Bleeding Effect during Proctor Testing 1

[Snagglespree]

When performing Modified Proctor Testing of soils, I am sometimes seeing odd results for the wettest test point. For example, I will add water for ~10.0 moisture to obtain . When pounding the Proctor "pumping" will begin to occur. Usually by the 4th & 5th lift, muddy water will "bleed" out the bottom of the mold. This water is essentially lost and isnt accounted for when plotting the moisture-density relationship and after oven drying the sample it indicates that the sample contained only 8-9% moisture as opposed to the original 10% that was added to the soil. This reduction in moisture, in turn, yields higher than expected dry densities and seems to imply that maximum density is at this moisture content. Visually, the material is not stable and you would expect low density values to get a true "curve". Instead, it produces a one sided curve and adding over 10% moisture would turn the material into mud.

Is there a method/accredited source that provides a way to account for this moisture loss or is that not necessary?

Also, whats the most appropriate way to determine the MDD in this situation?

 

[Ron]

What you are seeing is relatively common in granular soils wet of optimum. If you take your moisture content from the center of your sample, it has no effect on the moisture-density relationship curve...it is just another moisture-density point for you to plot. I infer from your comments that you are using a pre-compaction moisture content for your M-D relationship curve or you are considering your moisture input to the dry soil to be correct. If so, your procedure is incorrect.

Your curve should represent post-compaction density and moisture.

 

[Snagglespree]

Thanks Ron, the moisture content that I am using to plot the curve is strictly what is determined through oven-drying the sample.

The pre-compaction moisture contents are being used as only an estimate for what to expect after oven-drying for quality control.(i.e. if a tech obtains actual moisture contents that are drastically different than expected, the procedures for adding & mixing water would need to be reviewed). I understand that there will be differences between the estimated w/c and the actual w/c obtained, but they should be close to the same value for specimens on the dry side of optimum.

For the wettest point, I believe the moisture input is correct, it just doesnt come out as expected because of the loss of water at the base of the mold.

Whats happening is I'll put say ~8.0% moisture for my 4th point, ~10.0% moisture for the fifth, but when I determine the actual oven dried moisture contents I get two points both near 8.0% with noticeably different corresponding dry densities.

 

[Snagglespree]

If I'm understanding you correctly the excess water that escapes the mold is a result of over-saturation and the soil cannot absorb the excess water. If this is the case, is there any reason to try and pound a point on material overly saturated?

 

[HSIII]

Yes to the pounding, in order to complete the curve series. Don't forget, ASTM D1557 requires a minimum of 4 points for your moisture-density curve.

If you're that worried about bleeding, perhaps try placing a slip of filter paper on the bottom of the proctor mold prior to pounding your sample. Or, cut your next point to, say, a 9% instead of a 10%.

What do the fines look like in your sample? is it below 5% (clean)?

 

[Snagglespree]

The material does have less than 5% fines (clean).

 

[HSIII]

usually you won't be able to run higher moistures without some degree of bleeding if the sample is that clean. Especially if it is a gravelly-based material as opposed to a sandier one.

 

[Ron]

Check your ZAV curve and specific gravity. If you're getting that much bleeding and you haven't lapped the ZAV curves, then you probably don't have enough specific gravity range on your ZAV curves.

Keep in mind...it is what it is. As I'm sure you know, not all materials behave as we sometimes expect.

I would probably try to shift my moisture brackets downward a bit and re-run the test.

 

[Snagglespree]

Thanks for the input everyone, my main goal here is to improve the quality of our proctor curves to account for these situations where the materials dont behave as expected. My supervisor wants to see perfect curves on everything, which in my opinion isnt always obtainable for certain material types.

On free-draining fine grained sands I will get flat lines or the density will seem to go up & down repeatedly with increased moisture. Or sometimes the density just rises and seems to peak where the material exhibited excess moisture and this dumbfounds my supervisor. I've found some threads on these forums that have mentioned these behaviors, but I want to have answers to back up my proctor values to my supervisor and to the occasional client civil engineer who claims the curve should ALWAYS be concave down.

Any good books out there that discuss these issues, besides Holtz & Kovacs Intro to Geotech...

 

[HSIII]

Y'know...I've always wondered about performing r&d for proctors on different soil classifications in 1% increments just to see how they'd play out for proctor curves, specific gravities, total saturation, etc.

However, I know that's not really feasable because even within the different classifications the proctor curves could vary greatly.

Still, always thought that'd be cool to do.

 

[Stoph]

Snagglespree, we encounter this often when we test ODOT #304 (a well-graded granular base material). While I believe that this type material should only be tested with a roller pass test section, we still provide a starting optimum moisture content to the contractor using AASHTO T-99. I have found that vacuum grease around the bottom of the mold where it meets the base of the mold seals it nicely if the wing nuts are adequately tightened.