FEM Modelling
FEM Modelling
(OP)
The geotech on my project is curently FEM modelling a 35 foot tall 30" diameter secant pile wall system adjacent to an existing structure with pile foundations (within 2 to 3 feet). My design specified 5 levels of tiebacks on the secant pile (so its nice and stiff and minimizes soil dilation to maintain the existing pile soil friction). The geotech on the other hand took the EI stiffness per foot of the secant pile wall and modelled it without tiebacks. i.e. a 35 foot cantilever. The geotech also indicated that the top of this 35 foot cantilever only deflects a 1/4 of an inch. This is with W21x93 spaced at +/-54" OC. He also says he will provide me with the surcharge loads of the piles based on this cantilevered model.
So here are my questions:
1) Is it correct for the geotech to model the secant pile as a cantilever, without the tiebacks? In my mind I understand that he says that if the movement works as a cantilever than the tiebacks are insurance to minimize movement. In my mind, if a tieback is present it attracts load to the shoring and changes the secant pile movement behavior.
2) Is it correct for the geotech to provide a surcharge load based on the cantilever case without tiebacks?
So here are my questions:
1) Is it correct for the geotech to model the secant pile as a cantilever, without the tiebacks? In my mind I understand that he says that if the movement works as a cantilever than the tiebacks are insurance to minimize movement. In my mind, if a tieback is present it attracts load to the shoring and changes the secant pile movement behavior.
2) Is it correct for the geotech to provide a surcharge load based on the cantilever case without tiebacks?





RE: FEM Modelling
So except that you by whatever the reasons whish to keep your project for the construction, it might be reasonable to hear their arguments and if you want, further clarifications.
It must also be ascertained if through the change he wants or enters a process where undue differences in cost go -or risk to go- unwarrantedly to their pockets at no benefit at all to the works or the owner, taking amidst your professional stature.
RE: FEM Modelling
Wedlmic
RE: FEM Modelling
RE: FEM Modelling
RE: FEM Modelling
Note that traditionally cantilever walls are designed based on developing full passive pressure. FEM generally uses soil springs. Depending on how good the model is depends on two things. First was the model appropriate? Did it capture the nonlinear nature of the soil or was it just a linear soil spring with subgrade reaction modulus or was it a spring with values that matchede passive pressures? Note that passive pressures develop on both sides of the wall. The other problem is data quality. FEM for wall analysis seems to be greatly by influenced by the quality of the soil data used in the model.
I would suggest being sure everyone has the same soil profile and soil values and then run some hand solutions for a few different support conditions.
RE: FEM Modelling
There's nothing wrong with this approach, but getting the input data "correct" is where the body is burried. My colleague used this exact method in 1980 or so, so it's proven technology, if properly applied. I think Prof. Duncan may have received his PhD on this at Berkley.
f-d
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