1 INTRODUCTION

1.1 Definition of the term ‘quarry’

“Quarry: An open air excavation from which stone for building or other purposes is obtained by cutting, blasting, or the like.” — Oxford English Dictionary.

There is sometimes a problem concerning the ex-act meanings of the words ‘quarry’ and ‘mine’, i.e. whether the underground extension of a quarry is still termed a quarry or whether such an extension should be called a mine. In the literature, both terms are used, and so there is no need to be pedantic about the precise usage. But note that this paper relates to the planning and operation of surface and under-ground quarries, but not to remediation of disused quarries.

1.2 The purpose and content of this paper

When designing any rock engineering project, we have to be concerned about risk: we need to be able to predict the future when designing a facility but the reality related to the design assumptions may be dif-ferent—and hence the risk. For quarry design there are many issues, ranging from quality of the stone, through excavation techniques, transport systems, safety and financial aspects to environmental issues.

In this paper, the emphasis is on the significant influence of geological factors on quarry design. The most fundamental geological consideration is wheth-er the rock is suitable for its intended purpose and whether there is sufficient grade of this rock type to enable extraction over the required life of the quarry. But there are many other geological and related rock mechanics aspects, such as the influences of

intact rock, rock fractures, in situ rock stress, and water flow through rock fractures.

These geological factors are reviewed and suita-ble approaches to reduce the related risks are rec-ommended. The awareness of these factors and the implementation of appropriate extraction procedures to avoid or at least mitigate their effects will enhance quarrying operations and have associated financial benefits.

1.3 The context for the paper

This symposium, the 1st International Conference on

Applied Empirical Design Methods in Mining, is an ISRM Specialised Conference organized by Dr An-tonio Samaniego through the ISRM Design Method-ology Commission. In the ISRM Presidential period 2007–2011, the Design Methodology Commission’s work was concentrated on rock engineering design and construction and how these aspects could be au-dited through the use of Protocol Sheets (Feng & Hudson, 2011). In the current Presidential period 2011–2015, the Design Methodology Commission’s work is focused on rock engineering risk and hence the emphasis is on the risk elements occurring in de-sign and construction. One of these is awareness of the many geological issues that can cause risks in design, whether the project is surface based, under-ground based, or both.

So, although there is a wide spectrum of potential geological influences, it is beneficial here to concen-trate on the four geology-related aspects of intact

The influence of geological factors on quarry design J.A. Hudson Department of Earth Science and Engineering, Imperial College London, UK

ABSTRACT: Geological factors on both the small and large scales are of utmost importance in quarry design: in particular, the intact rock anisotropy, the rock fractures, the in situ rock stress, and the hydrogeolog-ical regime can have major impacts. Examples of the influences of these factors on quarry geometry are pre-sented, including the effect of microstructural anisotropy in granite, sliding on pre-existing joints and faults, and the development of exfoliation joints during quarrying. Knowledge of these factors and appropriate ad-justment of the quarry design to account for them leads to significant safety and financial benefits.

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rock, fractures, stress and water—because these gov-ern the majority of risks.

2 INTACT ROCK MECHANICAL PROPERTIES

The nature and suitability of the intact rock for its in-tended purpose will have been established when the quarry was first envisaged and it may not appear at first sight that that mechanical properties have any significance in relation to the overall geometry of the quarry. However, note the quarry geometry in Figure 1.

Figure 1. Influence of the microscopic rock anisotropy on the macroscopic granite quarry geometry, i.e. walls at right angles.

Figure 2. Illustration of the grain, rift and hardway planes in granite. This diagram is from Fujii et al. (2007) and also shows orthogonal cores taken for anisotropy testing.

Granite tends to have a 3-D strength anisotropy, characterised by the rift, grain and hardway planes (the hardway being the strongest) as illustrated in Figure 2. This means that excavation is optimised if the procedure takes advantage of the anisotropy.

The excavation method used at the Grabinex granite quarry in Strzegom, Poland, is shown in Fig-

ure 3. Closely spaced small diameter boreholes are drilled in the vertical rift plane and the horizontal grain plane. The vertical side planes of the block are first released using flame cutting (Figures 4–5) and then detonating cord is used to blast the rift and grain planes. The high velocity detonating cord is a thin, flexible plastic tube filled with PETN, i.e. pen-taerythritol tetranitrate.

Figure 3. The mining technique involves creating flame-cut re-lease slots in the vertical hardway plane at each end of the block and then blasting closely spaced boreholes drilled in the vertical rift and horizontal grain planes.

Figure 4. Flame cut granite block surface following the extrac-tion method illustrated in Figure 3.

Thus, in this granite quarry case example, the re-alisation that the intact rock has a threeway ortho-tropic micro-anisotropy and the utilisation of this feature have not only helped significantly in enabling easier extraction but also determined the overall ge-ometry of the quarry.

Figure 5. Removal of granite blocks following the excavation procedure illustrated in Figure 3.

3 ROCK FRACTURES

3.1 The nature of rock fractures

Rock masses contain pre-existing fractures: joints (which have been formed by tensile stress) and faults (which have been formed by shear stress). Hydraulic pressure may also have played a role in the for-mation of fractures. Additionally, the fractures have usually been formed during several episodes in geo-logical time. In particular, later fractures will tend to terminate at older fractures, see Figure 6. This means that the fractures are generally not randomly located and orientated (although the assumption of random-ness can be a useful in some types of modelling).

Figure 6. Carboniferous limestone strata (South Wales, UK) illustrating how four successive fracturing events (indicated by the white numbers) have created this fracture pattern. The long-er more significant fractures tend to occur first.

The influence of fractures can be significant in

terms of quarry design—because of bench and larger scale surface instabilities induced by the fractures: plane sliding, wedge sliding and toppling. Such

instabilities have been studied from the early days of rock mechanics and rock engineering, but there has been insufficient emphasis on linking the existence of the fractures with the geological setting.

In Figure 6, it can be seem that the earlier fracture sets (labelled 1 and 2) are more dominant and split the rock mass into blocks, which in turn are split fur-ther by the later fracturing (labelled 3 and 4). How-ever, a three dimensional fracture characterisation is required, in this case via the bedding. Figure 7 indi-cates how the dominant fractures may have been weakened by the long term action of flowing water.

Figure 7. Limestone strata in the UK illustrating the presence of both bedding planes and large fractures, the surfaces of which been affected by water flowing through the fractures.

However, the dolomitic joint trace pattern shown

in Figure 8 shows that the situation may well be more complex and the exact distribution of rock blocks difficult to predict. To what extent it is neces-sary to predict the discrete nature of the rock mass will depend on the purpose for which the rock is re-quired. In the case of a quarry for aggregate material, any pre-existing fracturing can be regarded as bene-ficial, assuming that it does not lead to instability problems; but the requirements in a quarry for build-ing stone are for essentially unfractured rock blocks.

If the geometries of the fracture sets are known, these types of failure can be minimised by suitable adjustment of the overall quarry plan. It is rarely that an exactly circular quarry plan is essential: so chang-ing to an elliptical quarry plan with a suitable orien-tation of the long and short axes of the ellipse can have a dramatic effect in reducing the occurrences of plane, sliding and toppling failures. Of course, this can only be done if the fractures have been mapped.

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Figure 8. Joint trace pattern (on horizontal surface plane) in the Niagara dolomite at Lannon, Wisconsin, USA, from La Pointe & Hudson (1985). The circle of dots was used for measurement purposes and has a diameter of 13.5 m.

Figure 9. Example of a large shear surface which has caused plane sliding at a granodiorite quarry in the UK.

Examples of the effect of the presence of rock

fractures are illustrated in Figures 9–11. Bench and even multi-bench failures can be tolerated in many cases but major slides in deep quarries can be disas-trous. However, these are caused by large frac-tures—which are easier to detect during the geologi-cal mapping. Considering the relatively low cost of geological mapping, it is well worth the investment to avoid the risk of unexpected rock falling, sliding and toppling events.

Figure 10. Example of two fracture surfaces which have caused wedge sliding at a granodiorite quarry in the UK.

Figure 11. Sub-vertical fractures which can cause toppling fail-ure at a granodiorite quarry in the UK.

In some cases, pervasive pre-existing rock frac-tures can have a significant effect on the quarrying of rock for sculptural and building purposes, see Figure

12 where rock fractures in marble (dipping towards the observer) have caused failure at the apices of the blocks being quarried.

Figure 12. Marble quarry in Portugal where pre-existing frac-tures have interfered with the block quarrying technique.

3.2 Exfoliation fractures

This type of fracture is created when the magnitude of the vertical pre-existing stress component is re-duced due to removal of the overlying rock, either through natural geological processes or anthropogen-ically, i.e. quarrying. In Figure 13, horizontal natural exfoliation fractures are clearly visible in the granitic rock mass and can be used as the bench surfaces. In Figure 14, the intersection of a major fault with the exfoliation joints is illustrated.

Figure 13. Horizontal exfoliation fractures in a granitic rock mass at the Grabinex quarry in Strzegom, Poland.

The combination of large (Figure 14) and small (Figure 15) faults intersecting the exfoliation frac-tures can cause a problem when large bock extrac-tion is required.

Figure 14. Major weathered sub-vertical fracture intersecting sub-horizontal exfoliation fractures in a granitic rock mass at the Grabinex quarry in Strzegom, Poland.

Figure 15. Exfoliation fractures and jointing/faulting in a gra-nitic rock mass at the Grabinex quarry in Strzegom, Poland.

4 IN SITU ROCK STRESS

4.1 Surface quarries

The stress in a rock mass is characterized by the three orthogonal principal stress components: the major, minor and intermediate components. Gener-ally, these components are orientated horizontally and vertically. The primary causes of the natural rock stress are the weight of the overburden and the effect of tectonic plate movement. In the majority of

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cases and perhaps counter-intuitively, one of the hor-izontal stress components has the greatest value.

This pre-existing, naturally occurring, stress field may have little effect on the quarrying operations, but sometimes it can have a significant effects, e.g. when buckling occurs on the quarry surfaces, i.e., breakage as the surface strata are removed and hence the overburden load is removed, Figure 16. This phenomenon is discussed, for example, in Steck (1999) and Damjanac & Fairhurst (2009), the former of whom states that, “Quarry floor buckles are con-sidered to be modern features, caused by the relief of confining pressure by the removal of overlying lay-ers of rock/overburden. Pop-ups, on the other hand, are generally considered to be older features, related to the removal of glacial ice loads.”

‘Buckling’ of surface slabs at a quarry floor is

caused by pre-existing in situ horizontal stress

after the overlying rock has been removed

and the vertical stress reduced to a zero value

Figure 16. When a new quarry floor is exposed and the over-burden load thus removed, a pre-existing horizontal stress can cause the surface slab to buckle.

4.2 Underground quarries

Figure 17. Underground rooms at a marble quarry in Portugal.

In the case of underground quarries such as that

illustrated in Figure 17, the effect of the in situ rock stress is the same as that in other underground open-ings, i.e. rock spalling when the concentrated rock stress around the opening reaches the in situ rock strength—and rockbursts when the pre-existing rock stresses have high values.

An example of the latter is the rockburst occur-rence at the Carrara marble quarries in Italy (Coli et al., 2010). In a similar way to the anisotropic granite which has the grain, rift and hardway as described in

Section 2, due to the Carrara marble’s tectono-metamorphic deformation history, it also has three principal planes of weakness: the verso, secondo and contro planes. Additionally, in some locations, the marble contains a high in situ stress field, especially at the Carlone quarry (Coli et al., 2010). The authors note that, “There are two types of rockburst, i.e. de-lay rockburst and instantaneous rockburst. The rock-burst which occurred in Carrara marble laboratory samples belongs to the delay rockburst correspond-ing to the stress concentration after excavation in the field. The laboratory test shows a critical stress of σmax= 50 MPa for the rockburst of Carrara marble.”

Figure 18. The Carlone quarry in Italy: (from Coli et al., 2010).

Figure 19. Rockburst during development work for the under-ground Carlone quarry in the Carrara marble, Italy, from Coli et al. (2010).

The topographic setting of the Carlone quarry is

shown in Figure 18 and the aftermath of a rockburst shown in Figure 19. In the case of underground quar-ries, where there is usually some flexibility in devel-oping the geometry of the openings and in the case of a high stress field in the rock mass, the potential for such rockbursts can be significantly alleviated by aligning the tunnels and caverns parallel to the direc-tion of the maximum principal stress component.

This procedure reduces the magnitude of the stress concentrations and hence the severity of rockbursts. Note, however, that such alignment for stress reduc-tion may conflict with the optimal alignment for block extraction, given the micro-anisotropy of the rock.

5 THE PRESENCE OF WATER

When an excavation is made in a rock mass, there are three primary effects: the rock moves inwards slightly because a cavity

has been created; this movement can be small if it is simply elastic unloading, or it can be large if rock blocks are released as has been illustrated in Figures 9 & 10;

the in situ stress field is considerably altered be-cause the newly excavated surfaces become prin-cipal stress planes (because these free surfaces do not sustain shear stresses); and

the excavation, being at atmospheric pressure, be-comes a sink for water.

From the last point above, the process of quarry-ing, i.e. creating excavations in a rock mass, will in-crease any water flow into the excavations. It is now possible to use computer models to simulate such flow (e.g. Zhang & Sanderson, 2002; Franciss, 2010), although this is only possible if the fracture systems in the rock mass can be adequately charac-terised.

A small-scale hydrogeological study was con-ducted by Lemieux (2006) in a fractured carbonate rock aquifer located in a quarry to relate groundwa-ter flow to the fracture network. The authors state that, “The field study in the St-Eustache quarry, which integrates structural surveys, well logging and hydraulic tests, showed that the most important fea-tures that affect groundwater flow in the sedimentary aquifer are high hydraulic conductivity horizontal bedding planes. Vertical fractures are abundant in the quarry and throughout the region, but they have a minor effect on groundwater flow.”

Horizontal flow in sedimentary rocks is likely to dominate, but the flow in igneous and more com-plexly fractured rocks is more difficult to model and predict. However, for both new and existing quar-ries, the geological information on the nature and occurrence of the fracture system is crucial for such analyses.

In a karst region, the prediction of water flow can become further complicated by the presence of ir-regular, large flow channels, although the under-standing of karst geology is now much improved. A recent book “Karst in China” by Lu (2012) provides a complete overview of the development and proper-ties of karst geology, plus defensive strategies when engineering on and in karst geology.

6 CONCLUSIONS

The geological factors discussed in relation to quarry design have been intact rock properties, rock fractures, in situ rock stress, and the presence of water.

The implications for quarry design are straight-forward, although the implementation of these will depend on whether the quarry is for aggregate mate-rial or for building stone.

For a quarry excavating building stone, anisot-ropy in the intact rock can have a major influence on the quarry layout and excavation techniques, as was illustrated earlier in the paper for the Grabinex quar-ry case example. However, for an aggregate quarry, any anisotropy in the intact rock is unlikely to be of major concern.

The potential for rock fractures to enable planar and wedge sliding or toppling to occur is important for bench and overall quarry stability. Once these possibilities have been evaluated, the plan shape of the quarry can be optimised to reduce major intabili-ties by adopting a defensive layout—in particular by using an elliptical plan shape, rather than a circular one. A circular quarry will have quarry faces in all directions; however, an elliptical quarry can reduce the instabilities by arranging for the worst potential cases to be located at the regions of greatest curva-ture, hence minimising their occurrence.

If the quarry is located in a region of high rock stress, a surface quarry can suffer from surface strata buckling (Figure 16), but this is a rather rare phe-nomenon. On the other hand, the rooms in an under-ground quarry can suffer from spalling and even rockbursts, as has occurred at the Cararra quarry in Italy. As a general principle, it is best to orientate the long axis of underground excavations parallel to the direction of maximum principal stress, and hence re-duce the stress concentrations around the openings.

The presence of water can be difficult in some quarry situations necessitating continuous pumping, but establishing the presence of any major water bearing fractures in the rock mass beforehand may enable the overall quarry geometry to be designed together with grouting of the major fractures to min-imise the water inflow.

All these factors are readily determinable from a geological study of a proposed quarry site and the consequential defensive strategies are straightfor-ward. It is therefore strongly recommended that the geological factors are established and the associated quarry design implemented to reduce the possibili-ties of major quarry problems. The use of the geo-logical information leads to good engineering prac-tice and the associated economic benefits,

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Acknowledgements

The author is very grateful to the Grabinex company

and especially the President, Ryszard Chęciński, the

Mine Manager, Mr Stanislaw Paluch, and Translator

Mr Janusz Kurc for their kindness in explaining the

Grabinex granite quarry operation in Poland and for

allowing photographs to be taken at the site.

REFERENCES

Coli, M., Livi, E., Berry, P., Bandini, A., He, M. & Jia, X. 2010. Rockburst prediction in the Carrara marble quarry, It-aly. International Society for Rock Mechanics News Jour-nal: 51–54.

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