CH Bearing Material
CH Bearing Material
(OP)
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.





RE: CH Bearing Material
RE: CH Bearing Material
RE: CH Bearing Material
RE: CH Bearing Material
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
RE: CH Bearing Material
RE: CH Bearing Material
Mike Lambert
RE: CH Bearing Material