Hi Madalin, Rotary ...

Hi Madalin,

Rotary Percussion or DTH drilling is the fastest and most efficient method for drilling in hard rocks. It is not good for non-cehesive or cohesive soils or unconsolidated overburden deposits, but if you are just casing through these (up to 30m deep) to get into deeper drilling in the bedrock formations that is fine.

In my experience as a consulting geologist and professional water diviner, I find drilling with air-flush DTH method is great for fast drilling and excellent borehole logging information (recording the geology and water flow profile of the boreholes). I have done over 2,500 boreholes this way and satisfied 97% of my clients with all the water they required.

In hard fractured igneous and metamorphic rocks and fractured sedimentary aquifers my survey methods are second to none in locating exactly where to drill for best fracture intersections to get optimum borehole productivity (high yield at least depth). With DTH drilling you can drill to hundreds of metres but sometimes get no water. This is expensive for you and the clients and sometimes the clients refuse to pay. I am the troubleshoot expert in such cases and will usually find plenty of water where other boreholes have failed.

Casing systems are many, and sizes vary from under 100mm to over 1,000mm depending on the type and purpose of borehole to be drilled. For water wells always size the casing and borehole diameter to accommodate the pump and borehole lining installations required. If a client requires only a small water supply boreholes of 4-inch to 6-inch (100mm to 150mm) diameter can accommodate 3.5-inch to 5-inch borehole linings / well-screen and 3-inch pumps for yields up to 4,000 litres per hour or 4-inch pumps for yields up to 16,000 litres per hour. This covers most private water supplies.

For some bigger farm irrigation wells and industrial or municipal water supplies in excess of 16,000 l/hr you need to drill 7-inch to 12-inch diameter wells to accommodate 6-inch or 8-inch pumps with suitable borehole linings. Sometimes it is best to drill an exploratory hole first at small diameter to test presence of water in good quantity without excessive cost then, if you find it, to drill out the hole to larger diameter.

In general doubling the diameter will double the yield (yield is proportional to internal surface area of borehole (pi x diameter x depth).

If you want to just have one system for all water wells, I find that for most boreholes typically in the 30m to 120m depth range, I prefer the 8-inch (200mm) diameter, which allows plenty of yield capacity and diameter enough to install any size of lining tube up to 6.5-inch with gravel filter pack if necessary, and 4-inch or 6-inch pumps for yields up to 75,000 litres per hour.

If you are buying drilling rigs you need to match the drilling rig capability to the size and depth of holes you plan to drill, and also need to match compressor capacity (pressure and volume) to the planned borehole depth and diameter.

I hope this advice is helpful. - DB.