Terzaghi & Peck, multlayer soil

[9prostock]

hi,

how do I use T&P emperical equation when there are different soils within the trench, say a sand on top of a clay. Also, water table is halfway down, and will be dewatered, so I'll have water pressure on bottom half of braced excavation.

Thanks

 

[GeoPaveTraffic]

What emperical equation from T&P?

You need to give us some context to have a chance of getting some help.

Mike Lambert

 

[9prostock]

thanks for reply, sorry about that. I'm referring to the one for uniform lateral soil pressure distribution in a braced excavation. .65*ka*gamma*H. Thanks again.

 

[GeoPaveTraffic]

You will need to develop a pressure diagram based on the soils and your experience.

I usually, figure the K0 pressure for reference for each soil type, then develope a pressure diagram using T&P to develope the shape for each soil.

A lot depends on how stiff a system you are designing. If the system is fairly flexible, then pressures will be K0 or less. If the system is very stiff and the struts are heavily preloaded, the apparent pressure diagram can be higher than that given by K0 pressures.

In other words, it get complicated and you have to develope the apparent pressure diagram based on how you think the system will be constructed and how stiff a system you need. If movement sensitive imporvements are nearby, then a very stiff system is needed which results in a much higher apparent pressure diagram.

Hope this helps some.

Mike Lambert

 

[mudman54]

give it to a soils engineer and let them prepare the diagram for you including surcharge at the top.
then you both can focus on how stiff to make the support system to limit movements nearby like mike says.

 

[fattdad]

Braced excavations typically are for short-term loading conditions - i.e., just to facilitate construction. If you are dewatering for construction, then I'm not sure how the water table factors into this analysis.

The T&P equations for braced excavations are different for cohesive and granular soils. You have both. However, in both cases the area of the load v. depth (integral with depth) is 30 percent greater than the integral of the Rankine active earth pressure. If this were my responsibility, I'd determine the Rankine force for the upper layer, multiply it by 1.3 and then form the rectangle. I'd do the same thing for the lower soil.

Actually, as a design engineer, I'd leave the whole mess to the contractor, 'cause it's the contractor that owns temporary shoring. (That is unless you are working for the contractor. . .)

f-d

¡papá gordo ain’t no madre flaca!