Installing the 3ESP
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.
On soft bedrock or subsoil
, you should install a pier as depicted below.
2.
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.
3.
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.
Alternatively,
if time allows and granite is not available, prepare a concrete mix
with sand and fine grit, and pour it into the hole. Agitate (“puddle”) it whilst
still liquid, to allow it to flow out and form a level surface, then leave to set.
Follow on from step 7.
4.
Puddled concrete produces a fine-textured, level floor for placing the
seismometer. However, once set hard, the concrete does not have the best
15
Issue G - February 2016
