Maximum soil compaction. 2

[kmatte]

I just have recieved a report of soil compaction test performed on a project I am working on. Two test report relative compaction of 106 and 111 percent. The geotechnical engineering firm used a Standard proctor to find the maximum dry density. The test were taken in sp soils with optimum moisture of 9.7 percent. The inplace moisture content was 4 percent. I feel that the numbers may be wrong, or a different proctor needs to be used. I seem to remember from my geotech classes( my memory may be a little fuzzy) that the maximum field density could only be 104.5 to 105 percent of laboratory desity. Should i be concerned. These test are under the building pad and in fill material used to correct poor soils.

 

[Ron]

kmatte...I agree..they are out of the range of the Proctor. 106% is on the outside edge, 111% is completely off scale, even for a Standard Proctor.

I would question the 9.7% optimum in an SP material. That doesn't fit either. For a Standard Proctor, the optimum moisture in an SP material is typically higher than for the same material under a Modified Proctor.

The Standard Proctor shouldn't have been used here. Even if specified by an ill-informed Civil Engineer or Architect, the testing laboratory/Geotechnical Engineer should have questioned it.

 

[kmatte]

Thanks Ron. I have brought it to the attention of my boss. What is an acceptable maximum compaction for a standard proctor. Like i said earlier i thought that 104 to maybe 105 was pushing it. I know that since soil is so variable that a person has to use a little judgement or they will be taking a proctor for every density test performed.
I have looked back at other test results and several 105% test results were reported.

 

[iandig]

One very big word of warning for you, if you have achieved over 100% compaction, the amount of compaction is only relative to the compactive effort, BUT the fact that the material is at such a low moisture, i.e. 4% compared to 9.7% OMC, then it is highly likely that you have excessive air voids.
Around a compaction curve for a material, there are three zones which are likely to cause failure of the compacted material. Too wet (i.e. bottom right of the compaction graph) and a cohesive soil can suffer long-term settlemt due to consolidation of the material and a loss of water. Too dry and this can result in overcompaction leading to swelling (cohesive soils with very low air voids, upper left hand corner of graph), or too much air causing collapse failure due to inundation of water (bottom left of curve. It sounds very likely that the wrong compactive effort was used to identify the maximum dry density, as you have achieved a 'relatively' high compaction at completly the wrong moisture. If when you re-test the material (with an increased compactive effort), please be sure to assess the previous results in light of the possibility of collapse failure.
I would also suggest that you detemine the particle density (specific gravity in old money), and calcualte the air voids of the previous tests and plot the air void line on the compaction curves.
Geotechnical fill, using cohesive soils, for under a building should be between 100% of the standard proctor to 100% of the modified proctor with 0 to 5% air voids, based on current guidance in the UK.

 

[Ron]

kmatte...we flag results at about the 104% to 105% level whether standard or modified. I like to keep them below 103% because you have less explaining to do!

You have a condition that might cause the value to go a bit higher, simply because you have standard proctor in a clean fine sand (SP). With good compactive effort (vibratory compaction at or near optimum moisture), you can get good compaction. Again, the moisture contents look out of whack. You are right to question these results.

 

[BigH]

One other point to good ones above: Perhaps, the material placed really isn't indicative of the material of which the test was performed. Maybe the field materials were a bit coarser with some gravel and the lab was a fine to medium sand. Just a thought. Do check out other threads on compaction - this is a topic that rears up from time to time and has had extensive previous coverage.

 

[euygar]

I agree, laboratory standard proctor test says that there should not be any particles retained at a No.4 sieve (according to ASTM standard sieve sizes). Therefore, to apply the procedure correctly in the laboratory they might sieved the material before applying compaction trials in several water contents. After sieving, the soil sample might passed into a more uniform gradation than the original, causing a lower degree of packing than it can actually have. So, even if you apply an identical energy level in the field, it's obvious that you'll have a better packing efficiency.

Eris Uygar

 

[iandig]

There is a correstion within the AASHTO(T 244: 1986), which allows for a 'corrected' maximum density to be calculated, based on the % removed, and the particle density/specific gravity of the different portions, this may help in the assessment.
D = (1-Pc)Df + 0.9 PcDc
where:
D = adjusted max density (Mg/m³)
Pc = proportion by weight of coarse particles removed
Df = max dry density of the fine material
Dc = particle density of the removed coarse material (Mg/m³)
I have used this in the past, and wrote my dissertation on this subject when I did my Degree in Civil Engineering.
However, it still comes back to the actual moisture content of the material determined in-situ, what will be the effect on the placed and compacted material if it becomes inundated?

 

[euygar]

the water content estimated in the field may be wrong, if the insitu soil samples contained gravel. This is because, most of the naturally occuring gravels with different mineral origins have very little absorption capacity (about 1-5 % at most, and you can achieve this only if you boil them in a water tank for several hours). Only light weight aggregates (can artificially be made by pulverization of cementing materials for example, or naturally occuring tuff is an example also) have absorption values comparable to pore water of the finer soil. Thus, if sampling included gravels it is probable that the insitu water content is underestimated. In such cases, samples having larger weight or representative borehole samples may be directly dried.

Eris Uygar

 

[DRC1]

I would request a second procter test. I often see problems with compaction due to an incorrect proctor test. It is a relatively chap test , but a bad test can cause a lot of field problems. Get a new sample and request an additional test.
Also be sure the person doing the insitu testing is doing it correctly. for nukes that means scraping and leveling the base and driving the pin.

Good Luck

 

[fndn]

Standard proctors with today's equipment gives very low maximum dry density. I am thinking of two scenarios. If the contractor compacted one spot with high vibratory equipment, like a vibrating plate compacted to the enf of the backhoe swing, this would give us a high density and lower moisture.

The other possible scenario is that if a large boulder is near the gauge, you would also get lower moisture and higher dry density.