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Foundation for a retaining wall.

Foundation for a retaining wall.

Foundation for a retaining wall.

What makes a good foundation for a retaining wall?
If bearing capacity of the soil is acceptable for retainging wall base, a foundation is not needed for any structural support, but just a flat base for the wall,  drainage under the wall (if needed), and to get under the frost line?
All the books I have regarding retaining walls say make sure there is a suitable foundation, but thats it.

I am designing a wall to get loaders up 12' to a rock hopper. The wall will consist of 30"x30"x60" concrete lock blocks with geogrid sandwiched inbetween layers and extending ~5ft into the soil.

My bearing analysis is based on the concrete block and retained soils. Do I need to take any special consideration (dispersing of load of concrete block) if the concrete with retained soil bearing analysis is ok? I would believe, this would be the whole point of tieing the wall back into the soil, but want to make sure.

(first engineering project after school and working at a geotech lab, thanks for any help)

RE: Foundation for a retaining wall.

Your blocks will exert about 2 ksf bearing pressure; so be sure the soil can support the load.

I'm concerned about the short length of your geotextile anchors.  Five feet sounds too short; I would have expected the topmost layers to be 15 feet or so, particularly with the proposed site use.  Shorter lengths for the lower levels.

Wall stability is your main issue; bearing capacity is a concern - but somewhat less important.  And you need to look at slope stability.

Good luck!

RE: Foundation for a retaining wall.

Oh yes, my length of geogrid was just what I needed for external conditions - overturning, stability.  I imagine the top layers will need to be longer when I look at how long I need the geogrid to be to gain the shear/pullout strength from the soil.

Actually, I do have some questions about how I look at the load. I was going to take a conservative weight of the full loader and divide it by the footprint. Then take that surcharge times 1.5 (live load to dead load). This computed surcharge is what I would use in calculations, however I would only use it where it would be an failing and not stablizing force. For example in sliding I would use the horiz force from the surcharge pushing against my retained soil and wall, but not use the surcharge added to the weight of the wall in the resistance to sliding (modelling the loader being right behind my retained soil, but not on my retained soil). Does this sound valid?


RE: Foundation for a retaining wall.

As a general comment, you should present the total length of your geogrid - not simply the length to reach the active failure plane.  I still think 5 feet is too short; was indirectly involved in a geogrid wall that failed in the mid 80's in the Clear Lake City area (near JSC.)  That wall was designed with all of the layers the same length, and their length about half the wall's cantilever height.  This is roughly the same geometry that you originally described.

Your design approach sounds conservative.  Just remember to do all of the stability calculations: full slope stability, bearing capacity, wall stability, geogrid pullout, etc.

RE: Foundation for a retaining wall.

British Standard BS8006 which is for reinforced earth retaining structures desire of at least 70% of wall hights for the length of the Geogrid. Beside, one must check for overall stability and might get longer geogrid. Also, foundation condition matter. Be sure to compact the subgrade and replace soil if needed.

RE: Foundation for a retaining wall.

Mirror the comments of Focht3 and Doresh.  Remember that you must go beyond the "failure" plane sufficiently that your geogrids (or strips as the case may be) are sufficiently beyond to be achored.  They need to be able to develop sufficient achor so that they act fully in tension.

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