Stability Analysis of retaining wall design. 1

[pob11646]

In the stability anlayis of a cantilevered retaining wall for a short roadway, with dirt on both the heel and the toe, can we use the soil above the toe in helping to provide sliding resistance?

The roadway is on the side where the toe is, and the dirt is almost up to the top of the wall on the heel side. I have heard differing opinions about this one, esp. if the footing is at least two feet below the top of paving.

One side says we cannot use it because the roadway may not be filled in when the dirt is filled on the heel side. Another side says it is ok to do so because the roadway will be in place. Thanks.

 

[jorton]

We usually ignore it if possible. Even though a road will be in place, there is no way to know what will happen to the soil at the toe. What happens to the wall if they decide to bury utilities along the edge of the road?

 

[pob11646]

Thank you so much for that opinion, jorton. I think I do agree with you because there is no way we can know or can control what happens to the soil at the toe, and I certainly would not want that uncertainty hanging over my design!

 

[kslee1000]

There are also concerns of future construction on the road, and toe side soil errosion, both can compromise wall stability. Provide a possitive shear key to increas sliding resistance if necesary.

 

[csd72]

I agree with kslee, it it not unexpected that workers may excavate a trench in front of the wall for services thus removing some or all of this soil.

I doubt it would make much difference to the overturning anyway.

 

[pob11646]

Thank you for all the responses, and I have got a follow-up question. I have some vehicular traffic on the backfill portion (heel) of my retaining wall. What sort of traffic surcharge do I need to use? I have heard about the AASHTO requirement of 250 PSF. Is this true? Do I need to use the full 250 PSF since the backfill is not carrying full fledged traffic, but more like parking, etc. Thanks for your feeback and please provide references, if possible.

 

[hollowstemhead]

You may want to look at temporary construction loading of the retained side of your wall. What type of equipment and associated loads will be applied as your parking/traffic area is constructed? In my experience, 250 psf is typical for general auto loading, in which case I would apply even if "just a parking area". Larger earthwork equipment, compactors etc. or even large truck traffic in the finish parking area may require an increase.

 

[PEinc]

AASHTO's 250 PSF is their minimum live load surcharge.

 

[DRC1]

How close will the parking be to the wall? If it is tight to the wall, then you will have to deal with full lateral pressure. If it is back from the wall, the load may be significantly less. 5-7 ft can make a big difference. Also the AASHTO is for live traffic, you may want to look at ASCE live loads for parking garages. A 2500 # car in a 10x15 space gives you 16 psf. Not that that is the number you should use, but if you are just parking cars, 250 psf may be a little conservative.
On the other hand, walls should be designed such that a 250 psf surcharge is not a problem. If it is, you may be riding a little close to the edge.
Yes I realize I contridict myself. Life (and engineering) are like that sometimes.

 

[PEinc]

Just because the wall is currently intended to support a parking area, that doesn't mean that there will not at some time be vehicles such as large fire trucks, delivery trucks, or construction equipment parked or operating immediately behind the wall. I would use at least the minimum AASHTO live load surcharge.

 

[BigH]

PEinc has the experience of a good practical and experienced wall designer - I'd use his advice.
However, let's throw some food for thought. What is the unit weight accuracy of your backfill? Are you using actual compacted unit weights - say 135 pcf moist - or are you "assuming" a unit weight? Your assumption of 125 pcf makes a big difference if the contractor uses well graded crushed stone that could be as high as 140 pcf (10% difference in lateral pressure for a given H and ka). How much difference in lateral pressures if you determine the pressures using log-spiral or Rankine/Coulomb? My point is that there is inherent inaccuracies in any "design" of a retaining wall. How do you handle or take into account such factors . . .?
Still, AASHTO is most likely relying on long time experience of 50 state and 10 provincial departments of transportation - so I would consider using their recommendations. In other countries, I have seen 20 kPa being used.

 

[PEinc]

Very good points, BigH. Especially the first (just kidding!).

 

[pob11646]

Thank you so much for all the comments. I can see myself being confronted with this question of "traffic surcharge on retaining wall" again and again, and I will be better prepared next time. Can anyone provide a pdf of the AASHTO, or let me know where I can get that particular AASHTO code? I do not have the AASHTO codebook.

 

[BigH]

It would be in their standard specification for the design and construction of bridges. Also, you can google any of a number of state DOT bridge design manuals and it would be in there as well.

 

[PEinc]

AASHTO's Bridge Design Manual is big, copyrighted, & expensive. BigH had a good suggestion about checking the loacal DOT on-line design manual.