PYWALL bending moments 1

[smulmi]

Hi,
I'm designing cantilever retaining walls with sheet piles as well as soldier piles as 2 alternatives for the contractor. I did my hand calculations first and checked my results (especially displacement) with PYWALL. I applied a load factor of 1.5 to active pressure load.
I noticed that PYWALL gives bending moment that are very low compared with hand calculations for both cases. Also the PYWALL shows safe results with very low embedment depth as compared to hand calculated embedded depth.
I realize that the two methods are based on different approaches but the variation is too much to make a good decision. Any thoughts and comments are very much appreciated.

 

[FixedEarth]

It is likely that your software program used Free Earth Support method or Rowe's moment reductions. Another case, if you have cohesion, the program will calclate the negative active pressure on the upper zone thereby having less thrust. On your hand calcs, did you use just friction angle or friction angle and cohesion. .

Why did you apply 1.5 safety factor to the active earth pressure? Usually we use unfactored active earth pressure values and either apply F.S. to passive resistance or increase calculated embedment depth by 130%

If you provide us a sketch & what method(s) you used, we can provide you more information.

http://www.FoundEng.com

 

[smulmi]

Thanks for the comments FixedEarth.
The cantilever wall is retaining silty sand to gravelly soil with no ground water and the wall has no tiebacks/anchors. The free height of wall is 15'.
For hand calculation, I used the conventional earth pressure diagram as attached and solved for the embedment depth required and then went on to calculate For soldier pile system, I used effective width=3*flange width for passive and active pressure below dredge line. For sheet pile system, I designed for per foot length. For the embedment depth, I used a FS of 1.5 on Kp. I got 20' and 22' of embedment depth for sheet pile and soldier pile system respectively.

But with the software, the soil is modeled with subgrade modulus using p-y curves. I used subgrade modulus of 90lb/ft^3 for sandy soil. Also since I couldn't find a proper way to incorporate FS in the software, I applied a multiplier of 1.5 on active pressure, a feature available in the software. The software usually gives me a high deflection value but a low bending moment value and a shallow embed depth. So I had to choose sections based on deflection criteria rather than bending moment.

 

[FixedEarth]

Your selection of soil parameters are on the dot! Are you sure you are also not a geotecnical engineer? If the shoring is temporary, I use F.S. on passive of 1.33 For permanent work your 1.5 value is more suitable.

Since friction angle of Silty SAND with gravel is more like 33 to 36 degrees, I kept the 30 degree but did not discount any distance for ignoring passive resistance. Passive wedge can be approximated as 0.08 x Fric. Angle. In your case it is about 2.5 which is close to your chosen value of 3.0 I always throw in a minimum uniform surcharge of 72 psf. Attached is the SBWall output.

Correct, your sheetpile is by the foot length and very similar in calculations.

On the horizontal subgrade modulus approach, I prefer to use this method only when the lateral load is shear applied at the top of a pier or pile as in the case of a laterally loaded pier or pile. In soldier beams, the loading is along the member and the classical approach works better. Your 90 pci is on the low end. If you don't have cyclic condition and no GWT table, 200 kcf or 115 pci is a good approximation at the top of the cut. This value often increses with depth, so you may choose, say, 5 kcf increase per foot depth of soldier beam shored height.

On the lateral SG values, Terzaghi was on the conservative side, Bowles was on the other end and R. F. Scott, I think, was on the middle ground. Obviously, you can use much higher lateral SG values than given above, but would you like your prediction to be on the safe or unsafe side of measured deflection?

Anyhow, you can check my Max. Moment and deflection & hope we are in the ball park. I put a red box in the PDF attached document, on the more important values.

http://www.FoundEng.com

 

[smulmi]

How do you come up with the balancing couple force or the resisting height and moment couple height? Also the third reference of Aifi, 2009, is it available?

 

[FixedEarth]

A design example showing hand calculations, 8 pages, based on the moment couple method and other related materials all come with SBWall program.

The moment couple method gives a deeper embedment. This method is based on extending the embedment below the point of zero shear such that deflection will be tolerable, i.e. less than 0.5 inch, if the appropriate beam section is chosen. You can also get appropriate cantilever wall equations from Foundation Engineering, Jumikis, 1986 and Foundation Design, 1962, Teng.

http://www.FoundEng.com

 

[smulmi]

Thanks FixedEarth for your help.

 

[FixedEarth]

Anytime.

http://www.FoundEng.com

 

[tre5205]

smulmi

how did you get your embedment depth from PYWALL? I do not believe it gives you any of that information.

 

[smulmi]

Tre5205,

I calculated my preliminary depth from hand calculation and checked the forces and deflection in PYWALL.

 

[pelelo]

Fixedearth,

I noticed SBwall only works for sandy soils.

What software do you use when dealing with cohesive soils or c and phi soils?

 

[FixedEarth]

Pelelo-
I use SBWALL software for cantilever shoring in all soils. By discounting cohesion in the backfill soils, you are on the conservative side and you eliminate the -ve earth pressure value in the upper zones where you can get tension cracks. Also by discounting cohesion in the passive soils, you are also on the conservative. Besides, in the long term due to creep there may be little cohesion in the soils. Lately, I have been using Blum's method for soldier beams and it is within 10% of SBWALL.

For laterally loaded piers or caissons, I use LATERAL FOUNDATION software. I take the maximum bending moment from LF program, factor it to get an Mu value and then use a column design program to obtain my required longitudinal pier reinforcement. Many times it is more than 1% of cross sectional area. In my firm we do both the geotechnical and substructural design for shoring and retaining wall on pier foundation projects.

Not sure what type of structure you are analyzing, a cantilever shoring or a drilled pier foundation.

 

[pelelo]

Fixedearth,

Thanks for your reply,

In SBWALL It seems you use the DRAINED condition (long term condition).

In my case I am analyzing a cantilever shoring (sheet piles). In my case, these are temporary shorings (pipeline installation). These are 3.5 m deep, I might use one row of braces, depending on the deformations.

With PYWALL I usually get high deformations, sometimes they don´t even make sense. SBWALL seems to be easier to use.

Thanks for your reply.

 

[FixedEarth]

For sheetpiles you can also use Blum's method set up inside a spreadsheet but SBWALL is only for soldier piles. As you know for sheetpiles, you can design it by the foot length like a cantilever retaining wall, where as for soldier beams it is per span distance. I use this sheetpile program-you can get their demo:

http://www.soilstructure.com/sheetpiledesign.html

A Cantilever system should work for the 11.5 ft cut.

 

[FixedEarth]

Forgot to attach it.