Minimum Fall Across Compacted Hardstand 1

[Wes-FA]

We are designing a central storage 'precinct' for a large farm, to contain sheds, silos, water & chemicals tanks, etc. All of this on a compacted hardstand (in this case it is not feasible to finish the whole area - 7000m² - with bitumen or concrete).

What is the minimum fall we should allow for drainage?

With thanks and kind regards,
Wes

 

[dik]

If your hardstand is similar to our Gran A, then it's almost as impervious as asphalt and I would normally like to see a slope of 1-1/2% with 1% being absolute minimum. Water will pond on well compacted Gran A.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KaBone]

I'm assuming the the site is going to be regraded and the exposed soil will be used as a road and maybe parking.
Sounds like you're calling the hardstand as impervious for the purposes of storm water quality. In that case, i wouldn't worry too much about minimum slope. I'd worry more about flow velocity and quantity.
Wind and water are the enemy of exposed soil. If not a road or parking, use landscaping to protect the surface. I'd try to grade the road crowned in the middle with 1% slope to each side and 0.56% longitudinal slope. You'd like the road to convey the least amount of water as possible and the slowest velocity possible.

 

[jgailla]

If you don't have 2% minimum you will get puddles.

 

[SwinnyGG]

Compacted or not, when it gets wet there will be puddles created. No farmer I know calls off all activity for rain.

I'd want to cover it with SOMETHING - crusher fines, gravel, whatever - that provides some level of drainage.

 

[Wes-FA]

Thanks for the input guys. The project design is progressing.
The pad is going to be fully regraded and our Civil Engineer has worked out the falls to be workable at 0.5% with 200mm topping of gravel or crushed limestone.
The whole pad (65m wide) will have a continuous fall from west to east where we have a v-drain with large rocks to slow the water down. We considered crowning it in the middle, as KaBone mentioned, but we need to control the runoff to avoid contaminating nearby paddocks, and we also have multiple sheds along the high side at west.

 

[SwinnyGG]

You will not get consistent 1/2% at install with gravel, and you certainly won't keep 1/2% for very long once it's in use. If the impervious surface is graded at 1/2" and whatever is above it is highly permeable (ie, gravel), that impermeable surface is going to have puddles on it, and once it saturates the gravel is going to disappear rapidly. If this is the approach you want, you're going to need a serious high quality geogrid or geotextile, and the right size rock whatever exact product is chosen. This will not be cheap.

I'd suggest you make the owner aware of the fact that there's a lot of gravel maintenance in their future if this area is heavy traffic volume, or light volume with heavy farming equipment.