Considering a hinge at the bottom of excavation for 1 level of tie-back shoring

[HanStrulo]

Hello,

One of the things i learned from various shoring books is that for soldier piles with 1 level of tie-backs, a common practice is to consider a hinge at the bottom of excavation to calculate the moment on the piles and the tie-back forces. This method improved my shoring design considerably compared to using free earth/fixed earth methods.

One of my colleagues informed me that i can only use this method if i have a cohesion of more than 30KPa. Can anyone tell me from experience if there are limitations to this method or if they know any reference that could help me understand it better?

Thank you in advance

 

[PEinc]

Terzaghi & Peck developed their empirical earth pressure diagrams using a hinge at subgrade. FHWA and AASHTO show using hinge at subgrade for both single-tier and multi-tier anchored or braced, non-gravity retaining walls, both permanent and temporary, with granular or cohesive soils. Ask you colleagues for a reference that is less than 30 years old.

http://www.PeirceEngineering.com

 

[HanStrulo]

Thank you very much for the answer.

To clarify, I am mostly interested in a one level of support system. The failure mode for those systems is by a big sliding circle (same as slope failure) as opposed to the failure mode of a multi level support system (a vertical column of earth going down and then pushing on the imbedded part of the shoring system upwards- see picture). for the one level of support assuming a hinge for soft clay might not agree with the failure mode.

For your other point, I understand Terzaghi and Peck just 'ignored' what happens below subgrade when establishing their apparent earth pressures.

I could not find a reference to the hinge at subgrade in the book "soil mechanics in engineering parctice". is my understanding correct?

for last point, my colleague said that a Fleming book had a commentary about the applicability of hinge at subgrade as well as using 9.Cu.d formula to get the embedment length. He will confirm the exact chapter to me later.

 

[PEinc]

You are talking about bottom heave in soft clay, which can happen regardless of the number of bracing or tieback levels. I am not in my office and don't cutrrently have access to T&P. Their empirical rectangular and trapezoidal earth pressure methods assumed a hinge at subgrade. Get your ground (hinge) reaction and then design your soldier beam or SSP embedment.

Re: Fleming, get a different book.

http://www.PeirceEngineering.com