Wheel loads on MSE walls 1

[dgillette]

We have a temporary highway detour soon to be built so we can take the road out of service for several months without the locals getting out the torches and pitchforks and coming after us. It is to be constructed with a MSE retaining wall formed by woven geotextile between layers of fill, with the outer edge of each layer of textile folded back over the fill (sometimes referred to as a burrito wall). The fill material will be pit-run SM material with variable amounts of NP fines, compacted to 95% of std. Proctor. The fabric is specified to have a design strength (reduced for creep, etc.) of 4550 lb/ft. The outer slope of the wall is 0.25:1. The top will be paved with 6 inches of untreated base course and 5.5 inches of asphaltic concrete.

Is there a rule of thumb as to how close the wheels of (approximately) street-legal trucks can get to the edge without concern about a local bearing failure under a wheel? The jersey barriers will keep the trucks >3 feet back from the edge of pavement, which should be roughly 1 foot in from the nominal edge of the MSE wall, and the stability analyses recommended by the FHWA manual all indicate fairly high FS with the live loads represented as strips set back from the wall. I doubt local bearing capacity will be a problem with ~4 feet of setback from the nominal edge of the burrito wall, but I'd like to be a little more confident. It could be grim if we rolled a truck, either the contractor's or a coal truck.

Thanks,
dgillette

 

[Gimbli]

If bearing capacity is not of concern, I would keep the traffic loads as far from the edge where the passive failure envelope caused by the wheel load intersect the first, or upper, geotextile. Nevertheless, I think the rolled truck will give you the answer.

 

[jdonville]

dgillette,

FWIW, the LRFD 2006 Interim spec design loads (equivalent height of soil) depend on the height of the wall and whether perpendicular or parallel to direction of traffic travel. If parallel, heq also depends on the distance of the lane edge from the crest. More deets if you want 'em.

Don't forget to look at the construction staging, too.

Jeff

 

[emmgjld]

For a number of reasons, I have not designed any MSE walls which have been required to support significant traffic.
So to continue the question by dgillette, is there a source of notes, cautions, rules of thumb, etc. which can be of assistance. I have scattered notes, mostly from papers or seminars, but nothing in a convenient source.

 

[MSEMan]

You are doing all the "right" things. Keeping the live load a few feet from the face makes all the difference. You are unlikely to get a bearing failure - at least on paper.

The practical concern is that compaction near the face of a burrito wall is very difficult so this area will usually be a bit soft. You won't get anything like 95% proctor unless you are using 57 stone or something like that.

Have you considered adding a wire basket facing to provide some rigidity? A black (ungalvanized) w4 w4 4x4 10' x 1.5' basket is available off the shelf and is pretty cheap - less than a buck a square foot. Struts will be about 30 cents each, say four per 15 s.f.

Another benefit of using the baskets is that you can get the slope you are looking for by offsetting rows of baskets in steps rather than trying to form a face angle.

Hope this helps.

 

[dgillette]

Thanks all. I feel a little more comfortable now.

Had a pre-con meeting with the contractor yesterday, and made a point of emphasizing to him not to let the jersey barriers get any closer to the edge than is shown on the drawings.

DRG