Lateral Pressure due to Point Load using Boussinesq Equation

[techiestruc]

Hi

I am designing a truck loading ramp having side walls as retaining walls with footing. The space between will be covered by fill (to save concrete) and the top 200mm will be composed of slabs to be designed as slab-on-grade.

Im designing these retaining walls using MathCAD and Im applying a lateral load due to point load (HS20 truck) using Boussinesq equation. I used the MathCAD to integrate the lateral pressure acting on the wall to obtain the shear and moment acting on the bottom of the wall for design. I assumed that the wheel loads are closest to the wall (distance = 0). Then, i realized that, using my template, having shorter walls results into larger lateral forces. When i input a very small height, the resulting lateral force increases greatly. Also, when i input a very large value for the height, the lateral force seems to diminish.

Am i using the equation correctly? Am i having realistic results based on the principles using Boussinesq equation? or am i just missing some limitations of using the equation? Im no expert in geotech eng'g and I really need your help. I will be attaching the equation im using and also a snapshot of the mathcad equation.

Thanks in advance!

 

[RFreund]

I will try to look more indepth at this later but I should have an example on my website. I will try to post later.

EIT

http://www.HowToEngineer.com

 

[PEinc]

If you are looking at truck wheels as point loads, don't forget to consider the cumulative pont loads from the truck's other wheels. However, usually retaining walls supporting traffic loads are designed for an area surcharge behind the wall, not individual point loads.

http://www.PeirceEngineering.com

 

[techiestruc]

Thanks RFreund! I'm looking forward to your findings.

You're right PEinc. I think I should consider the adjacent wheel loads in calculating my lateral pressure. Regarding traffic loads being idealized as area surcharge, is it specified in AASHTO or other references? because I haven't encountered any. If i will be able to find an area surcharge equivalent for traffic loads, that would make my calcs easier! Thanks!

 

[PEinc]

Yes, AASHTO and other State highway design specs allow using an area surcharge to account for traffic surcharges on abutments, wing wall, and other retaining walls. Usually, the surcharge is equal to 2 or 3 feet of eath surcharge. The lateral rectangular surcharge is usually Ps = gamma x Ka x 2' or 3'. Under certain conditions, a Boussinesq surcharge is accepted. Check AASHTO or Pennsylvania Department of Transportation's on-line Design Manual 4 (DM-4) for traffic surcharges.

http://www.PeirceEngineering.com