Stuart Cartwright
Waterview Connection Tunnels Engineering Geology Assessment of East Coast Bays Formation from Investigation through to Construction S. Cartwright, D. Koumoutsakos, B. Hill, and C. Morrison
Project Overview Location & background NZ Transport Agency project to complete Auckland s Western Ring Route ~5km of motorway NZ s biggest roading construction project ($1.4B budget) to date
Project Overview The Tunnels Cross passages are 9m-11m long and spaced 150m apart 15 no. standard cross passages 1 no. special cross passage for the low point sump Source: www.nzta.govt.nz/waterviewtunnel
Project Overview The Tunnels Typical Cross Passage Low Point Sump Cross Passage
Project Overview Geological Setting ECBF = East Coast Bays Formation and was deposited in the Early Miocene Waitemata Basin ~18Ma ECBF is contemporaneous with the Parnell Grit member Uplift and erosion Tauranga Group is Pleistocene age alluvial deposit that sits unconformably on Waitemata Group <1.2Ma 60,000 years ago Mt Albert eruption blanketed the site in basalt lava Oakley Creek eroded valley along margins of the basalt flow Evolution of site
Typical ECBF Characteristics From outcrop to tunnel
Typical ECBF Characteristics From outcrop to tunnel
Typical ECBF Characteristics Lithology and Unconfined Compressive Strength Very weak sandstone and siltstone (EU2) Typical ECBF; 1-5 MPa Igneous and Northland Allochthon provenance High in smectite and illite, prone to sticking and swelling Uncemented sandstone (EU1) Grain-locked sands, poor in volcaniclastic sediment; <1MPa Lacks the same clay binder as EU2 Weak, volcaniclastic sandstone (EU3) Parnell Grit member, greater concentration of reactive clays; 5-25 MPa
Ground Investigations to Ground Model Investigations PQ size borehole investigations with SPT s Inclined boreholes, Optical and Acoustic Televiewers Pressuremeters, packer and pump tests Triaxial testing, Atterberg limits, PSD s, abrasivity tests Probe drilling before opening cross passages 3D model of SPT N >50 (approximated slightly weathered rock head) surface Strata orientation Strength and stiffness characterisation of rock (EU1, EU2, EU3)
Ground Model Tunnel Reaches Reach 6 Reach 1 Reach 2 Reach 3 Reach 4 Reach 5
Construction Observations TBM Cutterhead Inspections GSI typically very high and in the 70-90 range Persistent joints were rarely observed Slabbing occurred where the bedding dip direction was into the face Sometimes lead to unravelling in the crown at depths >30m in uncemented sandstone Water bearing faults Sustained flows of 30 l/s in Reach 3, with unravelling of highly fractured siltstone adjacent to XP12,
Construction Observations XP17 EU1, 20m depth EU1 EU2 Thick uncemented sandstone beds with thin siltstone and carbonaceous beds GSI 80-90 RMR 67-75 Damp with minor seepage Small scale spalling of uncemented sandstone in the crown between the canopy tubes and on the heading face Fast excavation rates
Construction Observations XP17 EU1, 20m depth CPT TRACE qc = 10 MPa qc = 20 MPa SPT N=30 SPT N>50 19m DEPTH
Construction Observations XP09 EU1, 41m depth EU1 EU2 Uncemented sandstone and carbonaceous laminations in the crown Series of intersecting faults with small offsets GSI 60-70 RMR 40-60 Damp with minor seepage Wedges <1m3 and some spalling of uncemented sandstone An increase in strain readings on props supporting the opening during excavation (the only cross passage this observation was made)
Construction Observations XP14 EU2, 9.1m span, 34m depth EU2 Typical of most of the cross passages with thickly bedded sandstone and thin siltstone No significant joint sets (other than bedding) GSI 80-90, similar to M1 range of Hoek et al (2005) for Molasse rock types RMR 76-81, Class II rock (Bieniawski, 1989) Damp with minor seepage No rock failure observed
Construction Observations XP13 EU3, 29m depth EU3 Massive, coarse-grained volcaniclastic sandstone with steeply inclined bedding and very wide joint spacing GSI 80-90 (M1) RMR 77-82 (Class II) Damp with minor seepage Slow excavation rates No rock failure observed
Construction Observations XP12 EU3, grouted rock mass, 38m depth EU3 Massive, highly fractured siltstone (UCS >5 MPa) Highly permeable rock mass (see France, 2017) Pre-excavation grouting undertaken, wet to damp during excavation (see Maclean et al, 2017) GSI 35-50 RMR 35-56 (Class III-IV) No rock failure observed
Conclusions ECBF was typically an excellent tunnelling medium Slabbing of gently inclined to sub-horizontal bedded ECBF in the crown was the typical failure mechanism, but rarely occurred Lack of persistent and closely spaced joints meant wedges were rare Intact strength often governed the ground behaviour EU1 rock type: Spalling occurred when normal stress exceeded UCS. For the depth of the tunnels this was limited to locations of uncemented sandstone at >30m depth. The frequency of slabbing also increased. Tectonically undisturbed EU2 rock type: Encountered in most cross passages, typically had a high GSI of 60-85 and RMR of 70-80 and no over excavation GSI & RMR was highly variable in Tunnel Reach 3 where EU3 was prevalent, with RMR reducing to 35-60 in one cross passage
Acknowledgments My co-authors New Zealand Transport Agency Well-Connected Alliance Sumi Eratne Ewart Barnsley Geotechnical team and all the Engineering Geologists that contributed to the geotechnical investigation campaigns throughout the Project s lifespan Construction Phase Support team