Tiered Retaining Wall Surcharge
Tiered Retaining Wall Surcharge
(OP)
Can anyone tell me the surcharge load that I should be applying?
I am looking into the design of a tiered wall system. The lower wall consists of a 26 ft high concrete cantilver retaining wall and the upper wall is a 23 ft high MSE. The set back distance is 9 ft.
I have currently used the Boussinesq equation for area loading a distance away from the wall. The surcharge that I calculated is (height of the upper wall X unit weight of soil) 2.875ksf. The distance my surcharge load is applied I assumed as 17ft(26-9, 1V:1H).
This is resulting in extremely large loads and a footing size that is not economical. Is there another way I should be analyzing this?
I am looking into the design of a tiered wall system. The lower wall consists of a 26 ft high concrete cantilver retaining wall and the upper wall is a 23 ft high MSE. The set back distance is 9 ft.
I have currently used the Boussinesq equation for area loading a distance away from the wall. The surcharge that I calculated is (height of the upper wall X unit weight of soil) 2.875ksf. The distance my surcharge load is applied I assumed as 17ft(26-9, 1V:1H).
This is resulting in extremely large loads and a footing size that is not economical. Is there another way I should be analyzing this?





RE: Tiered Retaining Wall Surcharge
EIT
RE: Tiered Retaining Wall Surcharge
I created a spreadsheet for assuming both a 1V:1H and ~2V:1H and my resultant is 21.5klf at 13.0ft and 6.98klf at 14.89ft, respectively. This load is acting over a 30.5 ft panel so the moments are extremely large for either case. The footing sizes are impractical or not possible.
RE: Tiered Retaining Wall Surcharge
You did not say if the upper wall exists or both walls will be newly built concurrently. If both walls are new, you have few options to deal with the high lateral stresses.
RE: Tiered Retaining Wall Surcharge
I am looking for the lateral load that is produced from the upper wall soil load for the design of my lower wall. Using the Boussinesq approach I took the area under the curve for the total load that is applied over the 30.5ft panel. See attached.
Another thought, if I were to use Rankines theory for surcharge right next to the wall I would get a total lateral load of 20.2klf @ 13ft. Why would this be less than when I use Boussinesq's appraoach at a 1V:1H or any other greater distance?
RE: Tiered Retaining Wall Surcharge
If the wall can tolerate some movement the force from the boussinesq may actually be half of the value. The factor of 2 in the equation seems to be pretty debatable see this link which has been posted before:
http://
EIT
RE: Tiered Retaining Wall Surcharge
To compare exactly, I need the following 4 values:
Setback distance
Strip load width
Strip load intensity and
proposed lower wall height (including embedment depth)
RE: Tiered Retaining Wall Surcharge
If I lose the factor of 2 my loads are still very large, no problem though. I just want to make sure that the loads I am applying are realistic.
FixedEarth - My "a" distance was not variable. I calculated it looking at "a" going from the back of the upper wall to edge of the failure plane (~2V:1H) and 1V:1H. The values depicted represent a value for "a" of 4.53ft and 17.5ft respectively.
The setback distance is 9ft. The lower wall height is 26ft. The load intensity is 2.875ksf (23ft*125pcf). "a" would be 4.53ft assuming an active failure plane or 17.5ft for a 1:1. I guess this is where my question lies; what is the appropriate "a" dimension and what is the proper load intensity?
RE: Tiered Retaining Wall Surcharge
The other is to use the Boussinesq equation, like you did ina spreadsheet. The "a" and setback are fixed and you solve for your 3 angles and then the lateral stress directly. I prefer thsi method, since it is more direct approach. Attached is the solution I obtained.
The 3rd approach is to draw a 40 to 45 degree line from the upper footing and use an equation. This is how the older reference books solved it and they say if you are more than 1:1 influence away, then there no surcharges involved. This is not the case for strip, but is valid for line loads.
Try playing with your spreadsheet and get a feel for it.
RE: Tiered Retaining Wall Surcharge
I went about the Boussinesq approach the exact same way as you did, using slightly different values though. Modifiying my spreadsheet to match yours I would still get a 250kip force acting at 14.7ft from the bottom of the footing, producing a very large moment. That is fine if it is the correct approach. Just seems to me like it is too large as my footing size, pile arrangement and stem backface reinforcement are not very realistic.
RE: Tiered Retaining Wall Surcharge
See attached - Ignore the Sliding and Overturning analysis section.
EIT
RE: Tiered Retaining Wall Surcharge
And the spreadsheat is pretty crude as I always say that I'm going to refine it and make if more user friendly but I'm usually always busy. I can post if for you if you'd like.
EIT
RE: Tiered Retaining Wall Surcharge
I still am not confident in the way I chose to calc "a" but I'll have to dig into it more to see what is appropriate.
Thanks all for the input.
RE: Tiered Retaining Wall Surcharge
EIT
RE: Tiered Retaining Wall Surcharge
Or a butress wall with micropiles.
RE: Tiered Retaining Wall Surcharge
RE: Tiered Retaining Wall Surcharge