A small interesting & practical retaining wall project to discuss

[cclin]

Hi, I recently got a small project for designing a small 8" block retaning wall that is about 2' to 3' above driveway. The driveway is close to the edge of the slope.


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1 1 Driveway side
1 1
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/ 1 1
/ 1 1_____
/ 1_______1


Before I conducted any design, I reviewed the soil report. 45 active and 250 passive are suggested. Also pier foundation is suggested as alternate for using 200 bearing. My design considerations are the following.

(1) How deep is the portion of the wall that needs to be below the top of the paver driveway? Maybe 2' to 3' ?
(2) What surcharge load should I use? 200 ?
(3) What is the size of the footing? Maybe 1' deep x 2'-6" wide as shown above?
(4) Should I provide drilled pier to prevent the wall from tilting and sliding? Min. 12" diameter x 15' deep? @ 6' max spacing?
(5) How far should the slope be? Maybe 2 to 3 times the height? Say 4'. Slope may be 2.5 to 1 (H to V).


Any suggestions or comments or ideas or points to make and discuss? Thanks

 

[ishvaaag]

First with the type ... by putting CMU blocks both under and over the surface you risk the differenctial behaviour of the exposed and buried parts result, soon, midterm or longterm in some cracks. My advice is make a foundation in reinforced ocncrete, and then you may treat the exposed part in any masonry you want.

Respect how much deep to put the base of the footing, it depends on some factors such as on whether where you live the soil freezes some days along the year or not. Anyway for so weak structure most wouldn't go as deep as to forfeit entirely the expected (bad) effects of soil freeze, which for temperate zones is about 0.90 to 1.10 m deep (3 to 4 feet).

The layers composing your driveway also influence the top depth, but mainly on if you will end pushing away -to crack- your wall wen undergoing compaction. Many would build entirely the wall after making the paving layers themselves, or at least the exposed parts.

Respect surcharges many have been using for traffic from 400 kgf/m2 to 1000 kgf/m2. You may select any suitable one depending of the kind of vehicles you expect there.

Drilled piers (15 ft deep) sound as some luxury if you have reasonably strong soil. This is even more so for such secondary structures as this wall. Anyway, piling of a required depth would be considered where the soil is as bad that you want to put your wall entirely on a foundation that acts as a beam supported by the piles. This rarely will be the case for ordinary sites, but there are weak soils that may require these things if you want a long lasting structure. Even so for this case it is likely that the structure on piles might stay well but the soil continue separately moving and so some disgusting apparent dislocations appear with time.

Respect the slope it depends much on where it is placed, what goes above (in traffic) and of what material it is made. It is quite typical of public works where the apport material to build the sloped fill is mainly cohesionless and sound to go between 2:1 to 1.5:1 or so (H/V). There are circumnstances and fills that may require lesser slope. With very good fills and support at the base one may also go higher in slope. These things are analyzable, so if you think worth the effort, look for an able designer.

 

[Focht3]

You have a "soil report". Have you discussed this with the geotechnical engineer? Do that before you go any further...

Where is the site? What are the soil conditions? Groundwater?

Give yourself as much distance away from the slope as possible. But if the slope is only 4 feet tall, then slope "failure" isn't that much of a risk -

Why are you considering the use of drilled piers? If you go with drilled piers, I'd design the system as a cantilever wall supported on drilled shafts, and forget the "footing".



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[solutioninc]

If there is enough space between the end of the driveway and the slope (2 to 3 times the wall height) why would one want a wall there at all?
If a marking is needed a fencing as a non structural member will suffice.
Am I missing something?

 

[wallguy]

Solutioninc makes a good point. CCLIN help us by letting us know the need. It appears to be a barrier wall for trafffic, I am correct? If you were to consider a segmental retaining wall with a parapet wall on top, you would want to be a least 2 times the height away from the slope edge. You would want to use 100 psf for light traffic and 250 psf for heavy traffic in the design of the wall. If your foundation soils are strong you would only place gravel or lean concrete as your leveling pad and no "footing" would be required. I will suggest also that a design include global stability. I am not a licensed engineer, I have been around retaining walls for a while. Let my suggestions be from a constructability perspective.

ken

 

[cclin]

I really appreciate all the engineers who respond this question. I have a preliminary design already that is a little conservative. I will post my design concept later when I really finish. To make things clear, the entire driveway is new, it will be light to heavy traffic (for a 6000 sq ft customer built house). the base soil and the slop are new as well. The soil report suggests using drilled pier for the house itself but no mentioning using drilled pier and gives 2000 psf bearing capacity for good bearing soil. The report also suggests using 45pcf min. for active soil pressure (for drained condition) and 250pcf passive soil pressure. Need any more information, let me know. Thanks for all your suggestions. Every one really makes good points to ponder. Very good and interesting.

 

[Focht3]

Pretty low allowable bearing pressure for drilled piers; it implies the undrained shear strength of the soil is only about 1,000 psf...

Where is the project located?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.