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Braced excavation

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.

RE: Braced excavation

My understanding is that the distribution of soil pressure for braced or tied back excavations have been measured in-situ on many projects and the distributions you indicate are the ones recommended by AASHTO and other official groups to most closely match actual expected conditions.  Also, the distribution varies by soil type and by construction techniques, so pay very close attention to constuction sequence and site conditions.

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

Be careful on this one - there isn't as much supporting data as dison thinks.  And what does "closely matching" mean to a practicing geotechnical engineer? Plus or minus 30 percent!

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

The magnitude of the pressure diagram depends on the soil type.  

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 we are talking about sands, the iniital pressure distribution on the sheeting will be triangular. Once the first brace is installed, the pressure distribution will still be triangular with the waler adiing a point load for resistance at the level of the first brace. Active pressure is equal to Ka*gama*depth. Once the second brace is installed and the excavation continues, the actual pressure distribution becomes very funky and difficult to model, as loads shift from the upper brace to the lower brace to the toe. The distribution of the load is influenced by the deflection of the sheeting which is difficult to predict with sufficient accuracy to predict brace loaads. However, based on brace load measurements that have been done on excavations, an envelope of pressures has been devloped. For sand the envelope is defined by a uniform load equal to .65 * the height of the cut * effective unit weight of the soil * coeeficent of active pressure.
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

I stand corrected - 0.2 to 0.4 is right (that should remind me not to rely on memory when committing things to writing!)  Thanks, gibbycu.  Also remember that this load does not include any hydrostatic loading.

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 Ka in your design.  Remember that movement must occur for the soil to move from a K0 to a Ka 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 Ka 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.

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