Catagunya Dam restoration

Catagunya Dam is located on the Derwent River in the southeast of Tasmania, Australia. It is the third in a cascade of six dams forming the lower Derwent power development. Designed in the late 1950s and completed in 1962, Catagunya is a 49m high concrete gravity dam which relies on a large number of 200 ton capacity post-tensioned steel cables to provide the necessary structural stability against the stored water load. Its design was considered leading edge, being the highest post-tensioned dam in the world at the time of its construction, and the designers adopted a 50-year design life for the anchors. It was estimated that the use of post tensioning provided a saving in the order of 20% compared with a conventional gravity dam.

During construction of the dam in the early 1960s, 412 post-tensioned anchors were installed but the integrity of the original anchors can no longer be assured. The stability of the dam has been restored over the past two years using 92 modern, large diameter and corrosion protected, post-tensioned anchors that can be monitored for deterioration. These are the most highly stressed anchors applied to a dam at this time.

Construction challenges and solutions

A number of challenges were addressed during implementation, including:

• Installing more than half the anchors within an operating spillway, utilising a limited construction window over the summer months.

• Providing access for drilling equipment and installation of the anchors well below the spillway crest on a 54° degree slope, 25m above the riverbed, and demobilising these platforms sufficiently to allow floods to pass during the winter months.

• Replacing severed surface reinforcement with 9m long carbon fibre rods.

In order to gain access to the spillway of Catagunya dam for installation of the temporary access platform brackets on the spillway and the carbon fibre tensile reinforcing on the spillway face, a purpose designed and built travelling gantry was suspended from the spillway crest. This gantry had five working deck levels, with four of these providing access to the full length of the 9m long carbon fibre rods which were to be installed.

Two 200m long temporary platforms were constructed to access the spillway. The upper platform was designed to provide access for the drilling, installation, and stressing equipment, as well as to provide safe and efficient egress in times of flood. The lower platform was used to give direct access to the anchor hole locations for installation of anchorage assemblies.

With the unique cross-section of Catagunya dam, tensile steel reinforcing is required on the top surface of the downstream face of the spillway to support the large cantilevers overhanging the spillway. This steel was to be severed as the large diameter holes for the spillway headblocks were cored, so a replacement was required prior to the coring.

A major innovation of the project was the design and installation of carbon fibre reinforcing into the spillway face to provide tensile reinforcing for the cantilever section of the dam. Used widely in the bridge industry as a method to upgrade and restore structures, epoxy bonded carbon fibre strips are a very cost effective and simple solution. However, to date, these have not been used for a similar application in the dams industry.

The 9m long carbon fibre rods were installed in the face of the spillway by wall sawing a 9m long, 15mm wide, and 90mm deep slot in the concrete. Each of the six 8mm diameter rods were installed individually, guided into place on a bed of epoxy adhesive injected into the slot and bonded to the concrete. The overall solution took around four months to complete, installing over 10km of carbon fibre rod.

A requirement of the design of Catagunya dam was to maintain the hydraulic performance of the ogee crest spillway. This meant anchors installed within the spillway were required to be constructed within the current profile. Coring of 1.2m diameter holes into the slope, 3m into the dam was undertaken to install the precast reinforced concrete headblocks to distribute loads into the structure. Abutment anchors were installed onto a specially designed concrete beam, located above the level of the parapet walls.

The drilling was undertaken using a 350mm downhole hammer (DHH) from a Rotomech drilling rig. The longest drill hole was circa 78m through the concrete and into the dam foundation, and in total 6.2km was drilled throughout the project.

The modern post-tensioned anchors were all fabricated on site using specialist equipment to open and grease the cable, and then install into a 20mm HDPE sleeve over the free length. The bond length is cleaned back and left bare, with all 91 lengths of cable twisted into an hour glass shape to increase the bond with the dolerite rock in the area.

The complete anchor is installed into a part corrugated, part smooth polyethylene sheath within the anchor hole, and grouted into the hole using Class G Oilwell and GP cement. Once the grout reaches its strength, a purpose built 2200 tonne capacity hydraulic jack is used to load the anchor to over 70% of its minimum breaking load, making these anchors the highest stressed ground anchors in the world.

Long term maintenance

The newly installed post tensioned anchors will be monitored five-yearly by Hydro Tasmania, with the first of these checks to be undertaken between November 2010 and March 2011. To undertake these tests, the travelling gantry has been modified to carry all load monitoring equipment.

The tests for the first four anchors installed on the spillway have already been completed and found the anchors in excellent condition. The lift off tests performed on these anchors has found only a very small loss in load over the past year, well within a tolerable margin and below what has been observed in other anchoring jobs of this size.