Moisture content in soils
Moisture content in soils
(OP)
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.





RE: Moisture content in soils
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.
RE: Moisture content in soils
RE: Moisture content in soils
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.
RE: Moisture content in soils
RE: Moisture content in soils
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.
RE: Moisture content in soils
RE: Moisture content in soils
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!
RE: Moisture content in soils
RE: Moisture content in soils
f-d
¡papá gordo ain’t no madre flaca!
RE: Moisture content in soils
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.