MATH HELP: Culvert pipe capacity in gpm | Page 3 | Construction and DIY projects | Forums

UPDATE

After my last posting I was left with some reservations and concerns. Rather than summarize, I'm going to do a walk through of where that led. My concerns were:

• Soil type - It was looking like I had clay soil. I was still awaiting results of a soil composition test. I remembered that it advised against clay for this purpose. I was concerned about weight bearing which clay is said to be less ideal at. I was also concerned about impermeable damming vs permeable, and I was concerned about erosion. I wanted to know what the recommended soil content was and wanted to know how much it could cost if I needed to truck in aggregate.
• Slope - After calculating for a 4:3 slope I realized that while that is the steepest slope recommended, that I had not looked at the slope recommendation for the purpose of raising to driveway elevation combined with a culvert.
• Drainage - Even though I opted for the higher of the recommended culvert diameters, I was still concerned about capacity during 20 to 100 year flood events. I was also concerned about using only 1 culvert pipe and creating a pinch point. The channel is 50 feet across and the low point is on the north side and flows about 7 feet broad. There is a second smaller natural channel on the south side which runs when the water level gets high enough. I was concerned about pooling.

I messed up my first attempt at a soil composition test (link pending), but have my result from the second attempt. I have clay soil. It was 80% clay, 15% silt, and 5% sand.

Referring back to http://ucanr.edu/sit.....138029.pdf, page 2 recommends:

Bedding and backfill material for culverts is commonly specified as "select granular material" or "select mineral soil". Actually, most soils are satisfactory if they are free of excessive moisture, muck, lumps of frozen soil, roots, highly plastic clay, or rock larger than 7.5 cm. Bedding material beneath the pipe should not have rocks larger than 3.8 cm. Clay soil can be used if it is carefully compacted at a uniform, near-optimum moisture content. Ideal backfill material is a moist, well-graded granular or sandy gravel soil with up to 10 percent fines and free of rocks. The material should be well compacted, at least as dense as the adjacent ground, and preferably at a density of 90-95% of the AASHTO T-99 maximum density. It should be placed in 15cm thick layers (lifts). A dense, uniform backfill is important to structurally support the lateral pressure from the pipe, particularly with plastic pipes.

Uniform fine sand and silt soils can be problematic when used for culvert bedding or backfill material. These fine, non-cohesive soils are very susceptible to scour and piping from moving water (Photo 8.6). Thus their use is discouraged. If used, they should be very well compacted against the pipe. Ideally, a clay plug or anti-seepage collar, made of metal, concrete, or even geotextile, should be placed around the culvert pipe to force any water channel to flow in a longer path through the soil. Concrete headwalls also deter piping.

So while it says "clay soil can be used if it is carefully compacted at a uniform, near-optimum moisture content", I would be concerned that achieving an "ideal and uniform" compaction and moisture content may not happen. I figure a higher safe margin of error is preferable.

So I looked up "select granular material" and discovered on https://www.dot.state.oh.us/Divisions/ConstructionMgt/OnlineDocs/2009MOP/SS%20840,%20850,851,%20S-1015/SS-840/SS.htm that you should:

bury the highly erosive select granular material and erosion can be minimized

"Highly erosive" concerned me as well. So lots of cover by gravel then lots of rip-rap on top of that? Or might there be something else? So I kept looking.

So returning to http://ucanr.edu/sit.....138029.pdf, page 6 shows an illustration with a 2:1 slope, and then on page 8 shows an illustration with a 1.5:1 slope. Much more conservative than the 4:3 slope. I generally favor opting for the conservative choice with such matters to increase margin of error. But a 2:1 slope would mean much more fill, and twice the length of pipe (6 sections instead of 3). I did the math. I did not like the result.

If I trucked in aggregate for a 4:3 slope, I was looking at 3 sections of 48 inch arch pipe to span 24 feet from toe edge to toe edge, and 24 truck loads for 409 cubic yards of fill. A 2:1 slope would mean 6 sections of 48 inch arch pipe to span 44 feet from toe edge to toe edge, and 25 truck loads for 522 cubic yards of fill. The cost of trucking in fill for the 4:3 slope would be about $4700, while the cost of trucking in fill for the 2:1 slope would be about $5300.

Even if I opted for the on site (clay) soil as fill to save money by not trucking in aggregate, I would want to have it excavated from the pond site and that creates the problem of moving the soil from the pond site at the top of the hill down to the stream crossing down in the valley. The more soil to move, and the farther it needs moving, the more expensive things get... again. And I'd still be stuck with having to get twice the length in arch pipe.

I stepped away to clear my head in the hopes of figuring out a better solution. (To be continued.)

The Right-angled Triangles Calculator

http://www.sustland......walls.html

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How much concrete to fill 8x8x16 block wall?

I just had another thought. If I'm doing form work, rebar, remesh, and ready mix concrete, what if I did my own culverts and direct the money I'd spend on precast into that? Also, I wonder how much it would cost to pour steel reinforced ready mix concrete retaining walls vs cinder blocks?

Does it count as monolithic if done in multiple pours/drys/stages? As in slab first, then culverts, then retaining walls?