When the Bulk Water Alliance began construction of the Enlarged Cotter Dam in 2012, the project carried significant ambitions. At 87 metres, it would become Australia's tallest roller compacted concrete (RCC) dam, increasing water storage from 4 to 78 gigalitres for the ACT. But ambition creates complexity, and for the foundation grouting contractor, the complexity was fundamentally spatial.
The grout curtain - a 60m deep, 330m long seepage barrier extending into 200 MPa Rhyolite - had to be constructed from within the dam's access galleries. These galleries, designed for inspection rather than construction, measured just 3.5m wide and high. They ascended both abutments at a 42-degree angle, climbing 80m of elevation. Standard drilling equipment wouldn't fit, standard access methods wouldn't work, and the RCC construction above could not wait.
The Access Challenge
Moving a drill rig up a 42-degree incline inside a confined gallery presents challenges that don't yield to conventional solutions. The geometry alone ruled out most tracked equipment - anything too wide, too tall, or too heavy simply wouldn't navigate the space. But the rock required power, and Rhyolite at 200 MPa demands robust drilling capacity. The depths specified for the grout curtain, reaching up to 60m, ruled out any lightweight alternatives that might have been easier to manoeuvre.
The solution involved Geovert engineering a 5-tonne hydraulic drill rig small enough to navigate the 3.5m gallery width while maintaining sufficient power for deep, accurate drilling in hard rock. The rig was electric over hydraulic, eliminating diesel emissions in the enclosed space, a consideration that became important during the extended operations that foundation grouting requires. Engineering the rig was only half the problem however, moving it was the other half.
Large hydraulic spool winches were installed at the top of each gallery incline to haul both the custom tracked and a diamond coring drill rigs up and down the stairs. With work continuing on the dam crest above, including long drainage holes to intersect the gallery. The time pressure added a further level of complexity to operations that were already constrained.
Precision Under Pressure
In the confined galleries automation became necessity rather than convenience. The GIN (Grout Intensity Number) grouting method specified by the engineers required exact control of injection pressures, volumes, and flow rates across primary, secondary, tertiary, and quaternary grouting phases. In a gallery barely wider than the equipment, with operators working alongside active drilling and grouting operations, manual control systems would have been both unworkable and unsafe.
Geovert installed a Jean Lutz CINAUT system, a fully computer automated grout injection control platform that monitored and adjusted parameters in real-time, ensuring the GIN stop criteria were met and recorded accurately at each stage. This level of automation wasn't about technological sophistication for its own sake; it was about maintaining the precise documentation that dam safety requires while reducing risk in a space where human error could have serious consequences.
The drilling itself demanded similar exactitude, particularly for the pressure relief drains. These holes were drilled from the dam crest downward using down-the-hole hammer techniques, penetrating up to 65m to intersect a breakthrough target on the gallery roof just 1.5m wide. The allowable deviation was 2 percent, which in practical terms meant maintaining near-vertical alignment through dozens of metres of rock, with breakthrough accuracy measured in centimetres rather than metres. Missing the target would have meant re-drilling from the crest at considerable cost in time and resources.
Sequencing and Scale
While the access constraints and technical requirements naturally drew focus, the project's scale created its own pressures that shaped how the work proceeded. Approximately 8,350 metres of percussion drilling supported the GIN grouting program, with another 7,000 metres of drilling creating drainage paths through the dam and foundation. An additional 700 metres of NQ coring and 1,500 metres of ground anchor holes rounded out the drilling program.
This volume of work, conducted in restricted spaces with custom equipment and automated systems, required careful sequencing. At peak operations, five drill rigs worked simultaneously - some on the dam crest installing spillway anchors, others in the galleries advancing the grout curtain and drainage program.
The project's compressed duration for the grouting and drainage work alone reflected this complexity, but it was also a deliberate decision by the Bulk Water Alliance to engage a single contractor capable of handling the full scope. Restricted access drilling, foundation grouting, drainage works, depressurisation drilling, rock bolting, and rock scaling were all managed by one team, eliminating the risks that typically emerge when multiple specialists work in confined, interdependent sequences where coordination failures can compound quickly.
What the Work Required
Projects like the Enlarged Cotter Dam test capabilities that don't always appear in equipment specifications or company credentials. The custom drill rig worked because it was engineered for a specific set of constraints, not adapted from existing equipment with compromises. The winch system worked because it was installed and operated by personnel who understood rigging loads, angles, and safety factors in confined settings. The automated grouting system worked because it was integrated into a broader quality control framework that prioritised documentation and real-time monitoring throughout the extended grouting phases.
These aren't innovations in the breakthrough sense, they're applications of sound engineering to difficult constraints, which is often what complex infrastructure projects actually require - disciplined problem-solving applied systematically to geometry, geology, access and time.
Icon Water's Enlarged Cotter Dam now operates as designed. The confined galleries that presented such challenges during construction serve their intended purpose of inspection and monitoring, while the foundation grout curtain and drainage system continue to perform as specified.