CH Bearing Material

[geojosh84]

I am currently working as the geo on a new development. I have given initial recommendations to undercut 6 feet of the underlying CH material. The CH has PI's up to 27, and I am approximating 1800 psf swell pressures. The owner and SE have pitched the use of rammed aggregate piers, and we shot that down due to the possibility of transferring the shrink/swell forces from the pier to the shallow footings. Our response recommendation was to do additional borings to a greater depth to entertain the idea of micropiles. Shot down due to cost ineffectiveness. Our last recommendation was to use a 3' undercut in conjunction with a monolithic slab on grade and grade beam scenario, hoping that the 3' will provide enough cushion from swell pressures. I would greatly appreciate it if anyone could weigh in with some ideas and or other concerns. Thanks in advance.

 

[oldestguy]

You are expecting a lot here having said nothing about the loads, climate, etc. I believe it is not recommended here to ask for design assistance. I, for one, think this post is treading on thin ice expecting meaningful assistance.

 

[geojosh84]

Oldestguy, I am simply stating my current situation and thank you for your input. Wall and column loads 3 kpf and 100 kip respectively and the site is in the middle coastal plain in SC. Allowable bearing pressure of 2000 psf.

 

[oldestguy]

OK, here goes. I'd be more concerted with shrinkage. That has been my major experience, except where a well meaning engineer required 95 percent compaction where site grade had to be raised using local clays and a well meaning architect architect directed all roof water to perforated pipes alongside foundation leading to new ponds on site. Combine that with sand backfill to utility pipes in ad out from buildings. Talk about expansion, WOW. Sure raised holy hell with several university class room buildings. Most other problems were shrinkage of the clay to deep depths, when fast growing trees were located nearby. The shrinkage effect easily went to 20 feet below grade. The area is in eastern and southern Wisconsin where experience nearby was not known to the designers. I'd be very skeptical of any drainable undercut fill where possible water might some day enter, such as cracked drain pipes. What is the experience in the area? Sometimes problems are "lived with" and word does not get known to builders. I know of an engineer from Bureau of Rec, Denver who every 6 months adjusts the floor support posts in his house to allow for seasonal moisture changes and thus shrinakeg-expansion problems.

 

[GeoPaveTraffic]

Here is my two cents worth.

A 6 ft. undercut is a lot. I've worked on many projects with considerably higher PIs and never went that high. You don't say what you are replacing the CH soil with. If you are using a lean clay, at some point you have to assume that the undercut and lean clay fill is deep enough to prevent moisture changes. No moisture changes, no shrink/swell. In my opinion a remove and replace operation with lean clay of a couple of feet is enough to prevent moisture changes in most situations.

The other option that I have used once is a waffle slab bearing on pea gravel. The concept is that if the soil swells, the thin ribs on the slab combined with the pea gravel causes a bearing capacity failure and the pea gravel flows into the void created by the waffle slab. The system has a specific name that escapes me at the moment. Has worked like a charm, the one time I used it. Note that this system does nothing for shrinkage, so your soil needs to be very dry if you use this option.

Mike Lambert

 

[geojosh84]

Oldest guy and GeoPaveTraffic, thank you both for your input. I will say that the strata at the time of drilling was discovered be at moisture contents near to 10% below the respective PLs. Geopave, isn't a monolithic slab on grade with grade beams a similar concept to a waffle slab? I am leaning towards a 3' undercut and replacing with a lean clay with recommendations to pipe roof drains to adjacent storm drainage etc.

 

[GeoPaveTraffic]

Yes a monolithic slab with grade beams is similar. The difference is that if the soil swells the monolithic slab and grade beams will transfer the swell load into the structure. The concept with the waffle slab is that there is minimal, and predictable, load transfer to the structure.

Mike Lambert

 

[geojosh84]

GeoPaveTraffic, 10-4. Thank you for your input.