Soils testing. Previously compacted fill material is now saturated

[Jmorgans1122]

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.

 

[Ron]

Don't confuse compaction with stability. While the density of a compacted material doesn't change with an influx of moisture, not so with stability. The material can be properly compacted, but because of gradation and moisture issues, the material can turn to mush in the presence of excessive moisture.

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.

 

[Jmorgans1122]

Thanks for the response Ron. At the time of placement the material was compacted and was stable. I was wondering if a thoroughly compacted and stable engineered fill would be so susceptible to rainfall. I've never seen a properly place material turn to such crap, due to rainfall alone. It's a 50'x6'x4' area and it has all turned to mud.

 

[Ron]

Based on your description you are using limestone "tailings". These materials typically have a high percentage of non-plastic fines (calcareous dust or silt). They are susceptible to moisture instability because of the gradation and character of the material.

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?

 

[GeoPaveTraffic]

Ron,

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

 

[Ron]

GPT....Yes, I suppose that's an issue for some limestones/limerock. Limerock is our base material of choice in my area and the Florida DOT and consultants specify the modified Proctor. We have checked gradations before and after the Proctor and have found no significant difference.

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.

 

[Jmorgans1122]

The standard proctor was called for in the project specifications. The majority of my experience with crushed limestone had been with CR-610 (95% passing the 1.5") and it has performed well, regardless of weather conditions. However, this was my first experience with this fine of a mixture and it performed very poorly when exposed to moisture. I would not recommend this material in the future.

 

[Ron]

this fine of a mixture and it performed very poorly when exposed to moisture

This is my point exactly. See the attached curve for typical graphic of this issue.

 

[Okiryu]

Ron, I have seen a similar graph in the Holtz's book (Introduction of Geotechnical Engineering - Compaction Chapter, I think).

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?

 

[Jmorgans1122]

There is about 4' of the limestone material, below that is 3' of a compacted sandy clay (SC). The SC beneath the limestone is very firm. The SC was used to fill the majority of building pad, however, in the areas where grade beams were to be cast, no fill was placed to allow for gb construction. This essentially created a bowl with the bottom lined with Sc. By the time the gb was ready for backfill the contractor had exhausted the stockpile of SC and decided to use limestone. I would guess that the SC is keeping water from escaping the areas around the gb and the limestone is left to soak up moisture. Would removing the water from the area allow the limestone to "tighten up?"

 

[Okiryu]

Jmorgans1122, perhaps the compacted SC is not providing good drainage, so after several months of heavy rains water just accumulated. The grade beams are forming also a barrier there since they are lined on the SC. If weather allows, the saturated limestone can be conditioned/dried, but you already know that once this material gets wet, becames weak and you may need to think about providing good subsurface drainage. If you have high water table, this is more critical. Would also like to see other opinions from the senior guys in this forum...

 

[Terratek]

SC is the symbol for Clayey Sand. CL would be the symbol for lean (possibly sandy) clay.

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.

 

[Jmorgans1122]

I was using(SC) as an informal shorthand for the paragraph. The limestone material was only used around grade beams, but was only saturated in a couple of areas. Some of the limestone was removed and stockpiled on site, after a few days of sunny weather the material began to change to its original, grey color.
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!!

 

[newa]

Jmorgans1122,

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.