SYDNEY SANDSTONE AND SHALE PARAMETERS FOR TUNNEL DESIGN

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1 Robert Bertuzzi Pells Sullivan Meynink, Unit G3 56 Delhi Road, North Ryde NSW 2113, Australia 1 BACKGROUND Inherent in any set of rock mass parameters are various assumptions regarding, amongst other things the depth of cover, the proportion of materials encountered and the scale of the proposed excavations. The characteristics used to define rock mass parameters of strength and stiffness comprise: rock type (lithology) strength of the rock substance (intact rock strength) fracturing of the rock mass by defects (bedding, joints, shears, etc.) o persistence and spacings of defects o number of defect sets o infill material o roughness of defects groundwater pressures reactivity of the rock substance to environmental change (shrink/swell, slaking) The intention of this paper is to present typical geotechnical characteristics that can be used for tunnelling projects in Sydney s Hawkesbury Sandstone and Ashfield Shale based on the Sydney classification system (Pells et al 1998). It provides the following information. A brief discussion about the Sydney rock mass classification system A table summarising suggested classification parameters for tunnelling projects for the 10 classes Validation of these suggested classification parameters from recent tunnelling projects Tables summarising suggested rock mass parameters for tunnelling projects. Two sets of material properties are provided to cater for different scales as rock mass parameters are scale dependent. o Overall tunnel scale properties to be used in continuum analyses e.g. FLAC, Phase 2, PLAXIS, Abaqus. Specific geological structures, one or two at most, can be included in these types of models. o Approximately 1 to 2 m 3 scale properties to be used in discontinuum analyses where numerous geological structures are explicitly modelled, e.g. UDEC and potentially jointed network finite elements such as that available in Phase 2 Summary sheets showing examples of core photographs and typical Geological Strength Index (GSI) and Q values for: o Sandstone Class I to V; Faults and Shears o Shale Class I to V; Fault and Shears. This paper updates Bertuzzi and Pells (2002) with data from recent tunnelling projects. The database now includes information from the Ocean Outfalls, Sydney Harbour Tunnel, M2, Eastern Distributor, M5 East, Cross City, cable tunnels, Epping to Chatswood rail link, Lane Cove, CBD Metro, Wynyard Walk and the Northwest Rail link projects. Detailed borehole logging from the last three projects has been especially used. This paper also brings into consideration the rock mass behaviour types recommended by the Austrian Society for Geomechanics (2010). The example of a nominally 8 m diameter TBM at depths of up to 50 m is used. It is important for readers to appreciate that any set of design parameters also communicates the designer s assumptions regarding the ground conditions to those in the field responsible for implementing the designs. It is vital that those in the field are vigilant in observing, documenting and interpreting geological structures that may dominate support requirements at a particular location which would render the average rock mass class irrelevant and feed back to the designers where conditions are different from those anticipated. 2 SYDNEY ROCK MASS CLASSIFICATION SYSTEM The classification system for Sydney sandstone and shale - Pells et al. (1998) which updated Pells et al. (1978) - was intended to assist in the design of foundations on rock in the Sydney area. The classification system is based on rock strength, defect spacing and allowable seams as shown in Table 1. All three factors must be satisfied. Seams include clay, fragmented or highly weathered zones. Australian Geomechanics Vol 49 No 1 March

2 Sandstone Shale CLASS Table 1: Sydney Classification System (Pells et al., 1998) UCS [MPa] [mm] DEFECT SPACING (a) Description ALLOWABLE SEAMS % I > 24 > 600 Widely spaced < 1.5 II > 12 > 600 Widely spaced < 3 III > 7 > 200 Moderately spaced < 5 IV > 2 > 60 Closely spaced < 10 V > 1 NA NA I > 16 > 600 Widely spaced < 2 II > 7 > 200 Moderately spaced < 4 III > 2 > 60 Closely spaced < 8 IV > 1 > 20 Very closely spaced < 25 V > 1 NA NA (a) Defect spacing based on ISO/DIS & ISRM suggested methods replaced the degree of fracturing terms in the Pells et al. (1978) paper Pells et al. (1998) recommended that the zone of rock being classified be over a length of core of similar characteristics. That is, the classification system be applied to portions or units of rock mass having similar UCS, defect spacing and seam characteristics (Bertuzzi & Pells, 2002). An example profile of a mapped face was provided in that paper to clarify the correct method of applying the classification system. This is reproduced in Figure 1. Figure 1: The wrong and right way to classify (Bertuzzi & Pells, 2002) While the Sydney Classification System was not intended for tunnelling, it does represent a good method for communicating rock mass quality in Sydney sandstone and siltstone. It is also useful for linking values put forward for designs with measured and back-figured parameters from existing excavations (e.g. Bertuzzi & Pells, 2002; Clarke & Pells, 2007). Hence, it is often used as the basis for tunnelling projects in Sydney. However, the defect spacings which are appropriate for foundation problems have been found to cover a too narrow range for tunnelling, which needs to consider spacing in three dimensions. Typical spacing for bedding and for jointing is therefore suggested in Table 2. It is not intended that this replaces the Sydney Classification System (Pells et al., 1998) but rather used as a guide as to what to expect in the tunnelling environment. Table 2: Suggested conditions for tunnelling projects. 2 Australian Geomechanics Vol 49 No 1 March 2014

3 Sandstone Shale CLASS UCS [MPa] DEFECTS DEFECT SPACING [m] BEDDING TYPICAL JOINTS ALLOWABLE SEAMS [%] I > 24 > 0.6 > 1.5 > 2 < 1.5 II > 12 > 0.6 > 1 > 1 < 3 III > 7 > 0.2 > 0.5 > 0.5 < 5 IV > 2 > 0.06 > 0.2 > 0.2 < 10 V > 1 NA NA I > 16 > 0.6 > 1 > 1 < 2 II > 7 > 0.2 > 0.5 > 0.5 < 4 III > 2 > 0.06 > 0.1 > 0.2 < 8 IV > 1 > 0.02 > 0.1 > 0.1 < 25 V > 1 NA NA 3 FIELD DATA VALIDATION Recent tunnelling projects have provided a large database of drill core logs, comprising 6 km of logs containing over 7200 records of defect characteristics as well as field estimated strengths, point load index tests and laboratory UCS tests. The data has been collected by several different geotechnical organisations and various geotechnical engineers and engineering geologists. Hence, any bias of particular geotechnical organisations or individuals should be balanced out in the collated data. The database has been accessed to assess the variability in intact strength and the spacing, characteristics and orientation of defects for each particular class of sandstone and shale. The variability is presented in Figures 2 and 3 and in Tables 3 and 4. The terms used in these tables are from the following standards and guidelines. Field estimate strength terms are those of AS Geotechnical site investigations Defect spacing based on ISO/DIS & ISRM suggested methods Defect characteristics follow AS Geotechnical site investigations It is submitted that this data validates the bedding and joint spacing for tunnelling projects suggested in Table 2. In terms of orientation, the recorded data shows bedding is obviously sub-horizontal (to 10 ) in both the sandstone and shale whereas joint patterns differ (Figure 4). In the sandstone, two sub-vertical joint sets are apparent - striking northnortheast and east-southeast. While these two orientations are also apparent in the shale, other joint sets do occur dipping between 30 to 50 in various directions. Australian Geomechanics Vol 49 No 2 June

4 Figure 2: Distribution of logged spacing and aperture for bedding planes and joints for Hawkesbury Sandstone Figure 3: Distribution of logged spacing and aperture for bedding planes and joints for Ashfield Shale 4 Australian Geomechanics Vol 49 No 1 March 2014

5 CLASS Table 3: Variability in strength and spacing as recorded in the drillhole database. FIELD ESTIMATE STRENGTH AXIAL Is(50)[MPa] TESTS RANGE AVERAGE TYPICAL SPACING RANGE [m] BEDDING JOINT TOTAL CORE SAMPLE LENGTH [m] I High > > Sandstone II High III Medium to High IV Low to High V Very low to High 41 < I Medium to High II Low to High Shale III Low to High IV Very low to High 31 < V Very low to High 10 < Table 4: Variability in the defect characteristics as recorded in the drillhole database. CLASS I DEFECT TYPE DEFECT CHARACTERISTICS ROUGH SHAPE APERTURE (mm) INFILL DEFECTS RECORDED BG Rough Planar Undulating Clean to 1-5 Clay 1066 JN Rough Planar Undulating Clean Sandstone II III IV BG Rough Planar - Undulating Clean to 1-10 Clay 952 JN Rough Planar Clean to Veneer Clay - Fe 230 BG Rough Planar Clean to 1-50 Clay - Fe 1369 JN Rough Planar Clean to Veneer Clay - Fe 511 BG Rough Planar 1 50 Clay - Fe 897 JN Smooth - Rough Planar Clean to 1-5 Clay - Fe 376 V I II BG Rough Planar - Undulating Clay - Fe 117 JN Smooth - Rough Planar - Undulating 1 5 Clay Fe 73 BG Smooth - Rough Planar Clean to Veneer Clay 106 JN Smooth - Rough Planar Clean BG Smooth - Rough Planar Clean to 1-5 Clay - Fe 314 JN Smooth - Rough Planar Clean to Veneer Clay - Fe 467 Shale III BG Smooth Planar Clean to 1-50 Clay - Fe 420 JN Smooth - Rough Planar - Undulating Clean to Veneer Clay - Fe 411 IV V BG Smooth Planar 1 50 Clay - Fe 205 JN Smooth - Rough Planar - Undulating Clean to 1 5 Clay - Fe 108 BG Smooth Planar Clay - Fe 124 JN Smooth - Rough Planar - Undulating Clean to 1 5 Clay - Fe 18 Australian Geomechanics Vol 49 No 2 June

6 Based on the field data presented, the typically observed defect characteristics and those characteristics which can occur but represent adverse conditions are summarised in Tables 5 and 6 for Hawkesbury Sandstone and Ashfield Shale, respectively. Table 5: Typical and adverse conditions of defects in Hawkesbury Sandstone. Class I to V Bedding Cross-bedding Jointing PARAMETER TYPICAL ADVERSE Persistence [m] > 20 > 20 Roughness Rough Slightly rough Shape Undulating Planar Aperture [mm] < 1 > 10 Infill None / limonite Sandy clay Persistence [m] > 10 > 10 Roughness Rough Slightly rough Shape Undulating Planar Aperture [mm] < 1 < 1 Infill None / limonite None Persistence [m] 5 > 10 Roughness Rough Slightly rough Shape Planar Planar Aperture [mm] < 1 > 10 Infill None / limonite Sandy clay Class I, II & III Class IV & V Table 6: Typical and adverse conditions of defects in Ashfield Shale. Bedding Jointing Bedding Jointing PARAMETER TYPICAL ADVERSE Persistence [m] > 20 > 20 Roughness Slightly rough Smooth Shape Undulating Planar Aperture [mm] < 1 > 10 Infill None / limonite Silty clay Persistence [m] 3 > 5 Roughness Slightly rough Smooth Shape Planar Planar Aperture [mm] < 1 > 10 Infill None / limonite Silty clay Persistence [m] > 20 > 20 Roughness Slightly rough Smooth Shape Undulating Planar Aperture [mm] < 1 50 Infill Clay coating Silty clay Persistence [m] 3 > 5 Roughness Slightly rough Smooth Shape Planar Planar Aperture [mm] < 1 50 Infill Clay coating Silty clay Notes: Persistence of cross-bedding is controlled by sandstone bed thickness Geotechnical models should consider and include adverse conditions, particularly that of defect persistence 6 Australian Geomechanics Vol 49 No 1 March 2014

7 Figure 4: Equal area projections for Hawkesbury Sandstone on the left (4675 defects) and Ashfield Shale on the right (1338 defects) 4 ROCK MASS PROPERTIES The rock mass properties presented in Tables 7 and 8 are derived following the GSI approach (Hoek & Brown, 1997; Hoek et al., 2002; Hoek & Diederichs, 2006) tempered with field measurements (e.g. Pells, 1990; Pells, 2004; Clarke & Pells, 2007). Values of GSI and Q for the various classes are presented in the accompanying summary sheets. Table 9 lists suggested Mohr-Coulomb strength and stiffness parameters based on defect shear tests carried out for the tunnelling projects. In order to calculate the Q value an estimate of the in situ stress is required, or more correctly, the ratio of intact rock strength to stress. Enever (1999) collated in situ stress data that suggested the following stepped profile for the upper bound major horizontal stress ( H): Approximately 2.5 MPa above the vertical stress ( V) for depths less than 20 m Approximately 6.5 MPa above V for m depths Approximately 15 MPa above V for m depths Rather than adopting this profile with seemingly arbitrary steps, Pells (2004) suggested H can be related to V to simplify the design process as: H = (1.2 to 2.0) V Adopting this same approach to the data presented in McQueen (2004), the relationship, H = 2.0 V + 2.5, is proposed as shown in Figure 5. This means that the typical in situ principal stresses for depths less than 50 m (stresses at specific locations may be different) can be assumed to be: σ v up to 1.2 MPa σ H σ v MPa, i.e. up to 5 MPa. Australian Geomechanics Vol 49 No 2 June

8 Figure 5: Major horizontal stress versus depth with the design line H = 2.0 V shown (after McQueen, 2004). 8 Australian Geomechanics Vol 49 No 1 March 2014

9 Table 7: Hawkesbury sandstone Substance PARAMETER Class I Class II Class III Class IV Class V Uniaxial compressive strength, UCS (MPa) Young s modulus, E (MPa) Unit weight, (kn/m 3 ) 24 Poisson s ratio, m i m 3 scale Mass Young s modulus, E mass (MPa) GSI m b Hoek-Brown (1) s a cꞌ (kpa) Mohr-Coulomb (2) Ꞌ ( ) t (kpa) Not applicable See tunnel scale parameters Tunnel scale Mass Young s modulus, E mass (MPa) GSI m b Hoek-Brown (1) s a cꞌ (kpa) Mohr-Coulomb (2) Ꞌ ( ) t (kpa) < 300 < 100 < 40 < 10 0 Table 8: Ashfield shale Substance PARAMETER Class I Class II Class III Class IV Class V Uniaxial compressive strength, UCS (MPa) Young s modulus, E (MPa) Unit weight, (kn/m 3 ) 24 Poisson s ratio, m i m 3 scale Mass Young s modulus, E mass (MPa) GSI m b Hoek-Brown (1) s a cꞌ (kpa) Mohr-Coulomb (2) Ꞌ ( ) t (kpa) Not applicable See tunnel scale parameters Tunnel scale Mass Young s modulus, E mass (MPa) GSI m b Hoek-Brown (1) s a cꞌ (kpa) Mohr-Coulomb (2) Ꞌ ( ) t (kpa) < 100 < 60 < Damage parameter D of 0, assumes minimal disturbance to rock mass surrounding tunnel 2. Normal stress range of 0 to 2 MPa assumed Australian Geomechanics Vol 49 No 2 June

10 Table 9: Defect properties. DEFECT TYPE Sandstone Bedding, joints Shale Bedding, joints Faults, shears, erosional contacts THICKNESS (mm) INFILL TYPE SHEAR STRENGTH cꞌ (kpa) INFILL MODULUS (MPa) Ꞌ ( ) NORMAL STIFFNESS (MPa/m) kn SHEAR Tight Clean or hard mineral 0 40 NA >10000 > Firm clay Soft clay Tight Clean 0 30 N/A >10000 > Firm clay Soft clay Equivalent to 50 mm Soft clay ks 5 ACKNOWLEDGEMENTS The author is indebted to his colleagues Andrew de Ambrosis, Derek Anderson, Mark Eggers and Ben Rouvray, for their constructive comments and opinions; and Dan Sandilands and James Smith for their manipulation of spreadsheets. It is hoped that practitioners will find this paper useful in their work in Sydney. 6 REFERENCES AUSTRIAN SOCIETY FOR GEOMECHANICS, Guideline for the geotechnical design of underground structures with conventional excavation. Salzburg, Austria. BERTUZZI, R. & PELLS, P. J. N Geotechnical parameters of Sydney sandstone and shale. Australian Geomechanics Journal, 37, CLARKE, S. & PELLS, P. J. N A large scale cable jacking test for rock mass modulus measurement, Lucas Heights, Sydney ENEVER, J. R Near surface in-situ stress and its counterpart at depth in the Sydney metropolitan area. Australian Geomechanics, HOEK, E. & BROWN, E. T Practical estimates of rock mass strength. International Journal of Rock Mechanics & Mining Sciences, 34, HOEK, E., CARRANZA-TORRES, C. & CORKUM, B. Hoek-Brown failure criterion edition. North American Rock Mechanics Symposium, 2002 Toronto HOEK, E. & DIEDERICHS, M. S Empirical estimation of rock mass modulus. International Journal of Rock Mechanics and Mining Sciences, 43, INTERNATIONAL SOCIETY FOR ROCK MECHANICS, Suggested Method for Quantitative Description of Discontinuities in Rock Masses ISO :2003. Geotechnical investigation and testing -- Identification and classification of rock -- Part 1: Identification and description MCQUEEN, L. B In situ rock stress and its effect in tunnels and deep excavations in Sydney. Australian Geomechanics, 39, PELLS, P. J. N Stresses and displacements around deep basements in the Sydney area. 7th Australian Tunnelling Conference. Sydney, Australia PELLS, P. J. N Substance and mass properties for the design of engineering structures in the Hawkesbury Sandstone. Australian Geomechanics, 39, 1-21 PELLS, P.J.N., MOSTYN, G. & WALKER, B.F Foundations on Sandstone and Shale in the Sydney Region. Australian Geomechanics Journal, 33 Part 3 10 Australian Geomechanics Vol 49 No 1 March 2014