Hi,

the geostatic step is required to estimate the initial stress state for an undisturbed soil resulting from its own weight (and the geological history), called the "geostatic stress state". By this, the initial strength and stiffness of the soil can be determined. Note that zero stress means zero strength for cohesionless soil.

The installation of structural elements such as piles changes the geostatic stress state locally. Since this change of stress state is hard to determine, a common but very rough approach is to model the structural elements as "wished-in-place". This is achieved by placing resp. activating the structural elements in a time step after the geostatic step.

Hope this helps.

Regards,

Daniel

Hi Daniel,

Thanks for your explanation, it helps a lot.

However, I still have some questions which you may give further explanations and I will be very grateful for that.

(1) What do you mean by saying 'wished-in-place'? Does that mean I deactivate the pipe elements at the geostatic step and then reactivate them in the next step?

(2) If my understanding is correct, then how to do that by modifying the keywords or the other way? And how about the hole inside the pipe? Should I assign soil elements to form a whole solid cubic or just leave it while assign constraints (U1=U2=U3=0) at the interface (though the interface should be deactivated, as I considered a surface-to-surface contact) ?

Regards,

Yadong

Hi Yadong,

(1) Yes.

(2) Don't know how deactivating/reactivating elements resp. material can be achieved in ABAQUS. In ANSYS, for example, the EKILL/EALIVE commands will do the job. EKILL multiplies the stiffness of the selected elements by a tiny number so that they do not affect calculations. EALIVE uses the original stiffness value.

Basically, you have to prepare the computational domain to enable a "staged construction". The part of the domain which is filled with soil in the geostatic stage and with a different material (pipe material or void) in other stages has to be meshed twice. Both meshes are present throughout the calculation. Boundary and contact conditions can be set as well. In the geostatic stage, however, only soil is active and the other material is deactivated (e.g. by multiplying the stiffness with a tiny number). In subsequent stages the soil is deactivated and the pipe material or void (inside the pipe) is activated.

A word of caution: Note that element deactivation/reactivation can produce unphysical results in non-linear analyses involving path-dependent or time-dependent materials. However, it is still a common approach in computational geotechnics.

Regards,

Daniel

Hi Daniel,

Thanks for your in detail explanation, and it helps a lot!

Let's see whether there is a similar operation in ABAQUS to kill/activate particular element sets.

Regards,

Yadong