91-year-old drawing solves Lock One’s mystery damp patch

Work has just wrapped up on the repair of an area of seepage in the weir section of Lock One at Blanchetown, the cause of which was until recently unknown.

SA Water's Senior Manager of River Murray Operations Garry Fyfe said a combination of engineering expertise and an exhaustive search of construction drawings, photographs and other historic records led to the discovery.

"The eureka moment came from a single note on an original concept design drawing from 1927," Garry said.

"The note revealed that one section of timber sheet piles, which protect the foundation of the weir, was for reasons unknown, not installed as part of the original construction in the 1920s.

"Instead, a section of steel piles had been fitted around the area, in an apparent attempt to reduce the potential impact of the missing section of timber material.

"A series of photographs, which clearly show the gap in timber sheet piles, back-up our findings and also matched with the location of the seepage."

The increased water seepage was first observed during a planned weir pool raising event in late 2014, on the downstream side of the weir supports, where the concrete structure meets the riverbank.

"Under normal weir pool operation, there is a small amount of seepage, which our local teams routinely monitor and it doesn't impact the operation of the weir," Garry said.

"On this occasion however, we immediately put measures in place to ensure the safety of the weir wasn't compromised. This included lowering the weir pool level by around 100 millimetres and installing a protective layer of rock and sand material over the seepage area.

"Parallel to this, investigations were undertaken into the cause of the increased seepage, including underwater inspections using divers and sonar surveys, dewatering of the fishway, 3D computer modelling, and the drilling installation of several instruments into the edges of the weir structure.

"The seepage hasn't impacted on the lock's ability to continue operating safely, but it was important to resolve the issue to ensure we don't encounter any issues with the structure in the longer term, and so the weir could be brought back to stable pool levels."

Repairs began in early March and were completed in late April, with the weir now raised back to normal upper pool levels, which indicates the success of the work.

The work involved the installation of a series of cement-filled columns, which were drilled to a depth of around 15 metres below the top of the weir, creating a barrier to the seepage.

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