Installing the 40T
Ideally, the sensor should rest directly on the bedrock for maximum coupling to
surface movements. However, if bedrock cannot be reached, good results can be
obtained by placing the sensor on a granite pier on a bed of dry sand.
1. Prepare a hole of 60 – 90 cm depth to compacted subsoil, or down to the
bedrock if possible.
2.
On granite or other hard bedrock
, use an angle grinder to plane off the
bedrock at the pit bottom so that it is flat and level. Stand the instrument
directly on the bedrock, and go to step 7.
3.
On soft bedrock or subsoil
, you should install a pier as depicted below.
4. Pour a layer of loose, fine sand into the pit to cover the base. The type of sand
used for children's sand-pits is ideal, since the grains are clean, dry and
within a small size range. On top of the sand, place a smooth, flat granite
plinth around 20 cm across, and shift it to compact the sand and provide a
near-level surface.
Placing a granite plinth on a sand layer increases the contact between the
ground and the plinth, and improves the performance of the instrument.
There is also no need to mix concrete or to wait for it to set, as in step 4.
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Issue F - June 2019
