Settlement of Compacted Soil, Wet/Dry of Optimum? 3

[mohanvenn]

Hello:

My question is does the clay soil compacted wet of optimum settle less or more than the clay soil compacted dry of optimum?

For example, a soil has a MDD of say 100 pcf at say 15% OMC.
The soil is compacted in lifts to a in-situ dry density of say 95 pcf (95% Compaction) at 12% mositure (-3% of OMC); and, the same soil is compacted in another area to 95% of MDD at 18% moisture (+3% of OMC). Will these fill soils settle differently (Both, Magnitude and Rate of Settlement)?

I ask this question b'coz, in my practice I came accross a debate with a fellow practioner, who insists that both soils should settle same. He believes that since the void ratio of soil at -3% or +3% of OMC is one and same, they should settle one and the same. The void ratio being the same is true because the degree of saturation of both these soil conditons would be different. The equation; wG = Se would be balanced in both cases only when S is different and; when g and e are same to balance with w.

Having said that, I belive they should settle differently both in magnitude and rate. The starting point (initial void ratio, ei) of the material could be same, but, the ending point (final void ratio, ef) may not be the same and the path (e log p) they take to get a final ef during saturation would different. Also, the path (e log p) they may take after saturation may be different, as there would be some particle rearragment during saturation.

Any thoughts? Any references to read up on this matter?

Thank You.

 

[cvg]

Placement of compacted structural fill on flood control dams in this area is typically done dry of optimum - again, to prevent cracking and potential for piping / internal erosion failure. The material is typically low plasticity, granular material and tends to form longitudinal dessication cracks.

 

[Ron]

BigH...I agree that volume changes can occur in moisture sensitive clays, but with regard to consolidation settlement it makes no difference whether compacted wet or dry of optimum.

The large volume swings associated with some moisture sensitive clays are necessary to consider, but separate from the consolidation settlement issue.

 

[dirtguy4]

It's been I while since I last visited the forum, I've just gotten too busy, but it is nice to read some interesting discussions again. Just adding my thoughts and just trying to get the situation correct on the Case Histories. We have a 30 ft embankment that will carry no load, is built on bedrock, is not a dam and is not a road, is extremely well drained and will have zero moisture changes after construction (more or less) because it is well drained. Sounds like an extremely well drained pile of dirt, not sure as to where this application would fit any engineering design, but there we have it.

My first thoughts are that we can take swelling issues out of the equation at least for the first three to five years, extremely well drained and water won't get into the fill. So if we believe what mohanvenn believes, that dry soils settle more than wet soils, then we must pick Cases III, IV, I, II.

If I understand our boundary conditions correctly the fill should remain in an unsaturated state, high and dry. Therefore, I would partially agree with mohanvenn's initial selection. The high expansive clays have higher water contents and on upon draining would settle more than the non-expansive clay. Thus we pick Case III, IV, I and II. So compacting over optimum does us no favours, we don't have a swelling issue and excessive water added during compaction will just drain out and allow for more settlement.

Geometry and positive site drainage are very important to this big pile of dirt. Slope issues may also be very critical, especially with high soil suctions and the possibilty of heavy seasonal rains.

In this case why not compact at optimum using a non-expansive clay? Also, what really is the purpose of constructing such a highly engineering pile of dirt that serves no purpose? Just wondering what the application is that's all.