Heave

[ianab]

I am trying to calculate the elastic heave within a deep basement 8 m deep and with an area of approximately 50 m2. The underlying strata is Gault Clay.

I believe that this is done by calculating this as elastic settlement. Using traditional stress distribution methods appear unrealistic for this type of structure.

Can anyone tell how to calculate stress distribution/depths of influence for a structure of this size or raft foundations?

 

[jdmm]

50 m2 is not very big so that there will be edge effects. There are probably a number of closed form solutions to this problem that date back to the 60's but with all of the new programs available, if it is important to know this then it is worth purchasing a finite element program. We used SIGMA and found it to be very helpful in 2-D analysis. This would help in future problems as well

 

[fattdad]

Get the rebound curve from an odometer test and look at the change in vertical effective stress using Bousinesq distribution. You will see greater stress change in the center, less at the edge and even less at the corner. Take this and solve for your measure of heave.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[mudman54]

i agree with jdmm that this might be a finite element model solution. you can get a better approximation of the "stress relief" for an embedded structure. shape factors for raft footings always seemed like black art to me, so be careful how and where you use them. once you feel good about the effective stress distribution, divide up the clay into thin layers so you can see what the heave is at each depth, sum them up, and compare to the one-dimensional solution for a sanity check.

 

[DRC1]

Heaving in an excavation is really a factor of safety against a bearing capacity failure rather than elastic settlement. See Pile Buck Steel Sheet Piling Design Manual (or I assume the new manual), US Steel Sheet Piling Design Manual, or other excavation design guides.
The factor of safety against heave is (5.7C)/ ((gama*H)-(1.41*C*H/B)) where C= cohesion, gama= unit weight, H= depth of cut & B=width of cut. FS sould be>1.5
There are some other formulas that account for end effects etc. These values are probably somewhat conservative, but have been used for many years.
Movements due to elastic relief are not large and probably do not effect construction. Heave on the other hand can be significant and can disrupt construction. If you do look at FE solutions, be sure you have high quality data and be sure to look at bearing capacity as well as elastic movement.