Physical and Mechanical Characteristics of Bringelly Shale

 

Ezzat William

Department of Civil Engineering, University of Sydney,

NSW, Australia

ABSTRACT

In the Sydney metropolitan area the Wianamatta group is a major geological sequence in the Sydney Basin. It is comprised of two shales known as Ashfield shale and Bringelly shale. Both shales are comprised predominantly of claystones and siltstones with occasional sandstone layers. Bringelly shale, which is the top layer in the Wianamatta group, is highly compacted, weakly cemented, and contains significant amounts of swelling clay minerals.

Bringelly shales have very low porosities and in their fresh state high strengths, but it contains swelling clay minerals, swells and disintegrates rapidly on immersion in water and is generally less durable. With conventional drilling techniques core recovery of this shale is low and it is difficult to obtain samples for mechanical testing.

The method of sample preparation involved samples reconstituted to low and high void ratios. Shale with different degree of weathering was tested for durability and strength. Natural water content, Atterberg limits, uniaxial compressive strength, and point load index tests have been investigated. The results were then correlated to mineralogy and internal structure of the rock.

Isotropically consolidated undrained triaxial tests have been performed on specimens for a range of stress levels, over-consolidation ratios and initial porosities. The reconstituted material is seen to exhibit brittle behaviour that is not typical of normally consolidated reconstituted samples sheared from isotropic conditions. The undrained secant stiffness was found to vary with strain level and also to be independent of the consolidation pressure.

Keywords: Sydney, shale, claystone, siltatone, mineralogy

 

INTRODUCTION

Shale, in general is one of the most problematic and least understood geological materials due to the wide variation in its engineering properties. Engineers have often encountered significant problems involving shales and other argillaceous rocks.

Bringelly shale is a formation of a group called Wianamatta, a soft rock of Triassic age within a geological structure known as the Sydney Basin in New South Wales, Australia. The Wianamatta group is an abundant geologic sequence in the Sydney basin dominated by argillaceous rocks that are characterised by wide variations in their engineering properties. In this group, Bringelly shale is the more problematic engineering material because of its higher content of swelling clay minerals. However, There is relatively little data on the engineering performance of Bringelly shale, and before the current study only a few test results had been published by Won (1985). Being a main source of brick making, Bringelly shale was extensively investigated by Chesnut (1983) and Herbert (1979). The investigation has only covered the geology and mineralogy of the rock.

In the current study the performance of Bringelly shale including physical and mechanical properties of the rock is the main concern of this paper. There is relatively little data on the engineering characteristics and behaviour of this rock.

Due to the apparent weak bonding of the rock constituents, the free swell tests in water have shown a volume strain of 8% that was measured within 5 hours. The shale specimens were almost totally disintegrated within 24 hours (William & Airey, 1999a,b). As a consequence of these properties, difficulties were experienced in obtaining sufficient numbers of core samples with dimensions agree with the universal standards.

To solve this problem, the author has adopted three approaches. First to use the intact rock samples in carrying out all index tests. Secondly to perform a series of tests on reconstituted samples of crushed shale to determine lower bound values for strength and stiffness. Thirdly to explore an alternative drilling fluid that might prevent wetting and disintegration of Bringelly shale.

Hence there is no evidence of induration or bonding in the intact shale, it is believed that reconstituted samples could provide mechanical responses of normally consolidated (NC) and over-consolidated (OC) samples for a range of porosities.

This paper gives some results from the tests on intact and reconstituted shale. The objectives of these tests were to determine the physical and the mechanical properties of Bringelly shale.

Standard index tests, scanning electron microscopy and x-ray diffraction analysis were conducted and the results of selected tests are discussed in relation to mineralogy, weathering, and the internal structure of the shale.

Standard isotropically consolidated drained and undrained triaxial tests have been performed. Different techniques were used to prepare specimens with a range of porosities, so that for some tests the very low porosity of the natural shale was approached. The sample preparation method had a significant influence on the mechanical response and reasons for this are discussed.

 


Figure 1. Location map of the study area

EXPERIMENTAL PROGRAM

Specimen preparation of the rock samples, setting up laboratory tests, and adopted procedures are stated. Also, investigation of mineralogy, structure, and physical and strength properties all are described and illustrated below.

Rock and specimen preparation

Blocks and core samples of Bringelly shale were collected at depth of 7 to 10 metres from four sites located in the south west of Sydney (Figure 1). The four sites are active quarries being used to provide material for brick making. The core samples were obtained by diamond drilling below the quarry floor while block samples were obtained from ongoing excavations in the four sites.

Specimens varying from fresh to extremely weathered Bringelly shale were crushed and material passing a 0.425mm sieve was used in preparing samples. Core samples were also prepared for other index tests in accordance to ISRM procedures.

Index and physical properties

Bringelly shale is an over-consolidated, compact shale. The water contents range between 1.2 and 4.5% based on the state of weathering where water content increases with degree of weathering and this appears to be associated with increasing numbers of micro-cracks within the rock. The average unit weight is 26.5 kN/m3. The back calculation of saturation is in the range of 60 to 90%. The lower saturation value of the shale is believed to be relative to the unloading history of the formation (Crawford, 1977) and also to the fact that the water table is at a depth of about 30 m in the vicinity of the sites.

The plastic limit ranges from 15 to 22% and the liquid limit lies between 50 and 30% based on the state of weathering where higher plasticity indices were all obtained from extremely weathered shale (Figure 2). The porosity of fresh specimens ranges between 5 to 12%. The lower value may reflect the high degree of compaction that the formation has subjected to following the time of deposition.

 


Figure 2. Atterberg limits of fresh and weathered Bringelly shale

Two cycle slake durability test were performed on specimens from block of shale with different state of weathering. Test results have indicated that the effect of increasing number of cycles of wetting and drying has resulted in decreasing the percentage of the sample retained, this reduction is proportional to the degree of weathering of the specimens (Figure 3). Based on the classification by Franklin and Chandra (1972), Bringelly shale can be classified as a medium durability-low plasticity to low-durability-high plasticity shale.

 


Figure 3. Effect of 2 cycle slake-durability (%retained) on Bringelly rocks

Unconfined compressive strength and point load index tests were carried out on core samples from Bringelly shale. Measured UCS values ranged from 2.4 to 88.2 MPa, and the mean strength for the samples tested in this study was 31.2 MPa, with a standard deviation of 10.3 MPa. Samples were also tested to determine their point load index. Tests were performed on fresh core specimens in two directions parallel and perpendicular to the laminations.

The laboratory preparation and procedures were in accordance with the recommended ISRM (1985) methods. Significantly lower strengths were measured from diametral point load tests. An average anisotropy ratio of 2.5 was measured. The high strength anisotropy can be explained by the frequent micro-cracks in the plane of the laminations. These micro-cracks by in large is a result of the pre-historic deposition and subsequent stress relief.

Correlations have been made between the axial point load strength and the corresponding values measured from the uniaxial compressive strength tests. The data points (Figure 4) show a straight-line relationship with a mean correlation factor of 20.7. Regression analysis shows that the correlation coefficient is 0.95. This correlation value agrees with results obtained by Romana (1995) during his comprehensive studies on different type of sedimentary rocks in an effort to establish a common correlation factor based on the relationships between axial point load strength and unconfined compressive strength of all sedimentary rocks.

 


Figure 4. UCS versus axial point load strength index

The durability of Bringelly shale was assessed by performing a series of slake durability tests. Test results revealed that the durability of shale at different state of weathering is not constant but varies slightly from one site to another. The influence of number of cycles on the durability of Bringelly shale was investigated at different state of weathering. The durability indices of four cycles were recorded and results were summarized in Table 1. Based on these tests, durability was assessed in accordance to the classification scheme suggested by Gamble (1971) and Franklin and Chandra (1972). The result revealed that durability of Bringelly shale varies from medium for fresh intact shale to very low for extremely weathered shale.

 

Table 1. Average clay mineral composition at
different degrees of weathering in the Bringelly shale
Mineral

% clay mineral in each state

Fresh Moderate High Extreme
Kaolinite 30 30 31 33
Illite-Smectite 40 44 50 56.5
Montmorillonite - - - 2.5
Illite 18 19 16.5 14
Chlorite 6 5 2.5 -

Mineralogy and structure

Specimens varying from fresh to extremely weathered Bringelly shale were examined using x-ray diffraction and scanning electron microscopy. X-ray diffraction patterns of the less than 2 microns fraction of the four sites is dominated by the mixed-layers clay mineral (MLC) as evidenced by the altered illite peak at 10Ǻ to become symmetric and give rise to broad diffused peak between 10 and 14Ǻ. This peak profile is mainly due to irregularly mixed layers of illite and smectite. The commonest non-clay minerals present in the clayey fraction are quartz, feldspar, and siderite. Illite has a strong 10Ǻ peak and a rather weak 5Ǻ peak on the x-ray diffraction pattern.

Based on theinterpretation of x-ray diffraction patterns of the less than 2 microns fraction, mixed layers illite-smectite and kaolinite are the dominant clay minerals in the Bringelly shale, and that on average, mixed layer clay is more abundant. In order to differentiate between the first order kaolinite 7.1Ǻ and second order 7Ǻ chlorite, representative samples were x-rayed after heating to 450o­­­C. The subsequent x-ray diffraction patterns (Figure 5) has shown all peaks in the 7Ǻ area indicating that the 001 kaolinite had collapsed, and confirming the presence of chlorite in the samples of Bringelly shale.

 




Figure 5. Typical x-ray diffractogram, 2m fraction of Bringelly shale Untreated and treated samples

The illite is detrital in origin with a fragmental shape as evidenced by the polarized transmitted light microscope. In this study, interpretation of mineral abundance was carried out using quantitative analysis method adopted by Carver (1971). The results were in agreement with that obtained after running the Siroquant software program.

Mean mineral contents using these methods indicated that the clay minerals comprise 54% of the shale samples while the quartz constitutes about 37%. Clay mineral analysis has indicated that montmorillonite has formed due to the increasing degree of weathering. It also suggests that the absence of chlorite is a result of chemical and physical weathering that washed away the mineral (Table 1).

 



Figure 6. Major structure and basic mineralogy of the Bringelly shale

The microstructure and nature of the cementing material in specimen of Bringelly shale were investigated using scanning electron microscopy (SEM) and optical microscopy. Samples examined under the scanning electron microscope are presented as micrographs in Fig 6. High resolutions surface images at magnification of 400-5000x were produced via SED and BED respectively. The images represent the basic mineralogy and the major structure of Bringelly shale. The examination of the images suggests that the white spots in SE mode are mixed layer clay minerals. It also suggests the general characteristics of the rock structure is demonstrated by the presence of diagonal micro-cracks. This linear fabric’s characteristics were observed most clearly on surfaces cut normal to the lamination.

Despite the very low porosity of Bringelly shale, no evidence of induration is present. However, based on the structural analysis, it is believed that mica is the major cementing agent while carbonate compounds are a secondary cementing agent. Planar micro-cracks, associated with the horizontal particle alignment of clay particles, were particularly evident in specimens with higher porosity and in more weathered samples, and appeared to be responsible for the increase in porosity with weathering

Engineering properties

In order to investigate the strength of Bringelly shale, two approaches have been followed.

Firstly, to perform a series of tests on reconstitute samples of crushed shale to determine lower bound values for strength and stiffness, and

Secondly, to explore alternative drilling fluids that will enable core samples to be obtained for subsequent comparative tests.

Because there is little evidence of bonding in the intact shale, it is believed that the reconstituted soil could provide useful information. This can be achieved through the determination of the mechanical response of normally consolidated (NC) and over-consolidated (OC) specimens for a range of porosities. Prior to testing, different techniques were used to prepare specimens so that the very low porosity of the natural shale can be approached. A pulverised material from block samples was used in preparing the samples. The material was comprised of fines with 60% finer than 2mm. Two methods were adopted prior to triaxial testing.

A. Slurry method

Was used for relatively high porosities. The crushed shale was mixed with water to form slurry at moisture content close to the liquid limit. The mix was then placed into a brass cylinder mould of 38 diameter. The mould was then compressed one dimensionally by loads applied to a hanger to give a vertical stress  of 80 kPa. Forty-eight hours later, samples were extruded and transferred to a triaxial cell. The sample was then subjected to a saturated under 30 kPa effective stress and 500 kPa back pressure.

B. Dry pressed method

This technique was used to prepare samples with low porosity. A pre-determined mass of dry crushed shale was placed into a greased split-cylindrical steel mould where 47.5 kN axial load was required to reach a target void ratio of 0.15. The sample was then set up dry in the triaxial apparatus where effective confining stress of 30 kPa was applied. In order to saturate the sample, the drainage lines and samples were flushed with carbon dioxide and then water.

The responses in isotropic compression mode of samples produced by both methods can be showing in Figure 7. The figure shows that on removal of the axial load, the specimens expanded so that the void ratio prior to installation into the triaxial cell was 0.26. The final void ratios of the dry pressed samples are much higher than that of the slurry samples compressed isotropically to a similar void ratio. Extrapolation of the reloading response of the dry pressed samples to the NCL indicates a previous maximum stress of about 30 MPa, which agree with the pressure applied when forming the sample.

 


Figure 7. Isotropic compression of reconstituted shale

Samples were normally consolidated to a range of confining pressures from 100 kPa to 1000kPa. Deviator stress, axial strain responses and the corresponding pore pressure responses are shown in Figure 8. It is believed that samples are approaching a critical state at the peak. However, due to low void ratio and hence high stiffness, subsequent reduction of the peak is a result of non-uniform deformation in the samples. The effective stress paths of these samples (Figure 9) show that samples appear to be approaching a critical stress ratio, with slope M= 1.14, equivalent to an effective friction angle28.5o.

 


Figure 8. Typical stress, strain responses of normally consolidated material

The peak deviator stresses of over-consolidated shale samples of the two prepared types are greater than for normally consolidated samples at the same confining pressure. This was associated with a generation of negative pore pressure during shearing. In order to examine the relationship betweenand the behaviour of the material, the effective stress paths of all samples were normalized (Figure 10).

 


Figure 9. Stress, strain responses of over-consolidated samples

 


Figure 10. Normalised effective stress paths of all samples

Normalisation was based on the values ofanddetermined from the normal consolidation line (Figure 7). The critical state line is reduced to a single unique critical state point indicated by, and the normally consolidated samples describe a unique curve. The normalized effective stress paths for the slurry samples are typical of clay behaviour (e.g. Atkinson & Bransby, 1978). The over-consolidated samples reach higher stress ratios than at the critical state, and the locus of their peak strengths defines a Hvorslev surface. The failure of dry pressed samples to attain the critical state ratio suggests that the location of the critical state line in theplot is influenced by the method of sample preparation, and also that the method of normalising the results is not appropriate for all of the samples.

DISCUSSION

The lack of data on engineering behaviour of Bringelly shale was noted by Won (1985) who urged the need for more testing to provide a more rational basis for design and better correlation with engineering properties. Since 1985 there has been very little additional testing on Bringelly shale rocks. This paper summarises the new data available and attempts to develop a link between the geology and various engineering properties of the shale.

The important factors controlling the behaviour of Bringelly shale are its clay fractions and mineralogy, particularly the presence of swelling clay minerals, and its extremely low porosity. Due to its low porosity, the unconfined compressive strength has reached values greater than 80 MPa. However, despite these high strengths there is very little evidence of induration. The rapid disintegration and the very poor recovery after drilling are indications that the bonding in Bringelly shale is rather weak.

It is believed that the shale was buried to considerable depth to produce its low porosity and this is supported by geological data (Hubble, 1998) although the subsequent location of the estimated 2-3 km of overburden remains a mystery. It was important however to prepare samples under high pressures so that the engineering behaviour might be mirror the behaviour of the in situ rock.

This was investigated through the results from the dry pressed samples that do not appear to be explainable within conventional soil mechanics theories, as the responses show aspects of normally and over-consolidated behaviour. One possibility is that dry pressing the soil creates a different structure, so a different compression line is appropriate. This is contrary to Cotecchia and Chandler (2000) who argued that different structure would result in different compression line.

The greater compressibility of the dry pressed samples was evident from theplot and also from the difference in their patterns behaviour compared to those of other reconstituted soils. However, the suggested explanation for the behaviour of the dry pressed samples is speculative, and further microscopic and mechanical tests are required to understand it more completely.

CONCLUSIONS

A variety of index, mineralogical, and mechanical tests have been performed on Bringelly shale to investigate its likely behaviour as an engineering material. These test have shown that the shale can have a high compressive strength even though it has low durability and can swell significantly on immersion in water.

Bringelly shale has low porosity and has a higher clay content that contains swelling clay minerals. A correlation factor of 21.0 is indicated between the axial point load and the uniaxial compressive strength. Slurried samples showed the expected patterns of behaviour for a clayey soil, this was indicated by a well defined normal consolidation line for which similar normalised responses were obtained.

Dry pressed samples showed unexpected and curious responses. Despite the low void ratios, the specimens showed some aspects of normally consolidated behaviour. It is believed that the unloading of the sample at the end of preparation led to yielding and created a state of an inhomogeneous soil structure. However, further tests would be recommended to explain such behaviour. Dry press technique is not a preferable method for creating homogenous low porosity samples.

ACKNOWLEDMENT

J&K Pty Ltd, Coffey Partners International Pty Ltd, Douglas Partners. The assistance of A. Sikorski. Australian Research Council is also gratefully acknowledged.

REFERENCES

1.   Atkinson, J.H and Bransby, P.L., (1978). The mechanics of soils. McGraw-Hill, London

2.   Carver, R.E., editor, (1971). Procedures in Sedimentary Petrology, Wiley Inter-science, 541-69

3.   Chesnut, W.S., (1983) “Geology of the Sydney 1:100,000 sheet” New South Wales Geological Survey, Report No. 9130, pp 182-99

4.   Cotecchia, F. and Chandler, R.J. (2000) A general framework for the mechanical behaviour of clays Geotechnique 50, 4, 431-47

5.   Franklin, J.A. and Chandra, A., (1972). The slake durability test, International Journal of Rock Mechanics and Mining Science, 9, pp. 325-41.

6.   Gamble, J.C., (1971). Durability-plasticity classification of shales and other argillaceous rocks, Ph.D. thesis, University of Illinois at Urbana-champaign

7.   Herbert, C., (1979) “The geology and resource potential of the Wianamatta Group”, New South Wales Geological Survey, Bulletin 25, 203 p

8.   Hubble, T., (1998) Personal communication

9.   International Society of Rock Mechanics (1985). Suggested methods for determining water content, porosity, density, absorption and related properties.

10.   Romana M., (1995). The use of the point load test as an index for the strength of sedimentary rocks. Universidad Politecnica de Valencia, Valencia, Espana

11.   William, E. and Airey, D.W., (1999a). A Review of the Engineering properties of the Wianamatta Group hales. Proceedings of 8th Australia New Zealand Conference on Geomechanics, Hobart, 2, 641-46

12.   William, E. and Airey, D.W., (1999b). Influence of swelling Strain on Selected Engineering Properties of Bringelly Shale at South West region of Sydney. Australia. Electronic Journal of Geotechnical Engineering, Volume 4.

13.   Won, G.W., (1985). Engineering properties of Wianamatta group rocks from laboratory and in-situ tests, Engineering Geology of the Sydney Region, (Ed. P.J.N.Pells), pp, 143-61

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