Post-tensioned rock anchors|a practical solution to the problem
of near-surface rock structures
Post-tensioned rock anchors are a practical solution to the problem of near-surface rock structures when constructing tower foundations for high tension transmission lines. These foundations must be capable of resisting massive overturn moments created by ice and snow loads or wire breaks. Rock anchors can provide the stability required while eliminating the extensive rock excavation which would be needed for a gravity type foundation. The anchor is designed for two stage grouting. The lower portion is left bare, without duct covering and the strands are separated from one another to provide maximum bonding surface for the first stage grouting. The upper portion of the tendon is encased in a semi-rigid duct with a grout seal at its lower end to separate the second stage grouting from the first. This duct ends just below the top of the preformed hole in the foundation to permit the strands to assume their proper positions as they pass through the splay plate. The second stage grouting operation is done through a tube that reaches to the bottom of the grout seal. Enough grout is injected to reach to the top of the foundation hole. To complete and anchor the extensions for strand stressing are burned off and concrete covers cast over the anchorages. Backfilling over the foundation restores the terrain.