ODEX System and soil disturbance

[Bigft64]

For those who are familiar with the ODEX-type drilling system, is there any infomation regarding the amount of soil disturbance it causes and relation to blow counts? We just recently used this system for the first time in an alluvial area containing poorly graded sands and gravels. The drillers claim there is no soil disturbance caused by the downhole hammer but there is obviously some level of disturbance.

 

[emmgjld]

I have not considered this problem, as I only use the ODEX system to drill very rocky soils which cannot be augered or cored. Talking with my driller, we believe the soils may be disturbed 1 to 2 feet below the casing face. Some cases may be worse. We believe the sample disturbance and probable soil compaction would be about the same as driving a split spoon 18 to 24 inches and using the last 6".

 

[soiledup]

I have seen the ground literally bubble surrounding a rig because the driller had too much air going through the hammer. This was also through similar materials. So, I would say yes, there probably is alot of disturbance. Sorry I cannot be any more empirical than that.

 

[dgillette]

If you are using air below the water table, the gentlest of drilling techniques can cause loosening and heave of the bottom of the hole. As soon as the air pressure is let off, there is atmospheric pressure inside and hydrostatic outside, which can cause a very large gradient and heave (or collapse if there is no casing). I don't believe there is any way you can measure heave with the ODEX. With heave or loosening, you would measure a blowcount that is lower than what it "should" be.

If you are below the water table and need a precise blowcount (for liquefaction assessment for example), insist on water or mud.

 

[Bigft64]

This is all good information. I was surprised about the bubbling soil but it makes sense. How deep did they have to drill before the bubbling stopped? Our site was covered with asphalt so any bubbling wouldn't have been visible.

On this job that we used the ODEX, we had tried to use rotary wash but the soil was so gravelly and had so little supporting matrix that even the mud couldn't hold it up and was caving the hole...and this was all happening even before we hit the ground water. We didn't want to use hollow-stem due to heaving issues (although it sounds like ODEX isn't any better with heaving). Our blowcounts were reasonable though for the type of material we were in so it's hard to say how much effect the jetting air/pounding/heaving really had on blow counts. Ideally it would be nice to punch an ODEX hole right next to a hollow-stem hole and be able to compare the differences in blow counts.

It sounds like in the future we'll avoid using ODEX if possible as the blow counts and sample quality are both in question.

 

[soiledup]

Yes, I would suggest maybe cable percussion drilling throught the alluvial deposits to obtain a better quality of SPT's. You can always use rotary follow on if necessary to obtain cores.