An aerial photograph of Paradise Dam, which is currently at 75 per cent.
An aerial photograph of Paradise Dam, which is currently at 75 per cent.

Hearing focus on Paradise’s RCC practice, apron design

THE final day of this week’s Paradise Dam Commission of Inquiry hearings saw two witnesses questioned about the quality practice of roller-compacted concrete, the apron design calculations and the dam’s due diligence report.

Roberto Montavlo, who worked as a project engineer responsible for RCC materials and quality control of Paradise Dam, appeared via teleconference at the hearing yesterday.

While Russell Paton who was working for SunWater throughout the Paradise Dam project, as part of the preliminary design stage of the dam, took the witness stand in person.

Mr Montavlo was thoroughly questioned about whether good construction practice of RCC was achieved in light of the “expected” unbonded lifts.

He said Paradise Dam was the first time he had worked with RCC and watched it being laid “very closely”.

While a lot of time was spent in the field, Mr Montavlo said part of his role required him to compile reports so he couldn’t be watching 100 per cent of the time.

Mr Montavlo said the construction practices were good and when issues arose, they were dealt with.

“At the end of the project, I think it was 2006, we extracted dam cores from the secondary spill,” he said, which was believed to have been part of the plan from the beginning.

“It was co-ordinated along with Dr Schrader, the design team and the construction team.”

After much discussion about bonding, Mr Montavlo maintained the number of cold joints was not necessarily the result of poor construction practice, not treating them would be bad practice, which “was not the case”.

“The lift joint not being bonded, what I remember back then, it was not a concern because there was an expectation that some of them were not going to be (at the location of the cores),” he said.

“The expectation was good if we get bonding, and if not then that was expected.

“The bonding is expected on the upstream section of the dam.”

Mr Paton said the preliminary design report did propose a dam constructed of RCC, and would have indicated a medium-high mix, with a proposed apron size of 20m.

“As part of the design of the dam you obviously need to consider site conditions, one of the most relevant factors for the design of an energy dissipation structure is the tailwater downstream of the dam.

“The elevation of the water downstream as a particular flow passes over the dam.

“For this site, the tailwater appeared to be particularly high, unusually high.”

Mr Paton said the tailwater for the larger events exceeded the spillway crest level, which was not something the design team had come across before in a dam.

After testing and ruling out the use of a flip or roller bucket energy dissipation option, he said his calculations considered elements of water velocity, tailwater and the level of erosion resistance of the riverbed.

The inquiry heard a simple horizontal apron, nominally 20m wide, was adopted to the preliminary design.

Mr Paton said the word ‘nominally’ was used in the report because it still needed to be demonstrated to be appropriate.

The Commission will continue next week in Brisbane. The next Bundaberg hearing date is April 6.

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