Creating Queen’s Marque Part II: Inclined Rock Anchors

Aerial view of Queen's Marque

The steel sheet pile (SSP) cofferdam has been designed to resist external soil and hydrostatic pressure. This was accomplished by installing a series of internal structural steel walers and 63 rock anchors that are inclined at 45 degrees down towards the underlying bedrock. Additionally, 162 king pin piles were also installed on the north, west and south sides of the SSP cofferdam. These piles will comprise pipe piles socketed into the bedrock to provide additional resistance from external soil and hydrostatic pressure.

The inclined rock anchors are installed at the SSP waler, which is a heavy structural steel beam located on the inside of the cofferdam and which is installed at approximately elevation + 4.0 feet (or approximately four feet below existing grade). From the cofferdam waler elevation, the post-tensioned rock anchor is installed at a 45 degree angle down to bedrock. The inclined rock anchors were installed generally as follows:

  • The overburden soil is drilled and cased down to the bedrock level (at 45 degrees from the waler level).
  • The bedrock is then drilled/socketed and cleaned to accept a post-tensioning tendon. There are a total of nine tendons making up the rock anchors.
  • The tendon, made-up of seven 0.6 inch post-tensioning strands, is then placed through the cased hole and into the rock socket.
  • The tendon is grouted solid into the rock and the casing in the overburden soil is extracted.
  • Once the grout has achieved its required strength, the tendon is then stressed using a conventional hydraulic post-tensioning jack and the waler end of the tendon is locked-off (i.e. anchored).
  • Once the building foundations are advanced to the point where the SSP cofferdam is no longer required, the strands in the tendons are cut and the upper four to five feet of the steel sheet piles are cut-off and removed.
  • The cut/destressed tendons remain in place; however, they are essentially benign. At this point the inclined rock anchors have no stress and they are no longer required from a structural perspective.

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