In 1973, a Clutha Valley Development Commission was set up to evaluate potential hydro-electric dam sites along the Clutha River. Test drilling in the Cromwell Gorge confirmed what local people already knew, that the gorge was highly unstable. A few years later in 1976, the National Government convened a Clutha Valley Advisory Committee to assess all the available information and to make a decision regarding proposals for high and low dams.
The Advisory Committee acknowledged that there was a serious instability issue, and finally voted to recommend the low dam option (Scheme H), however three members of the Committee who knew the gorge well, voted against any dam at all, referring to the prospect of inevitable landslide issues.
Surprisingly, gorge instability was given little consideration when the high dam option (Scheme F) was chosen. But in 1982 dam workers discovered a faultline directly under the dam and spillways. Investigations revealed that this was a River Channel Fault branching from the main Cairnmuir-Dunstan Fault crossing the gorge some 3kms above the dam. Vast amounts of slurry concrete were pumped into tunnels across the fault called “shear pins” to supposedly lock the fault, even though the fault was 12-15kms deep and such “dental concrete” would be instantly broken in a large earthquake.
It was acknowledged that a fault directly through the dam and spillways warranted some attention. The dam was redesigned around the fault and the "slip-joint" was invented to sit over the fault. The dam was literally built in two "halves."
As time passed, more problems came to light. In April 1989, an intense investigation began into landslide issues, involving an international team of up to 40 geologists. This investigation revealed large numbers of highly permeable loess underlying large areas of broken rock slides, throughout the gorge. It was feared that when the reservoir was raised, the water would permeate through the toes of the slides, triggering landslides into the reservoir, creating waves that could overtop the dam.
Fourteen major slide zones were identified in the Cromwell Gorge, including one beside the dam itself - the Clyde landslide. Another three major slide zones were discovered in the affected part of the Kawarau Gorge, including the Ripponvale landslide. Water was the primary issue, since water entering the permeable loess (fine layers beneath the slides) from above or below, would literally lubricate them, resulting in accelerated movement or a sudden failure.
The rate of landslide "creep" was difficult to measure since there was insufficient data available upon which to accurately assess the amount of movement in each slide. The new road cutting above the old road had actually increased the rate of movement in many slides by removing material from the toes of the slides. Geologists soon installed instruments and estimated movements ranging from millimetres to centimetres per week or per year, subject to rain/water and earthquake induced movement. The massive Nile Mile slide, seven kilometres long and 200 metres high, was moving several centimetres per week. The extent of the problem was vast. The international team of geologists described many of the slides as "potentially catastrophic" and "very dangerous." The cost of the proposed stabilisation measures kept going up, but the work began.
Major remedial work was undertaken at nine of the seventeen landslide zones, involving toe buttressing, pumped drainage, gravity drainage, capping to reduce infiltration from above, and the drilling of "grout curtains" to reduce leakage from the reservoir into the toe area of slides.
Eighteen drainage tunnels were drilled into the sides of the gorge. From these "drilling stubs" drainage holes were drilled further into the slides. A total of 140kms of drilling was undertaken for drainage, and 6,500 measuring and monitoring instruments were installed.
To install "grout curtains," holes were drilled at regular intervals to varying depths into which concrete and water was pumped under pressure. Many of these holes collapsed during drilling because of the loose nature of the material, and each time this happened, the drillers pumped in concrete and water, and later re-drilled it. This was an extraordinarily time-consuming and expensive exercise.
Buttressing rock was also placed across the base of some slides to provide some frictional resistance. A total of 5 million cubic metres of rock was used in buttressing work.
The Cairnmuir landslide posed a significant challenge, since the upper material was particularly loose and permeable, and riddled with rabbit holes. A network of drainage tunnels combined with buttressing at the toe of the slide failed to stop it moving, so it was eventually decided to pave and terrace the top of the slide to seal out water, creating a bizarre amphitheatre. The rate of movement slowed, but the additional weight means that any increased infiltration could result in an even greater failure.
In the end, the investigation and stabilisation work cost a staggering $936 million. Work on the Nine-Mile landslide alone, reportedly cost $60 million.
Landslide "creep" has been reduced, but monitoring indicates the continuing potential for slide zone failure. Monitoring has shown that movement of a known "active" part of the Brewery Creek landslide is triggered when the water level exceeds a critical threshold in a key piezometer (instrument for measuring hydraulic pressure). Records also show that movement of part of the Ripponvale landslide increases following prolonged rainfall, and that it is highly sensitive. Data indicates that the rate of movement of the Ripponvale landslide increases when the cumulative rainfall during a period of 3 to 4 months exceeds a total of about 300 mm. It has been suggested, alarmingly, that a failure of the seven kilometre long Nile Mile landslide could form a debris dam causing a catastrophic wave event, followed by widespread inundation in the Cromwell area.
Despite all that has been done, and the mind-boggling cost, major landslide zones in the Cromwell Gorge are still prone to failure under the impetus of heavy rain events, and of course earthquakes. The landslide risk has not been removed, and no one knows when the next major landslide will occur.