Longitudinal Grade for Access Road Over a Berm

[Trackfiend]

I'm drawing a blank and hoping someone could guide me in the right direction. I'm designing the grading and drainage for a certain area that has a containment berm surrounding the entire site. The berm in approximately 4' high from natural grade with 3:1 side slopes and a 4' wide crown. The site will need access on both sides by large truck traffic. The client has specified a minimum width of 24' for the access road. Currently I'm using a maximum grade of 10% for both sides of the ramp going over the berm with a widened crown width of 10' (giving an overall toe to toe dimension of 34' long).

My only concern would be the undercarriage height (from ground to truck or trailer clearance). Does AASHTO have info on truck clearances or does anyone know of any other publications that could help?

 

[cvg]

10 percent is fine, your 4 foot wide crown is not. You should have enough room at the top of the berm for the entire vehicle to sit level.

 

[Ron]

The trade-off, as cvg noted, is the crown width. The narrower the crown, the flatter the approach grades must be. If you can't widen the crown, then you will have to have a shallower approach grade.

For dimensions, go to the websites for trailer manufacturers and get the dimensions of the expected trailers to access the site (40', 45', 50', 53', etc.)

Triangulate between the forward rear tandem, the bottom of the trailer mainbeams, and the dolly wheels. Change your approach grades until they will all clear the crown.

 

[cvg]

you can mess with your approach grades, but if you start going flatter the ramps get extremely long. So, I would recommend a wide spot in the berm as the better solution. See attached for a typical detail.

 

[CarlB]

If you got 34' toe-to-toe, looks like you used 3:1 access slopes rather than 10%. For a 4'rise, 10% slope, one side of ramp is 40', so 90' toe-to-toe.

 

[Trackfiend]

cvg/Ron,

My 10' crown width is acceptable for smaller trucks, but I guess I'll have to just compare the dimensions (as noted by Ron) to see what the clearances are. I'm waiting to hear back from the client to confirm if any "lowboy" trailers are being used. The biggest issue with widening the crown or extending out the ramp with a flatter slope is permitting. The COE (Corps of Engineers) has approved the site dimensions (including access ramps) already and being that this project is in a swamp (protected land), the issue becomes having to resubmit for extending the footprint (if needed). They should have had an engineer look at this before the permitting stage, but I'm sure the client thought he could save some dough by having his resident field manager draw up a sketch .

I didn't know if there were any publications that covered "in general" most truck and trailer types with clearance dimensions.

CarlB,

The 34' toe-to-toe is the berm width. However the ramp width is 90' (using a 22' crown elevation and an 18' natural ground elevation). Thanks for the reply.

 

[beej67]

Can you approach the berm at an angle? Might help if it's tight.

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