## Braced excavation

## Braced excavation

(OP)

I am reviewing some braced excavation pit calcs (piles, horiz beams, and wood lagging) where in one case, there is a single brace support (the horizontal beam ). In this case the regular triangular backfill load is used to determine the brace force. The other case is where there are two brace levels and a full height UNIFORM load is used for the backfill which is=25xheight of the wall.

My question is should the first case with only one brace be checked for a full uniform load as well and should the

second case (2braces and full uniform load) the only way the braces can be loaded due to a different kind of failure wedge or should the normal triangular load be checked for this case also.

My question is should the first case with only one brace be checked for a full uniform load as well and should the

second case (2braces and full uniform load) the only way the braces can be loaded due to a different kind of failure wedge or should the normal triangular load be checked for this case also.

## RE: Braced excavation

To answer your questions:

Depending on the soil type, a uniform distribution may be applicable to the single brace case. However, the triangular distribution is common and normally these lateral pressures are computed quite conservatively.

The triangular distribution would probably only be applicable to the two brace case during construction. As excavation proceeds, the wall could be in various stages of loading. For instance, a typical top-down construction would include a short portion of the wall as a cantilever prior to installation of the first brace. Then, the wall would have a single brace as the excavation continues until the depth at which the second brace could be installed. Each load and support condition case must be checked for construction and final state of the shoring.

~dison

## RE: Braced excavation

Note: [gamma] denotes the Greek symbol; in the context of this message, it represents the soil total unit weight. H is the depth of the cut to be braced. Ground surface is assumed to be flat around the cut.

The triangular case doesn't really apply to a braced cut - remember that "load seeks resistance." A triangular distribution applies to a cantilever section only. You should get a triangular pressure above a single brace, and uniform below. Ralph Peck's 1969 ICSMFE State of the Art lecture in Mexico City included a triangular distribution above the top strut and below the bottom strut. The maximum pressure was proportional to [gamma]H - I think it's 0.3 to 0.5*[gamma]*H. (You should check to be sure.) The value of 25 pcf times wall height is entirely too low - it should be about 0.4*125, or 50 pcf.

That's a big difference.## RE: Braced excavation

For Sand - Peck, '69 suggested using a rectangular distribution of 0.65*Ka*gamma*height, where ka is the active earth pressure coefficient [1-sin(phi)]/[1+sin(phi)]

For soft to medium clay - (Terzaghi and Peck '67), suggested triangular dist. for top 0.25*height then rectangular for the remaining height. The max value on the triangular and rect. dist. is 1.0*ka*gamma*height, where ka is 1-(2*qu)/(gamma*height).

For stiff fissured clay - (Terzaghi and Peck '67), suggested a rect. distribution of 0.2 to 0.4 *gamma*height with the top 0.25*height and bottom 0.25*height a triangular distribution.

Remember that the bottom of the excavation acts as a strut when you are distributing the forces. The above distribution are described on page 469 of the "Foundation Engineering Handbook" by HSAI-YANG FANG, 1991

If there is water above the bottom of the excavation, add a hydrostatic water distribution to the above mentioned distributions.

## RE: Braced excavation

If you are reviewing sheeting designs, I recommend "Pile Buck Steel Sheet Piling Design Manual" from Pile Buck, Jupiter Fla. (www.pilebuck.com) It does a good job of explaining the design process for sheeting.

Good Luck!

## RE: Braced excavation

DRC1's comments remind me of a critical aspect of choice of parameters. Understanding the construction sequence is crucial in developing an appropriate design.

Be careful in using KRemember that movement must occur for the soil to move from a K_{a}in your design._{0}to a K_{a}condition. NAVFAC DM-7 (available as a pdf file online) has guidelines for how much movement is required. If your retention system is installed in a manner that severely restricts soil movement, then the use of K_{a}is not appropriate. Also remember that we have been discussing loads appropriate during a "reasonable" construction period. These are not appropriate for long-term loading conditions on finished structures.