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soil structure interaction for underground structure

soil structure interaction for underground structure

soil structure interaction for underground structure

(OP)
Hi Everyone,
I am wondering what would be the best approach to consider the soil structure interaction:
CHBDC specifies that the reaction pressure on the bottom of the box shall be considered uniformly distributed,
but I tend to do a spring analogy, I think it reflects more the real thing for this 8.7 c2c span box culvert.

What do you guys think?

Thanks a lot

Mixtli

RE: soil structure interaction for underground structure

The springs are generally a better approximation, but simplifying a soil medium down to a series of linear-elastic springs is still a gross assumption in most cases. I would recommend a parameter study of a couple different spring stiffnesses. Also, if you are deep enough, and depending on the internal loads, a uniform reaction distribution might be perfectly acceptable.

RE: soil structure interaction for underground structure

A uniform pressure distribution would normally be conservative, because the deflection of the base will tend to re-distribute the pressure towards the walls. The problem with using springs is deciding the spring stiffness. The spring stiffness can not be directly related to a soil elastic modulus, and in my experience the typical values quoted in foundation text books are often way too high. I would recommend a 2D plane strain FE analysis, using plate elements, which with modern hardware and software takes hardly any longer than using springs, and is much closer to modelling what actually happens.

Whether the extra analysis time over a uniform distribution is worthwhile depends on whether your 8.7 is in inches or km.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: soil structure interaction for underground structure

(OP)
Thanks guys,
I think a uniform pressure distribution would be conservative on the structure, but I don't think it is realistic from the soil perspective: as Doug pointed out at the end the deflection of the base will re-distribute the pressure towards the walls, but that pressure will exceed the minimum allowable bearing capacity.
Now, Doug, the strain FE analysis would render non uniform pressure distribution and I would end up with more pressure under the walls than in the center (structure is 8.7 meters, btw) right?
I guess my point here is trying to figure where CHBDC is coming from when it specifies the reaction of the soil to be considered uniformly distributed.

Cheers

M.

RE: soil structure interaction for underground structure

The pressure under the walls will be higher than in the middle, which will significantly reduce the bending moment in the base slab. Bearing pressure is not a problem because the average pressure under the culvert will be less than the pressure in the surrounding soil. The worst that can happen is there will be some local yielding under the walls which will re-distribute the pressure back to the centre. To handle that in the analysis use a Mohr-Coulomb model for the soil elements and use a conservative low stiffnes value for the soil (with an uncracked stiffness for the concrete). Taking the soil pressure as uniformly distributed is the worst case for the actions in the structure, and will not cause a problem for soil failure because the overall pressures are reduced by the culvert.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

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