Clay Moisture Approaching Liquid Limit

[insitegeo]

Looking at a foundation design of a stiff clay that has a moisture content of +/- 20% near the surface (swelling active zone), and then changes abruptly to ~ 40% around 7 feet down. However, below about 18 feet, the clay moisture content is ~ + 60% (plastic limit is about 30% and liquid limit is about 70%). A glacial till is found below this at about 25 feet. Building a 4 storey structure with underground parkade. Does anyone have any guidelines on bearing / settlement of a clay as it approaches the liquid limit?

 

[BigH]

Do you not have any more information than some Atterberg limits and water contents? Any undrained shear strengths? Any consolidation tests? Any SPT values? Where, in general, is the site?

 

[BigHarvey]

You have a highly compressible clay layer ( void ratio should be around 1.6 and Cc around 0.5). Pile your buiding down to the till.

 

[BigH]

Big Harvey - but he is building a basement, say, 10 ft deep which unloads the site considerably. If he builds the structure with the basement as a floating or buoyant foundation he probably will be able to compensate the weight of the building with the soil removed and be able to achieve the parkade below grade and the building without piles. (See Winterkorn and Fang's Foundation Engineering Handbook, First Edition, 1975, Chapter 12 by Hugh Q. Golder)

 

[BigHarvey]

Sorry BigH, I missed the basement. Constructing the basement will certainly be a nightmare because he will be excavating in a clay with w=40%++ at the level of the mat.We should be above water table but I would definitely consider overexcavation and put a nice crushed stone mattress underneath the mat, say 1 meter. Most of the settlement will come from the remolded materials just underneath the foundation.

 

[BigH]

With clay you should be able to excavate below - but you are right in that the base may be a bit soft - so throw in a working platform. There was a case history in Guyana (very soft clay) where the buoyant raft foundation was used. (I think that R.M. Hardy was involved.) I'll find the details tomorrow at work.

 

[DRC1]

One other thing to keep in mind is that calys at or near LL are sensitve to vibration, such as sheetpile instalation. the effects of construction should be considered in your design.