Wednesday, 11 May 2011

Ground tile anchor condition

Another thing that we are quite keen to check on is the condition of the anchors. On a LOFAR HBA tile, these are a combination of rods, elastic, nylon and duckbill anchors. Around each (external) tile, there are 16 (5 on two sides and 3 on the other two).

The rods are about 10cm or so long and are incorporated into the edge of the black UV-resistant tile cover. A small carefully punched hole exposes these and the elastic ring goes through this.

The elastic then is stretched down to where it meets a nylon line that is embedded in the ground. At the top of the nylon line is a loop knot (a bowline to be precise). At the other end (about 50cm underground) there is a duckbill anchor. This is inserted into the ground with a long metal driving rod. When tension is applied, the anchor twists out to lock into place.

When we deployed the test tile in October 2010, the sub-zero temperatures caused the elastic to be very stiff. We were concerned that in the warm spring, there would be a major loss of tension and, as a consequence, significant tile shift.

As it turns out, the anchors have held remarkably well. There was only one which could be termed as 'loose', although several didn't have the sort of tension that we would like. What has been clear from this part of the exercise is that the decoupling of the anchors from the tile cover by use of the intermediate timber frameset also significantly helps in the anchoring of the overall tile.

We'll be looking at the raised tile later, to see how its anchoring system has fared.

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