How much does dirt settle?

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  #1  
Old 05-01-10, 06:34 PM
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How much does dirt settle?

I dug a 12 inch wide trench to run two 4 inch pipes, one perforated and covered with a fabric sleeve for a french drain the other non-perforated to carry water from my downspouts. I dug about 10 feet of the trench too deep by about 5 inches and now must put dirt back in the trench to main a proper slope to the dumping end of the pipes. My questions: How much will that 5 inches of dirt that I add settle? I have a 5 foot 4x4 which I am planning to use to tamp it down, lifting the 4x4 vertically and pounding a 4x4 end down to the ground. Does wetting the area down help compact it better?
 
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  #2  
Old 05-01-10, 07:21 PM
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Do you plan to put a permeable fill (sand and rock mixture) under and around the pipe since the "sock" is not a cure-all?

This would minimize the amount of compressible "dirt" that causes settlement.

I assume the perforated pipe is to collect more water, since a solid pipe would do a better job to carry water away.

Dick
 
  #3  
Old 05-02-10, 05:49 PM
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I was told the sock was sufficient to keep dirt out. The perforated pipe would then capture the surface water that filtered down through the dirt and rock.

This captured water would then run down to the dump point in the pipe. That made sense to me since it seemed to me that if the area beneath the perforated pipe was permeable any water that filtered down from the surface into the trench would be below the pipe and would not get into the pipe through the perforations.

I have to admit I don't quite understand how the french drain is suppose to work but a lot of people and web sites claim they do.

I have seen drawings of cross sections of french drains that showed stone beneath the perforated pipe. It seems like there would have to be an awful lot of water in the trench to reach the perforations in the pipe if the pipe were on top of the stone.
 
  #4  
Old 05-03-10, 08:19 AM
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Back fill

Use gravel to fill in the low spot. Pack once with the 4x4 and you are done. Gravel settles much less than dirt.
 
  #5  
Old 05-03-10, 11:51 AM
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Based on the web site All Gravel and Stone Sizes I am going to try to find #10 gravel and use it.

I do have a concern that gravel under the perforated pipe will lessen the functionality of the french drain but I'm more concerned about keeping the slope on the non-perforated pipe carrying the downspout water to the storm drain catch basin.

Are there any french drain experts out there that can explain the physics behind exactly how the french drain works? It would seem like the only water it is going to capture would be that water that filters down on top of the perforated pipe. It doesn't seem like it will pick up any water from below the pipe, even if the trench bottom is solid dirt, unless of course there is a lot of water filtering down which is not the case with me. The slits of the perforated pipe are at least a 1/4 of an inch or more above the trench bottom.,
 
  #6  
Old 05-03-10, 12:31 PM
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Generally gravel around a French drain pipe is what you want and it improves it' function. Water can easily flow through clean gravel so the stone alone is helping to transport water. The stone also acts as a filter to keep dirt out.

French drains are very simple. They are basically a covered ditch to move or redirect water. Usually they are filled with stone but many (like yours) also contain a pipe. Your situation with a perforated pipe is much like a septic system leach field where the water goes through the pipe and leaks out through the holes wherever possible and is absorbed into the ground. The gravel around the pipe helps keep the soil further away, making the trench walls larger with more surface area which absorbs more water. French drains can also operate in reverse, catching water either from the surface or water moving underground and redirecting it to a different location.
 
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Old 05-04-10, 07:28 PM
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The final puzzle in my mind about how the French drain will work with stone or gravel under the perforated pipe has to do with getting the captured water into a catch basin if I go through the concrete wall of the catch basin with my pipe.

I have a couple drawings on my web site at FRENCH DRAIN QUESTION

Question #1: Does Figure #2 illustrate the proper way to get the water from a French drain to go through the concrete catch base to the storm drain?

Question #2: will the ground around the catch basin eventually get washed out with this design?
 
  #8  
Old 05-05-10, 04:59 AM
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I've always seen the gravel depth stay the same with the gravel ending outside the catch basin. The pipe going through the wall of the catch basin is mortared in place.

Don't sweat this project too much and don't worry about water in the bottom of the trench. Yes, there will be water trapped in the lower gravel layer that does not make it up into the pipe. Since it is underground and not high enough to make it to the pipe it is insignificant. It will soak into the soil over time reducing the amount of water you put into the storm drain.
 
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