Please advise on French drain renovation/replacement

[autopoiesis]

My property is on a gentle north slope, with a paved patio on the south side (added, I believe, after construction), the patio area is ~10 x 5m.

View media item 59971
During particularly heavy rain, the south wall (which can be seen from the crawl space) becomes saturated up to the DPC, and rainwater trickles through it into the crawl space where it soaks into the ground. Puddles also form outside the house, at the east end of the drain where the downpipe joins it; this section flows freely.

The ground floor is made of 20cm slabs of reinforced concrete (a bit like Ytong with rebar). There's no damp problem within the house, but the drain clearly isn't functioning.

View media item 59973
Taken after removal of uppermost layer of large rocks, photo shows there's no screening material at all, and soil pervades the whole volume. The pipe itself is resting on the footing.

NB: I'm not trying to tank/waterproof anything - if there's a little moisture (not running water!) in the crawlspace, then fine (I think).

But, I am confused by conflicting info/advice:

1) Should the drain be on top of the footing, or next to it?
View media item 59970According to the attached diagram, there are a number of approaches, but which to choose? What are the dis/advantages of each?

2) How is one supposed to achieve the recommended slope if resting on a horizontal footing? If laying adjacent to footing, same problem: one must start higher, or end below footing level.

3) If the patio was added following construction, is it possible that the original drain's capacity is too small for the fast-flowing surface it's draining? Is there some formula for calculating capacity?

4) Some sources state that the drainage holes/slots should be face up, and others the opposite (including the diag in the album). How to decide?

View media item 59972
My current thinking:
- install a drainage membrane (Isostud or similar) against the wall and footing
- position correct-sized pipe adjacent to the footing
- fill appropriately, including filter material

Am I on the right track? What's the best approach?

Best wishes,
autopoiesis

edit: images

 

[pinenot]

I will have to make assumptions here, so bear with me.

You'll note from your pics that your drain/pipe is in actual fact perforated, that's to say it is made to allow the water to flow out of it's sides, it's called a perforated distribution pipe (PDP), what you have uncovered is actual effect a long soak-way. The principal behind the operation of such a system is more like an underground irrigation system than a drainage system, per say. Without a drawing of your system I'll do my best to try and explain.

First off the pipe should not have a run to it, only the closed wall pipes feeding it should have a run. The later feed the collected rainwater to the soak-way (perforated pipe) which should be thought of more like the main sewer connection, than a drainpipe. The feeding pipes direct the water into soak-way, which may outflow into a suitable ditch/burn or may bunged end. Once the rainwater arrives in the soak-way the pressure from the following on rainwater flow pushes that which is already there out via the perforations. AS you can see the PDP you have unearthed requires run, indeed it can have an negative run with no end outflow or indeed have a feed to both ends.
Your system appears to collect rainwater from above and feed to the PDP below the hill-face with the PDP sitting on the footings. The dispersal should be away from the foundations to prevent soaking.

Hope this helps explain...pinenot

 

[pinenot]

You say soil pervades the whole volume, that may, in itself, suggest a solution.
If soil has pervaded that perforated pipe it may be causing a problem, it would reduce the volume of the soakway and if disturbed i.e. during heavy rain, form a slurry. Indeed just as you describe "soil pervades the whole volume" could that be coming from inside the pipe?
The only way of really knowing would be to have a drain inspection camera down there have a look. However if i'm right you could employ a rodding scraper of the correct diameter, to scrape out any mud/residue. I'm presuming you can hire these where you are. View media item 60013

 

[stuart45]

4) Some sources state that the drainage holes/slots should be face up, and others the opposite (including the diag in the album). How to decide?

On a French drain it's best to have the holes at the bottom, as when the water table rises it start to run away when it reaches the pipe, whereas at the top it takes longer.
However most people use the flexible pipes now which have the slots all round, so they go in anyway.

 

[autopoiesis]

Hi Pinenot,

Thank you for your response.

You'll note from your pics that your drain/pipe is in actual fact perforated, that's to say it is made to allow the water to flow out of it's sides
Your system appears to collect rainwater from above and feed to the PDP below the hill-face with the PDP sitting on the footings. The dispersal should be away from the foundations to prevent soaking.

I'm pretty sure in that position it's not supposed to be allowing water out - after all, it's positioned uphill of the house. I think stuart45 is on the right track: it should be collecting any surface water running downhill towards the house (or downward-seeking water within the ground), and channelling it to the pipes running to the north.

Best wishes,
AP

 

[autopoiesis]

On a French drain it's best to have the holes at the bottom, as when the water table rises it start to run away when it reaches the pipe, whereas at the top it takes longer.
However most people use the flexible pipes now which have the slots all round, so they go in anyway.

Thank you stuart - that's very pertinent here: the bricks (breeze block) on the footing are only cemented horizontally, not on the vertical joins (where there are gaps of a few mm).

Is this is a standard method, or just corner cutting? Could there be some particular reason to do it (eg stop frost damage from a accumulated water)?

Obviously, water flows through as soon as it starts to accumulate above the footing, hence my thinking that any drainage should be adjacent to to the footing, not on it.

Should I consider waterproofing from the footing up to the DPC?

Thanks again,
AP

 

[pinenot]

My apologies AP I didn't note the position of the drain as on the uphill side, was describing what I thought was on the downhill side.

My only thoughts on the perforated pipe collector, is that at best it's a bit hit and miss. Placing it on top of the footing acts as a catchment dam and only demonstrates this was considered as a boost to the perf-pipe, in my opinion.
Better would be to hang a geocomposite catchment panel on the vertical surface of the sub build wall, with perf-pipe at the bottom providing the run off.
Something like this -

Untitled

• pinenot
• 15 Apr 2013
• 1

 

[autopoiesis]

Better would be to hang a geocomposite catchment panel on the vertical surface of the sub build wall, with perf-pipe at the bottom providing the run off.
Something like this -

Untitled

• pinenot
• 15 Apr 2013
• 1

Thanks again pinenot.

Can you recommend any specific geocomposite panel names?

Given that there are holes in the wall between the bricks, I assume the catchment panel will need to be pretty much waterproof to avoid ingress. Right?

Best wishes,
AP

 

[pinenot]

DRAIN AWAY™ PANEL DRAIN -

Untitled

• pinenot
• 15 Apr 2013
• 1

DMX Drain Geocomposite Drainage boards

Your best bet may be to speak to a local road builder who will know about geocomposites or vertical drainage as it's sometimes known, although in your instance, you are looking for the domestic version.

Good luck...pinenot