Undisturbed Samples

[jjsim]

Anyone has any idea how to collect undisturbed clay samples for the lab 1-D consolidation test? I was told by the soil investigation team that they could only collect undisturbed samples up to SPT-N = 4 using the hydraulic method. For higher SPT values, they could use the Mazier Core Barrel using the rotary drilling method.

I am not sure how to measure the level of 'disturbance' in a soil sample? What is the acceptable criteria for an undisturbed sample. How reliable is the Mazier core barrel method... Appreciate your comments.

Thanks

 

[Ron]

Use a Shelby tube

 

[LRJ]

You'll definitely need to do some rotary drilling rather than a percussion technique. And like Ron said, you'll need a standard sample tube; the particular tube to use depends on the ground you are in.

There are a few methods to check sample disturbance. The one I use most often is the Lunne et al. (2006) method which relates initial void ratio in the oedometer (e_i) to the in situ void ratio (e₀) based on particle density and water content. It's very simple to use and seems very reliable.

The full reference is: Lunne, T., Berre, T., Anderso, K.H., Strandvik, S. and Sjursen, M., 2006. Effects of sample disturbance and consolidation procedures on measured shear strength of soft marine Norwegian clays, Canadian Geotech. J., 43, pp. 726-750.

 

[fattdad]

So, if the N-value is 6 the drilling subcontractor WILL NOT even attempt pushing a Shelby tube? Find another contractor!

The only thing that will break is the Shelby tube! The owner buys the tube when it's affixed to the drill pipe. Why would they even care if it's a failed attempt!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

We use double tube piston samplers.

 

[Mad Mike]

We most commonly use Shelby tubes since they compliment our standard N-size boreholes.

The large diameter U-100 tubes provide far superior, damn nearly undisturbed sample but because they require a larger borehole diameter, are only used in our highly sensitive jobs where budget is in abundance.

Per fattdad's remark, the contractor's objection is petty- Shelby tubes can usually be effective in cohesive soils up to about N = 10, but this is just an indication - as the clays get stiffer so the sample recovered becomes less and less. The contractor must demonstrate some effort on his behalf and to this degree, I would instruct him to attempt a tube sample under my supervision. Only when he has pulled up a distorted tube with bugger-all sample in it, would I be satisfied that alternative sampling methods are required.

Gravelly clays are notoriously difficult to sample using tubes.

Good luck,
Mike