HS-20 surcharge on an underground pit wall

[cessna98j]

I am having trouble determining the best way to distribute an HS-20 surcharge on a wall. The structure I'm designing is a square underground pit with a grate on top, which HS-20 trucks will drive over to dump their load into. I tried distributing the load on the wall by converting the two 16k wheel loads into a strip load, and using a Bowles, 2nd edition (equation 11-20) which is sigma h = (2q/pi)*(Beta - SIN Beta * COS 2 Alpha). This gives the surcharge on the wall as a function of depth. The problem is, since the trucks will be driving over the wall, I tried calculating what the surcharge will be just as the strip load approaches the wall, and obtained a very high local load on the top (about 2500psf on the top 1' of wall) which then drops off significantly as you progress down the wall. This high load concentration on top of the wall significantly increases the required wall thickness due to shear. My question is, am I being too conservative? Is there a reasonable cut-off point in using this equation as far as the strip load approaching the wall? My thickness is fine if I just use the standard 300 psf uniform surcharge, but I just want to find out if such a high localized surcharge is reasonable when using this equation as the axles approach the wall, changing the thickness substantially. Any advice would be greatly appreciated.

 

[miecz]

ASTM/ANSI Standard C890 "Minimum Structural Design Loading for Monlithic or Sectional Precast Concrete Water and Wastewater Structures" has a provision for lateral earth pressure due to wheel loads, including the effect of impact. ASTM/ANSI Standard C857 has the same provisions for Precast Concrete Utility Structures.

 

[kslee1000]

Another location to look for information is the State Department of Transportation (D.O.T). Get their design manual on Culverts and Underground Trenches, Utilities.

 

[RJMurray]

Does anyone have a specific reference for the lateral load effects/distribution of concentration point-load surcharges behind a retaining wall? I remember reading and using some classical formulas that were developed years ago....but can't find them.
Thanks.

 

[SlideRuleEra]

RJMurray - "Naval Facilities Engineering Command Design Manual 7.02" has nice coverage of this on page 7.2-74
Here is a link

http://www.ce.washington.edu/~geotech/courses/cee523/manuals/NAVFAC72.pdf

http://www.SlideRuleEra.net

 

[MichSt]

I would think the design of a bridge abutment wall is similar to your situation. For the design of bridge abutments and retaining walls the AASHTO standard specs section 3.20.3 say to add a 2’ surcharge to account for highway traffic. This is how I always consider truck loading for bridge substructure design. This is similar (actually a little less) than using the 300 psf surcharge you mention.

 

[ItsOnlyDirt]

And dont forget you walls are effectively propped at each end by the other two perpendicular walls!!!

 

[ItsOnlyDirt]

.....And the base of course