TAS – ANCOLD Study Tour to Rowallan Dam

Author: Chris Topham.  Hydro Tasmania, in conjunction with Shaw Contracting is undertaking invasive excavation and rebuilding works to address piping risks within Rowallan Dam, a 43m high earth and rockfill embankment, near Cradle Mountain in northern Tasmania.

The work comprises:

•              Removal and rebuilding of the upper 7m of the embankment; and

•              reconstructing the dam from crest to foundation adjacent to the central spillway

The unusual aspect of this work is the very deep excavation on each side of the spillway to remove old material that was damaged during a piping incident in 1968 and replace with materials installed to modern geometry and construction techniques.

Flood risks during construction are being managed by a sophisticated flood warning system that forecasts lake levels 7 days in advance and specifies lake level triggers at which emergency backfilling operations would need to commence, to avoid inundation of the works.

On 13th February 2015, ANCOLD hosted a study tour to the project.  The tour was well attended by 33 participants from around the country, who were given a presentation on the genesis of the project, the outputs from the flood warning system, photos for the maximum extent of excavation, and pictures of the evidence of piping (actual “pipes”) from the original piping incident that were found during excavations.  The group then toured the site works and saw placement of contact clay, core and filters for the deep excavation and top 7m rebuild (refer to photos below).

Maximum-extent-of-deep-excavation-on-the-right-hand-side-of-the-spillway.

Maximum extent of deep excavation on the right hand side of the spillway. Note the sheetpile wall that was used as extra protection for the excavation against floods during construction. (Full supply level is at the top of sheetpile)

Deep-excavation

Deep excavation works had to progress 24 hours per day 7 days per week in order to minimize exposure to flood during construction. The photo shows night shift on the left side of the spillway.

The-ANCOLD-Study-Tour-Group-on-the-partially-rebuilt-right-hand-side-embankment.

The ANCOLD Study Tour Group on the partially rebuilt right hand side embankment.

The organisers would like to thank Hydro Tasmania for their support of the tour, site staff, and “bus captain” Angus Swindon for his entertaining tour-guiding on the day.