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I'm putting in a drainage system on my property including 12" drain basins in low spots on one side of the house and under downspouts. Probably going to use a light weight solid drainage pipe like, http://m.homedepot.com/p/4-in-x-10-ft-Triplewall-Pipe-Solid-4550010/100135310

I often see people laying a gravel need under the pipe and over. Is that only useful for perforated pipe or is there a reason to do it on solid pipe as well? Buried PVC for instance doesn't require a gravel bed? Should the drain boxes be on a gravel bed too?

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    If it is solid other than bedding the pipe it really won't matter but at the exit of the pipe a gravel bed will help prevent erosion.
    – Ed Beal
    Commented Oct 9, 2016 at 21:10

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What does the gravel provide: The gravel helps allow you to build the drainage slope for the pipe and provides a back-up drainage medium for leakage. In commercial projects we spec the drainage starts with perforated pipe then truncs into main drainage that is non-perforated and we require the gravel to be carried throughout, as a secondary drainage path.

Why do I need the extra drainage in gravel: If the pipe connection has a flaw or if the ground shifts a pipe crack could form. If the pipe is backfilled directly with soil the water will collect against the pipe and soil without somewhere to go. This collecting of water will make the soil saturated, cause additional issues and water infiltration. The gravel will allow the water to drain away from the pipe and down the path of least resistance in the direction of the pipe to the outfall.

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  • I assume this is why I see some lay a gravel bed under the drainage boxes as well.
    – Hashamyim
    Commented Oct 10, 2016 at 19:07
  • That, and in some scenarios where it would not be necessary for drain certain types of rock are considered self compacting and its cheaper for the contractor to fill the base up to grade with that rock than send someone in the hole with a compactor for soil. Commented Oct 10, 2016 at 19:28
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There are two schools of thought. One is yes use filter rock to bed the pipe for ease of grading and to prevent build up of water under the pipe. The other is to use impervious soil material because the water at the inlet may flow under the pipe instead of through it. It depends on the situation and the engineer.

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I believe the recommendation is to surround all buried pipes with pea shingle or similar aggregate to avoid puncturing or crushing them when compacting, when the soil settles over time or the ground is loaded in some other way (traffic, building...). The pea shingle will "flow" around the pipe and fill any voids underneath better than some other options.

From PavingExpert.com:

Vitrified clayware is much stronger than the plastic equivalents, and is less susceptible to deformation when buried; therefore, clayware relies less on the competence of the granular bedding material to withstand external loading. Clay pipes can often be laid directly into a trimmed and formed trench (known as Class D bedding), whereas plasticware MUST be surrounded by a selected small gravel or pea-shingle. This essential bedding material can often cost as much as, or even more than, the pipes and fittings needed for the job.

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