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South African dam anchors set new testing record

Grouted anchor testing on the Hazelmere Dam in South Africa has set a new record with the world’s largest sacrificial test anchor programme.

The testing was essential to prove the anchor design for the Department of Water and Sanitation’s plan to raise the height of the gravity dam structure by 7m.

The 45m high, 478m long dam was built on the Mdloti River in KwaZulu Natal Province of South Africa in 1977 to supply water for irrigation but the need for more storage led to the work to raise the height.

22 mn jack

22 mn jack

The 22000kN is the largest anchoring stressing jack ever manufactured

According to Geosolve Global managing director Devon Mothersille, who provided advice to the project, the unprecedented 7m dam height raising called for required technology never before seen in the region.

“Extensive feasibility studies established that the optimum solution to facilitate the raising was to install 83 high capacity rock anchors with working loads varying from 6835kN to 12694kN through the concrete superstructure into the underlying rock,” said Mothersille. “The anchors, installed in 355mm boreholes, ranged in size from 49 to 91 strands, which were each15.7mm in diameter and were up to 95m long.”

With test loads up to 19042kN are among the largest ever installed in a post-tensioned dam, according to Mothersille.

“In order to quantify in situ rock/grout bond stresses, a comprehensive test anchor programme was commissioned and this involved the world’s largest sacrificial test anchor programme,” said Mothersille. “The test anchors consisted of four sacrificial anchors with 61 strand tendons which were 45m long, installed in the stilling basin, and one dress rehearsal anchor with 61 strands and 80m long installed through the dam crest.

“In order to facilitate the cyclic load testing stipulated in BS8081:1989 two hydraulic stressing jack were commissioned with capacities of 15000kN and 22000kN; with a 1000m stroke length and a weight of nearly 10t - the latter was the largest anchoring stressing jack ever manufactured.”

In addition to the anchor testing records, the dam redevelopment also features the highest piano key weir in the world. The 9m high structure effectively increased the length of the spillway by almost seven times the original 103m length.

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