Borehole

Today has been quite interesting. I've never witnessed a borehole being drilled and today was a first.

What was most surprising was the rate at which the shaft is sunk. It moves rapidly, especially through the soil section. When it finally hits rock it slows quite a bit, but it still continues at an even pace. Throughout the process, a compressed air and water mix powered by a massive compressor is blown out of the drilling bit forcing all the debris up and out of the hole. As it gets deeper, the increasing ground water makes the mix wetter and a fine mud slurry develops. The noise of this all is quite intense, especially when the hydraulic hammer kicks in.

Samples of the ejected material are kept and it gives quite an interesting view of whats below ground. Initially the soil is very red, which is the red tacky clay we have everywhere. Below that is a decomposing granite which is yellow in colour. This was the material that the engineer recommended we build the house out of, except that it's inaccessible. Then there is granite where the slurry colour changes to a blue grey. This is apparently what we want to be boring through with the hope of encountering a fracture or fault line, which is where all the water is.

One of the problems we had was that the wall started to collapse into the hole. Apparently they like to get some sense of the volume of water that the borehole will deliver before they put in the steel sleeve. This way if the hole ends up being dry, which happened to my neighbour and others in the area then the costs have been to some extent mitigated. However, this was not an option and the operator had a feeling that he was busy eroding a cavern below his rig and was concerned about the risk of a major collapse. So the decision was made to install the sleeve that protects against collapsing side walls before there was any guarantee of water. The sleeve (or casing) is a steel pipe that comes in 6m sections. As each length is lowered into the ground, the next length is butt welded to the buried section and this is done as many times as is required until the bedrock is reached. In total we installed approximately 30m of sleeve. The bottom three sections of sleeve had been perforated to allow water to seep into the pipe. To prevent these clogging, several bags of fine stone were finally poured into the gap between the sleeve and the drilled hole. Apparently this also promotes movement of water in and around the borehole.

The plan is to drill until the hole is 70m deep. By the end of day one, we were at 50m and the feeling was positive with evidence of water and a good 'recovery' - I suppose that means that water is flowing fairly freely into the borehole and that we have a sustainable supply. However, this is a guess and without a proper test the yield will remain a mystery. What is important is that it delivers water and in sufficient quantities to tide us over the building period and then through any periods where we run out of harvested rainwater.

Next on the agenda is to get the pump installed and operational. That includes an automatic switch that will keep our reservoir full and all the various protective circuitry. Hopefully then our immediate water issues will be resolved.