries between different quality groups in a deposit are not clearly distinguished. Moreover, quality also depends on a company's infrastructures for transformation of the blocks, and also on its commercial strategies.

Introduction

The ornamental rock mining sector in Galicia (NW Spain), and particularly slate extraction for use as roofing material, has developed spectacularly in recent years. This development has led to an increasing need to update and modernise working methods, which, in many aspects, are antiquated. The perspective for the future for the sector is to produce slate slabs of maximum quality at the lowest cost possible, taking advantage of quarry reserves using non-aggressive extraction methods for the rock masses and ensuring an optimal extraction yield (García-Guinea et al., 1997).

An increasingly popular and advanced technology used in slate extraction is in-quarry mechanical cutting using diamond wire machines, ranging arm shearers or disk saws. This approach increases the deposit recovery rate but is more costly than the traditional method using explosives.

In order to optimise the profitability of the mechanical method, it is crucial to know a priori, i.e. prior to planning cutting, the intrinsic quality of the rock in the quarry benches and to evaluate the economic value of each area. Failing to detect, in advance, surface indications of discontinuities or other faults in benches may result in poor planning and affect the quality of the blocks obtained, with the consequent financial repercussions.

Quality grading of quarry benches is generally made using visual procedures, i.e. qualitative methods. To date no quantitative method is available that permits the quality of the interior of extraction benches to be calculated, with minimal or at least predictable uncertainty, from the measurement of specific parameters in the rock face.

The elaboration and practical application of a methodology that effectively resolves this question is of great interest to the slate industry in general, since it means that a quality grading system can be established for quarry benches, making it possible to plan extraction using scientific methods and thereby optimising yields and recovery and minimising the possibilities of disagreeable surprises in the bench. It would also permit the exploitation to be planned financially (Bastante et al., 2004).

No system that resolves this problem for slate quarries exists as yet. There have only been tentative efforts to assess the level of discontinuities using purely geometric methods, which, however, fail to take into account the other geotechnical and aesthetic characteristics that affect the quality of slate (García-Guinea et al., 1998).

In terms of evaluating the quality of a mineral deposit, the main difference between traditional extraction activities (metals and coal) and ornamental rock quarrying is that, for the former, a single parameter – ore grade – indicates the quality of the deposit, whereas for the latter a range of variables affect overall quality.

Section snippets

Slate grade

The 1960s and 1970s saw considerable advances in terms of mining technologies. One of the main reasons for this was a new concept, developed in the Colorado School of Mines, of mineral as a material whose exploitation generated a benefit. On the basis of this concept, scale economies were applied to mining, and it was quickly demonstrated that large mines (exploitation of large deposits with low mineral content) was more profitable than the exploitation of smaller, richer mines. These changes

Aims

Our first aim is to define each and every one of the parameters that condition both the quality of the rock mass and of the slate slabs. These parameters will be either mechanical — affecting recovery of blocks from the quarry — or aesthetic — indicating the ornamental quality of the slate that will ultimately be used in the construction industry.

Our second aim is, from the defined parameters, to create two quality indexes – geotechnical and aesthetic – indicative of the characteristics of the

Slate grade conditioning variables

We first define the variables that condition the exploitability of a slate deposit. In general terms, the most important requirement for a sufficiently fissile slate mass to be considered exploitable is for it to have satisfactory geotechnical quality, in other words, it is capable of yielding sufficiently large blocks to make the roofing slate elaboration process viable. This is a factor that affects all ornamental rocks (Taboada et al., 1997).

The variables considered are as follows:

  • RQD (Rock

Field data compilation

To characterise the quality of a slate deposit, chosen was a slate quarry located in the Truchas syncline in the Western Asturias-Leon region (NW Spain). This is an area in the Hesperic Massif of the Spanish northwest (Pérez Estaun, 1978).

The exploitable layer of slate is located within the Rozadais formation. Towards the base, the slate is dark grey in colour, is fine-grained, and contains silica, or has fine sandy laminations, pyrite cubes and good fissility. Towards the top there is a level

Design of the fuzzy expert system

An expert system is a computer program that uses knowledge on a particular domain and reasoning techniques to support activities performed by human experts. No matter what approach is to be used, the main task in the development of any expert system is to obtain and represent the specialized knowledge to be processed by this particular system. Different applications of expert systems are described in Liao (2005).

Fuzzy expert systems are assumed to behave like humans because they handle concepts

Sensitivity analysis

A sensitivity analysis was performed with a view to determining the variables with the greatest weight in determining slate quality.

The sensitivity analysis was performed using the Monte Carlo method. First of all, the theoretical distributions for each of the variables were adjusted from the observed data and the correlations between variables were determined. Next, 1000 trials were generated according to the adjusted theoretical model, for each of which the fuzzy expert system determined

Conclusions

The definition of the quality of a slate deposit depends on a range of variables and is usually assessed by experts. However, the boundaries between the three quality grades habitually employed – top, medium, and waste – cannot be defined precisely. Indeed, classifications of the same set by different experts often fail to coincide. To avoid the need for always having an expert on hand to classify slate deposits, we have designed a mathematical model based on fuzzy logic which, using variables

References (19)

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