Ks (coefficient of subgrade reaction) in sheet pile design 1

If you are doing relatively simple hand calculations, k_s probably is not needed. If you are doing the sheet pile design with a slope stability program or a program like LPile and you need an accurate wall deflection value, you probably need k_s.
Often and commonly, simpler design programs just calculate the wall deflection as a structural member deflection without regard to soil movement.

@ PEinc: Thank you for your reply. We have a simple sheet pile software that does use ks as an input. We can not find any theory or example of sheet pile design where ks has been used.

Most text books and design manuals for sheeting walls are modelled as structural calculations for the support member (pile or sheet) using on soil, water, and surcharge loads. The calculations are essentially hand calculations programmed for the computer. They usually do not consider soil movement. What program are you using?

PEinc said:

What program are you using?

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We are using SPW911. This program does not seem to take Modulus of Subgrade Reaction ks as an input. So we think it uses the classical equilibrium approach that you described.

The Geotechnical Report we received recommends Finite Element model using soil springs having ks stiffness.

We have consulted Bowles' Foundation Analysis and Design (Section 13-3.3) and understand that in FE model, ks is to be used in the passive pressure region below dredge line. We are thinking of using SAP2000 - a common structural engineering FE software.

Geotechnical Report has recommended ks values at different depths. It's not quite clear to us whether these ks values already include a Safety Factor (like Bowles indicated).

As Structural Engineers, we are used to Limit States Design, where we increase the applied loads by certain multipliers, and reduce the resistance by certain other multipliers. So, we are wondering if these ks values are already reduced, and whether we need any multiplier on the load side as well. We don't want to double count safety factors.

I would say it sounds like a FEM parameter for software like Plaxis. You won’t need that for SPW911. The Geotech should provide boring logs and possibly soil parameters like unit weight and friction angle, and cohesion for you to run a typical sheet pile wall design. The only reason to use FEM is if the design is unusually complicated or if it’s the preferred method of the designer.

I personally would just ignore the recommended k values and proceed with the standard design procedure for SPW911.

@MTNClimber, thank you. Yes, Geotech has provided boring logs and soil parameter. We have already done a design using SPW911. But the Geotech also emphasizes that "passive resistance of the sheet must be assessed by FE analysis due to very weak nature of the soil".

I may be out of it, but I would not use the modulus of subgrade reaction for cantilever sheet pile design. I've only ever used active and passive pressures.

-----*****-----
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-Dik

tmalik3156 said:

the Geotech also emphasizes that "passive resistance of the sheet must be assessed by FE analysis due to very weak nature of the soil

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That's a first for me. I don't understand why they cannot provide soil strength and density parameters to base the design on.

Sounds like they may hamstring you into using FEM. You'll have to ask the geotech if the ks provided includes a factor of safety. I've only used Plaxis for FEM of sheetpiles, so I can't help you with SAP2000.

I like MTNClimber's first response but I don't like SPW911. I always consider, but then frequently ignore, Geotech recommendations for SOE design.