Soils testing. Previously compacted fill material is now saturated
Soils testing. Previously compacted fill material is now saturated
(OP)
The contractor backfilled an area adjacent to a plinth beam with a 3/16 minus limestone material and will be supporting a slab on grade (I'm not sure if the fraction is exactly correct but the material had no large aggregate and was mostly fines). The material was placed in 8" lifts, compacted, and tested with a nuclear gauge. All test exceeded 98% compaction, using a standard proctor and had a moisture content of 1 to 2 percent below optimum. The area was untouched for a couple of months, exposed to several heavy rains. After the months of exposure to the weather, the contractor was preparing the subgrade for base rock when I noticed a soft spot in the area. After further investigation, the area was found to be completely saturated. The material went from a typical grey color, to almost a dark brown and lacked any stability. The entire depth of the limestone fill is basically mud at this point. Any ideas on why this happened? I know limestone is a porous material but would that explain why the area filled is now completely saturated? Thank you for any help.





RE: Soils testing. Previously compacted fill material is now saturated
Your material was likely only required to meet a compaction specification. This is common for static applications like a building structures; however, for pavement applications, corresponding stability is necessary.
To answer your basic question.....the material does not have an appropriate gradation to provide adequate stability.
RE: Soils testing. Previously compacted fill material is now saturated
RE: Soils testing. Previously compacted fill material is now saturated
Look at a CBR curve of the material (ask around...you'll find one). You'll see that on the dry side of optimum, the material is stable, while on the wet side of optimum, its stability drops significantly.
Also, why are you using a standard Proctor instead of modified?
RE: Soils testing. Previously compacted fill material is now saturated
We normally spec Standard Proctor with limestone as our experience is that the modified hammer energy can break cause the limestone to breakdown more than is seen in the field.
Mike Lambert
RE: Soils testing. Previously compacted fill material is now saturated
Not sure of the character of the limestone in the OP's area, but without regard to whether it is a standard or modified Proctor, the material and its gradation are obviously moisture sensitive which would show clearly in a CBR.
RE: Soils testing. Previously compacted fill material is now saturated
RE: Soils testing. Previously compacted fill material is now saturated
This is my point exactly. See the attached curve for typical graphic of this issue.
RE: Soils testing. Previously compacted fill material is now saturated
Also, I remembered that you can check soil stability based on the coefficient of uniformity of your soils. If you have Cu less than 3, particles are tightly interlocked. In other words, such a soil has a maximum internal stability. When a soil has a Cu greater than 3, the coarsest particles are not in contact (they are "floating" in the soil matrix) and they do not form a continuos skeleton that entraps other particles. So for Cu>3, soils have lower internal stability. However, the OP mentioned that the backfill does not have large aggregate and was mostly fines, so the Cu approach may not be applicable.
Jmorgans1122, just curious to know how thick is your backfill and what is below it? What is the depth of the water table?
RE: Soils testing. Previously compacted fill material is now saturated
RE: Soils testing. Previously compacted fill material is now saturated
RE: Soils testing. Previously compacted fill material is now saturated
Perhaps the material was not as clean as you had suspected? By your comments that the material was brown, it sounds like there may have been some clay mixed in, assuming your limestone is grey. This is quite possible if the limestone was mined onsite. I can't think of any reason why the limestone would change colors.
Also, I agree with Ron, if you have limestone tailings, that is essentially silt which is very sensitive to water movement and pumps easily.
Is it just one area of the pad, or the whole pad? The most likely solution would be to strip material until a firm enough base was exposed for a proof-roll and then re-place the over-excavated soils.
We use crush limestone for road base in my area, but the limestone is varied and it must pass specs for abrasion and angularity, in addition to gradation, because some of the limestone is susceptible to degredation.
RE: Soils testing. Previously compacted fill material is now saturated
The issue has been resolved. We decided to excavate an area to the sandy clay elevation and place a vertical piece of pvc, approximately 10" diameter and perforated in the lower 12", in the bottom of the excavation extending above the final grade. We then backfilled the pipe with 18" of #57 stone, followed by compacted lifts of borrow material. After backfill, the water was pumped from the area, via the pvc pipe, until there were no signs of water accumulating. The area was allowed to sit for several days before we dug a few test pits. The test pits were probed with a static cone penetrometer and found to be virtually impenetrable. With satisfactory conditions observed, via the test pits, the contractor placed the base rock and poured the slab.
THANKS EVERYONE FOR YOUR HELP!!
RE: Soils testing. Previously compacted fill material is now saturated
Have you carried out shrink-swell test for that material or looked at CBR swell.
If CBR swell is more than 2.0%, the soil is likely to suck water and get wet in the rain. In Australia some State Road Authorities reject fill material with CBR swell more than 2.0% due to the potential for moisture susceptibility.