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Old 07-18-2010, 10:56 AM   #1
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gravel for drainage


what kind of gravel should I use behind a block retaining wall for drainage?

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Old 07-18-2010, 01:10 PM   #2
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gravel for drainage


we use #57 or # 89 HOWEVER we also provide a 4" pipe installed in filter cloth w/pvc drains thru the wall,,, depending on height, we may also apply either sonolastic OR roofing cement protected by delta-drain hdpe fabric.

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Old 07-18-2010, 04:49 PM   #3
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gravel for drainage


dcberg -

You should have well draining sand/gravel behind the wall. After that, the type of wall and height become bigger factors since some types of walls are more forgiving.

Is the wall a rigid block retaining wall (mortar reinforcement and a concrete footing) or is it segmental retaining wall that cannot be mortared or put on a concrete footing. For most walls (especially in cold or variable climates) the segmental wall is more forgiving up to 4' high. After 4 feet high, all walls usually must be engineered and the designer will specify the type of backfill, considering the available local choices and whether a mesh is needed around a perforated pipe to prevent plugging. The segmental walls can be built up to 40' high, but need engineering and soil reinforcement.

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Old 07-18-2010, 08:18 PM   #4
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gravel for drainage


The word gravel unfortunately has many different meanings to different people. In the world of geotechnical engineering (foundation design), gravel means the fragment of soil larger than sand size (there is a specific cutoff limit in mm, but for most purposes it is suitable to say larger than sand grains). In the world of material salesmen, gravel probably means a mixture of sand, silt and gravel (larger than sand size) material.

What you want behind your wall is free draining material free of fines. The best material which is well defined is 1/2 inch or 3/4 inch crushed stone, with less than 5% passing the #200 sieve (less than 5% silt). If your supplier does not understand what this means, find a different supplier. You want clean material behind the wall to allow for drainage.
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Old 07-19-2010, 03:30 AM   #5
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gravel for drainage


dick, can you believe we're got 1 county just north of us who requires a stamp on retaining walls OVER 2' high ? ? ? now i realize guys like you & dan have to eat & i don't mind paying for 5' but give the h/o a break then, when the h/o sez ' nutz ! ' to the stamp'd dwgs, contractors lose out on the work & the county gets all these unapproved walls.

ok, lemme expand for our op,,, picture a vertical section behind the wall,,, we get wall, waterproof coating, pipe, #57 clean stone, filter fabric all on vertical layers,,, we also have fabric installed under the pipe by 2" in a 1' x 1' area,,, the use of pipe w/fabric sock causes silt to infiltrate too close to the pipe thereby contributing to early blockage,,, at my house, there's also a cleanout just for giggles on a 6' standpipe.

those segmented walls you mention are damn'd near another walter mitty wonder of the world to me,,, we've got some down here 60' tall holding back $ 2 M homes,,, f'n amazing to me
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Old 07-19-2010, 09:02 AM   #6
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gravel for drainage


In much of the Midwest, the break between enigneered and unengineered is either 4' or 5' depending on the municipality. This is irregardless of the type of wall construction. That height break is generally the point where the walls are no longer treated as gravity walls, but are considered walls where the soil and site conditions become a large factor. Under some conditions a segmental wall (SRW) may need little or no engineering for few feet more height, but a reinforced concrete wall needs either additional steel and a wider footing. The slight batter to the face of a SRW wall is a great help in working with the soil and not against it. The open joints in the pattern of a SRW wall aid in reduces moisture in the soil behind the wall, while concrete walls are notorious for retaining the moisture, which causes more pressure and random, unsightly cracks.

A higher concrete wall sees more pressure, which requires a bigger (wider, deeper) footing. Since a SRW wall is not allowed to have a concrete footing, the overturning is eliminated and the soil is reinforced with engineered geogrid similar to that used for years in soils and site work.

Since there is there no structure on a soil retaining wall small movements that cause cracks are tolerated and may be reversed, such as frost heaves. In cold climates, all roads freeze and heave somewhat and then return and as long as there is no differential heaving and it is expected and tolerated.

Some people are not aware that many government organizations operate with a set of standard plates for lower wall construction. This permits the use in specs and drawing and for use by agency crews and contractors in the construction of street widening and grade changes along private property. The allow the construction using standard units for straight, inside or outside curves and elevation changes on a routine basis without any formal engineering for each application. They often are available. Some organizations make these standards available for the public if they are used in a permitted job that may be inspected.

Those 60' highs are probably tiered walls with a set-back for each higher tier. These must be engineered because of the global soil failure concern in order to be economical. The most impressive SRW wall installation was in a foreign country where the walls were about 8 miles long in a curving route. The highest single wall was 40' and there were others that were each smaller and tiered back if the site permitted. There wher many low segments and some areas where the retaining wall was suppoerting a bit of the highway.

Dick
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Old 07-19-2010, 12:35 PM   #7
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dick, GREAT answer, but in cherokee county, i call it the ' professional engineer's retirement act ',,, then again, i also call rain my ' PERSONAL wealth builder w/o buying tony robbins' cd's '

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