Basement floor drainage

Hi there,


We're in the process of slightly lowering our basement floor (nothing drastic like underpinning, just the remove of the cracked slab and some substrate and relaying it slightly lower with insulation and DPM, all based on a structural engineer's report and going 300mm further down the footings but not below them of course).

All going well so far, but in taking up the old slab we discovered an old (likely 1890s) 8" ceramic drain pipe, with a series of weeping tile terracotta pipes leading into it. These 4" terracotta pipes were not joined together or sealed, just laid next to each other to provide multiple channels for drainage in the substrate. They all lead into a little gully into the 8" pipe which is sealed with collars and runs out of the property and into what smells like a surface water sewer.

Because of the lower level that the new slab is going to, the plan is to remove this 8" main pipe, and run connected land drain perf pipe in some channels of 20mm gravel (under the newer lower floor and under a DPM) and lead it to a central point - probably a sump. There is an existing steel sump basin in the old basement floor, and this is going to be replaced by a deeper, modern plastic sump in the usual surrounding of 20mm gravel, which will have an automatic pump fitted to it. But we're not sure the best option/practice when it comes to where to channel the drainage.

We are in south London so we're sitting on clay, and water doesn't seem to come up through the clay (as you'd expect it wouldn't) but pools on it as it comes through the substrate just above it.

I am wondering if we should:

1. Run the new land drain perf pipe into the new sump, and connect the sump pump output to the main sewer line (the foul sewer line is not below the floor, it runs out of the basement at shoulder height at the opposite end of the basement to the surface water sewer).

2. Run the land drain perf pipe into the existing surface water drain. Although the main pipe is being removed, the part that leaves the property could be used (even though the floor is being lowered) and adapted to take the land drain and for it to drain out of the floor passively - although not sure if there could be a future problem if the surface water sewer floods and backs up (I was surprised that there was even an inlet to it that low, it's already two meters below the street level outside, but that shows what I know!)

3. Run the new land drain perf pipe into the new sump, and run the sump pump output into the surface water sewer pipe by adapting the fittings. There will be a check valve on the sump pump waste output as standard.


Any thoughts and advice very welcome, thanks in advance.

 

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