Foundation on collapsible soil

[hetgen]

Hey folks,

When designing light weight structure on moderately collapsible ground, one recommended solution is to remove in situ material below foundations to a depth and width of 1,5 times the foundation width and replace with material compacted to 93% Mod.
But if the collapsible soil horizon is way deeper than the 1.5 times the foundation width what good is it to have compacted soil above it?

Thanks.

 

[CELinOttawa]

The intent of removing materials to a depth below the footing is to create a raft under which the stresses will be less than were the foundations directly onto the natural materials. But, frankly, this all sounds wrong: I've never heard of 93% modified proctor compaction, but 95% or 100% is typical (not necessarily for this application).

You need a good Geotech.

 

[dcarr82775]

My understanding is that it spreads the loads from the building out more and will lessen any local effects. Whenever I see this type of requirement there is an additional requirement that the foundation be capable of spanning some long gap. Don't think it works terribly well.

 

[emmgjld]

Need a lot more information, to include what part of the world.
1] How deep/thick is the collapsible layer & is a deep foundation system even an option?
I use overexcavate/replacement w/watersoaking for thin deposits (usually up to 10 feet), depending on the degree of potential collapse. Also for thicker if the overex & watersoak depth is increased quite a bit.

2] What soils are involved?
If loess or debris fan deposits (Silty Clays, Clayey Silts, Lean clays), the 93% modified proctor is reasonable to provide lessened permeability in the upper fill surrounding the structure. The majority of collapse failures I deal with is mostly water intrusion at the foundation backfill (roof downspout discharge, near foundation landscape irrigation, poor/adverse surface drainage).

3] What is the Geology?
Thick Loess and debris fans can be structured that water is introduced on the side and seeps toward our structure. Can the water be intercepted?
I also deal with diapirs (salt intrusions and domes). The is very little' bottom' to these. A deep foundation is not an option.

 

[hetgen]

emmgjid,

The collapsible soil horizon is more than 5m deep and it is naturally transported loose to medium dense fine grained sand with a collapse settlement % of ~10.

Most of the structures to be supported are not heavy; the required bearing capacity would be less than 30KPa if we use 1m wide strip footings. My questions is, beside the permeability you mentioned above, what good is it to replace the soil beneath the foundation?

CELinOttawa, we do have geotech on board, he also recommended soil replacement.

Thanks for your response guys/grls

 

[emmgjld]

I ask the following to get discussion going, The subject is VERY SERIOUS and very seldom really thought out:
-- Can the structure and anticipated uses tolerate movement?
-- What is the actual history of the area?
-- Is it reasonable to keep water away? To keep the subgrade soils unsaturated? Is it common for saturation to occur the full depth or part? If full depth saturation, does the laboratory estimate reflect reality?
-- If a deep foundation is chosen, how are slabs & pavements to be dealt with?
-- If a deep foundation is chosen, how are the utilities supported.
Think out the ramifications of a foundation & structure performing near perfectly (a Structurals' goal) & yet all adjacent & attached elements are undergoing extreme differential movement, Broken utilities, broken slabs, continual tripping hazards, access problems etc.

-- If a shallow foundation is chosen, how much overex/compaction is actually called for? Is the explanation logical?
-- Is the overex/compaction primarily used to decrease the potential collapse or only to improve bearing capacity, (improved bearing is implied by the 1.5 times the foundation width rule).
-- If a shallow foundation is chosen, does the overex seem to be enough to reasonably deal with interior/exterior slabs & pavement transitions, utility connections?

 

[hetgen]

Thank a million for you quick response emmgjid, I will try to answer you questions one by one:

-- Can the structure and anticipated uses tolerate movement?

- Collapse settlement won't have a serious structural consequence, but the building cladding is clay brick with plaster finish and undesirable cracks will form even with small differential settlements.

-- What is the actual history of the area?

- I don't know the geologic history of the area; the geotech report only mention that the deposit is naturally transported.

-- Is it reasonable to keep water away? To keep the subgrade soils unsaturated? Is it common for saturation to occur the full depth or part? If full depth saturation, does the laboratory estimate reflect reality?

- the area surrounding the building will be paved with impermeable pavement at 2% gradient falling away from the building. A full depth saturation is unlikely.

-- If a shallow foundation is chosen, how much overex/compaction is actually called for? Is the explanation logical?
-- Is the overex/compaction primarily used to decrease the potential collapse or only to improve bearing capacity, (improved bearing is implied by the 1.5 times the foundation width rule).
-- If a shallow foundation is chosen, does the overex seem to be enough to reasonably deal with interior/exterior slabs & pavement transitions, utility connections?

- a shallow foundation width 'B' with an allowable contact stress of 100KPa above 1.5xB X 1.5xB compacted feel is recommended by the geotech. But as I mentioned earlier the bearing requirement is quire low and the added 1.5 width is not much of an improvement.

You said "...Is the overex/compaction primarily used to decrease the potential collapse....,..." My question is in what mechanism does overex/compaction decrease the potential collapse?

 

[emmgjld]

Hetgen, sorry for the delay, It has been a busy couple of days.

-- What is the actual history of the area? I was interested in the performance of other structures. How much collapse has been experienced in the past. What is the usual foundation system in this area?

Is it common for saturation to occur the full depth or part? If full depth saturation, does the laboratory estimate reflect reality? Again, what is the area history?

You said "...Is the overex/compaction primarily used to decrease the potential collapse....,..." My question is in what mechanism does overex/compaction decrease the potential collapse? If collapsible soils are removed and replaced by non-collapsible fill, even native soils properly compacted. This should be obvious. The obvious query would be -- how much collapse potential has been mitigated by the overex/compaction? The history of the area would provide some of this answer.

In my opinion, 1.5xB X 1.5xB compacted fill seems like minimal subgrade improvement. I would want some idea of the history of this amount of improvement.