epping to chatswood rail link_soft ground tunnel

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SOFT GROUND TUNNEL, CHATSWOOD, SYDNEY 1 July 2006 EPPING TO CHATSWOOD RAIL LINK, SOFT GROUND TUNNEL CHATSWOOD, SYDNEY 1.0 INTRODUCTION Most of the 13km long Epping Rail link in Sydney comprises twin 7.2m diameter tunnels bored by TBM through Hawkesbury Sandstone. However, where the tunnels emerge to the surface to join the North Shore rail line north of Chatswood, they pass through weathered Ashfield Shale that forms a capping over the sandstone. There is approximately 170m of twin tunnel within residual clays and extremely to highly weathered shale, with a depth of cover as low as 5m. This length of tunnel is called the Chatswood Soft Ground Tunnel and a key feature (see Figure 1) is that the tunnels had to be constructed under the embankment of the operating North Shore line. The initial design was undertaken by specialist tunnel consultants from Austria. The design comprised 7.8m diameter tunnels with typical support as follows (Reference 1): Initial Lining Split headings, bench and invert excavation sequence 170mm to 250mm shotcrete two layers of welded mesh lattice girders at 0.75m to 1m centres, full round for first 35m and beneath Boundary Street bridge. Waterproof Membrane 10mm thick, continuously welded full parameter Final Lining 200mm or 300mm cast in situ concrete reinforced for most of the length with 16mm bars at 200mm both ways. Pells Sullivan Meynink Pty Ltd was requested by the Thiess Hochtief John Venture to take design responsibility for this length of tunnel. This note summarises the final design and construction performance. 2.0 GEOLOGY Figure 1 summarises the geology at the portal. Ground conditions are poorest at the portal but steadily improve to the north as the tunnel descends and the ground surface rises. At the portal the crown is at the boundary between residual clay soils and Class V Shale (as defined in Pells et. al. 1999). The residual soils in the crown have unconfined compressive strengths <1MPa and at mid height of the face the substance strengths are < 2MPa. Defects comprise near horizontal bedding and joints dipping at between 40° and 90°. The quality of shale improves with depth, such that at the portal, the floor of the tunnel comprises slightly weathered shale. From about 100m inbye, the entire tunnel lies within Class II Shale. Figure 1: Cross section at the portal showing geology and existing North Shore Railway.

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Page 1: Epping to Chatswood Rail Link_Soft Ground Tunnel

SOFT GROUND TUNNEL, CHATSWOOD, SYDNEY

1 July 2006

EPPING TO CHATSWOOD RAIL LINK,SOFT GROUND TUNNELCHATSWOOD, SYDNEY

1.0 INTRODUCTION

Most of the 13km long Epping Rail link inSydney comprises twin 7.2m diameter tunnelsbored by TBM through Hawkesbury Sandstone.However, where the tunnels emerge to thesurface to join the North Shore rail line north ofChatswood, they pass through weatheredAshfield Shale that forms a capping over thesandstone. There is approximately 170m oftwin tunnel within residual clays and extremelyto highly weathered shale, with a depth ofcover as low as 5m. This length of tunnel iscalled the Chatswood Soft Ground Tunnel anda key feature (see Figure 1) is that the tunnelshad to be constructed under the embankmentof the operating North Shore line.

The initial design was undertaken by specialisttunnel consultants from Austria. The designcomprised 7.8m diameter tunnels with typicalsupport as follows (Reference 1):

Initial Lining Split headings, bench and invert

excavation sequence 170mm to 250mm shotcrete two layers of welded mesh lattice girders at 0.75m to 1m centres,

full round for first 35m and beneathBoundary Street bridge.

Waterproof Membrane 10mm thick, continuously welded full parameter

Final Lining 200mm or 300mm cast in situ concrete reinforced for most of the length with

16mm bars at 200mm both ways.

Pells Sullivan Meynink Pty Ltd was requestedby the Thiess Hochtief John Venture to takedesign responsibility for this length of tunnel.This note summarises the final design andconstruction performance.

2.0 GEOLOGY

Figure 1 summarises the geology at the portal.

Ground conditions are poorest at the portal butsteadily improve to the north as the tunneldescends and the ground surface rises. At theportal the crown is at the boundary betweenresidual clay soils and Class V Shale (asdefined in Pells et. al. 1999). The residual soilsin the crown have unconfined compressivestrengths <1MPa and at mid height of the facethe substance strengths are < 2MPa. Defectscomprise near horizontal bedding and jointsdipping at between 40° and 90°.

The quality of shale improves with depth, suchthat at the portal, the floor of the tunnelcomprises slightly weathered shale. Fromabout 100m inbye, the entire tunnel lies withinClass II Shale.

Figure 1: Cross section at the portal showing geology and existing North Shore Railway.

Page 2: Epping to Chatswood Rail Link_Soft Ground Tunnel

SOFT GROUND TUNNEL, CHATSWOOD, SYDNEY

2 July 2006

3.0 DESIGN

The poor ground conditions together with theneed to limit settlement of the overlying rail lineconstrained the construction sequence andsupport design.

The tunnels were initially excavated with smallexcavator. An AM 105 Alpine Minerroadheader was used from about 15 m inbye,which imposes some constraints on the tunnelgeometry and construction sequence, notably:

the shape of the head dictates a flatcentral portion of the crown

a maximum cut height of 6 m, whichmeans a heading and bench excavationsequence

The finite element package Phase2

wasused to assess the induced stresses withinthe rock mass and the tunnel lining, as wellas surface settlement. The ground reactioncurve from this analysis is shown in Figure 3.

It was concluded from this curve that it wasessential that support be installed close tothe face so that the support would actuallycarry a high proportion of the overburdenstresses thereby avoiding excessive crownand sidewall displacements.

Figure 2: Tunnel geometry.

Under the contract for the “soft ground” tunnel,the lining was required to carry the fulloverburden weight. Yet in practice this isimpossible, as some displacements will alwaysoccur prior to the installation of support. Hence,while the structural capacity of the lining wasdesigned for the overburden load, theconstruction sequence was designed to controlsettlement.

The analysis package FLAC3D was used tomodel the three-dimensional geometry thatexists at the portal, as shown in Figure 4.This package enabled modelling of the effectof canopy tubes and steel sets.

Page 3: Epping to Chatswood Rail Link_Soft Ground Tunnel

SOFT GROUND TUNNEL, CHATSWOOD, SYDNEY

3 July 2006

Figure 3: Ground reaction curve for thewestern tunnel, showing a collapsemechanism occurring after about40 mm crown displacement.

Figure 4: FLAC3D model of portalshowing steel sets and canopytubes.

Figure 5: Steel set being placed underneath previously installed spilling bars.Note residual clay in the tunnel crown.

A series of support types was designed toaccount for varying overburden loads andground conditions. The various primarysupport types included canopy tubes, spilingbars, soil nails, rock bolts, steel sets, andstructural shotcrete.

The permanent lining comprises a 260mmthick steel fibre reinforced shotcrete shell witha formed concrete invert. A sprayedmembrane in the crown acts as an umbrellafor any groundwater percolating from thesurface. The groundwater table is drawn downto approximately invert level.

Page 4: Epping to Chatswood Rail Link_Soft Ground Tunnel

SOFT GROUND TUNNEL, CHATSWOOD, SYDNEY

4 July 2006

4.0 MONITORING

The design analyses indicated that groundmovements above the tunnels were verysensitive to the proportion of overburdenpressure actually carried by the groundsupport – this in turn being very sensitive tothe proximity to the face of support installation,and effective contact between the steel setsand the rock. With tunnel support in thetunnels carrying 80% of overburden, theground surface settlements were predicted tobe as little as 8mm. However, when the liningcarries between 50% and 80%, settlements ofup to about 60mm are calculated. Based onthe FLAC3D analyses it was concluded thatwith near perfect construction of the steel sets,canopy tubes and spiling bars, that the liningwould only ever carry between 60% and 70%of overburden pressure (Reference 2). Onthis basis it was predicted that when thewestern tunnel was excavated the surfacesettlements would be 8mm to 10mm,increasing to 30mm to 40mm with excavationof the eastern tunnel.

Surface settlements monitoring just in-byefrom the portal gave settlements rangingbetween 15mm and 45mm.

REFERENCES

1. D2 Consult, Parramatta Rail Link, Chatswood Portal Design Report, 5 March 2004.

2. Pells Sullivan Meynink Pty Ltd, Internal Memorandum PSM649.TM31 of 28 June 2004.