Wheel loads at top of MSE walls

[dgillette]

What is typical and/or preferred practice for wheel loads on the top of MSE retaining walls? I'm reviewing the design and stability analysis of a temporary road fill that doubles as a cofferdam. We could get some pretty large, possibly overweight trucks close to the edge, and I'm concerned about stability. The embankment would have 1/4:1 slopes, and be constructed as a "burrito wall," with woven geotextile reinforcement. The outside edge of each layer of geotextile would be folded back over the lift of fill overlying it before the next layer of geotextile, giving it the appearance of a stack of burritos - hence the nickname.

Most of what I've seen in literature and a quick search of eng-tips was simply a uniform live load of a few hundred psf. With heavy wheels close to the edge, however, I'm not sure that is appropriate. What I'm considering is taking the weight on the drive tandem (up to maybe 45,000 lb for the trucks involved - not necessarily street legal) and, for the purpose of 2D stability analysis, modeling it as being spread over two 2.5'x6' patches close to the edge of the crest. Thus, on the crest of the 2D model, there would be two areas, each of which is 2.5 feet wide, with surface pressure equal to 45,000/(2x2.5x6) = 1500 psf.

Suggestions? I'm interested in both the amount of reinforcement required, and whether we need to move the Jersey barriers back from the edge.

Thanks in advance.

DRG

 

[MSEMan]

We did a failry large job in Africa a few years back for a haul roads and crusher headwalls for a mine operation. They had 150t dumpers running on the walls.

By all means do the numbers - but I'd suggest distributing the load over a larger area - assuming your using a reasonable base for the haul road there's no reason why you shouldn't treat the load as a uniform surcharge between the barriers (something like 200-300 psf). Using whatever distribution you feel appropriate (geometric or boussinesq) work out the wheel load at depth and multiply by your K to get a horizontal load per unit length. Add this load to the normal retaining loads. Since its a temporary condition, you can reduce all your partial factors, as well as your creep rupture factor on the geosynthetic.

Definately keep the trucks back off the face. The area immediately behind the facing is extremely difficult to compact properly. As a minimum I would say put the Jersey barrier 1.5 metres away from the edge.

 

[DirtJockey]

dgillette:

There's a way to do it. If you have Koerner's book "Designing with Geosynthetics," there's a figure used to show how to distribute tire loads. Unfortunately, I don't have the page number to share with you (the book is at work), and I'm sure you have access to the book. I have the 1st and 4th editions, and I'm almost certain its in both of them.

The method uses Boussinesq's pressure distribution theory to distribute the loads horizontally to the wall. Part of this can be found in the NAVFAC DM 7.2, pg. 7.2-74, Figure 11, right half of the figure. You'd have to have an idea of the axle and tire spacings to solve your unique solution, as well as the axle weights.

Hope it helps,

Gary.