Foundation on high plasticity clay 2

[Rakra]

Can someone share their experience of foundation types for a 4 storey (with basement) to be built on a site with about 5 to 6m of alluvial high plasticity clay overlying weathered calcareous rock (7 to 10m) followed by HW calcareous rock. The clay are being tested for shrink/swell properties. We are thinking raft foundations at this stage.

Thanks for your input in advance.

 

[hokie66]

Depends on the consolidation and swelling characteristics of the clay, but I would be thinking bored piles on the rock, with the lowest level slab suspended and cast on void formers.

 

[BigH]

With such a short depth of the clay and a structure having a basement, why not just excavate to the rock and be done with it? The basement would be 4 m deep or so below ground level so you are only talking of overexcavating 2 m or so - good insurance.

 

[crcivil]

What BigH suggests makes a lot of sense to me, not big deal to invest a few more dollars to get rid of the remaining of the clay. Regards.

 

[hokie66]

Excavating to the rock is one way, but installing relatively short bored piers only requires excavating part of the material, so not as much to haul away. Then you have to replace that with borrowed material. You would have to do the sums in your location, but where I am, the bored piers and suspended slab would be more economical.

 

[fattdad]

without knowing the undrained shear strength, the consolidation characteristics and other engineering properties of the clay, who knows. . .

If the foundation is 12 ft deep, who cares if the clay shows shrink-swell behavoir. There will be such little moisture variation at that depth that the chance of swell pressures developing is moot. I'd think there'd be a real opportunity to use conventional spread footings if the clay has supporting strength. Then again, we don't know.

Regarding the calcareous bedrock, I'd be looking at regional geology and landforms too. Is this an area of sinkholes or dissolution features that could surprise the owner? I'd also be interested in knowing whether there is an "enhanced weathered zone" where the residuum (clay) at the bedrock interface is much softer.

I've designed many foundations on clay. Heck, in Richmond, Virginia our tallest buildings are on cassions that bear on a sensitive marine clay. So, we just can't blame "clay." So, it's time to do real geotechnical engineering, look at the strength, look at the geology, look at the risks, screen options and make an informed decision in concert with the owner.

Otherwise, it's piles to bedrock!

f-d

¡papá gordo ain’t no madre flaca!

 

[Mccoy]

I've designed many foundations on clay. Heck, in Richmond, Virginia our tallest buildings are on cassions that bear on a sensitive marine clay

fattdad, that's interesting, do you have any references on that (details on the foundations adopted and the geotechnical model with relevant parameters)

 

[fattdad]

Mccoy,

I do have some details. I have old publications from Arthur and Leo Cassagrande (c. late '50s) as well as buildings I've worked on directly. The old publications are just fascinating! Two dudes carving out blocks, taking tubes and returning the samples to Harvard University and such. Belled cassions as well as (more recently) double-belled cassions have been successfully used. (The double-belled caission offers some end bearing, but creates a much larger shear resistance in the cylinder of soil between the upper and lower bells.)

So, we have preconsolidation pressure, we have much lower remolded strength, we have fat clay fines (LL in the range of 70 to 90), so it does offer good bearing. I know you asked for relavent parameters, but that's like me writing a term paper. Ray E. Martin (former CEO of Schnabel Engineering) published in ASCE on the application of double-belled caissions. If time allows, I may scan some of the early work by the Cassagrande Brothers.

My most recent foundation design on the stiff clay was for Virginia's State Lab building. In this instance, there was a basement structure (parking and mechanical) the resulted in 25 ft of below-grade construction. Turned out that was the elevation of the top surface of the marine clay. So, I just used spread footings. Biggest design problem was the design of the sump capacity to keep the perched water from the overlying terrace deposits from flooding the basement. That was a fun excercize! I ran some calculations, based on correlations to grain size had a few observation wells and calculated 60 gpm. Well, when the job went to construction we monitored the contractor's dewatering flow volume and voila it was 60 gpm. Folks thought I had a crystal ball - ha! But, it was just flow to well equations and some judgement. I let them think I was smarter than I am though!

f-d

¡papá gordo ain’t no madre flaca!

 

[Rakra]

Thanks everyone for your comments and discussions. I am awaiting some lab results so will come back with more once I get those. sorted. At this stage, what hookie66 and BigH have suggested (bored piles) seem applicable in this case.

 

[Mccoy]

My most recent foundation design on the stiff clay

KK, so we're talking about some stiff OC clays like, say, the well-known Boston blue clays or the London clay...
Never heard of double belled cassoins previously, it just sounds redoubtable!

 

[Rakra]

Hi all, lab results indicate the clay with shrink/swell of 0.5, c'of 14kpa, effectivef riction angle of 22°. Clay from 0 to 4.5m followed by XWvery low strength mudstone. Clay is stiff to very stiff. Groundwater depth possibly 3m below ground. Any comments about foundation type to.use? Am thinking strip footing or bored pile with suspended slab.

 

[FixedEarth]

Raft foundations, because of their width (size) will settle quiet a bit, given your fat clays are soft to firm in consistency. Why not run a settlement analysis and see if you need piled raft?