It's not compaction, but moisture conditioning the expanisve soil which will reduce the expansion potential. Expansive soils swell when water is introduce. The theory is that if the expansive material is excavated, broken up to clumps smaller than 2" or 3", moisture conditioned to optimum or even a little above, and then compacted. This should result in a less expansive soil deposit within the zone of moisture variation. This has to be done by a contractor experienced in working with expansive soils and compacting material at or above optimum. The soil shouldn't be left to sit uncovered for long though. The expansive soil could dry out, shrink, and regain some or all of it's expansion potential. If the site is left to dry out too long, the contractor might have to redo some of the moisture conditioning.
After the fill is in place, it is prudent to drill more borings to test the fill after this is done to make sure the contractor did it correctly (or the field technician didn't use the wrong proctor/opt. moisture content).
Conversely, compacting the soil will make it MORE expansive. It's a balancing act as you need to compact the soil to reduce settlement but also don't want to over compact the soil which can make it more expansive. The trick is not to over compact the upper portion of expansive fill. I've seen specs of 95% of the standard proctor instead of the modified proctor for any moisture conditioning and compaction of expansive soil within 10' or 15' of a foundation. Any fill deeper than 10' or 15' should be compacted to a higher degree, i.e. 95% of the modified proctor to limit settlement concerns.
I should note that this method is relatively new and not used by a lot of engineers or on a lot of projects. I've only seen this used for large residential projects with basement slabs that the owners accept there could be some movement and distress in the basement slab. MEP lines at transition points are typically flexible.
]It's not compaction, but moisture conditioning the expanisve soil which will reduce the expansion potential. Expansive soils swell when water is introduce. The theory is that if the expansive material is excavated, broken up to clumps smaller than 2" or 3", moisture conditioned to optimum or even a little above, and then compacted. This should result in a less expansive soil deposit within the zone of moisture variation.
Will you elaborate more this theory..advance thanks
It's pretty simple. Expansive soil is sensitive to moisture. It expands as it absorbs free water. If a contractor excavates the expansive soil X-feet below the proposed structure, adds water to the soil to make it swell, and then places it back into the excavation in compacted lifts, then in theory the soil is significantly less likely to swell from getting wet in the near future.
What is the application of the project? What is the swell percent and swell stress ?
If it is for a roadway application (I'm presuming this since you mentioned CBR) would replacement of the expansive soil be an option? Or, would soil modification with lime stabilize it sufficiently? Does your highway department indicate depth of treatment based on Atterberg Limits ?
If it is for a building application would placing the foundation on deeper, more stable ground be an option?
I'm doing research into soils with LL = 400+, PL = 200+, and free swells on the order of 70% plus. These are obviously extreme results (they caught me by a bit of surprise!) Anyone out there have thoughts, ideas or responses?
Hi steve... Actually i am a bit confuse in swell and shrink relation with compaction..i mean how the swelling force is controlled by compaction..10 blows cbr mold more swell 65 blows mold less swell how?
Compaction is not used as a way to limit or reduce swelling potential. Its not a remedial option, IME.
I would expect that there is some relationship between compaction and swelling potential however, I am not sure on how relevant or useful it is though.
There's a school of thought that compacting on the wet side of optimum reduces swell potential, which I think makes sense theoretically but I'm not certain the effect is big enough to really mitigate the issue completely.
Shrink-Swell is really about the movement of water into and out of a partially saturated soil. In a dry or partially saturated soil there is alot of capacity to absorb water into the soil matrix (and also a high suction creating a tendency for the water to travel into that dry soil). With certain types of clay minerals this additional water causes swelling. So logically if you have to samples of exactly the same soil compacted to 95% SPMDD, one dry of OMC and one wet of OMC, the one wet of OMC will have less swell potential since it's already somewhat more swelled.
Higher compaction levels in the CBR with expansive soils will cause a reduction in swell because the permeability is decreased with higher compaction. Thus for the time period that you are allowing for expansion in the CBR test, you are likely less inclined to get full saturation of the soil to allow full expansion.
