elastic settlement

[Bgeob]

I'm designing a shallow foundation of 3X6m and with 900tons I have SPT and CPT values of the layered soil. The soil's first 0.3 m is sandy to silty sand, 1.7m clay, 0.2m silty clay to clay, 1m clay, o.5m silty clay to clay. There is no water table. Where do you think I should place the foundation, and with what formula should I calculate the elastic settlement, and to what depth )depth of influence? Thank you all, any advice will be helpfull.

 

[Ron]

I wouldn't be too concerned about the elastic settlement....I'd be concerned about the consolidation settlement.

 

[grabens]

I agree with Ron your soil profile suggests that consolidation settlement needs to be evaluated for this project based upon the foundation size and load. You need to get some samples for consolidation tests too.

 

[Mccoy]

Now, sorry for the late post but I'd like to introduce here the issue of unsaturated soil.
Since the surveys rule out the presence of water (it is implied for a certain depth below the foundation) how do we deal with consolidation?
If we get samples they'll be at least partially saturated in the lab, which is not the natural condition (assuming again that the groundwater level is well below the foundation depth).
In such a case the consolidation moduli meausred in the lab would be overconservative.
Do you guys ever reason in such terms? Do you evaluate the saturation ratio to estimate wheter the soil will behave more as a saturated or unsaturated material?

 

[BigHarvey]

Totally agree with Mccoy. The settlement will be more an elastic type of settlement. A good tool would be pressuremeter tests to evaluate the settlement.

 

[Mccoy]

A good tool would be pressuremeter tests to evaluate the settlement.

Or dilatometer tests where the pressuremeter is not widely used. In some instances it is observed that the edometric moduli calculated from the dilatometer surveys are significantly larger than the edometric moduli from lab tests. An explanation may be that, in unsaturated soils, the edometric modulus is closely related to the elastic modulus since consolidation phenomena do NOT occur. Unfortunately, I couldn't find any comments on that in the literature, even professor Marchetti, the inventor of the dilatometer, says nothing about these observations.
I wonder if it is the same with the pressuremeter (frequent disagreement between lab tests and pressumeter edometric moduli in similar soils).

 

[BigHarvey]

Pressuremeter tests are based on the elastic theory, it works generally very well in all soils but for soft clays where they give an optimistic result and generally in this case an oedometric method gives closer results.

 

[Ron]

McCoy...I agree with your premise that if the clay stays dry, elastic settlement should be considered; however, even with a low water table, the clay can become saturated or nearly so from rainfall. Even if only a temporary soil condition, the building load condition is constant so during the partial saturation period (who knows how long that might last?), consolidation is likely to occur, or if the clays are expansive...another issue.

 

[fattdad]

Is the "clay" residual or fluvial? Is there no water table, 'cause you set a well and it read dry or did you just not see water in the hole during drilling? Is the natural moisture content near the liquid limit?

For unsaturated clay that's fluvial in nature and when the natural moisture content is well below the liquid limit, I'd just use elastic theory and move on. . . A DPT or pressure meter would be a good tool for sure.

Disturbed clays that dry out can show a dry strength that's misleading. So, fills can be a problem if there is no compaction control and n-values are obtained in the summer. Especially for soils in the upper 5 ft or so.

Please recognize that your data only extends to 3.7 m and you are designing a footing that's 3 x 6 m. I'd think this is not sufficient.

f-d

¡papá gordo ain’t no madre flaca!

 

[Riggly]

Are you missing soil information? You have a total of 3.7m of soil data. Considering embedment and the influence soil below the foundation, which is about 6m in that case, you may need more soil information, or disclose what is below the soils given. That's a huge load, so it is worth additional soil data if you don't have it available.

 

[aeoliantexan]

Unsaturated soil can be compressible in its natural state. I would estimate the undrained strength from both the SPT and CPT data and assume that the preconsolidation presure is about 4 times the undrained shear strength. If the preconsolidation pressure is not well above the foundation pressure, run some consolidation tests at natural water content.

 

[BigH]

I think Riggly hit it on the head - the soil profile (as given) is insufficient to make a valid and reasonable estimate of which is more important - elastic or consolidation.