Well we do have some holes - but they are 36-38m deep only so there may be a 'void' everywhere around here! My geologist brother-in-law will be consulted this weekend! Anyway we now need 8 holes, and there has been some tree-trimming to accommodate this.
The existing holes have plastic pipes in - with blue end caps - and are both in the grass at left. The machine is starting on hole 3.
Note that 'voids' may well be full of water rather than air, but the drilling fluid still flows away. There are granular materials that are put down in the fluid that can block small cracks - but they did not work here so we may have a more fractured-rock (limestone) environment. They used to use compressed air instead of water - much messier but you do not run out of it if you hit a void! We will now be OKish provided that the remaining holes run to 37.5m or thereabouts (the existing ones are 38 & 36). The current 3rd hole is unlikely to get to this depth today anyway (they stop drilling at lunchtime).
The drillers blocked off the hole base with grout as they put the pipes in - and the hole then fills with water from a gravel/sand layer that is quite close to the surface.
The top picture is of the actual drill bit.
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