soft to stiff clay layer for foundation design

[oengineer]

I have a bid I am working on were I am given a specification that states the top 4 feet layer of soil is classified as firm to stiff clay with shell and gravel which is underlain by 12 feet layer of soft to stiff clay. It then says that foundations for major structures should be supported on Piles. I am providing the foundation Bill of Material estimates for the bid. I would like to verify that the 12 feet layer of soft to stiff clay is not good soil. I am writing the assumptions I used when preparing my engineering designs for the Bid and would like to verify.

 

[LRJ]

So what is your question exactly? This probably ought to be in one of the geotech-related sub-forums too.

 

[Retrograde]

Soft clay is not able to support major structures. They are correct in specifying piles.

 

[BigH]

Pretty poor description of the site soils. Did they give any borehole logs? The Soils Report? The upper 4 ft is obviously desiccated crust - then the soft to stiff clay for 12 ft . . . the term "soft clay" implies that the clay is normally consolidated and has an undrained shear strength of between 12.5 to 25 kPa (20 to 40 kPa if working under British Stds)). Stiff is between 50 to 100 kPa (75 to 150 kPa in BS). This is a huge range - how thick is the "soft" soil? What would be good is if you had a graph of undrained shear strength with depth. What materials is located below 16 ft (5 m)??

Does the spec say that you, as the contractor, are responsible for confirming the soil conditions?

How this could be handled depends, too, on the nature of your structure. A single to two story house could possible be founded on spread footings if you don't have a deep frost or moisture change depth. Heavier structures might require a new set of "toys" to use. What strikes me is that a stiff clay lays within, perhaps, 8 ft to 10 ft - One could consider various methods other than piling - use of stone columns underneath the foundation locations - or at worst, removal of the material to a depth of suitable undrained strength and then replacing with an engineered fill. In this way, you could design to have your footing's depth of influence totally within the engineered fill or if not, within the engineered fill and stiff to likely very stiff soils below.. This might be cheaper than bringing in rigs for piling which, to be honest, will not be that deep.

I guess what I am trying to say - depending on your structure and other criteria, a number of options exist that could be used. This might not help you at the bid time; it is something that the geotechnical engineer should have studied and provided alternative solutions to foundation support.

 

[Leftwow]

Are you sure it's shell and not shale?

 

[dik]

Can you use the softer clay skin friction values and ignore the top 4' or more? We typically ignore the top 10' in this area due to the possible desication of the highly plastic clays.

Dik

 

[BigH]

Again, we don't know your structure - but . . .

For minor to medium structures such as strip malls, one or two story commercial, for me the scale is all wrong for driven piles - what? to 10 ft or so?? and then how deep below? We do not know what is under the stiff clay - presumably it could be rock. Again - the soil report or recommendations should be made available for you to look at - if I were to figure this, a telephone or hydro pole auger machine that would reach to 12 ft or so to form a cast in place shaft.

I had a problem like this back in Indonesia and for the 3 to 4 m, we just took out the material to the stiff clay and replaced with crushed rock (that was readily available). But this would be for more major structures.

We really need better information on the nature of the project and other issues involved.

 

[MotorCity]

You should ask the geotechnical engineer that provided the information. Obviously we are not familiar with the project or the soils report, so all we can do is speculate. I would assume that since piles are recommended, that the 12' clay layer is not sufficient to support the required loads and you need to reach some layer of soil below it (hence the need for piles). If that 12' layer were sufficient, then the recommendation probably would have been a shallow foundation, such as spread footings, bearing on it.