Swell tests on very stiff clays (SPT-N > 15 BPF) 3

[pelelo]

Engineers,

The original geotechnical report of a site mentioned that there are shallow clays with swell and collapse potential (conclusions obtained based on the lab tests).

There is a second round of a geotechnical investigation for this site in which it is required to perform swell tests (ASTM D4546) on these clays.

The blowcount of the clays in general is over 20 BPF and few between 10 and 20 BPF.

In my experience, the only way to perform ASTM D4546 is by grabbing shelby tubes and preparing the "undisturbed sample". Shelby tubes are usually extracted on soft to very soft clays (SPT-N < 5 BPF), therefore in this case whenever the drilling contractor tries to push the shelby tube, the tip might bent because of the consistency of the existing clays (SPT > 20 BPF, in general).

I was wondering if there is any other way to perform the swell tests on these clays?. Do you have any suggestions?

Or maybe the question is: is there any way to perform SWELL tests on stiff clays (SPT-N> 15 BPF)?

 

[MTNClimber]

You need to collect deep samples of stiff clays using a California modified sampler with brass liners. It collects intact samples but not undisturbed since you have to drive the sampler. The sample liners are capped and sent to the lab to have the soil samples extruded and cut down to size for 1D swell tests.

For shallow samples, you can dig a test pit and collect similar samples by driving the brass liner into the ground with a driving attachment and a 3-pound hammer.

 

[TigerGuy]

I've collected Shelby tubes on 30 blow clay. What's the issue? Granted, it takes a little patience to push and retrieve the tube.

 

[MTNClimber]

When the driller has been willing the try to sample very stiff clays, I've seen Shelby tubes bend. Often the driller doesn't even want to try.

 

[pelelo]

Tigerguy,

I am impressed your drill crew being able to extract shelby tubes with N around 30. I have never seen that before. In my experience, SPT-N of 10 or less is ok to extract shelbys.

I have heard of other firms to extract shelbys if N values are less than 15.

 

[TigerGuy]

Don't get me wrong, we've left plenty of tubes in the ground when the top tore out and only the tool returned to the surface.

Usually, it's the gravel that likes to appear at the wrong time that destroys most tubes for me.

 

[pelelo]

Other than the california sampler, any one recommends any other sampler?

Cali sampler is a disturbed sampler as MTNclimber mentinoned.

Even though there are several types out there (denison, pitcher, etc), i was wondering which one you recommend based on your experience.

Thanks,

 

[TigerGuy]

I'll play a card from one of my professors. Extract a "block" sample, where you carve out a vertical column of soil and transport it intact to the lab. Has anybody on here done this? I haven't and would like to keep that record.

 

[pelelo]

I have heard of this.

This is a reasonable approach if the sample is shallow (< 10 feet).

What happens if the sample is 50 ft below ground?.

 

[MTNClimber]

Pelelo,

What are you trying to do with the lab test results? Are you trying to calculate the magnitude of heave accurately? Good luck! I have yet to find a method that is accurate or not disputed by another "expert."

If a Shelby tube doesn't work, the California sampler is the next best bet for deep samples.

The intact samples from the cal sampler will likely be more expansive due to slight compaction during driving. This provides a conservative value for estimating the risk and magnitude of heave.

If you are trying to check if some expansive soil has been effectively mitigated (soil conditioning, stabilization, etc.), then you will want to use a Shelby tube to avoid compacting the sample.

Remember, the 1-D swell test can be conservative, too, since it fully saturates the sample, which may or may not happen in the field.

 

[pelelo]

MTNClimber,

Thanks for your response.

California sampler is considered an disturbed sampler (due to the driving).

If you want to run swell tests or any other advanced test, you want to extract undisturbed samples.

Have you tried the pitcher and denison samplers?. I have not, that't why i ask.

 

[MTNClimber]

pelelo - I have not. All of my Shelby tube samples have been collected on soft to very soft clays and organics or moisture conditioned clayey fill.

I have some initial thoughts about the pitcher and Denison samplers in expansive clay. The Pitcher and Denison samplers require drilling fluid through the sampler, which could be problematic for sampling moisture sensitive soils such as expansive clay. Additionally, I know drillers don't always use drilling fluid where expansive soils are prevalent, often relying on solid or hollow stem augers. This may be overly conservative, but is it an undisturbed sample if the soil is allowed to come in contact with expansive soil?

 

[theCorkster]

Denision sample was designed for stiffer materials, Pitcher was designed to push inner barrel though softer material and avoid drill fluid contamination. Performance is dependant on samples seals so drill fluid does not enter sample.