Excavation Support 1

[deejay]

I am reviewing a design calculation for the Excavation Support. The loading diagram used by the designer is in the attached link. This is the standard loading diagram provided by NYC Sewer Design Standards for Non-Watertight conditions. Can anyone tell me how the reaction at the dredge line (as shown in the attached) is computed?

I worked the numbers in the attached backwards and forwards. The system does not appear to be in equilibrium.

I took the moment of passive and active pressures about the top and I come up with a completely different number.

Any help in understanding how Rsg at dredge line is computed would be appreciated.

http://files.engineering.com/getfil...d-d0508ecb66c3&file=Timber_sheet_-_sample.pdf

 

[PEinc]

Break the sheeting wall at the lower brace and calculate the ground reaction assuming a lower, simple span of 6'.
There is no safety factor shown in the typed equation for the embedment depth. D = [(R x SF/(0.5 x gamma' x (Kp - Ka))]-2

Please study up on this type of design. It's not really fair for you to review another's design if you don't know how to do the design yourself. You could be accused of practicing outside of your area of competence. Did the contractor submit his engineer's design calcs? If so, that should tell you how he calculated the ground reaction and required embedment. If no calcs were submitted, ask for them.

Although I have not checked the design completely, it appears to be reasonable. However, with vertical sheeting boards of at least 18' in length, it may be hard for the contractor to dig this narrow trench. At least when starting the excavation, the vertical boards will be projecting well above the original grade and the excavated grade inside the trench. If the contractor uses shorter, overlapping, vertical boards, the design approach may need to be revised.

http://www.PeirceEngineering.com

 

[deejay]

Thanks PEInc. I am trying to understand the design as an exercise.

I ignored the location of the lower brace while trying to compute the R. I will try again with that.

Appreciate your help.

 

[PEinc]

Analyze from the lower brace up as a simple span with a 3' cantilever above the upper brace. Get the cantilevered and interior moments, the upper brace load (A), and the partial lower brace load (B1). Analyze the lower span from the lower brace to the hinge at subgrade. Get the moment, the B2 reaction, and the ground reaction. For the lower span moment, use 80% of the simple span moment to account for continuity. Top Bracing load is A. Lower bracing load is (B = B1 + B2). Ground reaction is R. Use R to determine D.

http://www.PeirceEngineering.com

 

[deejay]

Thanks a lot PEInc. I was approaching the equilibrium by balancing the active and passive pressure. I lost sight of the bracing.
Your response cleared my confusion.

 

[PEinc]

Remember, the walls push against each other, hopefully with equal and opposite loading. If so, design one wall. The other should be the same.

http://www.PeirceEngineering.com