Following the slab collapse on Middle Lake in December, divisional head of communication and media relations in Ekurhuleni Nhlanhla Cebekhulu provided the City Times with the following report:
“The Middle Lake dam was inspected by the metro’s appointed approved professional person, who is also approved by the National Department of Water and Sanitation as an approved Dam Safety Professional in May/June 2018,” said Cebekhulu.
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“It was not known that internal erosion took place, which caused the December 2018 failure, due to the concrete slabs covering the earthfill spillway and water continuously spilling over the spillway.
“The differential vertical displacements have been visible since 2003 when a similar situation was detected and repaired in 2004.
“Intensive capital remedial work was done during 2003/2004 when the following was done:
• A vertical cut-off concrete beam was installed on full supply level to prevent seepage through the earthfill dam.
• Cracks were sealed and all joints were sealed.
• All cavities were grouted.
“There was no new settlement visible during the last inspection.
“The collapse on December 23 was therefore unforeseen.
“The inspection report recommended the following be done:
• The low embankment crest point on the left flank be reinstated.
• All shrubs and trees with a trunk diameter of less than 100mm be eradicated.
• All vegetation in the spillway be cleared.
• Weeds growing at full supply level be cleared.
“This was not identified as a structural defect.
“The inspection report was submitted to the Dam Safety Office.
“The following tasks are in the process of being undertaken in order to enable the repair of the whole of the spillway, including the stilling basin.
“These tasks will include:
• Rubble to be cleared in the spillway area, including the stilling basin and on the embankments and immediately downstream of the embankments as appropriate in terms of dam safety considerations. This task is complete.
• An investigation to determine the extent of the erosion damage to the rest of the spillway, which will include details surveys and ground penetrating radar scans. This task is complete.
• Following the investigation, the necessary rehabilitation by means of grouting the known cavities (as determined by the ground penetrating radar scans) will be done. This task has not yet started as the results of the scans are not known yet.
• The sealing of joints and cracks and small concrete repairs on the spillway. This task will be done concurrently with the grouting exercise.”
