Determining swell potential from Consolidation Reports

[Drock99901]

Hi everyone,

I am a Geotech engineer who has been practicing more as an owners rep for longer than I got to play with the really technical stuff. My client in Southeast Michigan has an existing structure that was built on deep foundations and grade beams in '01; however, the interior partition masonry walls were just thrown on thickened slabs. Starting in '02 they started showing signs of settlement (substantial cracking at the interface of the exterior walls that were on grade-beams and the interior walls) jammed doors, drains no longer-draining in showers. Total settlement to-date amounts to approximately 3.5" of total settlement. Based on the Cc 0.39 E-naught of 1.312, Cr 0.05 values for the CH clays from 5'-42', we're working with normally consolidated material. Atterbergs LL: 57, PI:35, Dry density: 73.9, MC: 51.6%. For funding reasons, they just received a proposal from an Engineering firm claiming the issue is not just settlement related, but also due to the "expansive nature of the clays." Being from Southeast Michigan, my experience in expansive clays is severely limited. My question is if anyone has any good methods for determining swell properties based on the existing lab data, which does NOT include distributions or Swell index or swell % data?

I am having a hard time agreeing with the assessment that soil expansion is of any bearing here whatsoever. Maybe I am over-looking something monumental, but any insight is appreciated! In-situ moisture contents ranged from 28-53%. It's a completed occupied building that has not had any flood events/water infiltration. Can't figure out why the design team is citing expansive soils as our issue -- to me its a basic over-sight on the ramifications of primary settlement in a highly compressible clay strata.

-Drock99901

 

[EireChch]

What foundations are the structure on, you deep foundations? but internal walls are on thickened slab. Are these internal walls load bearing.

What is the soil profile, you say clay from 5' to 42'? Is the CLAY layer varying in thickness across the site from 5' at one location to 42' at another?

You have a CLAY material that is highly susceptible to primary and secondary compression. If build in 01, its likely that a lot of your primary is complete and secondary is on going. What is your CV value.

Your CLAY is also very susceptible to shrink/swell. From an initial look it does appear to be a very valid concern, whether its contributing to 5 or 50% of your settlement is a different story.

If there is changes on the moisture condition of the CLAY then it may be swelling and causing heave or could be shrinking and settling. Both could cause jammed doors and an uneven floor plate.

 

[Drock99901]

We have a fairly uniform CH strata from approximately 5'-42' in depth. The superstructure is on drilled piers down into the hardpan and the interior non-load bearing walls (masonry construction ranging from 15'-30' in height) are on thickened slabs. My first take was to just remove the masonry walls and replace with cold-formed steel to reduce the load and then just mud-jack the slabs back square. Based on preliminary calcs, the reduction in load from replacing the masonry with cold-formed should be enough to counter the added load from the mud-jacking. I am aware we are well into secondary consolidation at this point. There has been no substantial changed in moisture content. We opened a small section of the slab to correct a drain that had settled and was flowing back into the building and hand-augered down into the CH clay strata, MC was at 46%... the original borings from '01 and the borings from a previous investigation back in '16 show a consistent 30-55% MC. I know expansive soils can mimic settlement to some extent, but I am still struggling to accept its our primary issue here. This highly compressible clay is a known issue and I still can not understand why the building wasn't put on a structural slab to begin with - especially considering they already put the superstructure on drilled piers and grade beams...

I guess my main hang-up is that in my experience, if the in situ MC is approximately the same as the LL then the probably of swelling is dang near nill. Maybe this is a poor assumption to hold.