PVC question on Retaining wall project

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Old 10-30-14, 02:12 PM
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PVC question on Retaining wall project

We're removing an old 6x6 wood retaining wall along our patio and replacing with a CST 6" Terrace wall. On the up slope side of the new wall I was told to lay the landscape fabric from my local stone center against the new wall, then backfilling with 3/4 clean and PVC for drainage prior to backfilling with dirt. It was suggested I use 4" PVC with the holes in it.

Should the holes be facing up or facing down on the PVC? I have gotten a few different answers this past week and now not sure which is correct. So I wanted to ask the professionals. Thank you in advance for any help you can provide. AW
 
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Old 10-30-14, 02:45 PM
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Holes up, slightly towards the backfill (away from the stone). You want water to run down through the soil and stone, then go into the pipe and be carried away in the un-perforated portion.

You also wrap the fabric up and over the pipe before back filling. And ensure you have some slope in the pipe.

No expert, I've only helped on one job, but that's what we did.

I think holes down might be when collecting groundwater around a basement wall? Maybe?
 
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Old 10-30-14, 02:46 PM
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Holes on the PVC pipe should be facing down. The bottom of the pipe should be the lateral space between the holes.
 
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Old 10-30-14, 03:06 PM
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LFW...can you explain for us that have (or may do it wrong)? I thought you were trying to catch water running down from above. Will it just pool in the area of the pipe and run out? I guess that makes sense too.

Oh wait...is there an area between the holes that runs the length of the pipe, not just the mostly solid section on top?
 
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Old 10-30-14, 03:44 PM
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A perforated pipe is a 24/7 system that collects the free water from short term storms to allow the soil to absorb the temporary surges from short term weather an provide long term structural stability.

The bottom of the PVC should slight below or at the bottom of the footing (or wall in the case of a SRW system). There should be about 2" inches of perveous soil/rock below the bottom or invert of the pipe. This allows a long term dry-down and less hydrostatic pressure. The holes always are faced toward the bottom half.

The fabric directly behind the wall can be good just as a layer between the perveous fill and the native soil is as it does wonders to prevent washing out the fines that are necessary for compaction.

It is a classic system used for many years that consitas of providing short and long term stability if the appropriate local materials (and fabric) are used in the proper locations.

For a very low wall 2-3', you can be sloppy and get by, but do not stretch what works for a low wall into a serious wall (5' to 25' high).

Dick
 
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Old 10-30-14, 03:55 PM
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Thank you guys. I guess I should tell the guy back in VA we did it wrong...lol.
 
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Old 10-31-14, 07:44 AM
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holes down,,, not to worry as water will also run downhill the soil fabric's purpose is to keep as much silt out of the system as possible,,, this is why our pipe trench is usually 3x pipe width x 3x pipe height,,, much more filter area

dick, isn't ' invert ' the elevation of the pipe as measured @ the lowest point ? as opposed to the ' crown ' elevation ? are not both interior measurements ? eg, 12" rccp w/2" thick walls would have an overall height of 16" but invert & crown would be 2" less ?

happy halloween !
 

Last edited by stadry; 10-31-14 at 08:02 AM.
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Old 11-02-14, 07:14 AM
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I agree with stadry. I was taught that the invert is the lowest elevation interior of the pipe. The holes are set 120 along a portion of the pipe. The area between the wholes is for the water entering the pipe to flow out. The holes allow the water to flow into the pipe and once the water is past the area of infiltration, the water will not flow into the holes and saturate the rest of the stone. The projects I have worked on have followed the 3X solution that stadry mentioned. The contractor would place fabric in the trench, add clean stone beneath the pipe, then backfill with clean drainage stone wrapping the stone with the remaining fabric. The bottom of the trench would determine the slope of the pipe and the pipe would daylight into a drainage area or sump.
 
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