Offset Surcharge on Broken Back Slope - Boussinesq / Elastic Methods

[RFreund]

When you have an offset surcharge on top of a broken back slope and using elastic methods, such as boussinesq, do you adjust the angles to the load accordingly and apply the formulas as normal? or do you apply the surcharge at the top of the wall elevation and apply the soil as an overburden pressure? Or model the slope to get an active pressure of the soil and model the surcharge at the elevation of the wall basically disregarding the fact that it is on top of the slope?

Thanks!

EIT

http://www.HowToEngineer.com

 

[dcarr82775]

I do a trial wedge to get the load in that situation.

 

[RFreund]

This actually relates to my other post and those retaining walls. Do you think they (the reviewing engineers) would except a trial wedge analysis?

EIT

http://www.HowToEngineer.com

 

[dcarr82775]

I don't see why they wouldn't accept it. It is a perfectly valid analytical tool. It does give active loads so I might bump them up a bit simply because it is a railroad up there. I have found the BNSF reviewers to be pretty reasonable folks when I have dealt with them.

 

[Doctormo]

The UPRR and BNSF shoring manuals will provide some guidance. Any excavation within a 2:1 influence line needs to be shored per their guidelines which will be a pain. The 1.5 influence line is for the RR loading so you will be close on that if you can not get gravity walls to work. Their E-80 loading is something like 1778 psf over a 9' tie width as I recall so it will be appreciable if you have to account for it.

I have used other methods but you need to use their loading or get ready for a big fight. I have not found RR people to be reasonable as a rule so it really depends on who the approving engineer is.

 

[RFreund]

ahh, I see. The UPRR distributes the strip load due to the rail down at a 2V:1H to the top of the wall and then uses the modified boussinesq equation. So I guess the general answer to my question is to distribute the vertical load down at some angle then recalc your strip load and apply the appropriate equation (i.e. Boussinesq).

For example:
Strip load; q=1000psf
distance to strip load from wall x1 = 10ft
width of strip b = 10ft
height to strip load above wall h1 = 4ft
Slope of surcharge through soil; S=2V:1H


New distance to strip load = x1new = x1 - h1/S
New width of strip load = bnew = (b+2*(h1/S)
Find new strip load
P=q*b => qnew = P/(bnew)
Use 'new' variables in elastic method equation (i.e. Boussinesq)

EIT

http://www.HowToEngineer.com