The Ross River Dam is located 20km southwest of Townsville, Queensland an is not only the major water supply for Townsville and Thuringowa, but also protects these cities from flooding. The dam was built in 1971, with a catchment
area of 750km2 and a capacity of 219,000 mega litres. Beside the 30m high, 40m wide concrete spillway structure, the dam has an earth embankment of some 8km and averages a height of 27m.

As a result of detailed investigation, with reference to local and international dam standards, the entire dam was deemed to need upgrading to achieve compliance at a cost of approximately US$70m. The improvements to the concrete spillway and installation of radial gates have increased capacity by 9% (+21,000 mega litres) providing an additional four months' water supply, as well as allowing better flood control. significant embankment works, including 1,900,000m3 of earthworks, have upgraded its safety to an acceptable level.

Upgrading the spillway involved lowering the crest some 3.8m and providing a new curved "ogee" crest to the original height. In addition, two new piers and three radial gates were fitted. Structural Systems has undertaken a significant share of Australian dam stabilisation projects - including the longest and largest capacity permanent anchors ever undertaken in Australia (91 x Ø15.2 x 142.2m) at Canning Dam - and were the preferred choice of the client , JHMJV to undertake this critical work at the Ross River Dam. The company was engaged to complete nine permanent ground anchors - three in the top of each of the three ogee crests - and around 13m spillway bays, plus additional post-tensioning work to tie back the radial gates, at the trunnion mounting, to two piers and two abutments.

Anchoring works included the fabrications, transportation, installation, grouting, stressing and monitoring of the permanent ground anchors. The anchors were up to 54m long, each providing a force of 875t. An anchor consisted of 50 15.2mm strands, individually greased and sheathed over the free length allowing the strand to extend without restraint, while the remaining 11m of bare strand, in the bond length, provides fixity via grout to the surrounding rock.

Heavy duty corrugated polyethylene sheathing in the bond zone and smooth sheathing over the free length form a flexible, but impermeable membrane to protect the strand tendon against corrosion over its 100+ year design life.

The anchors were fabricated at site on a specialist bed, using a purpose-built strand pusher and greasing machine. This machine opens each strand, separating the individual seven wires, fully greases each wire, then reforms the strand and pushes it into the 20mm HDPE sleeve.

After fabrication, the anchors were transferred into a storage rack. Sheath for the main tendon, was prefabricated as a complete unit with spacers and grout tubes, prior to installation into the dam by crane.

The anchors were individually loaded and transported on a specially designed trolley system and installed into pre-drilled holes in the dam crest, through into the underlying bedrock. This was followed by a meticulous grouting process with 0.36w/c ratio Oilwell cement grout. After the grout had achieved strength and an age of 21 days, anchors were stressed to their test and lock-off loads using a 1,250t capacity hydraulic jack. Each anchor's residual load was checked a minimum of 48 hours after stressing to verify the residual load. Upon confirmations of the correct loading, the tendons were trimmed, corrosion protected and fully capped.

Initially the trunnion tendons were detailed in 75mm bar but, due to installation difficulties, an alternative was proposed to convert the tendons from bar to strand. The works were spread over two piers, each with 4-26 15.2mm diameter tendons, and two abutments with 2-26 15.2mm diameter and 1-31 15.2mm diameter tendons.

At the piers which were newly constructed, heavy duty HDPE pipes were cast in during forming. A small window was left at the base to allow fitting of the four dead end anchorages and, at the top, the trunnion mounting incorporated provision for the stressing tendon anchorages. The existing abutments required careful drilling from the top to intersect the lower anchorage locations, which was excavated into the side of the abutment. HDPE pipe was then installed and later grouted. Through detailing was necessary in order to maintain the corrosion protection systems.

Works commenced in December 2006 - and were two days from completion, when a 4m high major flood engulfed the site. It was six weeks before we could gain access to the work area again and unfortunately the stressed, but ungrouted abutment tendons were damaged by corrosion and required replacement which we completed in May 2007.

The flood event loaded and submerged the permanent anchors, demonstrating their effectiveness in providing additional stability to the dam. ON close inspection the anchorage zones were 100% dry, even after being submerged for a significant time, confirming their reliability, the quality of the system and the workmanship of the team.