Tensile Strength of Geological Discontinuities Including Incipient Bedding, Rock Joints and Mineral Veins
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Geological discontinuities have a controlling influence for many rock-engineering projects in terms of strength, deformability and permeability, but their characterisation is often very difficult. Whilst discontinuities are often modelled as lacking any strength, in many rock masses visible rock discontinuities are only incipient and have tensile strength that may approach and can even exceed that of the parent rock. This fact is of high importance for realistic rock mass characterisation but is generally ignored. It is argued that current ISRM and other standards for rock mass characterisation, as well as rock mass classification schemes such as RMR and Q, do not allow adequately for the incipient nature of many rock fractures or their geological variability and need to be revised, at least conceptually. This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents results from a laboratory test programme to quantify this parameter. Rock samples containing visible, natural incipient discontinuities including joints, bedding, and mineral veins have been tested in direct tension. It has been confirmed that such discontinuities can have high tensile strength, approaching that of the parent rock. Others are, of course, far weaker. The tested geological discontinuities all exhibited brittle failure at axial strain less than 0.5 % under direct tension conditions. Three factors contributing to the tensile strength of incipient rock discontinuities have been investigated and characterised. A distinction is made between sections of discontinuity that are only partially developed, sections of discontinuity that have been locally weathered leaving localised residual rock bridges and sections that have been ‘healed’ through secondary cementation. Tests on bedding surfaces within sandstone showed that tensile strength of adjacent incipient bedding can vary considerably. In this particular series of tests, values of tensile strength for bedding planes ranged from 32 to 88 % of the parent rock strength (intact without visible discontinuities), and this variability could be attributed to geological factors. Tests on incipient mineral veins also showed considerable scatter, the strength depending upon the geological nature of vein development as well as the presence of rock bridges. As might be anticipated, tensile strength of incipient rock joints decreases with degree of weathering as expressed in colour changes adjacent to rock bridges. Tensile strengths of rock bridges (lacking marked discolouration) were found to be similar to that of the parent rock. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and that this can best be done with reference to the tensile strength relative to that of the parent rock. It is argued that the science of rock mass characterisation may be advanced through better appreciation of geological history at a site thereby improving the process of prediction and extrapolating properties.
KeywordsGeological discontinuities Incipiency Persistence Rock bridges Uniaxial tensile strength Laboratory testing Rock mass classification Fracture development
The International Society for Rock Mechanics (ISRM 1978a) defines ‘discontinuity’ as the generic term for mechanical fractures in a rock mass that has zero or low tensile strength. It is applied to such geological features as open joints, weak bedding planes, weak schistosity planes, weakness zones and faults. In practice, the term is also often applied to any visible geological planar features (including joints and veins) that can be observed in exposures, in tunnels, in borehole walls, on photographs or measured by ground radar scans even where these features might and often do retain high strength and therefore strictly fall outside the ISRM definition of a discontinuity. In a similar way, whilst only open natural discontinuities should be counted when determining Rock Quality Designation (RQD) as defined by Deere (1968) and Deere and Deere (1989), in practice incipient discontinuities are also included in the assessment, especially where dealing with rock exposures rather than logging core. This might be a conservative approach in some situations (rock mass would be stronger and stiffer than that assumed on the basis of the assigned RQD value) but certainly would give an inaccurate representation of rock mass quality. This will of course have ‘knock-on’ consequences when using RQD as part of assessment of rock mass quality, likely behaviour, say, in tunnel stability, and in determining numerical parameters on the basis of empiricism. Where the presence of open fractures is relied upon, say for open excavation or tunnelling using a roadheader, misrepresenting incipient discontinuities as if they lacked tensile strength in site investigation descriptions, can lead to misunderstandings, poor selection of equipment and contractual disputes (Hencher 2014, 2015). A separate but important conceptual point is that incipient discontinuities will propagate and weaken over geological and engineering time as the result of processes such as unloading and weathering (Hencher 2006; Hencher and Knipe 2007); the tests reported in this paper need to be considered in this context of a developmental process for discontinuities. Any discontinuity observed in the field is at a particular stage of development and if we are to characterise such discontinuities we need to appreciate that their properties reflect long and often complex, site-specific histories.
In the view of the authors, relating the nature and development of incipient discontinuities to their geological histories is an exciting research area with major potential for impacting many rock-engineering problems. These include such diverse areas as rock slope engineering, the characterisation of large volumes of rock mass for nuclear waste disposal and for oil and gas reservoir engineering. This paper describes preliminary attempts to measure directly and to interpret the tensile strength of different types of incipient discontinuities.
2 Tensile Testing of Rock
Laboratory investigations have been carried out into the tensile strength of incipient geological discontinuities including bedding, mineral veins and joints using a uniaxial pulling arrangement (ISRM 1978b). Numerous authors have conducted uniaxial tension tests on intact rocks (Hawkes et al. 1973; Okubo and Fukui 1996; Li et al. 2013; Liu et al. 2014; Erarslan and Williams 2012). Saiang et al. (2005) carried out uniaxial tension tests on the bond between shotcrete and rock. A number of authors have tested samples incorporating incipient discontinuities in a general way. Barla and Innaurato (1973) and Dan et al. (2013) found that tensile strength decreased with the increase of the discontinuity orientation relative to the uniaxial loading direction (from 0° to 90°). Khan and Irani (1987) investigated the dynamic tensile strength of samples that were cored perpendicular to bedding planes. In their tests, the split Hopkinson bar was used to apply dynamic loads.
3 Test Set-Up
3.1 Specimen Preparation
Large rock blocks containing incipient discontinuities were collected from two locations. Samples of siltstone with incipient mineral veins were taken from Dry Rigg Quarry, Horton-in-Ribblesdale, North Yorkshire, UK. The Silurian, Horton rocks are typically medium-to-dark grey sandy siltstone with carbonaceous debris (King 1934; Arthurton et al. 1988; Aitkenhead et al. 2002). Samples of medium-grained sandstone containing incipient bedding planes and joints were collected from Blackhill Quarry, West Yorkshire, UK. The sandstone is from the Carboniferous Midgley Grit formation and ranges from fine- to very coarse-grained sandstone (Stevenson and Gaunt 1971; Waters et al. 1996).
3.2 Testing Apparatus Set-Up and Verification
Tensile strength of homogeneous sandstone samples from uniaxial direct tension and Brazilian splitting tests
Tensile strength (MPa)
Average tensile strength (MPa)
Uniaxial tension test
4 Direct Tension Tests on Incipient Discontinuities
4.1 Samples with Incipient Bedding Planes
Tensile strength of incipient bedding planes
Tensile strength (MPa)
Percentage to UTS of intact rock (%)
Average tensile strength (MPa)
4.2 Samples with Incipient Joints
Tensile strength of incipient rock joints
Tensile strength (MPa)
Percentage to UTS of intact rock (%)
UTS of rock bridges (MPa)
4.3 Samples Containing Mineral Veins
Tensile strength of incipient filled veins
Tensile strength (MPa)
Average strength (MPa)
X-ray diffraction analysis was used to identify the mineral composition of the veins. Three samples were prepared by scraping from pyritised and white vein segments, respectively, from sample DR50C1 (see Fig. 14a) and from the orange-tinted coating from sample DR70C1. Test results confirmed that calcite is the major mineral infill for all samples. The orange discolouration on the 70-mm-diameter samples is probably from iron oxide staining. The pyritised area from sample DR50C1 was confirmed as pyrite but there is also calcite and a small amount of dolomite and quartz. For samples DR50C1, DR50C2, DR50C3 and DR50C4, failure surfaces were completely coated by calcite. As discussed above, samples DR50C5 and DR70C4 had rock bridges of intact siltstone with no mineral coating. Evidently these rock bridges have survived intact and unchanged from the original propagation of a fracture around them, long ago in geological time, leaving them as remnant intact rock surrounded by the mineral infilling what have been open fracture segments. The fracturing and mineralisation probably accompanied a mountain building episode in the late Silurian to early Devonian (Soper and Dunning 2005) as evident from the observed relationships between mineralisation, folding and faulting within the quarry.
The cohesion of an incipient joint can be caused by mineral bonding over areas of distinct rock bridge and by secondary cementation (Hoek 2007; Hencher 2012). For intermediate unfilled incipient joints (the type of incipient joints tested in the homogeneous sandstone samples reported in this paper), joint planes can be broadly divided into areas that comprise discrete rock bridge segments and persistent fracture segments (persistent areas). Hencher and Richards (2015) report shear test data from an intact rock bridge, revealed after shearing an incipient joint, with a cohesive strength of around 750 kPa for the rock bridge area; persistent sections of joint contribute to shear strength but only in terms of friction and roughness interactions. Regarding the relationship between UTS and modulus of incipient joints (Fig. 15), as for mineral veins, no clear relationship can be found between these two parameters. Modulus of tested samples of incipient joints ranged between 0.5 and 5 GPa, while UTS varied between 0.5 and 2 MPa.
Different sections of incipient discontinuity in the field may have different characteristics with some areas characterised by discrete rock bridges, others with mineral infill or precipitated weathering products and some sections open and with zero tensile strength. The rock bridge sections might be essentially large and discrete pillars or might comprise more diffuse areas with multitude small zones of intact contacts. A methodology is anticipated whereby strength contributions from the different segments of the discontinuity can be used to predict the overall shear or tensile strength and stiffness of the feature. This would be supported by geological analysis as to the origin and geological history that have resulted in the discontinuity characteristics in conjunction with strength tests. This subsequently might allow some predication of overall parameters for incipient joints with the same lithology and geological history based on visual inspection. That might be optimistic but must be better than current attempts at modelling rock masses that disregard such aspects completely leaving a situation that is best described as ‘geological guesswork’.
In the tests reported here, colour variations of joint surfaces were associated with different degrees of weathering and/or mineral alteration products and hence potentially different tensile strengths. In this particular case (for the tests on joints), the strengths of rock bridges overshadowed contributions from coatings on persistent sections but there will be situations where contributions will come from various factors and sources. Procedures for back-analysing the contributions from each segment will be discussed in a separate paper.
Brzovic and Villaescusa (2007) produced a classification of mineral infill based on strength, and calcite (the major component of vein infill tested here) was graded as ‘soft’. This is supported by the results of vein tests from this paper (Table 4), which show that the tensile strength is much lower for samples with calcite veins than for the intact rock. Where veins were combined with rock bridges of intact rock then the strength was much higher and in one case apparently much higher even than might be anticipated based on the rock bridge areas (DR70C4). For this case, the rock bridge areas are approximately 1555 mm2 (about 42 % of the total cross section area). The tensile strength of these rock bridges of intact rock was calculated as 17.8 MPa assuming zero strength elsewhere. This compares to the measured Brazilian tensile strength (12.5 MPa) for the intact rock. This might reflect some internal cementation and strengthening within the rock bridge area compared to the parent rock.
Tensile strength of geological discontinuities is usually neglected or underestimated by current standards (e.g. ISRM) and rock mechanics practitioners. Incipient rock discontinuities, however, may retain intermediate or high tensile strength—approaching and even exceeding that of the parent rock and will consequently strongly influence rock-engineering performance.
In this study, a series of uniaxial tension tests were carried on incipient geological discontinuities at laboratory scale including incipient bedding (bedding laminations), mineral veins and incipient rock joints. Tensile strength of these incipient features was identified.
The direct tension test is useful to measure and research the UTS of visible and discrete incipient geological discontinuities. Incipient discontinuities can indeed have high tensile strength. In this series of experiments some discontinuities had tensile strength approaching that of the parent rock. Some are far weaker of course. The degree of development/incipiency of geological discontinuities can be expressed by their UTS relative to that of intact rock as suggested for broad characterisation by Hencher (2015).
Tests on weathered but incipient rock joints (with rock bridges) showed a clear relationship between UTS and area of rock bridges. The open, mineral coated sections of joint contributed little to the tensile strength.
For tests on calcite veins the UTS was considerably lower than that of the intact rock. Where rock bridges were present as well as mineral infill, then the rock bridge strength dominated measured strength.
Sandstone samples containing incipient geological fractures always break through these weaker planes under direct tension. Geological discontinuities tested in this paper all exhibited brittle failure after direct tension at a low axial strain below 0.5 %.
The tangent elastic modulus increases as weaker bedding planes are progressively removed, i.e. the higher strength, the higher modulus. For mineral veins and incipient joints, however, no clear relations can be established between UTS and modulus.
The China Scholarship Council and the University of Leeds provided financial support to this research. The support and practical assistance of the managers of Dry Rigg Quarry and Blackhill Quarry is gratefully acknowledged. Kirk Handley and John Martin are thanked for their support and constructive suggestions regarding the laboratory testing at the University of Leeds.
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