Exploration Drilling

I had a look at the Drilling information from CCAG, which is very detailed, and in my opinion correct.

I have a “Heavy Civil Engineering” background where I spent a good part of my life in deep foundation “piling” work.
I was employed at the time by the world largest Piling company which operated in over 60 countries.
During my time I was responsible for over 400 individual projects, including all foundation work on the “New parliament house” , Sydney Entertainment Centre, Bridge foundation and large Industrial work all over NSW.

In piling work with similar ground conditions as we have in the Mount Vincent/Cherry tree hill I have found general the following.
Drilled piles would range from 600mm to 1800mm diameter. The Basalt is in most cases very hard for the size of piling holes.
The alluvial material above the Sandstone is often of “gravelly-sandy” nature
The Sandstone ranges from weak to very dense and hard with intermitted layers of iron stone “extremely hard” and or layers of faults/cracks of soft sandy material. Often the deeper the harder it gets.
In weak Sandstone and alluvial material it is the norm to use drilling fluids. The drilling fluid prevents the hole from collapsing and prevents the ingress of water.

In our area I would expect water in all strata’s, i.e. crakes in the Basalt which ones exposed will decant into the drill hole.
Large water flows in the alluvial layers, which without drilling fluid will have strong flows into the hole and bringing with it large amounts of sand and gravel, i.e. the hole would become considerable larger in this strata.
In the Sandstone considerable amount of water can be expected  trapped  in various layers of soft rock and cracks up to 30mm wide. Yet in some Sandstone there is almost nil water ingress.

During my time the majority of drilling fluids were bentonite mixed with water, however some chemical mixtures started to appear which at the time were highly toxic, and very costly to dispose of.

On many cases I have noticed  considerable loss of drilling fluids, for example a hole used 12 m3 of concrete to fill, but absorbed 13 m3 of drilling fluid.
Where did it go?  Well it clogged up crakes in the rock formation and travelled considerable distances in porous water bearing layers, which could affect springs a long way away.

Please note I am not a geologist, the above is soly based on experience and actually dealing with water in the foundation work.
In the piling game water is a “nuisance” which makes the job more difficult, therefore little attention is given  to its value.

Rolf Vogt

Posted for Rolf