Scrivener Dam Anchor Bolt Remediation

The NCA completed major engineering works on Scrivener Dam on 11 April 2014. The works involved the remediation of the Dam's original 'hammerhead' anchor bolts which were corroded. The works ensure public safety is maintained and provide continued effective function of the anchor bolts for the next 100 years. Scrivener Dam was originally commissioned in 1963.

On 7 September 2012, the NCA referred the remediation works to the Parliamentary Standing Committee on Public Works (PWC). On 13 September 2012, the PWC endorsed the works proceeding for all flap gates. On 29 October 2012, tenders were called for Head Contractor services. On 21 January 2013, Structural Systems (Western) Pty Ltd was engaged to undertake the remediation works. Remediation works commenced on site on 18 March 2013. The anchor bolts were remediated at each flap gate, working from east to west across the dam wall, and each flap gate returned to service on the following dates:

• Flap Gate 5: 11 July 2013
• Flap Gate 4: 4 September 2013
• Flap Gate 3: 6 November 2013
• Flap Gate 2: 16 January 2014
• Flap Gate 1: 14 March 2014

Example Bolt and Hinge assembly (left: previous, right: new)

Background

Scrivener Dam is a 33 metre high concrete gravity dam incorporating five custom-designed, steel 'fish belly' flap gates. Each flap gate is approximately five metres high by 30 metres wide and pivots about six hinges arranged along the gate's lower horizontal edge. The hinges were connected to the dam wall with four hammerhead anchor bolts, each up to two metres in length. There were 30 hinges and 120 hammerhead bolts in total evenly distributed across the five flap gates.

When required to accommodate flood events, the flap gates are lowered to release water into the downstream concrete spillway. The ability to pivot the flap gates also facilitates programmed maintenance activities. Maintenance works sometimes require a relocatable 'floating' gate to be positioned on the upstream side of the gate to keep the work area sufficiently dry while the gate is lowered and works are undertaken.

The NCA became aware of corrosion issues related to the original anchor bolts in late 2011. During 2012, engineering investigations were conducted and a design was developed and documented, statutory approvals obtained and a head contractor procured to undertake the works.

The works involved remediating the original hammerhead anchor bolts one flap gate at a time.' Anchor remediation works included retrofitting two anchor bolts immediately adjacent to each hinge, or 60 new bolts in total. The introduction of custom-designed hinge bracket extensions enabled the transfer of the full structural load at each hinge to the new bolts. The new anchor bolts at each hinge are longer and stronger than the originally installed anchor bolts. The existing anchor system has been largely left in place and is redundant as the new anchors take up the required load.

The design for the anchor bolt remediation meets current design standards, has proved to be efficient, cost effective and has had a low impact on the dam wall and its surrounds during construction.