Braced Excavation - Opposite Top of wall at different elevations - Uneven Braced Excavation

[RFreund]

I'm wondering how others would approach this:

A braced excavation with different heights on each side. See attached sketch.
I'm thinking you could design the taller side as you normally would with apparent pressure diagrams. Then apply the forces of the struts as reactions on the shorter side. These reactions will the dictate the magnitude of the apparent pressure diagram (so there is equal and opposite forces). Then check an 'overall' stability by comparing the active and surcharge pressure from the taller side against the available passive pressure from the shorter side.

It looks though like the force on the top strut would be very high.

Any thoughts?


EIT

http://www.HowToEngineer.com

 

[FixedEarth]

Load path, equations and a design example are all shown on p 391-407 of FOUNDATION DESIGN by W.C. Teng.

http://www.soilstructure.com/

 

[RFreund]

FE - Thank you, I appreciate the reference.
I read through the section and Teng discuss the stability of this scenario. He essentially says that you should:
1. Check slope stability using the swedish circle method and
2. Check internal stability and suggests carrying the reaction of the upper tiers to the lower tiers where "adequate soil reaction is available."

My question:

How would you determine if there is "adequate soil reaction?" My initial thought (in the original post) was just to enlarge the apparent pressure diagram based on the applied forces. HOwever this total force could not be greater than the available passive pressure as determined by a rankine or coulomb equation. My justification for this method is that this is essentially how the apparent pressure diagrams by Peck, etc. where developed except using the active pressures (i.e. see page 394 section B just above equation 13-1 in W.C. Teng. They use the total force from Rankine and change the shape of the pressure diagram).

Another thought is that I adjust the apparent pressure diagram magnitude as discussed above however I limit the pressure at any point to the passive pressure at that depth below grade.

Any thoughts?

Thanks again!!



EIT

http://www.HowToEngineer.com

 

[FixedEarth]

R.F. - Braced excavations are very complex so find a seasoned designer to help you. So with that in mind, What type of soils are you dealing with? SPT, qu, consistency, GWT, etc. Unless SPT < 10 or qu< 1.0 TSF, you should have "adequate soil reaction".

Also is your example excavation a limited dimension(Rectangular Pit) or a Continuous Trench Excavation? The analysis is different.

Your worst case scenario is the loading on the left side of your sketch, so you may start there. Since the struts limit horizontal deflection, you may consider designing for k closer to At Rest values. So yes, enlarge your apparent pressure diagram but no where closer to passive levels. At this point, you can start the soldier beam design, sizing the struts and walers.

You will then need the geotechnical firm to do a stability analysis of the excavation including heave, piping & rotational failure. See attached.

http://www.soilstructure.com/

 

[RFreund]

FE - thanks for staying with me on this one.

I don't have the soils report with me but I do believe the SPT > 10 and qu > 1. The trench is continuous.

I may be misunderstanding a couple things so I wanted to clear a few things up.

I will start on the left side and get the reactions on the structs. Then when it comes to the right side this is where I'm wondering the best approach. The pressure applied to the wall is going to be due to the struts pushing wall pushing on the soil (and the soil pushing back of course). However this would mean that the pressure diagram would need to be larger than that used of an 'active' case or your standard diagrams. So what I'm thinking is that (maybe) I can use the apparent pressure diagrams but 'scale them up' to match the total applied force (see attachment). However I was thinking that the total value of the resistance applied by the lower wall should not be greater than it can offer assuming a Ranking passive pressure? Or maybe it should be limited to the unconfined compressive strength of the material (or allowable bearing capacity) I am not sure about this part.

If I follow the above approach then there is a possibility that the right side would govern the design of the wall/sheeting because the span between struts is the same for both sides but the right side has a larger applied pressure. Having said that, the cantilever shown on the left side at the top will most likely govern.

When you say that there will be "adequate soil reaction" for STP > 10 and qu > 1. How are you determining this? I mean I'm sure it is based on experience somewhat but you still are considering something. Is it just that the bearing capacity / compression strength of the soil should be high enough?

https://dl.dropboxusercontent.com/u/65240332/Eng-Tips/Uneven Braced Excavation 2.pdf

Thanks again!


EIT

http://www.HowToEngineer.com

 

[FixedEarth]

I sent you email.

http://www.soilstructure.com/

 

[umrce]

Someone in our office is performing a similar analysis to what RFreund is discussing and I'm trying to learn as they design. I've been reading you're discussions here. RFreund would it be possible for you help me follow the end of the discussion by reposting your document posted in your digital dropbox? The file has expired (I'm assuming).

Let me see if I understand what FixedEarth is saying. The left side will not achieve active pressures (and will be somewhere between active and at-rest) because as it tries to deflect, the right side will help resist deflection of the left wall. In turn, this will cause the right wall to achieve a little more than at-rest pressure because the wall will be pushing the soil wedge behind (but not so much so that it reaches passive pressure).

Conceptually that is how I see it, but I'm not sure I have a good understanding of how to approach the design. Could you conservatively design for at-rest pressure on the left side and then as Rfreund recommended determine the apparent pressure on the right and ensure that it's less than the passive pressure? And if so, is that too conservative?

Thanks for any help.

 

[RFreund]

Sure thing:

http://files.engineering.com/getfil...0ab208bc6&file=Uneven_Braced_Excavation_2.pdf

Your summary sounds pretty accurate to me or atleast that was my line of thinking.
I ended up checking to make sure that I had enough passive resistance based on a tributary area approach) for the reaction at the strut. However global (slope) stability could also be an issue as well.

EIT

http://www.HowToEngineer.com