Elastic settlements on Clays

[pelelo]

Gents,

I was wondering what is the best approach that you use to compute elastic settlements on clays?.

There is a job site which is composed clay in the top 40 ft. Water table was no encountered by any of the of borings.

They plan is to build a 2 story home, therefore I was wondering what is the best approach to compute immediate settlements?.

After consulting several references, schmertmann's approach is available but it is for granular soils.

I wanted to follow the elastic theory approach and use the E values based on PI and Su (By Duncan and Buchignani) but those E values are for saturated clays.

Please advise.

 

[geotechguy1]

If you have 40 feet of clay with no water I'd be more worried about shrink-swell than settlement.

>I wanted to follow the elastic theory approach and use the E values based on PI and Su (By Duncan and Buchignani) but those E values are for saturated clays.

You could do this as an initial check and see what you get. How heavy is a two story wood frame house, really?

 

[pelelo]

Thanks,

The expected pressures are 4000 PSF.

One of the borings encountered 8 ft of very soft clay (SPT-N < 2, Su = 0.1 kPA).

I wanted to find an approach to proof analytically (e.g with numbers, using an approved approach), that any footing could have settlement issues around this area.

At the end, I want to recommend a mat, but before reaching that conclusion, I need to show to the client why the footing option is not safe.

 

[geotechguy1]

>The expected pressures are 4000 PSF.

Assume some footing dimensions and then convert this value into how many pounds the house weighs, and then compare that weight to other objects. Does the answer make sense to you? Does a timber frame house actually weigh this much?

>One of the borings encountered 8 ft of very soft clay (SPT-N < 2, Su = 0.1 kPA).

It seems suspicious to me that the clay is 'dry' but also very soft. With these SPTs and undrained shear strengths, I would think it is very saturated. The liquid limit is defined as Su = 1.7 kPa, so your clay is apparently wetter than LL. Are you sure it makes sense?

 

[BigH]

With clay - not encoutered during drilling does not mean that a water table isn't there . . . is your Su value right?? 0.1 kPa = 2.1 psf . . . somthing is wrong - with N ~ 1, even Su ~ 6 kPa . . . (rough guide)