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Proctor in earth dam

Proctor in earth dam

We have a earth dam and we need to know some values of Modified Proctor because it doen´t exist any data before. Is useful to extract samples from the current earth dam for determining it?
I think that it could be useless cause that material is already compacted.

Thank you.

RE: Proctor in earth dam

First question is why do you want the data?

Assuming that you are trying to verify that the material was placed and compacted to some specified criteria, then the steps would be as follows:

1.  Drill borings taking undisturbed samples.
2.  Drill more borings to collect disturbed samples of each material type.   Collect enough sample of each material to allow a laboratory Proctor test to be completed.
3.  Determine the dry density of the undisturbed samples and compare to the Proctor values.

I ask again, why do you want the data?  This a very expensive program for a dam of any real size and could require many borings.

Mike Lambert

RE: Proctor in earth dam apparently do not understand the use or method of the moisture-density relationship (Proctor).  The laboratory test can be done even after is used to compare the field density test data to the laboratory data (Proctor value) to determine the percentage of compaction achieved in the field.

The only way to determine the Proctor value is to test a sample of material taken from the site.  Proctor values can be variable, depending on the soil, so using historical values from another location would be of no value.

RE: Proctor in earth dam

Thank you for your answers.

Geopavetraffic, firstly, we need some data about compaction characteristics because there are no data; secondly, the earth dam is very small, so the program wouldn´t be very expensive (5 borings). I had thought to follow a similar steps like yours.

Ron, I know it is possible to do Proctor after compaction, but I don´t know if the values in the method are representative, as the soil has already been compacted (with the Proctor method we are recompacted the sample again, I think).  

RE: Proctor in earth dam

Unless your material is a soft, friable rock material, previous compaction of the material is not an issue.  You can obtain a sample and do the Proctor test, then compare to your in-place density values....unfortunately, if the compaction is lower than required, you can only affect the near-surface compaction with further compaction.

RE: Proctor in earth dam

I wouldn't recommend using Modified for constructing or evaluating a dam, especially a small one.  OMC and gamma-max in standard Proctor are closer to what you want the dam embankment to be like, and that's what our experience is "calibrated" with.  Typically, you would want to have the placement moisture OMC to 2 points wet, or possibly 1 dry to 2 wet according to standard Proctor.  With Modified, you would need it to be some larger number of points wet of OMC, and the offset between modified and standard OMC is not consistent for all soils.  If you built a dam at Modified OMC, it could be more brittle and more pervious, which you don't want.

So, be sure that it's really the Modified that you want to compare the sample densities with.

RE: Proctor in earth dam

Quote (dgillette):

it could be more brittle and more pervious

Not sure I agree with this statement, particularly the "more pervious" part.  The void ratio will be reduced with greater compaction, thus the permeability will be decreased.

I also wouldn't consider compacted, granular soils to be "brittle" unless cementation occurs.


RE: Proctor in earth dam

Hi Ron.  In a low dam, there is generally no need to have more than 95% or so of Standard Proctor density, and it is probably going to be constructed with smaller equipment, not Cat 825s.  Therefore, there is really no point in using the modified, which is more work.  (In a high dam, there are obviously advantages to greater density, but there is generally no need to go to Modified, although occasionally it's done.)

Here's where the more pervious part comes from:

If you compact a fine-grained soil to a given density much dry of optimum, you get get a little different structure, and the individual clods that make up the loose fill prior to compaction don't get smooshed (technical terminology) together quite as well by the sheep's feet.  Hence, density within the lift is less uniform, and there is higher permeability (primarily in the horizontal direction).  Even if you get a higher density (averaged within the lift) with a lower water content, the soil structure can still be the dominant factor.  UNLESS, the dam is high, in which case wetting and the overburden pressure in the lower part of the fill could destroy all of the structure and it won't matter.

Ordinarily, the higher permeability is not a make-or-break for a dam, because it is not all that much higher.

The brittleness is a function of water content and density.  If you put in a sample at some specific density (say, 90% of Mod, or 95% of Std, which are comparable), the drier it is, the stiffer and more brittle and prone to cracking it is until it gets saturated (which might not occur until the water is already flowing through transverse settlement cracks).  The brittleness is observable in the lab or with a mattock on the fill.  Hence, 90% of Mod at Mod OMC would be more brittle than 95% of Std at Std OMC.  Also, wetter fill is easier to compact against irregularities in the foundation, structures within the dam, etc.

I have assumed that the OP is referring here to core material, not granular material, because he is talking about sampling to measure density and using the Proctor test.  If he is using Proctor for granular material, he may have the wrong test, and he wouldn't be able to get samples that hold together enough for density tests.  Proctor is generally limited to stuff with >15% fines, although with very fine sand (Florida sugar sand, for example), it can work OK with less.  


RE: Proctor in earth dam

DRG...your comments make sense; however, trying to get "clods" to develop is hit or miss.  I agree with your statements about compacting dry of optimum; however, it is often difficult to achieve appropriate compaction dry of optimum in soils with greater than about 10-15% fines.

Again, have a problem with this statement:

Quote (dgillette):

Proctor is generally limited to stuff with >15% fines, although with very fine sand (Florida sugar sand, for example), it can work OK with less.

The Proctor is commonly used in Piedmont and coastal plains soils alike...Standard for Piedmont, Modified for coastal plains.  While we sometimes see a "double hump" Proctor curve in some of the fine sand materials, most of the time we get a clearly defined Proctor curve in materials that have 5% or fewer fines.  

RE: Proctor in earth dam

I generally agree with the comments about compacting wet of optimum using standard proctor.  But that's usually a consideration for new embankments to prevent brittle behavior during settlement of the core, etc.  Really have no idea from the initial posts though on what sort of work is being envisioned for this existing embankment.  Why do you care about density, what are you analyzing, what work are you performing (repair, modification, raising, etc, etc)?

You also need to be really careful if you plan to drill in a dam.  You need to know what you're doing and so does the driller or you can create some major problems where none currently exist.  You can hydraulically fracture materials, create preferential seepage pathways, etc, etc.   

RE: Proctor in earth dam

What are the sands like where you get the nice Proctor hump?  When I worked in central Florida, we got decent results from Proctor even with ~5%, maybe less, but only if the rest of the material was fine sand (mostly between #50 and #200 or #30 and #100).  That might qualify as borderline free-draining, as used in ASTM D698.  Coarser than that, I haven't seen it work well.

D698 says "This test method will generally produce a well-defined
maximum dry unit weight for non-free draining soils. If this
test method is used for free-draining soils the maximum unit
weight may not be well defined, and can be less than obtained
using Test Methods D4253."

USBR 5500-86 (nearly identical procedure) says "This procedure is normally performed on soils that contain more than 15 percent fines.  It may be used, under certain circumstances, for soils containing 5 to 15 percent fines."


RE: Proctor in earth dam

Regarding compaction wet of Optimum & Modified vs Std. Proctor:

Recommend reading ASCE JGGE May 1999, vol 125, no.5  Field Performance of Compacted Clay Liners, Benson, Daniel & Boutwell.
 They introduce the Line of Optimums, which is basically a line following the Zero Air Void,& connects, for instance, the Std & Mod Optimum points.  The goal is to set a minimum compaction and keep the field results at or over the Line of Optimums, when the density results are plotted on the Proctor Diagram.
Depending on the size of equipment which is available, the material can be worked until the  desired results are obtained. I have found this concept to be very helpful in the field.

RE: Proctor in earth dam

dgillette...all are SP, SP-SM or SP-SC....very few medium sands in Florida.  Standard Proctor is sometimes used on the SP-SC materials and soil-cement, but Modified is used otherwise.  FDOT uses standard Proctor for embankment soils and requires 100% compaction, but Modified otherwise.

When were you in Central Florida and with which firm?

RE: Proctor in earth dam

I was there in the mid 80s, and worked for what is now BCI (part of AMEC) in Lakeland.  I can't recall doing anything for FDOT (and very little on other roads), so I don't know much about their requirements.  I was primarily working for the phosphate mines (dams, tailings management, some enviro) or on reclaiming mined land owned by others.  Even my little drilling projects all seemed to involve mined land somehow.  (We had mining and dredging work elsewhere, too.)

RE: Proctor in earth dam

dgillette...see Foundations by Terzaghi (the forum...not the

RE: Proctor in earth dam


You really have not answered why you want the data.  You say you want compaction characteristics for the dam, but why?  Compaction is an easy field test that with proper planning can be assumed to be related to important engineering properties such as strength, consolidation, and permeability just to name a few.

If you are trying to determine if the dam is will perform for its intended purpose, I suggest taking samples and running the appropriate laboratory tests to determine the engineering properties of interest.

Mike Lambert

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