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What Size Gravel Behind Retaining Wall?

10/29/2010 3:12 AM

I've got a little problem outside my area of expertise. I'm putting in a retaining wall, about 2m high and I'm looking to backfill behind it with gravel, to provide drainage. The ground's a mixture of clay and degenerated rock. Crumbly sort of stuff

I've three options for stone size; 6 - 10mm, 20 - 25mm and 40 - 50mm. I've consulted various experts and they all give me different recommendations. Some say the coarse stone will flow better. Others say the coarse stone will clog fast so the fine stone is the way to go. Again others say the medium stone flows well and doesn't clog.

I'm totally confused, so I've come to the home of true expertise in search of informed opinion! This is really a civil engineering question but I'll give it a run in "General" as many people encounter french drains.

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#1

Re: What size gravel behind retaining wall?

10/29/2010 7:15 AM

if it were me i would use a mixture of fine and coarse, possibly starting with coarse on the bottom to allow water to flow around wall ,

http://www.google.co.uk/search?q=backfill+for+retaining+walls&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7GGLT_en&redir_esc=&ei=0azKTM26Ac6g4Qay47X7CA

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#2

Re: What size gravel behind retaining wall?

10/29/2010 7:31 AM

Put a filter fabric between the gravel and the top-soil to mitigate pore clogging.

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#3

Re: What Size Gravel Behind Retaining Wall?

10/29/2010 7:59 AM

I agree with guest (above) about using a filter fabric. I would also make sure I've provided drainage at the bottom of the wall (drainage tubing surrounded by gravel) with a route for the water; and for a 2 meter high wall I'd be sure to use 'deadmen' to tie the wall into the ground behind it for stability.

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#4
In reply to #3

Re: What Size Gravel Behind Retaining Wall?

10/29/2010 8:12 AM

what about dead women lets not be sexist hehehehehehe

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#5
In reply to #3

Re: What Size Gravel Behind Retaining Wall?

10/29/2010 8:58 AM

I also agree. They also sell socks to fit over the perforated drainage tube to prevent dirt from entering. Yet another lesson I learned the hard way.

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#20
In reply to #5

Re: What Size Gravel Behind Retaining Wall?

11/01/2010 11:10 AM

You don't need the socks if you encapsulate the drain rock in filter fabric properly, which he should do to keep the drain rock from clogging.

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#6

Re: What Size Gravel Behind Retaining Wall?

10/29/2010 5:14 PM

I would use crushed stone for most of the filling with gravel for the top foot or so with fabric in between. I would slope the interface of the stone & gravel and the soil, the water seems to run down the slope much better than vertically through the stone, there is room for the air to escape along the wall.

A slight overkill but I would do this on my own lot.

What kind of wall is it? We put a plastic or glass bottle in to reserve each hole, then stripped it out when the mortar was set.

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#7

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 12:06 AM

Thank you all. You have given me much food for thought. I was intending to use a layer of filter fabric but the idea of socks on the drainage tube sounds excellent. I'll have to look into what's available in that area. The combination of coarse and fine sounds good sense too. Put the coarse in first for capacity, then the fine above to filter.

The wall's steel. Mainly because I've a quantity of 3/16 steel shuttering that's too beat up for any other use. What I'm doing is building a horse round yard. We've excavated back into a hill to give the flat area. The yard's 12m diameter so I'm reckoning on welding panels of steel to give the basic circular shape, then dropping some heavy plastic down against the steel, some filter cloth against the excavation and then filling the gap in between with stone. There's already a drain round the bottom, so providing the water can't build up against the steel I reckon all will be well.

The actual yard will be timber built inside the steel. I imagine the steel will eventually rust away but it's leaning back at 15 degrees so by the time the steel has no strength I reckon the ground will stay put through force of habit!

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#8

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 12:24 AM

a few questions....! drainage, drainage and drainage.

....ideally, an L shaped footing...if the ground is unstable and or shifts with water seasons....the L should be on its side with more bed than height, thus the load down on it holds it up-right, a footing like this requires drain tile, ang gravel cover, depending on water drainage again, weep holes every4 to six feet,probably 2 inch Pvc.

iève done a lot of them,brick and stone faced....theyère still standing. rpaired a lot....more costly than doing it right

because some-one could do it cheaper.

ièd go with about 1 inch rock because it will not p^lug ur weepèn tiles.

donèt be shy on compacting.....lotsa luck

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#9

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 11:38 AM

If you use stone - use a minimum 2" up (50mm) and I would use it behind
a vertical layer of 4" land drainage pipes. (perforated, as they will clog.)
Breather holes are not a lot of use, and can look ugly. (IMO)

Just because it's steel does not mean it cannot be "pushed" along from
the base. (or buckle) I have seen whole walls move "forward" from the base
due to only soil pressure from behind it. To avoid movement attach several
wall ties (steel rods) to deeply buried "dead men" blocks of wood >10ft +
plus away from the wall. Old railway sleepers are excellent for these.

You accept the steel will rust, but a good painting of anti-rust paint (red oxide)
followed by a thick layer of bitumen, should take it's life time beyond normal needs.
Steel is expensive to waste, have you considered shuttered concrete?

Note: it will be far more costly to repair or renew in a few years time, than it
will be to do an "over the top" perfect lasting job the first time. Good luck.

jt.

Pass me the shovel, but if I stop leaning on this wall, it will fall over...

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#15
In reply to #9

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 8:35 PM

Thanks JT. You've put your finger on a dilemma I have. I agree totally about the red oxide and bitumen paint. I've been soul searching whether to do it or not. On the one hand, it'll last for ever. On the other hand, I've seen similar steel shuttering pulled out after twenty years and whilst it was rusty, it was still sound. It'll also be a heck of a lot of work cleaning the steel up. Wire brush on a grinder will do it easily enough but there's a lot of area to cover and if I don't do it properly, I might as well not bother as it'll rust under the paint.

I'm expecting the shape of the wall to give strength. It's based on a hexadecagon with each panel 2.5m and leaning back at 15 degrees. The panels have ribs so should be very strong. We're down to hard base so the plan is to jackhammer some holes in and concrete in some substantial posts. The posts to stop the structure being pushed along and also to support the internal wood panelled structure.

On the drainage front, I'm thinking the larger stone (40-50mm) for the bottom 800mm or so, then finer stone on top. I'll run some lengths of drainage tube vertically through the middle of this. Some might crush a bit but the tube's cheap enough that I can put in quite a few of these. The sock idea sounds just the ticket for these if I can find some.

I'm thinking that I probably won't back fill right to the top with gravel. We've got a good collection of substantial rocks (> 600mm dia) so once it's well up I might bed some of these around the top to make an edge. One row on top with another banked in behind should stop anyone falling over the edge and also save me a lot of gravel as the top section will fairly eat it due to the increasing gap between the wall and the excavation.

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#16
In reply to #15

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 11:22 PM

As far as the rocks go, what ever rock is used the sock on the pipe AND the land scape cloth are a must. The "dead man" tied into the backside is needed at at least one but more likely 2 or 3 levels, your local building code will specify. Stainless cable is the way to go for the tie. What ever you use for a cross member to bury remember to coat the ends of it if it is treated wood.

You have seen steel last 20 years and I have seen it crumble in 5. Murphy's law says you shouldn't gamble.

Try some phosphoric acid on the steel. Any decent paint store should have it. I just spent a fortnight at Frisco, NC on Cape Hatteras. 9 days to prep and 5 days to spay, prime and paint the frame. After scarping and wire brushing the frame my friend and I used the acid to turn the micro rust to black and then primed over it with a good industrial anti-rust primer. The next coat was a heavy duty industrial solvent based paint. If you prep it right it will outlast you. These are exposed to the salt air 24/7/365 but I expect this to last at least 20 years. The tar that U.S mobile home mfrs use lasted 17 years, since 1993, and that was applied over all the "yard dirt". (they do not prep the frames before painting) So that is another alternative, seal the surface with tar or spray on polyurethane truckbed liner. Anything to seal out the air and moisture.

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#17
In reply to #16

Re: What Size Gravel Behind Retaining Wall?

10/31/2010 3:36 AM

I hear what you're saying David but I'm just not sure that the labour, and it'll be considerable as these are rough panels, is worth it. I scrounged these panels ten years ago, rusty and beat up. I brought them home on the back of a 7 ton truck, tipped them under a tree up the back of the farm and there they've sat for ten years. I dragged them out last week and in all honesty they don't look any more rusty than when I put them there.

I reckon I'll put them in as they are. There's a gap between the steel and the timber and if years down the track the steel starts to let go I'll fill the gap with concrete. I suspect that, unless something happens to kick the rust on, things will just stay the same. Our ground's reasonably reactive but with 20 ton of gravel between the ground and the wall I think the steel's got a reasonably easy life ahead

Still pondering on dead men. The shape of this wall is a half circle, with the top leaning out. Providing my welds hold, it should be very strong and the only risk I see is the whole lot moving as an assembly, or perhaps tipping up. I'm reckoning on the overall mass, along with the boulders on top stopping any tipping. These panels have 100mm ribs welded on so they'll grip in pretty well. Dead men would be not be hard to add but they are best at controlling forward leaning which is important in a straight wall but not so much of a worry to me with a welded half circle design.

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#10

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 11:42 AM

Sorry - was not signed in for the last entry. jt.

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#11

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 12:13 PM

I used 1 & 1/2 stone. What I bought was crushed concrete as it is just for drainage between the wall and retained earth with no crushed concrete (stone) showing. I put about 6 to 8 inches thick between wall and earth with a good commercial grade weed barrier separating the stone and earth, this keeps the earth from filling the stone (crushed concrete). Working well so far. Joe

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#12

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 1:18 PM

Would seem that the drainage characteristics of the area/site should be taken into account.

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#13
In reply to #12

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 2:35 PM

It's always best have the contour of the land being the primary drainage scheme, with french drains being secondary.

There may be other considerations when building a round pen [arena]

the round pen may be set in a hillside to improve the view of spectators..

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#14

Re: What Size Gravel Behind Retaining Wall?

10/30/2010 3:08 PM

Mirra grid every 3 courses,3/4 stone 2' from the back of the wall,perferated 4'' sock pipe with 4" out lets in the bottom face of the wall.There is a calc for this for how high you go versus how far back you go with the mirra grid,ect. www.ephenry.com

The Company I work for specs out this type

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#18

Re: What Size Gravel Behind Retaining Wall?

10/31/2010 9:54 AM

Yes, (thanks to David) - I forgot about phosphoric acid.
I use it automatically - so assumed you would "clean up" the metal first.
Then the red oxide followed by bitumen. (last "forever" done correctly)

It is possible to buy the stone in rectangular cages (similar to chicken wire)
and to place them before back filling. This avoids any sudden pressure and
delays the onset. Fronted by drainage pipes should give a perfect job.

Sounds a nice project. Good luck.

jt.

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about 2000 years old - but it looked strangely sad and miserable.

Excited by the find they carefully unwrapped the body and found it was in fact
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The face was strained and looked so sad; with fists clenched in rage.

Carefully prising the fingers open they saw a slip of parchment and written on it
was, 200 on Goliath to win.

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#19
In reply to #18

Re: What Size Gravel Behind Retaining Wall?

10/31/2010 6:45 PM

Yeh, I'd be keener about painting if it was a simple matter of phosphoric acid. Trouble is this steel is rough. I've got 40 or 50 sqm of this stuff and it was free. It was painted once but that's half gone. Then there's the various layers of dirt, oil and who knows what that's found it's way on over the years. Really the best way to clean it up would be sandblasting but I can't justify the cost. Really, if I have to put too much into the steel, I'd be better off using a different material.

Reckon I'll be backing the truck up and tipping the gravel in. Too much of it to be clever. If the sudden pressure breaks anything, it wasn't strong enough in the first place!

Anyway, thank you all for your input. You've helped me solidify my plans. I'll try to remember to post a follow up when it's all together. Might be a few weeks, might be a few years (hope not!). Depends what comes along to drag me away from my little project

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#21

Re: What Size Gravel Behind Retaining Wall?

12/13/2010 2:29 AM

i have worked on commercial buildings where the specs called for pea gravel on the up-hill side. the reasoning was, if ground shifted, the pea gravel would act as ball bearings to distribute the pressure evenly across the wall. i'ld go with small stone.

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#22

Re: What Size Gravel Behind Retaining Wall?

12/14/2010 4:37 AM

any size stone will be fine if water drainage is your only concern. make sure that you have weep holes near the bottom of the wall or a french drain to prevent water build up behind the wall. this is the cause of most wall failures. cover the stone on the back side and top with landscape cloth to prevent soil intrustion. i think pea gravel would be the best choice for back fill. it prevents pressure points that my arise if the ground shifts

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