Contractor Doesn't Want to use Water 10

I have a max curve in hand which showing the optimum water content amd max dry density. I told the contractor million times to add more water to the loose soil prior to compaction but they would rather have the compactor running back and forth for the whole morning than connecting the hose to the fire hydrant. Results? They achieved the required 90% max but the moisture content is low. Is this acceptible? Was I looking at the same max curve in hand for they have added more compaction effort? Please provide your opinion!!

 

[georam]

It is very typical for contractors who do not understand the significant of compaction effort using optimum moisture content. However, sometimes it is more convenient for them for not adding/reducing water to meet the OMC. Perhaps tehy do not want to add water truck on their job, or do a proper mixing of the added water in the soil. In other occasion, I noticed that, when the inspectors are not around, they just put 1m lifts instead of 15cm ones, and then compact the top of the lift excessively !
You said that your density test is OK... however, if you compact the surface hundred of times or with excessive weight of heavier compacters, the soil/aggregate structure may change.... hence the characteristic of the compaction curve. You may start compacting sand and gravel, but end up with silty sand with broken gravel pieces (which may have actual lower MDD than the original soil) !!.
In clayey materials, excessive compaction may result in change of structure or broken down of the clay particles... resulting of more swelling as it absorbs water, or loss of strength of the soil.

 

Thanks GEoRam. The thickness of loose lift were OK, 15 cm or 10 to 12 inches. But boy! they did spend a lot of time pounding and rolling these lifts just to get 90%, instead of add some water to it. Per Spec, they met the requirement, what more can I say. Can anyone comment more per engineering point of view. Thank you.

 

[bb29510]

they do it that way on my jobs too. We have a spec that required 2% of opt moisture content. So fail them anyway.

 

[dirtman]

Both density and compaction water content affect the engineering properties of the soil being compacted. If the soil in this project were being compacted for a clay liner, even though it met density, it would not meet permeability because of the poor structure that results from compaction at low water contents. The structure achieved by wet compaction is very favorable to reduce permeability. See numerous articles in the ASCE geotechnical journal on this topic.

 

[Wbajwa]

What's your opinion for moisture content +8% of OMC and compaction 93%, for the construction of clay liner? Contractor is arguing that with these the required permeability has been achieved. Is this acceptable i.e. moisture content well above the 2% of OMC and compaction below the 95% mark.

 

[jheidt2543]

Just a suggestion for your future jobs. At a project meeting BEFORE the work starts, go through the compaction specifications and make sure everyone understands the parameters required for a passing test. It also helps to discuss the why behind the requirements.

Another thing to keep in mind is that the parameters you put in the specifications affect the contractor's and the OWNER'S costs. Make sure that you are specifing what is REALLY needed for that specific job (don't just take a spec. off the shelf). Then, if you have specified what is REALLY needed, stick to your spec. That is the basis of the design, the cost estimate, the field practice and, God forbid, the court case! Too many engineers over specify, make sure you know what you need.

 

[cbosy]

Need to be careful with cohesive soils being overcompacted on the dry side. Can get high dry strength that will deteriorate when saturated.

 

[emmgjld]

A very real problem with compacting well below Optimum Moisture is that some soil may undergo collapse if saturation ever occurs. There may also be significant problems with slope stability on high slopes if saturation occurs.

A potential problem with compacting excessively wet of optimum is that silts, silty clays, lean clays and fat clays will shrink if some to large amounts of drying occurs. The silts are USUALLY least affected and the fat clay are usually most affected.

Loss of Integrity of clay covers or liners due to shrinkage cracks are common in smi-arid and arid climates. I have observed shrinkage cracks over 6 feet deep (2 meters) occur in less than 2 weeks of hot, dry weather. The Engineering Geology literature has recorded Shrinkage cracks onver 30 feet (10 meters) and believe I can make a good argument for cracking up to 50 feet (16 meters).

Associated problems with 'piping' in the shrinkage cracks should be obvious. Of course, if the soils have any tendency to being dispersive, the results can be extraordinary.

Yes, it is important what the soil moisture is during compaction, just as it is very important that the correct Maximum Density Criteria (Standard or Modified) be utilized. The issue is NOT trivial.

 

[DRC1]

For clays, water content effects permiability and long term perfomance. For Sands compaction is compaction. The OMC allows you to acheive maximum compation at a specified energy. If you increase the amount of energy, you can increase the amount of compaction with less water, so although he is meeting 90% of compaction at the energy required by the procter test, he would be significantly above that if he added water. What does the designer say about the issue? Ask the contractor why he would rather compact than add water, he may have a verry good reason. Anyway, my answer is : yes water may matter if it is a clay, but no it probably does not matter if it is a sand, but check with the designer to be sure.

 

[dirtsqueezer]

I have been with a soils testing company for over two years, and here is my take on optimum moisture:
Optimum moisture is the moisture content required for one to achieve a certain compaction. It has absolutely no structural value.
Let's say a contractor places fill at exactly the optimum moisture, and you test that fill at over 95 %. Later that day, a huge rainstorm saturates your compacted fill, so that you are well over optimum. You then go back and take another test on your now oversaturated fill, and again test the fill to over 95% compaction. Has your fill been at all comprimised structurally by the excess rainfall after it has already been well compacted?
Let's go further and say you are placing fill in a seasonal water table in the summer. All tests for the fill come up to over 95%, and you are placing on glacial till hardpan that will permit the flow of water, while remaining firm and unyielding. You know that in 6 months, your fill will become oversaturated to the point where it will most likely be underwater. Will this make the fill any less compact in 6 months? Can your fill possibly settle any more after it has been compacted all the way?
I think the concept of optimum moisture is a tool to use if you are having trouble achieving compaction. I use it often, but I do not require that fill be within optimum if it is being placed properly and compaction is being achieved.
The one exception to my rule is if the client decides to place clay-dominant materials. Clay has a nasty habit of expanding when saturated, and I have seen compaction drop dramatically to the point of necessary removal after passing compaction standards.

 

[Ron]

Since your requirement is only 90 percent compaction, you are not likely worried about stability. In that case, the moisture content at time of compaction is somewhat irrelevant, provided the contractor wants to do it the hard way. He could achieve a similar result with less effort by adding water....but sometimes logic just doesn't prevail!

 

[BigH]

Original question is similar to a lot of those put on this site (which I happen to think is a great site). What is the similarity? The lack of details in the original question. To achieve valid and succinct replies, the original question should have included the type of soil being compacted, the type of fill structure - e.g., under foundation, general backfill, water dam, etc. Then, respondents can give solid points to a specific question.

I always suggest to contractors to compact a bit on the dry side of OMC as energy of most compactors is higher than that of the laboratory proctors (modified (heavy) or standard (light)) unless there is solid need to do otherwise (e.g., dam core). I do not suggest that such be 3 to 4 percent less, but somewhat less. If there is no compelling need to add water, don't. Most granular fills can achieve compaction on the dry side. What I have noted for several years now is that with the placement of WMM (Water mixed macadam - a well graded crushed granular material - such as Ontario Granular A - typically used for base courses), if the OMC is exceeded, there is a great chance of noting pumping/sponginess during compaction with tell-tale wheel paths and cracking between dual wheels. Why? I suggest that by being over OMC, with vibration, the forces cause excess water to rise and carry with it the finest materials of the mix. The water evaporates leaving a thin film of fine silt over the top. With further compactive effort, additional water will rise but will not be able to escape through the low permeability silt skin. This leads to short term sponginess (which clients, if they see, get paranoid about). On drying, things will be okay. The biggest problem seems to be (when there is a water truck available) that contractors actually provide no control on amount of water to add. Some simply leave it to the truck driver to do it . . . whenever. As a result, I have seen good granular bases and subbases turn to sponges.

With clays - if you want strength, go slight dry of optimum - if you need to achieve low permeability, go wet. Kneed the clay. If clay saturates after compaction, strength (e.g., CBR) reduces. But this should all be taken into design. Clays need careful attention.

Comment on tailoring specification to the job is very important - off shelf specs are the scourge of the constructing engineers. For instance, if you have a 4m embankment of soil being placed atop 6m of soft to firm clay that will consolidate significantly, what is the sense of specifying 95% heavy compaction (modified)? No 4m embankment will fail due to nominal compaction done in a professional manner - settlement will take place; but is is not caused by "low" compaction.

Keep up the chats on topics such as this. They, as one put it, are not trivial. Compaction is an art, still. What really gets to me though, is that EVERYONE (read that clients, lay people, etc.) are all experts in such when most don't know more than a buzz word or so.

Best regards to all.

 

[Focht3]

AMEN, BigH! Great post!

DirtSqueezer: Two years is not enough experience. I'll bet that your projects are all of a similar size / type / fill. Or perhaps a limited geographic area. Keep in mind that compaction requirements for landfill liners are different than the requirements for airfields or deep fills are different than buildings are different than...

You need a lot more experience - be open to the possibility that while your current experience indicates that OMC is not terribly significant, other circumstances may arise in your future practice where OMC is crucial.

 

[abarker]

I agree with Focht3. As an example, I've been in materials testing business for about 12 years now and until a year ago, I would not have thought collapse of sand soils would be a significant consideration. However, our firm became involved with a forensic investigation of a site having over 10' of silty sand fill (bottom ash material from a power plant) placed without moisture control (much of it very dry of optimum moisture content). A structure placed on it ended up settling differentially over 1" after rains saturated the soils. Although the collapse potential of the sand was low, the thickness of fill compounded the potential (it takes less than a 1% collapse potential to result in 1" of settlement upon saturation for a 10' fill thickness). Needless to say, I now have a much greater appreciation for moisture control during compaction of deeper fill systems!

 

[Focht3]

abarker -

Another example for you: Orange County, CA toll road design around 1990: Bee Canyon (where the ECTC was to tie into SR 91 (Riverside Freeway.) [I left California before the project was constructed.] We dug a test pit in aolian (wind blown) silty sand deposit 15 feet thick, took densities every two feet to a depth of 14 feet. Dry denssities were low - about 70 pcf (as I recall.) When we backfilled the hole with all of the removed soils, the hole was still six (6) feet deep!

The project specifics? An earthen embankment some 250-300 feet high for the new toll road...and within a few miles of the San Andreas fault system.

An extreme, and natural, example - but I hope that it illustrates and emphasizes abarker's point.

 

[IslandGEO]

An interesting progression of opinions. I hope the information viewed from these discussions is taken as just that, opinion.

I agree with some and not with others but I am concerned with the lack of understanding that anyone reading them has regarding the qualifications of those providing the opinions.

How many opinions are generated from a licensed geotechnical engineer versus a field technician? I'm not saying tech's cannot have a good grasp of the fundamentals of soil mechanics.

I read a comment from someone that moisture has absolutely no bearing as long as you get compaction. That is obviously not a comment from an experienced geotechnical engineer. Compacting a highly expansive clay or even a slightly expansive clay dry will inevitably lead to a sever increase in moisture content with time and unanticipated expansion.

Keep it up but I suggest anyone interested in these subjects review published material for their final study.

 

[Focht3]

Welcome, IslandGEO!

Your caution regarding review of the published literature is on target and good advice. I, too, am often concerned about how well a reader can discern "truth or fiction." But the point of the forum is to provide - and encourage - an open discourse on problems and how to solve them. The presence of bad opinions or advice is the price we must pay for the openness of the "conversation."

Again, Welcome! And I encourage you to fill out the 'My Profile' option so that other members can better evaluate your opinions...

 

[TJWATKINSPG]

Hooray!! even though I am a P.G. in a strange land. Please check the references of the information. Even L. Ron Hubbard has an opinion. The truth will set you free. Best of luck. Geodude