Figure 1. Layout of inclined bar anchors in the invert of Burnley Tunnel hydrostatic head was greatest, the ground anchors were installed in downward inclined arrays at 1.1 m (3.6 ft) centres along the length of the tunnel (Figure 1). At either side of this section, where there was a reduced hydrostatic head, the engineers installed vertical twin anchors at centres varying from 1.1 to 1.3 m (3.6 to 4.3 ft) along the length of tunnel (Figure 2). Gun Barrel Tests Grouted Anchors Help Seal Tunnel Joints The Burnley Tunnel, forming part of the Melbourne City Link project, was constructed between 1996 and 2000. The three lane east-bound tunnel is 3.4 km (2.1 mi) long comprising 2.9 km (1.8 mi) of driven tunnel, part of which runs 60 m (197 ft) beneath the River Yarra. The tunnel section, a modified ellipsoid shape 16 m wide x 9 m high (52.5 ft wide x 29.5 ft high), was driven using the New Austrian Tunnelling Method. The tunnel walls had a 300 to 450 mm (11.8 to 17.7 in) thick concrete lining, and to seal the tunnel against groundwater pressures an additional 1.8 m (5.9 ft) thick unreinforced concrete slab was placed over 2.6 km (1.6 mi) of tunnel floor. In late 1999, following a 10-month period of grouting to seal construction joints in the tanking membrane, concerns were raised about the ability of the tunnel to withstand the 60 m (197 ft) head of water. During subsequent water injection tests at pressures up to 600 kPa (87 psi), several invert slab panels (typically 12 m long and 12.7 m wide or 39.4 ft long and 41.7 ft wide) heaved up to 200 mm (7.9 in), and cracking was observed in others. To facilitate structural remediation of the invert, engineers installed some 5,200 permanent ground anchors in the underlying Melbourne mudstone to tie down 160 panels over a 2 km (1.24 mi) length of the concrete invert, the anchor lengths varying from 7 m to 13 m (23 ft to 43 ft) with working loads of 700 kN and 1,000 kN (70 tons and 100 tons). In the middle section of the tunnel, where the AUTHORS: Devon Mothersille, Managing Director, Geoserve Global Ltd & Single Bore Multiple Anchor Harrogate, England Stuart Littlejohn, Emeritus Professor of Civil Engineering, University of Bradford, Bradford, England 46 • DEEP FOUNDATIONS • JULY/AUG 2012 The concept of placing anchor tendons in grout confined by steel tubes to simulate a rock mass is well established and was used at Burnley Tunnel to demonstrate the performance of the corrosion protection system. The testing confirmed rupturing of the corrosion protection at the transition point between plastic corrugated duct in the fixed anchor length and smooth plastic duct in the free anchor length. This observation necessitated the implemen- tation of higher levels of quality control during tendon fabrication with particular emphasis on adequate 200 mm (7.9 in) lap lengths between the two types of duct and correct application of the heatshrink bonding sleeve. Grout Simulation Tests Preliminary grouting trials were carried out to demonstrate the effectiveness of the grouting operations by the execution of a standard grouting procedure with the tendon installed in a rigid plastic pipe to simulate a borehole. The grouted tubes were dissected after grout curing to inspect the integrity of the grouting particularly in the vicinity of the main tendon com- ponents, e.g., centraliser/spacer, compres- sible packer and coupler. Dissection of the cured grout simulation identified a 400 mm (15.7 in) long, 80 mm (3 in) wide zone of poor grout, including a void underneath the compressible packer. This unsatisfactory feature necessitated the introduction of a grout vent tube which allowed the passage of grout through the packer and into the free anchor length.