Moisture content in soils 2

[ryanmarquette]

In North Dakota the spec for earthwork requires that all soil be at or 5 percent over optimum moisture as determined by the 5 pt proctor. Also must meet 90 percent of the maximum dry density. Is this a bogus spec? The contractor is able to meet density even when the material is 1-2 percent under optimum moisture. I know other states have an over/under range of acceptable moisture content. Is this a big deal, enough to justify making the contractor scarify and water a compacted and hardened fill? I know they will make a big deal about it and i want some justification to make them do it rather than just adhering to a number in a book. None of this material is even being paved; its just for a widening of a gravel shoulder.

 

[Ron]

It depends on the soil. In some soils, 5% over optimum is so close to saturation that pumping occurs. In others, compaction cannot be readily achieve without having the initial moisture content higher than optimum at the time compaction starts and then allow the soil to dry back toward optimum as compaction proceeds.

Five percent over optimum seems a bit much, but then I am not familiar with ND soils. I would prefer to see a bracket of moisture, say +/- 2% of optimum. After all, if compaction is achieved, there's not much reason to worry about the moisture content.

 

[ryanmarquette]

Thanks Ron! I figured that moisture content shouldnt matter after compaction is achieved anyways. I'm thinking that the spec wasn't researched properly when it was written, because you are right, when you get to the +5% range, the material is very hard to work with. Most of the soil up here is just a basic silty clay. The material i'm talking about has a Dry density of 122.9 Max and 11.6% optimum moisture. The borrow site has been so variable with moisture content that it would be difficult to control anyway.

 

[BigH]

It depends on what the earthwork is for - if it is for a road, then the plus/minus 2% is reasonable although I don't like base course to be above optimum when starting compaction as the vibration brings up the water to the surface along with the fines - and then this blocks further water from escaping.

For dam clay cores, the +5% as a maximum might be desirable. Many specs say +2 to +5 - as this permits the clay lumps to be better "mashed" together than if the clay lumps were dry of optimum.

 

[FixedEarth]

It is possible the specs are meant to guard against heave of highly expansive soils. Usually we like to see +3% to 5% and 90% relative compaction, however, those soils have optimum water content well over 11%. If your area does not have highly expansive soils, your 2% over optimum is about right.

 

[dgillette]

First off, are we talking about 5% wet and 90% RC from standard or from Modified? That makes a whale of a difference in what I say below.

Big H - The following all assumes STANDARD. For dam cores, there is diversity of opinion on compaction moisture. Much wetter than +2% and you may have trouble getting density of 95% or 98% (standard Proctor), just because there aren't enough air voids. In a high dam, construction pore pressure can get very high if the compression of the lower fill occurs too fast for the water to bleed out, once the air voids have been compressed. Too wet, and trafficability and rutting become problems, and the feet of the sheepsfoot may not walk out very well. Some (like my outfit) have historically used Opt or 1% wet to 2% dry, for several reasons, including construction pore pressures, it being easier to get 98% on the dry side with tamping rollers than if it's bumping up against the zero-air-voids line, the lack of water at many of our sites. (We generally do use higher %w, select material with higher PI and less gravel, and thinner lifts at contacts with structures like spillways or rock abutments. I suppose you could make an argument for using dryer material at the bottom, then Opt to 2% or so wet in the top 15 meters.) I think USACE usually uses Opt to 2 or 3 wet in their dams. Peck gave a great Hilf Lecture on this subject in Boulder ~12 years ago, but there was unfortunately no paper on it, and nobody recorded it. He pointed out benefits of both wet-side and dry-side compaction.

There is never a simple answer to these things, and I could never give you a simple answer even if there was one.

 

[ryanmarquette]

Thanks all for the help, very useful information. dgillette, North Dakota does use the modified proctor with their soils. We ended up disregarding moisture if they were above density of 90 percent. Sometimes you just have to pick your battles with stubborn contractors, I suppose. The last few months have been very rainy so if the soil wasnt wet enough, im sure it will be enough now.

 

[fattdad]

I struggle with these conversations. Just recall that the optimum moisture content for 95 percent comapction (or 90 percent or 105 percent) is different than the optimum moisture content for 100 percent compaction. When a spec says, 95 percent compaction within 2 percent of the moisture content for 100 percent compaction it's inconstent with basic compaction theory. It's much more consistent to specify a band of saturation for compliance to a moisture content specification, but most folks can not intuit saturation from the proctor curve.

somebody posted that moisture content is not that relavent after compaction. I refer you to the work of Jim Mitchell (with Hooper and Campenola (sp)) where they show the saturated permeability of compacted clay can vary by two or three orders of magnitude when the sample is compacted over optimum - in this instance referring to the wet side of the line of optimums.

Just adding some of my thoughts.

I'd bet for 90 percent compaction the true optimum moisture content would be 5 percent greater than the optimum moisture for 100 percent compaction.

I think the earthwork contractors got a hand into this spec also. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Mitchell, J.K., Hooper, D.R., and Campanella, R.G. (1965), "Permeability of Compacted Clay," ASCE JSMFED, V. 91, No. SM4, pp. 41-65. I think it is discussed also in Fundamentals of Soil Behavior, by Mitchell, or by Mitchell and Soga for the most recent edition.

 

[fattdad]

Thanks Dave for completing this thread. Just a great publication for sure!

f-d

¡papá gordo ain’t no madre flaca!

 

[theCorkster]

Background?

Years ago the Bureau of Reclamation and Corp of Engineers had wet/dry perspectives, primarily based on the work environment. The COE experience was that building in wet environments made it impractical to be on the dry side, while the BuRec experience was that building in arid environments made it difficult to build on the wet side.

I recall H.B. Seed discussing this with respect to 1) the material properties required by the design and 2) the constructability and effectiveness of compaction. I took this to mean that the compaction specifications for a given material reflected some interaction (overlapping) of these factors in the design process - the amount of overlap depended on the critical function of the material.

In the past the BuRec accepted slightly more permeable core and accommodated rapid construction - the COE monitored and modified construction rate to address construction pore pressure instability.

The bottom line is whether the material in it's compacted state provides acceptable strength, compressibility and/or permeability, depending on it design function.