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Building in Expansive Clay

Building in Expansive Clay

Building in Expansive Clay

We are structural engineers working in a very small country with no geotechnical engineers.

Web have a client who wants to build a warehouse on a site with a very bad ground conditions.

We have a “fat clay” with a LL of over 100 and a PI of over 65. The thickness of this layer is about 6m after which we have a compacted tuff material. The soil lab in our country has significant limitations.

Site needs to be raised another 1m above existing grade for flood control which complicates matters even further.

Piling is not an option again due to in country limitations so we are looking at ground replacement and having a series of deep foundation walls on a grid of strip footings.

Would appreciate some advice to determine how much material we should remove. Thanks

RE: Building in Expansive Clay

What is the consistency of the fat clay?

Do you know the current moisture content of the fat clay?

If the fat clay has ok strength, then you may be able to just place the 1m of new material on top of the existing clay. In general moisture content changes below 1m are fairly small and moisture content changes are what you want to avoid.

Mike Lambert

RE: Building in Expansive Clay

May help to know if it's a tropical country or a frozen country?

I worked on a project in Panama and we had swelling clay too - altered volcanic rock. To some extent if you get these soils compacted they will not shrink when desiccated. They could swell; however. So, there's some need for confinement.

We did a building pad of the fine-grained materials, but ultimately used drilled shafts to minimize risk.


ípapß gordo ainÆt no madre flaca!

RE: Building in Expansive Clay

That added fill should be compacted. Expect it to settle. However the thicker and the denser the fill on top, the less you will see in differential settlement It has a form of "beam action". Building also should be capable of receiving some diffeential movement of foundations. It should not have utilities connected that cannot receive some differential settlement. I'd get rid of or do not add any trees. They draw water and there is more settlement near the tree than elsewhere.

RE: Building in Expansive Clay

Moisture content is 67-70%. Tropical country

There is a nearby stream and about 2-3m above sea level. I’m thinking of removing about 2 meters of material which will take me down about 600mm below the water level in the stream. Thinking it would be good if any fluctuations in the dry season.

There is a nearby hillside where we can get weathered rock. We will probably not be able to compact very well but I’m thinking of placing about 1.5m and compacting as best as we can and proof roll with a loaded dump truck.

Final layer will compacted well with grades crushed stone or other engineered fill.

Any comments? Have I removed enough clay?

RE: Building in Expansive Clay

The more clay gone, the better, but leaving some will still result in problems, but less. That below the water table likely not to be so serious.

RE: Building in Expansive Clay

If your removing clay and replacing with compacted granular fill. Unit weight of clay is less than compacted fill. Also raising grade 1 m will cause site settlement.

I think you may want to consider preloading and slab on grade foundation with minimal bearing capacity and design the building for a lot of movement.

RE: Building in Expansive Clay

As OG said, removing as much clay will help to reduce the potential effects of clay swelling. I would be careful though as it sounds like your ground water table could be close to the level of that stream. You dont want to dig too deep and then have a dewatering issue.

RE: Building in Expansive Clay

Hi do you have lime/cement supply for consideration of clay stabilisation?

RE: Building in Expansive Clay

what's the rock? Is it volcanic? Will it degrade as a fill (checking soundness and such)?

All the ideas so far seem to relate to something, unless the rock fill causes other problems.


ípapß gordo ainÆt no madre flaca!

RE: Building in Expansive Clay

How much to remove is very difficult question to answer since we don't have a lot of information. Assuming that your clay is expansive due to wetting, then there isn't a need to remove clay under the groundwater table since it should be relatively saturated (unless large seasonal fluctuations and drying periods are anticipated). You need to excavate enough material in the wetting/drying zone to address the expansive soil. This zone is location specific, land use specific, and likely hasn't been established in your area since there aren't any geotechnical engineers. Even if it was it would be highly contested like it is in my area of practice. After this zone was identified then you could do two things, excavate and replace all material in that zone or use swell tests to quantify how much anticipated swell will occur and figure out how much material you need to remove in the wetting/drying zone in order to reduce the anticipated swell to be within certain tolerances. Keep in mind that the bottom of the excavation should extend a minimum of 5' past the foundation footprint. Make sure the building location doesn't move at all after this has been performed. We recommend that our clients survey the bottom of the excavation because engineers or contractors have moved the structure after over-excavation has been performed.... hello differential heave.

Conservative irrigation and adequate site drainage are imperative to the performance of a building on expansive material. Make sure the owner understands the impacts of over-watering the site. Plants and trees should be located far away from the building so the owner doesn't have to irrigate next to the building. The ground surface around the building should slope away for the first 10'. I usually recommend 5% for landscaped areas and 0.5% for hardscape . Downspouts should discharge far away from the structure.

Lastly, which others have brought up, settlement of the compacted fill can be an issue. Make sure the material is non-expansive, durable, well graded, and well compacted.

RE: Building in Expansive Clay


I appreciate all the feedback. The rock fill is volcanic. Will try to get some more information on soundness to determine durability. We don't have lime.

I understand that there will be settlement once the fill is added. Will backfill and compact up to the underside of the slab and will excavate for the strip foundations afterwards. We probably not start construction for about 3 months after the placement of fill.

RE: Building in Expansive Clay

I just don't have too much experience in reusing volcanic rock rockfill. I am very cautious in using shale rockfill. So cautious that I require shale fill to be modeled using fully-softened strength. In that instance, we'd require the design engineer to take the shale, completely disaggregate the rock clasts, add water to the liquid limit and normally consolidate in the odometer. From there use the peak disaggregated strength for your long-term performance.

We base everything on 75 year design, which is long for commercial work.

Not sure if volcanic rock fill warrants such concern? But, maybe?

Durability is the key.


ípapß gordo ainÆt no madre flaca!

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