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Old 03-08-2012, 06:30 PM   #1
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


I am going to dig two french drains for two of my down spouts. I am trying to figure out how much pea gravel I need.

the trench will be 8" wide, averaging 18" deep and totaling 40' long. I will use 4" diameter solid corrugated pipe. Here is my calculation:

the cross section of the gravel = the square - pipe = 8"x18" - 3.14*2^2=131 sq-in.

Total volume for the gravel = cross-secion x length = 131*40*12=62880 cubic inches = 62880/36^3=1.35 cubic yards. That's like 2 tons of pea gravel, which seems a lot to me.

Is this correct? I thought I could just use my small trailer to haul. After all, this should be a tiny project.

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Old 03-08-2012, 06:49 PM   #2
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


Math seems right to me for 1.35 cubic yards.
One site I saw had: (1) Cubic Yard of Pea Gravel actually = 1.16 Tons
so, that would be 1.56 tons of pea.

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Old 03-08-2012, 07:15 PM   #3
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


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Math seems right to me for 1.35 cubic yards.
One site I saw had: (1) Cubic Yard of Pea Gravel actually = 1.16 Tons
so, that would be 1.56 tons of pea.
This is my first DIY project involving digging stuffs so I have no idea of the amount of materials I am up to. Guess whenever you touch landscaping digging and filling, you are talking about things in units of tons.

Before my calculation I was thinking I could buy the bags of gravel from Home Depot and haul it in my sedan.
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Old 03-08-2012, 07:19 PM   #4
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


If you are looking for that roof water to be temporally stored in the gravel until it's absorbed into the soil, then you need to look at the roof area drained and the amount of water to be dealt with.

Example: Roof area of 600 sq ft, with a 1 inch rain. 600 divided by 12 equal 50 cu ft of water. Gravel can hold in voids about a third of its volume in water. So you need 150 cu ft of gravel for the 50 cu ft of water. 150 cu ft = 5.5 cu yd. of gravel.

Got a 2400 sq ft house with four downspouts to be run into french drains/ dry wells? Burying 22 cu yd of gravel needs a BIG hole. Are ya starting to see why french drains are a stupid idea for roof water?
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Old 03-08-2012, 07:30 PM   #5
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


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If you are looking for that roof water to be temporally stored in the gravel until it's absorbed into the soil, then you need to look at the roof area drained and the amount of water to be dealt with.

Example: Roof area of 600 sq ft, with a 1 inch rain. 600 divided by 12 equal 50 cu ft of water. Gravel can hold in voids about a third of its volume in water. So you need 150 cu ft of gravel for the 50 cu ft of water. 150 cu ft = 5.5 cu yd. of gravel.

Got a 2400 sq ft house with four downspouts to be run into french drains/ dry wells? Burying 22 cu yd of gravel needs a BIG hole. Are ya starting to see why french drains are a stupid idea for roof water?
It's California. We don't have much rain here anyway. Plus, this gravel is to support the solid pipe, where ends in a blind well.
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Old 03-08-2012, 07:47 PM   #6
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


The calcs are correct as far as they go, however you need to account for the fact that you buy gravel loose, and you should compact the gravel when you put it in, so you should add about 15 percent to your total. Compaction is easy, you just tamp it down with a heavy object (your child works well, they like to stomp on gravel) in six inch or less lifts. If you do not compact the gravel, it will settle over time, leaving an unpleasant dip in your lawn.
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Old 03-09-2012, 08:29 AM   #7
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


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Old 03-09-2012, 11:06 AM   #8
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


Are you going to cover the gravel with soil and lawn or leave the stone exposed?
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Old 03-09-2012, 11:16 AM   #9
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


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Are you going to cover the gravel with soil and lawn or leave the stone exposed?
I am going to cover the whole thing with soil and lawn so that it looks just as everywhere else in the yard.
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Old 03-09-2012, 12:12 PM   #10
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Is my calculation correct? -- figuring the amt of cu. yard of gravel for french drain


Amount of stone for trench 8" wide x 18" deep, less pipe less 8" wide x ~4" of soil x length.

Added: I would not use pea gravel, it doesnt compact and your lawn will sink. Use clean #2.

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