Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676E683

Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683

Contents lists available at ScienceDirect Journal of Rock Mechanics and Geotechnical Engineering

journal homepage: www.rockgeotech.org

Full Length Article Rock mechanics for design of Brisbane tunnels and implications of recent thinking in relation to rock mass strength

L.B. McQueen a,*, A. Purwodihardjo b, S.V.L. Barrett c a Golder Associates Pty Ltd., 124 Paciﬁc Highway, St. Leonards, NSW, 2065, Australia b Golder Associates Pty Ltd., 147 Coronation Drive, Milton, Queensland, 4064, Australia c Golder Associates Pty Ltd., Building 7, Botanicca Corporate Park, 570 e 588 Swan Street, Richmond, Victoria, 3121, Australia article info abstract

Article history: This paper explores the potential implications of recent thinking in relation to rock mass strength for Received 31 July 2018 future tunnelling projects in Brisbane, Australia, particularly as they are constructed within deep hori- Received in revised form zons where the in situ stress magnitudes is larger. Rock mass failure mechanisms for the current tunnels 15 February 2019 in Brisbane are generally discontinuity controlled and the potential for stress-induced failure is relatively Accepted 18 February 2019 rare. For the road tunnels which have been constructed in Brisbane over the last 12 years, the strength of Available online 20 March 2019 the more massive rock masses for continuum analysis has been estimated by the application of the Hoek- Brown (H-B) failure criterion using the geological strength index (GSI) to determine the H-B parameters Keywords: ‘ Rock mass strength mb, s and a. Over the last few years, alternative approaches to estimating rock mass strength for massive ’ Hoek-Brown (H-B) failure criterion to moderately jointed hard rock masses have been proposed by others, which are built on the work Triaxial testing completed by E. Hoek and E.T. Brown in this area over their joint careers. This paper explores one of these Brittle failure alternative approaches to estimating rock mass strength for one of the geological units (the Brisbane Brisbane Tuff Tuff), which is often encountered in tunnelling projects in Brisbane. The potential implications of these strength forecasts for future tunnelling projects are discussed along with the additional work which will need to be undertaken to conﬁrm the applicability of such alternative strength criteria for this rock mass. Ó 2019 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

1. Introduction tunnel boring machine (TBM). Two double-shield machines were used for CLEM7 in the Tuff and NF. For APL, two earth pressure balance Three signiﬁcant road tunnels have been constructed in Brisbane, TBMs were used within the Tuff and Aspley formations. The TBMs Australia, over the last 12 years, namely the CLEM7 (formerly North- were supplied by Herrenknecht AG. The maximum tunnel depths South Bypass Tunnel), Airport Link (APL) and Legacy Way (LW) tun- approached up to w60 m for CLEM7 and w40 m for APL. nels, as a part of Brisbane City Council’s TransApex Plan. The location For both the CLEM7 and APL, the mainline tunnel/ramp ‘Y’ junction of the three projects, along with the Brisbane surface geology, is caverns were constructed by roadheader, with the bench excavation shown in Fig. 1 and their key features are summarised in Table 1. being accomplished by drill-and-blast method in some instances. For For the CLEM7 and APL projects, the geological units encountered these caverns, the excavation spans ranged from 18 m to 27 m, with include: (1) foliated metamorphic rocks of the Neranleigh Fernvale the caverns at Lutwyche having the largest span of 27 m. (NF) formation, (2) thin deposits of sedimentary materials on the palaeotopographic NF land surface forming the Unconformity be- 1.1. Geology of CLEM7 and APL tween the NF and the Brisbane Tuff (Tuff), (3) the Tuff, and (4) sedimentary rocks of the Aspley and Tingalpa formations. The Devonian-Carboniferous aged NF metamorphic rocks form the The LW project was constructed through the Bunya Phyllite and basement geology along the two project corridors. These rocks were the NF formations, and it is not discussed further in this paper. CLEM7 formedfrom asequence of deepwater marinesedimentsandvolcanics, and APL were constructed using a combination of roadheader and which have undergone low-grade regional metamorphism. The original sedimentary structure within the NF is indistinct or completely * Corresponding author. Fax: þ612 9478 3901. E-mail address: [email protected] (L.B. McQueen). obscured by the pervasive foliation, which has developed as a result of a Peer review under responsibility of Institute of Rock and Soil Mechanics, Chi- number of post-depositional periods of deformation. Dominant rock nese Academy of Sciences. types are quartz arenite and phyllite with quartz veining. https://doi.org/10.1016/j.jrmge.2019.02.001 1674-7755 Ó 2019 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under theCCBY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683 677

Fig. 1. Location of Brisbane road tunnels and mapped surface geology.

The NF basement rocks made up a former land surface upon 1.2. In situ stress which volcanic ash flows and falls were deposited in the late Triassic forming the Tuff. A number of episodes of volcanic erup- In situ stress conditions for CLEM7 and APL projects were esti- tions initially resulted in the formation of tuffaceous claystones mated from borehole measurements using hydraulic fracture and (unwelded or poorly welded tuff) in places. Later, more massive overcoring (two-dimensional (2D) mechanical strain measurement eruptions occurred with rhyolitic tuff deposited as a series of hot tool) techniques in the Tuff and NF rocks. Measurements are ash falls or pyroclastic flows, forming a welded tuff (or ignimbrite). influenced by the palaeotopographic effects of the NF irregular land Regional faults and altered zones also occur in the Tuff. surface and regional-scale geological faulting. Elevated horizontal A range of features was presented on the NF land surface, such as stress measurements of about 6 MPa and 12 MPa, at a shallow local deeply incised valleys with lakes and streams and areas of depth of about 40 m, could be attributed to these influences. In woodland. The volcanic material settled on this irregular land surface general, the major horizontal stress can be assumed to be about and progressively buried soils, lake deposits and stream sediments, twice the vertical stress calculated from the thickness of the over- which formed the Unconformity between the NF and the overlying burden in the NF rocks. Tuff. Rock types typically comprise of claystones (generally less than A lithostatic stress field was indicated for the overlying Tuff. This 500 mm thick at the top of the Unconformity) with conglomerates change in the stress field from the higher horizontal stress in the and breccias, and some siltstone and local thin coal lenses. underlying NF may be influenced by a number of aspects such as Unconformably overlying the Tuff in some locations along both the isolating effects of the lower stiffness Unconformity rocks be- alignments is the Late Triassic Aspley formation. The Aspley for- tween the two formations, the laterally discontinuous nature of the mation was formed from alluvial sediments deposited on the Tuff and the cooling effects of this volcanic rock. Elevated hori- irregular erosional surface of the Tuff. It consists of conglomerate zontal stress measurements of 5 MPa, however, were recorded at with coarse sandstone and siltstone interbeds. Local Quaternary less than 50 m depth. Local higher horizontal stress of about 3 times alluvial deposits cover parts of the stratigraphic profile. the vertical stress has also been suggested in the Tuff by McQueen

Table 1 et al. (2012), from back analysis of monitoring data obtained from a Key features of the CLEM7, APL and LW projects. low cover CLEM7 tunnel.

Project Construction Tunnel Tunnel Underground period length configuration ‘Y’ junctions 2. Rock mass properties of the Brisbane Tuff (km) CLEM7 2006e2010 4.8 Two-lane dual Shafston 2.1. Rock mass characteristics carriageways e APL 2008 2012 6.7 Two- and three-lane Bowen Hills The Tuff is typically massive to moderately jointed rock and dual carriageways Lutwyche Kedron LW 2012e2015 4.3 Two-lane dual None has three quasi-orthogonal discontinuity sets. A subhorizontal carriageways discontinuity set associated with partings between individual 678 L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683 tuff beds is typically the dominant set. Photogrametry ‘mapping’ Table 3 of cliff exposures at Kangaroo Point near the CLEM7 tunnel Discontinuity parameters adopted for the Brisbane tuff in the CLEM7. alignment provided a better understanding of these discontinu- Discontinuity Dip () Dip Trace Gap Spacing ities, particularly persistence. The Tuff is typically slightly set direction () length (m) length (m)a (m) weathered to fresh at shallow depths (within a few metres of the 1 3 122 5 61 5 rock surface). Locally, near the topographic lows (valleys) and 2 88 298 3 3 2 shear zones, the Tuff is more weathered and/or altered, and 3 88 216 3 4 4 b closely jointed. 4 45 127 7 6 5 Several rock mass domains were developed for the Tuff as part a Gap length is the length of rock bridge between two adjacent discontinuities of the geotechnical tunnel model for CLEM7 to describe the rock located in the same plane. b Mapping indicated that discontinuity sets 1e4 did not typically occur together. mass characteristics for design purposes (Table 2)(Funkhouser et al., 2010a, b). Variations in these domains were then used for the APL project. The domains were also employed during con- The results of a DFN analysis of the Tuff using FracMan (Golder struction as a tool to aid geological characterisation and assignment Associates, 2009) for a 30 m long and 12.4 m diameter TBM tunnel of various rock support classes. for one realisation of the generated discontinuity network are Table 2 summarises the input parameters, by domain, for the shown in Fig. 2. When evaluating key blocks using the DFN geological strength index (GSI) approach to the Hoek-Brown (H-B) approach, the analysis is carried out using the modelled disconti- failure criterion (Hoek et al., 1995) for the rock mass assigned to the nuity network with its fully defined spatial and geometrical pa- Tuff. The parameters shown are for the CLEM7 and APL projects. rameters. This contrasts with the approach used in UnWedge The rock mass rating (RMR) (Bieniawski, 1976) and tunnelling (Rocscience, 2018), which assumes a network of ubiquitous joints. quality index Q (Barton et al., 1974) classification systems were also As such, a more comprehensive probabilistic assessment of po- used as alternative approaches to classify these rock masses tential block geometries and sizes can be undertaken using Frac- (Funkhouser et al., 2010a,b). Table 2 indicates that lower values of Man, so long as there are sufficient data available to adequately GSI were assigned for CLEM7 compared with APL for most of the describe the three-dimensional discontinuity network forming the domains. This may reflect the fact that CLEM7 was the first road rock mass. In addition to assisting in the assessment of discrete tunnel constructed in this rock mass, and thus more weight was ground loads on the lining systems from key blocks, the results of given to surface exposures rather than underground observations the DFN analyses were also used to visualise the rock mass in the GSI assessment. bounding the proposed openings as another means of assessing Potential failure mechanisms for the Tuff are typically wedge, appropriate GSI values to be adopted for the continuum numerical block or slab fallouts in stronger rock masses and ravelling, analyses. slumping or minor squeezing in weaker rock domains and in zones which have been locally sheared or faulted. Stress-induced rock mass failures due to stress concentration at the excavation 3. Rock mass strength boundary rarely occur due to the strength of the rock mass relative to the in situ stress magnitudes. 3.1. H-B failure criterion

2.2. Discontinuity parameters Aspects of the H-B failure criterion, to support the discussion in this paper, are presented here. The original H-B equation for the Typical discontinuity parameters used for key block, discrete empirical, nonlinear, isotropic peak strength criterion (Hoek and fracture network (DFN) and discontinuum analyses for the Tuff are Brown, 1980a, b)is summarised in Table 3. These values are based on a combination of s0 0:5 acoustic televiewer (ATV) data from boreholes and mapping of s0 ¼ s0 þ s 3 þ 1 3 ci m s s (1) surface outcrop exposures along the CLEM7 alignment. This ci included a photogrammetric analysis of images which were taken s0 s0 for the exposures of the Tuff in the cliffs at Kangaroo Point. In where 1 is the major principal effective stress at failure; 3 is the s addition to discontinuity orientation, parameters also derived from minor principal effective stress at failure; ci is the uniaxial the photogrammetric images included spacing, persistence and compressive strength (UCS); and m and s are the H-B dimensionless termination characteristics for each of the identified discontinuity material parameter constants related to friction and cohesion, s sets. The surface mapping was also used to establish typical block respectively. The constants ci and mi (H-B constants for intact sizes and shapes for the rock mass domains. rock) are determined by statistical analysis of triaxial laboratory test data with at least five data points over a range of confining : s stress up to about 0 5 ci for a reliable estimation (Hoek and Brown, Table 2 1980a). H-B GSI parameters adopted for the Brisbane Tuff in the CLEM7 and APL projects. Bieniawski (1976)’s RMR classification was used before 1995 to scale parameters m and s for intact rock to those of a rock mass Rock mass Project H-B input parameters (Priest and Brown, 1983; Hoek and Brown, 1988). RMR was replaced domain s ci (MPa) GSI mi by GSI in the generalised H-B criterion proposed by Hoek et al. Tuff 1 CLEM7 100 70 18 (1995) with the following formulation: e APL 65 80 95 13 Tuff 2 CLEM7 80 60 18 0 a 0 0 s APL 50 60e80 13 s ¼ s þ s 3 þ 1 3 ci mbs s (2) Tuff 3 CLEM7 30 45 13 ci APL 25 45e70 13 Tuff 4 CLEM7 3 40 13 where mb (the H-B constant m for the rock mass) and a are the APL 8 20e35 13 material constants. The equations for the parameters in Eq. (2),

Note: sci is the uniaxial compressive strength, and mi is the H-B parameter. with the introduced GSI, and the factor D to account for the degree L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683 679

Fig. 2. FracMan predicted potential range of unstable block size distributions based on DFN modelling. (a) Distribution of calculated support pressure of unstable blocks; (b) One realisation of DFN model, looking northwest towards; and (c) Joint trace plane on Kangaroo Point Cliffs derived from photogrammetry, looking east towards. of disturbance to the rock mass due to blast damage (Hoek et al., the spalling limit (limit of axial splitting after which shear failure 2002), are expressed as dominates) and the strength at high confinement (taken as 0.8 of the laboratory strength). The reader is referred to Bewick et al. ¼ ½ð Þ=ð Þ mb mi exp GSI 100 28 14D (3) (2019) for the details. Although this equivalent H-B strength envelope approach was s ¼ exp½ðGSI 100Þ=ð9 3DÞ (4) recently developed with deep tunnelling and mining conditions in mind, where extensional and shear failure modes through intact 1 1 rock are anticipated, there is potential merit in its application to a ¼ þ eGSI=15 e20=3 (5) 2 6 future tunnels in rock masses at Brisbane, where the GSI approach may not be applicable. The recent approach includes an estimate of the spalling strength to better capture the behaviour of the rock 3.2. Rock mass strength e Brisbane Tuff mass at low confinement conditions of tunnel boundary. In these situations where elevated stress may occur, such as narrow pillars The original H-B criterion can be applied with confidence to and tunnels in close proximity, the possibility of extensional brittle rock masses with UCS greater than 15 MPa and GSI between failure will need to be considered. about 30 and 65 (Brown, 2008). Limits of the GSI approach (rock The mechanism of brittle failure has been developed and pro- mass strength estimated from sci, mi, and GSI based on an un- gressed by a number of authors such as Martin et al. (1999), Kaiser derstanding of the intact and rock mass properties, Eqs. (3)e(5)), et al. (2000) and Diederichs (2007). Near excavation boundaries, compared with a spalling brittle failure approach, were also brittle rock fails predominantly by extensional processes, leading to given by Diederichs (2007) with reference to the compressive to the progressive formation of thin slabs of rock referred to as spalls. tensile strength ratio. However, the GSI approach is often applied The spalling process occurs under tangential compressive loading to rock masses outside these limits including those better-quality conditions with spall formation resulting from tensile fracturing. domains for the Tuff in the CLEM7 and APL projects (Section 2.1). Due to the low confining stress conditions near the excavation For these projects, the continuum numerical stress modelling boundaries, tensile microcracks can propagate, creating spalls of adopted the GSI approach with the H-B rock mass strength rocks (Bewick et al., 2019). criterion. The better-quality Tuff domains, Tuff 1 and Tuff 2, represent Bewick et al. (2019) has presented a methodology to utilise the homogeneous massive to moderately jointed hard rock masses H-B GSI approach to estimate the strength of those less fractured with GSI typically greater than 65 (Table 2). For the purposes of rock masses with GSI 65. These are ‘homogeneous and hetero- making a preliminary assessment of the potential benefits of the geneous massive to moderately jointed hard rock masses’, where Bewick et al. (2019) methodology for future tunnels, the available the H-B GSI equations (Eqs. (3)e(5)) are not applicable. This triaxial data for the Tuff from the CLEM7 and APL projects were methodology obtains an equivalent H-B strength envelope, utilising sourced to develop the equivalent H-B strength envelope in com- laboratory tensile, uniaxial and triaxial strength test data, from a parison with the GSI approach for the strengths of Tuff 1 and Tuff 2. tri-linear strength envelope fixed at the tensile strength of the rock A plot of the available triaxial data from the CLEM7 and APL mass, the spalling strength (limit at which crack initiation occurs), projects is shown in Fig. 3. A division within the dataset is apparent 680 L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683

is of interest here to provide an understanding of the spalling limit. However, for the purposes of preliminary assessment presented in this paper, this limited dataset is considered suitable to provide some initial insights into the potential beneﬁts of applying the equivalent H-B strength envelope approach. A tri-linear envelope (red lines in Fig. 4a and b) has been derived using the mean H-B curve for the higher strength triaxial dataset (sci ¼ 87 MPa, mi ¼ 15, and s ¼ 1), as explained below. An equivalent H-B rock mass strength envelope (blue lines in Fig. 4a and b), to represent massive to moderately jointed Tuff (Tuff 1 and Tuff 2), is then ﬁtted to the tri-linear envelope using the estimates of the four points illustrated in Fig. 4a, according to the approach of Bewick et al. (2019). The four points are estimated from:

(1) The rock mass tensile strength. In this example, the value indicated by the equivalent H-B curve fitted to the tri-linear envelope is accepted. This is about 2 MPa, which is about 30% of the calculated laboratory DTS. (2) Uniaxial rock mass strength or ‘spalling strength’, which, in the absence of testing data on the crack initiation limit, is estimated at 0.4sci, based on Bewick et al. (2019) and other authors such as Martin et al. (1997) and Diederichs et al. (2004). This is about 35 MPa from sci of the higher strength tuff dataset. (3) Spalling limit, which marks the change in behaviour, as the confining stress increases, from failure dominated by tensile splitting to that dominated by shear failure mode. This limit ranges from s1/s3 ¼ 10 to 20 (Kaiser et al., 2000). Here s1/ s3 ¼ 15 has been adopted. Point 3 for the equivalent envelope is about midway along this limit line between the spalling strength and high confinement curve. (4) The high confinement curve for the tri-linear envelope is beyond the spalling limit and taken as 80% of the laboratory Fig. 3. Tuff triaxial test data with mean H-B curves fitted to the higher (diamonds) and strength. Point 4 for the equivalent envelope is selected, as lower (triangles) strength datasets, according to Hoek and Brown (1997) with tensile suggested by Bewick et al. (2019) for a practically meaningful cutoff based on Perras and Diederichs (2014). confining stress level of about 25 MPa (typically s1/s3 6). in the figure, which is believed to be related to the location where The equivalent H-B strength envelope (blue line in Fig. 4a and b), the samples were obtained (position in the volcanic sequence, as fitted to the above four points, has the rock mass compressive north or south of the Brisbane River) and the type of tuff (weath- strength (or spalling strength) of srm ¼ 35 MPa, and rock mass ered or poorly welded tuff). The division suggests a higher strength constants mrm ¼ 13, srm ¼ 1, and arm ¼ 0.65. It is then compared dataset (blue diamonds) with UCS (sci) of about 60e120 MPa and a with the strength envelope derived from the H-B GSI approach for lower strength dataset (green triangles) with sci of 10e40 MPa. A rock masses in the Tuff 1 and Tuff 2 domains (the parameters from statistical analysis to fit the H-B curve is shown for each dataset, the APL project were adopted, as listed in Table 2)inFig. 4b. The with the calculated sci and mi (and s ¼ 1 for intact rock), using the results are shown in Table 4. method of Hoek and Brown (1997). The equivalent H-B envelope is presented as an example of the The higher strength dataset is considered to be representative derivation of a rock mass strength envelope for the massive to of the Tuff 1 and Tuff 2 domains. The direct tensile strength moderately jointed Tuff. It is based on testing and considers the (DTS) cutoff for this curve fit was taken at about 6 MPa. This possible failure mechanisms that could occur at low confinement. value is 0.8 times of the mean of the Brazilian indirect tensile The rock mass strengths derived from the H-B GSI approach for Tuff strength test results, as suggested by Perras and Diederichs 1 and Tuff 2 are lower than that obtained from the equivalent H-B (2014). As a comparison, the DST would be 4.5 MPa according approach. The improvement at the low confinement part of the to the method of Hoek and Martin (2014). The lower strength strength envelope is of most relevance for future tunnelling pro- dataset may be more indicative of Tuff 3 with a tensile cutoff of jects in Brisbane. 2.5 MPa (Perras and Diederichs, 2014)or2.3MPa(Hoek and Martin, 2014). 4. Potential implications for numerical modelling Considering that the maximum confining pressure for testing in Fig. 3 for the higher strength tuff is less than about 0.5sci, and that The potential implications of adopting the equivalent H-B en- there are only a few data points over the narrow range of confining velope from the tri-linear failure criterion in Fig. 4 is illustrated in pressures (Hoek and Brown, 1980a), the derived constants sci and the pillar design for the Lutwyche cavern headwall for the APL mi should therefore be treated with caution and confirmed with project (Fig. 5)(Lagger et al., 2014, 2017). Tuff 1 and Tuff 2 pa- more triaxial testing over a larger range of confining pressures. rameters from the original design have been updated using the Although the main interest of this paper is in the low confinement proposed approach of Bewick et al. (2019). The parameters were range, the failure mechanism changes as the confining pressure then used in the Examine3D and Examine2D analyses for the cavern increases (Bewick et al., 2011) and observation of the failure mode headwall so that the two sets of results could be compared. This L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683 681

Fig. 4. (a) Tri-linear envelope (red line) and equivalent H-B rock mass strength envelope (blue line) for massive to moderately jointed Tuff (the Bewick et al. (2019)’s approach). Points 1e4 are explained in the text. (b). Equivalent H-B strength envelope compared with the strength envelope previously derived from the H-B GSI approach for Tuff 1 and Tuff 2.

Table 4 been improved, particularly for Tuff 2. The improvement is more Rock mass parameters for the equivalent H-B envelope compared with those of the pronounced at a lower in situ stress or lower confinement pressure, H-B GSI approach for Tuff 1 and Tuff 2. as indicated in Fig. 6. This suggests that the proposed approach has s Method cm ms a the potential to optimise excavation and support designs in these (MPa) materials at a lower confinement. Table 4 summarises the adopted Equivalent H-B strength envelope 35 13 1 0.65 H-B parameters for the analyses. H-B GSI approach Tuff 1 31 8.2 0.236 0.5 The results of the continuum model undertaken using RS2 also Tuff 2 9.4 4.5 0.0357 0.501 indicate very similar phenomena. To illustrate these phenomena, the shear strength reduction (SSR) method for the generalised H-B criterion discussed by Hammah et al. (2005) was employed. The analysis was a part of the numerical modelling for the cavern SSR is a concept which systematically reduces the shear strength design, which also included continuum and discontinuum plastic envelope of material by a factor of safety or by a stress reduction modelling. factor (SRF) in the finite element model (FEM). The results are The Examine3D and Examine2D results indicate that for Tuff 1 illustrated in Fig. 7, which indicates that the critical SRF increases and Tuff 2, the potential of material yield around the pillar areas has significantly (greater than 100%) when adopting the updated

Fig. 5. Plan and perspective views of the Examine3D model. 682 L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683

Fig. 6. Cross-sections of the Examine3D model showing calculated strength factors.

Fig. 7. RS2 results with SSR method. parameters for the Tuff 2 case, for an assumed initial in situ stress the massive to moderately jointed Tuff, with laboratory testing, ratio of 1.2. The improvement in the SRF reduces when higher in particularly triaxial data, as a basis. The benefits for Brisbane situ stress ratios are assumed in this model. In the same way, the tunnelling include a better definition of the low and high improvement in the SRF also reduces for Tuff 1. confinement parts of the rock mass strength envelope. The result of It is interesting to note that the encountered rock mass domain this preliminary assessment suggests a higher rock mass strength during the pillar excavation was generally Tuff 2/Tuff 1. The induced compared with the envelope developed from the H-B GSI approach deformations were less than 5 mm and there was no indication of for the better-quality rock mass domains, Tuff 1 and Tuff 2. How- cracks and spalling. These observations indicated that the strength ever, it should be viewed with caution, as the estimated tri-linear factors should be higher than 1.5, which are similar to the results of strength envelope for the Tuff still needs to be refined with addi- analyses using the Bewick et al. (2019) approach. tional triaxial testing at appropriate confining pressures. For future tunnelling projects constructed in the Tuff, greater 5. Conclusions emphasis will therefore need to be placed on specifying the necessary laboratory testing and interpretation methodologies, This paper provides some initial insights into the potential which will be required to further explore the insights and potential benefits of adopting an equivalent H-B envelope fitted to a tri-linear design benefits highlighted in this paper. Future tunnels will likely rock mass strength envelope for the Tuff, for the design of future be constructed within deeper horizons as Brisbane continues to tunnels in Brisbane, using the method proposed by Bewick et al. grow and develop, and as such, more of these deeper tunnels will (2019). A H-B curve is fitted to the estimates of tensile strength, need to be constructed in the heterogeneous foliated basement spalling strength, spalling limit and high confinement strength for metamorphic rocks. Triaxial laboratory data were not available L.B. McQueen et al. / Journal of Rock Mechanics and Geotechnical Engineering 11 (2019) 676e683 683 from the Brisbane tunnels for these rocks; therefore, future testing Hoek E, Kaiser PK, Bawden WF. Support of underground excavations in hard rock. and assessment work should also focus on the testing of these Rotterdam, Netherlands: A.A. Balkema; 1995. p. 215. Hoek E, Martin CD. Fracture initiation and propagation in intact rock e a review. rocks. Journal of Rock Mechanics and Geotechnical Engineering 2014;6(4):278e300. Kaiser PK, Diederichs MS, Martin CD, Sharp J, Steiner W. Underground works in Conflicts of interest hard rock tunnelling and mining. In: Proceedings of the international conference on geotechnical and geological engineering. Melbourne, Australia: Tech- nomic Publishing Company; 2000. p. 841e926. The authors wish to confirm that there are no known conflicts of Lagger H, Purwodihardjo A, Barrett SVL, Booth P, McKenzie N, Taylor E. Pillar sup- interest associated with this publication and there has been no port design for the Lutwyche caverns. In: Proceedings of the 16th Australasian fi fi fl tunnelling conference. Australian Tunnelling Society; 2017. signi cant nancial support for this work that could have in u- Lagger H, Purwodihardjo A, Barrett SVL, McKenzie N, Taylor E. Temporary support enced its outcome. design for the largest road caverns ever built in Australia e the Lutwyche Caverns. In: Proceedings of the 15th Australasian tunnelling conference. Mel- bourne, Canada: The Australasian Institute of Mining and Metallurgy; 2014. Acknowledgements p. 399e410. Martin CD, Kaiser PK, McCreath DR. Hoek-Brown parameters for predicting the The authors, as practitioners, wish to acknowledge Professor Ted depth of brittle failure around tunnels. Canadian Geotechnical Journal e Brown for his guidance and wise counsel in rock mechanics over 1999;36(1):136 51. Martin CD, McCreath DR, Stochmal M. Estimating support demand loads caused by the years and in particular for the advice he provided the authors stress-induced failure around tunnels. In: Proceedings of international sym- during the design and construction of both the CLEM7 and APL posium. On rock support: applied solutions for underground structures. Lille- projects. We are honoured to be included in this journal volume hammer, Norway; 1997. McQueen LB, Barrett SVL, Purwodihardjo A. Design of low cover road tunnels in dedicated to Ted. jointed rock in an urban environment: example from CLEM7 Tunnel, Brisbane. In: Proceedings of the 11th Australia-New Zealand conference on geo- References mechanics. Melbourne, Canada; 2012. Perras MA, Diederichs MS. A review of the tensile strength of rock: concepts and testing. Geotechnical and Geological Engineering 2014;32(2):525e46. fi Barton N, Lien R, Lunde J. Engineering classi cation of rock masses for the design of Priest SD, Brown ET. Probabilistic stability analysis of variable rock slopes. Trans- e tunnel support. Rock Mechanics 1974;6(4):189 236. actions of the Institution of Mining and Metallurgy A 1983;92:1e12. Bewick RP, Kaiser PK, Valley B. Interpretation of triaxial testing data for estimation Rocscience. UnWedge v4.0. 2018. www.rocscience.com. of the Hoek-Brown strength parameter mi. In: 45th US rock mechanics/geomechanics symposium. American Rock Mechanics Association (ARMA); 2011. p. 1979e88. Bewick RP, Kaiser PK, Amann F. Strength of massive to moderately jointed hard rock L.B. McQueen is a Principal of Golder Associates in Sydney. masses. Journal of Rock Mechanics and Geotechnical Engineering 2019;11(3): He became involved in tunnelling in the 1980s during his 562e75. 12 years at the Sydney Water Corporation. Since joining Bieniawski ZT. Rock mass classifications in rock engineering. In: Bieniawski ZT, Golder Associates in 1994, he has worked on major civil editor. Proceedings of the Symposium on Exploration for Rock Engineering, vol. tunnelling projects in Australia, Hong Kong and the 1. Rotterdam, Netherlands: A.A. Balkema; 1976. p. 97e106. Philippines. His expertise is in the fields of engineering Brown ET. Estimating the mechanical properties of rock masses. In: Potvin Y, geology, rock mass characterisation, rock mechanics, and Carter J, Dyskin A, Jeffrey R, editors. Proceedings of the 1st southern hemisphere rock tunnel support design and risk assessment. He international rock mechanics symposium. Perth, Australia: Australian Centre worked on the Brisbane tunnels and completed temporary for Geomechanics; 2008. p. 3e22. tunnel support design for roadheader caverns and ramps Diederichs MS, Kaiser PK, Eberhardt E. Damage initiation and propagation in hard for the CLEM7 tunnel and tender design for LW tunnel. rock during tunnelling and the influence of near-face stress rotation. Interna- tional Journal of Rock Mechanics and Mining Sciences 2004;41(5):785e812. Diederichs MS. The 2003 Canadian Geotechnical Colloquium: mechanistic interpretation and practical application of damage and spalling prediction criteria e for deep tunnelling. Canadian Geotechnical Journal 2007;44(9):1082 116. Dr. A. Purwodihardjo is a Principal of Golder Associates in Funkhouser MR, McQueen LB, Boultbee NL, Humphries RW, Carvalho JL, Stabler J. Brisbane. He began his career in Indonesia as a civil and Low cover considerations for the large tunnels on the north south Bypass structural engineer, and later completed his MSc and PhD tunnel project, Brisbane. In: 44th US rock mechanics symposium and 5th US- degrees in Tunnelling/Geotechnical Engineering in France. e Canada rock mechanics symposium. ARMA; 2010a. p. 1723 30. He worked in the United Kingdom as a Senior Tunnelling Funkhouser MR, Stabler J, McQueen LB, Boultbee NL. Construction aspects of Engineer with W.S. Atkins and moved to Australia in 2005 tunnelling and temporary rock support for the North South Bypass Tunnel. In: to join Golder Associates. He consults in the areas of soil/ Proceedings of ITA-AITES world tunnel conference 2010. Vancouver, Canada; rock tunnelling design, soil/rockestructure interaction, 2010. deep excavations, retaining walls, deep and shallow Golder Associates. Geotechnical interpretative report - driven tunnels, north south foundations, embankment design, soft-soil engineering, Bypass tunnel, Brisbane. Report reference NSBT-0802-GT-RP-055005[05]. ground improvement and geotechnical instrumentation. Golder Associates; 2009. He worked on the Brisbane tunnels and completed the Hammah RE, Yacoub TE, Corkum BC, Curran JH. The shear strength reduction detailed design of the temporary support for the Lutwyche method for the generalized Hoek-Brown criterion. In: The 40th US symposium caverns on the APL project, Brisbane and was the geotech- on rock mechanics (USRMS): rock mechanics for energy, mineral and infra- nical designer for CLEM7 tunnel. structure development in the northern regions. ARMA; 2005. Hoek E, Brown ET. Empirical strength criterion for rock masses. Journal of the Geotechnical Engineering Division ASCE 1980b;106(9):1013e35. Hoek E, Brown ET. Practical estimates of rock mass strength. International Journal of S.V.L. Barrett is a Principal of Golder Associates in Mel- Rock Mechanics and Mining Sciences 1997;34(8):1165e86. bourne and the leader of the Golder Global Tunnelling Hoek E, Brown ET. The Hoek-Brown failure criterion e a 1988 update. In: Curran JH, Technical Community. He is responsible for growing the editor. Proceedings of the 15th Canadian rock mechanics symposium. Toronto, global tunnelling and mine access practice. He is a rock Canada: Civil Engineering Department, University of Toronto; 1988. mechanics engineer with national and international ex- Hoek E, Brown ET. Underground excavations in rock. London: The Institution of periences in both civil engineering and mining projects. In Mining and Metallurgy; 1980a. these roles, he has worked on developing projects from Hoek E, Carranza-Torres C, Corkum B. Hoek-Brown criterion e 2002 edition. In: initial concepts to detailed design and construction. He Hannah R, Bawden W, Curran J, Telesnicki M, editors. Mining and tunnelling worked on the Brisbane tunnels and completed the innovation and opportunity, Proceedings of the 5th North American Rock detailed design of the temporary support for the Lutwyche Mechanics Symposiun and 17th Tunnelling Association of Canada Conference caverns in the APL project and the temporary support (NARMS-TAC 2002), vol. 1. Toronto, Canada: University of Toronto; 2002. design for roadheader caverns and ramps for the CLEM7 p. 267e73. tunnel.