Sonic Rig Question

[dBasement]

Sorry, for the cross-threaded post from mining engineering, but it is more geotechnical than mining.

We are using a sonic rig to look at overburden in a very dense / hard fine-grained deposit that I believe to be weathered bedrock. The problem is, we are getting SPT results just below the casing that show blow counts of 30 or 40. In the meantime, the driller says its hard as nails and we are mushrooming casing. It makes me think that we are imparting way more damage to the deposit because of the vibration of the sonic rig than we think. I want to do a CPT (Cone Penetration Test) when we return to the site to confirm, but, is that a reasonable assessment?

 

[Ron]

Either the CPT or flat plate dilatometer. If it is cemented, have you tried coring the material and checking RQD?

If you can actually retrieve a core from the material, there's probably not much point in the other methods.

 

[dgillette]

Is it even possible to push a dilatometer into 30-40 blow material without destroying the diaphragms?

On the other hand, what do you need to know about the material, excavation stability or rippability or suitability for tailings dam shells, or?

 

[coneboy]

I have seen this issue a few times.
Yes, the sonic drill will disturb the soils at least 1 to 1.5 feet ahead of the casing. A downhole hammer drill will also disturb the soils at least 1 foot ahead of the bit.

Your 30 - 40 blows/ft in sonically disturbed soils might actually be closer to a true value of 80 blows/ft.

On many occasions in the past we have gone back to sites with the CPT unit that intially had lower than expected SPT values.

If the soils are very dense like the drillers say they are then you won't be able to push the cone very far, but it will prove that the soil is at least very dense.

Check that the sonic drill that you are using can push the CPT.
Some can and some can't. In the past I got the drillers to build a small sub for the drill so that we could push right from the drill head.

I agree, the soils will most likely rip apart the membrane on a Flat plate dilatometer.

Coneboy