Land drainage - Advice

[craigdonaghy]

hi all
i am looking for some advice. i have a job to do at a family members house i am putting in some land drainage pipes and am intending to build a catch pit from some breeze blocks i have spare and discharge the water into the surface water drain but after some investigation work i have found that the land drain will need to be at 650mm below ground level but the surface drainage pipe is at the same depth meaning that there is no level difference when i build the catch pit and i dont think that would be a good idea to allow all the soil etc to go straight into the drain ?

[Tony McC]

There doesn't need to be a gradient between inlet and outfall on a catchpit. They can be at the same level or, less than ideally, the inlet can be very slightly lower than the outfall.

See this page

[craigdonaghy]

hi tony

thanks for the advice. next question if the inlet is lower than the outlet will the land drain pipes not start to back up with water ? also what thickness do you think the concrete should be for the base of the catchpit and what type would i use i know there is a few different batch types out there for me to mix up ?

thanks

craig

[Tony McC]

Having the oufall at or slightly lower than the level of the inlet will cause water to be held in the pipework on the upstream side of the catchpit, but it's a minor problem, as it (should be) relatively clean water within the system, and the important thing is the function of the catchpit itself.

If you have, say, 150mm or more of a drop from inlet to catchpit base, that should be adequate to trap sediment. Yes: it would be ideal to have an outfall at a lower level, but if it's not possible, increasing the linear distance between inlet and outfall will help ensure sediment and silt settles out before existing the pit, even if there is some backing-up within the land-drain section.

Catchpit base - min 100mm, which should be adequate for most domestic applications. If the cover is to be trafficked by vehicles, then I'd be tempted to increase the base to 150mm. C30 mix is ideal.