An Evaluation of the Role of Fabric on Stiffness and Shear Strength of Bringelly Shale
Department of Civil Engineering, The University of Sydney, Australia
Email: e.williams@civil.usyd.edu.au
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An Evaluation of the Role of Fabric on Stiffness and Shear Strength of Bringelly Shale
Department of Civil Engineering, The University of Sydney, Australia |
ABSTRACT
A series of stress paths tests are carried out for natural clay shale specimens from a geological formation in the Sydney basin, the specimens are reloaded to a given stress levels. The specimens were then subjected to a constant shear strain until approaching failure. The test results of the natural shale was compared to reconstituted shale compressed to different porosities. The test results indicate that there is a good hyperbolic connection between the mean normal stress and void ratio beyond a given effective confining stress. Data also indicates that at stresses < 1000kPa, the ultimate strength parameters approach residual strengths. At higher stress levels, the critical state friction angle is reduced awing to the fabric created by the high stress which in turns led to a reduction in the strength. To investigate the effects of cementation and de-structuring the behaviour of the reconstituted and natural material have been compared. It is found that the strengths of the natural and reconstituted specimens (at the same void ratio) are similar with both showing friction angles significantly less than the reconstituted material at higher void ratio.
This paper will present some results from the reconstituted material showing the important influence of porosity, and hence confining stress level, on the observed frictional response. Data from the natural samples will then be compared with the reconstituted samples to show the relatively minor influence of structure, which is consistent with micro-structural observations.
Keywords: Keywords: clay shale, stress paths tests, fabric, stiffness, shear strength
INTRODUCTION
As the urban sprawl of the Sydney metropolitan area reaches westwards, most of the residential, commercial and industrial development is taking place on land underlain by a major geological formation called Bringelly shale. This formation has long been known as a Wianamatta shale and often thought to be similar to its neighbouring formation, Ashfield shale. Recent study has confirmed that there are significant differences in the physical and mechanical properties of these two members (William & Airey, 2004). Bringelly shale which is a complex formation composed of different lithologies comprise claystone-siltstone (70%), laminite and sandstone (25%), coal and highly carbonaceous claystone (3%), and tuff (2%). The claystone units are a combination of several types of fine-grained sediments, namely light-grey leached claystone, dark-grey to black carbonaceous claystone and non-carbonaceous mid to dark-grey claystone and siltstone. The different sediments believed to reflect the alluvial flood basin environment where the shale probably was deposited.
There is considerable debate about the post-depositional history of the shales in the Sydney Basin, with estimates for the depth of over-lying sediments ranging from tens of metres to 4 km (Moore et al.,1986; Stewart & Alder, 1995; Bai et al, 2001; Keen, 2001). There is little information on the engineering geology of the Bringelly Shale (Won, 1985, Chesnut, 1991). More recently, an extensive study on Bringelly shale has revealed data involving mineralogy, durability, and strength characteristics that control the engineering behaviour of this material (William and Airey, 1999a, 1999b; William et al., 2001; William and Airey, 2004 ).
This paper will discuss basic laboratory characterization studies of the shale performed at Sydney University over the last 8 years with special emphasis on the stress paths behaviour of the material. Data will be provided on the mineralogy, micro-structure, durability, swelling, stiffness and strength of the shale. This study has been limited to claystone-siltstone sediments which are the predominant lithology in this shale. Large block samples and cores have been obtained from several locations represents Bringelly shale formation which has an approximate area of 750km2.
These specimens have been obtained from quarries used to extract shale for brick manufacture at Kemps Creek (k), Badgerys Creek (B), Horsley Park (H), and Mulgoa (M). The shale from all these sites could be described as a
non-carbonaceous mid to dark-grey claystone. Established data base on Bringelly shale has been widened by information supplied by several other industrial organizations.
Figure 1. Location map showing different sites
MATERIAL PROPERTIES
It has been found that the properties of the claystone/siltstone are similar at all 4 locations. The grain size distribution resulted from the crushed shale is shown in Figure 2.
Figure 2. Grain-size curve of tested material
The figure shows 55%, 41%, and 45 of clay, silt, and sand respectively. The material has a low plasticity index of 10 and relatively high liquid limit of 33. The material also shows high clay contents of 51.5% and a non-clay minerals vary from 38% for quartz, 3% siderite, 6% feldspar, and 1.5% organic matter. X-ray diffraction was used to explore the clay mineralogy of the shale at different degrees of weathering. Very similar results were obtained from all specimens and all sites. Only average values are shown in Table 1.
Table 1. Percentage of clay mineral species
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| Clay minerals | Fresh | Extremely weathered |
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| Kaolin | 33 | 30 |
| Illite-Smectite | 40 | 55 |
| Montmorillonite | - | 2.5 |
| Illite | 21 | 12.5 |
| Chlorite | 6 | - |
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Mixed-layer clays have properties intermediate between illite and smectite and thus the shale can be expected to be reactive, susceptible to swelling and changes in pore fluid chemistry.
STRESS PATHS BEHAVIOUR
In this investigation, triaxial tests were used to obtain data for peak shear strength parameters. Because it was difficult to subject the material to sufficient displacement on the shear surface, shear box and ring shear equipment were used to acquire data for residual shear strengths. An estimate of the stress required to produce the determined porosity of the natural shale was obtained by reconstituted the material to create a slurry. The slurry was then subjected to a range of stress levels from 100 to 60,000kPa while the natural shale was subjected to reloading from 20 to 60,000kPa.
The isotropic compression response of reconstituted and natural shale is shown in Figure 3.
Figure 3. Isotropic compression of shale results
The figure shows that at a mean effective stress less than 10,000kPa, the INCL can be represented by the equation
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(1) |
with the parameters l=0.07 and N =0.85.
For mean effective stresses higher than 10,000 kPa, a hyperbolic function was derived to give the best fitting for INCL and given by:
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(2) |
where A and B are constants that can be chosen so that the INCL is continuous with no change of slope at p’=10,000 kPa. The INCL and the reloaded response of natural shale indicate that an effective stress of about 60MPa is required to produce a porosity of 10%, similar to the natural material.
In order to determine the strength envelopes of the material, a series of standard drained and undrained triaxial tests were performed on saturated specimens with over-consolidated ratios up to 10 with effective stresses of ( 1000kPa. Another series of shear box and ring shear was also performed to determine the residual strength of the material. The triaxial test results revealed an ultimate friction angle of 28.50 and a normalised response consistent with many other reconstituted materials (Figure 4).
Figure 4. Stress-paths for reconstituted shale at low stress level
The ring shear and shear box test results have shown a residual strength equivalent to that of the peak strength. However, when the same material was saturated and tested drained at high stress levels a significant reduction in the peak strength measuring 170 was determined. Because of the in-situ partial saturation of the specimens, it was necessary to optimise the saturation condition of the material prior to performing triaxial tests. Susceptibility of the material to swelling (William&Airey, 2004), and possible partial disintegration under effective stresses less than 600kPa (Itakura, 1999) was also considered. Stress-strain behaviour (Figure 5) and their failure envelopes under different stress levels in drained conditions were investigated.
Figure 5. Stress-strain curve of natural shale
The investigation showed the very dramatic effect of saturation on the strength of the material (Figure 6). All specimens were saturated at 600kPa confining stress. The reduction in strengths upon saturation could be attributed to removal of suction due to reduction in effective stress and also could be a result of breaking of cementation bond due to saturation and reducing effective stresses. These observations were associated with a relatively low ultimate friction angle of about 16.5o close to that of the reconstituted specimens (Figure 7)
DISCUSSING THE RESULTS
Triaxial tests were performed on Bringelly shale. The rationale for these tests has been to investigate
Figure 6. Influence of saturation of strength envelopes
Figure 7. different envelopes at different pressures
how the Bringelly shale, which apparently has little cementation, can give UCS strength of up to 50MPa. Triaxial tests were performed on shale at the in-situ water content, on the natural shale after saturation, and on material that was reconstituted. The outcomes of this investigation have indicated that the reconstituted material at high level of stress (>6MPa) gives strength comparable with the natural shale. This was further confirmed from the test results on the stiffness of the natural and reconstituted shale (William&Airey, 2004) which showed a similarity in stiffness at and beyond a confining stress of 6MPa. Influence of porosity and fabric was more pronounced as indicated by the differences in the shear strength envelopes of the low and the high reconstituted specimens. The higher shear strength of specimens with higher porosity may explain their dilative behaviour during shear. A mechanism based on these observations may suggest that the reconstituted shale at high pressure has a low frictional resistance angle and that in addition to the existence of the internal microcracls, the role of fabric on the stiffness and shear strength is more pronounced in material with low porosity. It causes a reduction in the strength of the material hence at low porosity there must be locally high degree of alignment of the clay platelets and it is possible that failure surfaces could develop. These surfaces as they depicted by Figure 8 pass through regions where the clay particles are highly aligned.
On the other hand, when porosity is high, the effective friction angle is no longer controlled by the interparticle friction angle between the particles and the effective dilation angle. This may explain the reason for measuring a relatively high friction angle at low pressure hence strength is not totally controlled by frictional effects. An attempt was made to confirm the suggested mechanism by a close up examination of the clay platelets in the natural shale before and after shearing. The examination was carried out by using the scanning electronic microscope. The results of the investigation are shown in Figures 9 and 10.
Figure 8. suggested mechanism on the role of fabric
Figure 9. sliding surface in the natural shale
Figure 10. shear free zone of natural shale specimen
The first views an arrangement pattern at the rupture plane as indicated by the white colour at the foreground of the photo. The second photo taken at the shear free zone did not exhibit a particular pattern or trend to show evidence for partcle’s re-arrangement.
CONCLUSIONS AND RECOMMENDATIONS
Weak cementation may present in the shale. This was indicated by a high UCS of the material and also its disintegration at exposure to water. Peak strength of the material is the same as its residual strength when the material is subjected to effective stresses less than 1MPa. Some of strength and stiffness is lost when the shale is saturated. Pore water suction is believed to be the main cause. Effective friction angle which controlled by the degree of alignment of the clay particles has a very influential role in controlling the strength of shale. The behaviour of the shale is not consistent with the assumptions of critical state soil mechanics when effective stress reaches more than 6MPa. Saturation of shale causes reduction in its strength and stiffness due to the high pore water suction of the material. Use of Bringelly shale as fill material is not recommended as it deteriorates rapidly in the presence of water. Construction of the residual material of the shale will also require special attention
ACKNOWLEDGMENT
This work has been funded by an ARC project investigating the behaviour of structural soils. The authors are grateful to the supply of specimens, data and equipment from the following organizations: NSW RTA, Coffey Geosciences, Douglas Partners, Jeffery & Katouskas, UNSW, CSIRO.
REFERENCES
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