Garden path drainage

[jsmac]

Hello Team PPUK,

When we had the side and rear of our property excavated to take the significantly raised ground levels down to below DPC we had a retaining wall installed (which was done excellently) and a rather woeful attempt at drainage done at the same time

A trench was dug along the centre of the path and filled with gravel and a 100mm perforated pipe, topped up with more gravel, wrapped in a geotextile membrane and covered with Type 1 aggregate. Well, it was supposed to be a geotextile membrane but it turns out that they used a landscaping weed fabric of some variety and probably didn't dig the trench with the correct fall. When the first heavy rain came I popped round to see how they were getting on and watched them clamber about in several inches of water and stand around looking at puddled water. They made some noises about getting it sorted but basically couldn't be arsed to do it properly and subsequently sacked off the last bits of the job

I've attached some photos and would welcome your thoughts on what the best way to go about getting the drainage sorted before I have folks round to redo the drainage. The final path will be a block pavers. I'm guessing that the whole gravel filled trench will have to be dug up and reinstalled and/or a channel drain installed.

The perforated pipe is supposed to terminate at a gully trap which then connects to the downpipe drainage. I say 'supposed to' because I'm not sure that I ever saw the gully trap before they backfilled the path but assuming that it is there and installed correctly I'm hoping that the new drainage can connect to it.

Many thanks.

 

[Cubist]

Have you inspected the gulley trap to see if water is actually running into and out of it? Or is too flooded? All of the Darins I installed here were wrapped in weed fabric and have functioned properly and well for near 15 years.

In your case, either the perforated pipe is NOT connected to the gulley trap, OR the builders did not establish a sufficient fall toward the trap OR the exit pipe from the trap is blocked.

BTW - do you plan to cover the entire area of the path with block pavers?

 

[stuart45]

For French drains it's better to use a non woven geotextile. I've tested the 2 types, and it allows water to pass through a lot quicker. However, a woven one should still work.

 

[jsmac]

Thank you both.

I used some non-woven geotextile that was left over from the limecrete floor materials we purchased from Mike Wye for a small section of brick lined paving in front of our property. This area drains well and performs significantly better than the areas shown in the photos above. What is it that is said about if you want something done properly?

Cubist - What it is that they claim to have connected one end of the perforated pipe to is in the ground covered over with backfilled shingle and Type 1. Perhaps it is not called a gully trap? I am waiting for a period of somewhat drier weather before I get the shovel out. I suspect that you are right and that any and/or all of the variables you highlight could be the source of the problem. It is just so very irritating to have to contemplate having to do this all over again when we briefed in full what work we wanted done and have already paid for this work!

The plan is to use block pavers on the whole area but we are open to alternative approaches. That is why we asked for the soil to be excavated and 100mm of Type 1 laid - we planned to lay the pavers on 25mm of sharp sand and then brush kiln-dried sand into the gaps between the pavers to create permeable paving.

One landscaping firm that I have only spoke with over the phone and who are visiting this week suggested installing ACO drains along the edge of the property and connect these to the drains. Maybe this is the correct approach? Or this combined with the reinstalled French drain?

 

[Cubist]

If theres a means of connecting an ACO drain to the main waste/sewer connection then its should be as easy to connect the extant perforated pipe - maybe even easier.

Full width and length paving will block much of the surface rain water so it would be wise to ensure that the pavers are laid 'fall' away from the house and any ACO drains are installed on that side not next to house walls.

 

[stuart45]

You said there's been a retaining wall built. Is any water running through that into the area?

 

[Flyfisher]

Full width and length paving will block much of the surface rain water so it would be wise to ensure that the pavers are laid 'fall' away from the house and any ACO drains are installed on that side not next to house walls.

Definitely! I would say it's best to assume that any block paving is impervious. I say that because we have a fairly large patio area covered with clay pavers laid on 100mm of type 1 MoT and about 30-40mm of sharp sand (no geotextile membrane), all stabilised with kiln-dried sand swept into the gaps between the pavers. The patio is bounded on two sides by garden walls, one side by the house wall and the 'end' side is open to grass. When laying the pavers, I sloped them very slightly away from the three walls and towards the grass. Only very slightly and not noticeable by eye but when we have heavy rain I can see a definite flow of water onto the grass.

I'm not suggesting such paving is 100% impervious, just that whatever permeability it does have will not be adequate for drainage purposes except perhaps in the slightest drizzle.

 

[jsmac]

Thanks, all.

ACO drains are installed on that side not next to house walls.

Just so as I'm clear as to what you're suggesting - do you mean install the ACO drains next to the retaining walls? And then lay the pavers up to the brick/blockwork below DPC with a slight gradient running towards the drains away from the property?

You said there's been a retaining wall built. Is any water running through that into the area?

I did ask the bricklayers about weep holes in wall towards the bottom to drain excess water from the planting beds but the team doing that work said that it wasn't necessary. It is really chalky soil here so it looks like the beds drains well enough into the ground below.

clay pavers laid on 100mm of type 1 MoT and about 30-40mm of sharp sand (no geotextile membrane), all stabilised with kiln-dried sand swept into the gaps between the pavers

This is what I am planning on doing once the drainage questions are resolved. Given that your experience is that there can be noticeable water run off it sounds like ACO drains running parallel to the retaining walls might be the way to go.

 

[stuart45]

If the the soil drains really well you shouldn't have the water sitting on top where it is now.

 

[Cubist]

Just so as I'm clear as to what you're suggesting - do you mean install the ACO drains next to the retaining walls? And then lay the pavers up to the brick/blockwork below DPC with a slight gradient running towards the drains away from the property?

Yes, for best effect any ACO drains should be laid along new retaining walls and any pavers/blockwork must slope away from the house and down to the drain. Some might think that's counter-intuitive but siting the ACO drain beside the house and sloping the path toward it is more likely to produce pooling water against the house.

 

[jsmac]

If the the soil drains really well you shouldn't have the water sitting on top where it is now.

I should have been more clear - the photos I uploaded were taken after several hours of really heavy rain. You can see that the areas with the most water pooling is towards the area where the perforated pipe terminates. So what I think is happening is that the pipe is working correctly but that it isn't terminating into a drain effectively (or at all). So all the water that has been collected is just pooling in the pipe because it can't drain fast enough until it starts pooling above ground.

I did some rooting around yesterday afternoon and found the end of the pipe. As you can see from the photo it is an interesting and novel installation. The end of the pipe (in red) goes straight down at the end and is covered in about a cubic foot of concrete (in green) And some plastic sheeting has been used for... erm, reasons.

The landscaper who visiting suggested cutting the pipe off at the end so we can get a better look at the inside and low and behold there was about 2 inches of water sitting in the bottom of the pipe. It is unclear as yet as to whether it connects to something sensible or the bozos who did it thought they were creating a soakaway. He also commented that he wasn't sure what is going on and thought they might have 'blagged it' and then just covered it up. He's going to come back today with a small SDS breaker so we can gently break up the concrete lump and see what's below.

Does it drain into something other than the chalky sub ground? We'll see...

Whatever we find I am going to get the perforated pipe out of the ground since it's not working properly anyway. As expected the landscaping firm I met yesterday suggested installing ACO drains. Something that should have been contemplated from the beginning. But I suspect that like much of the work that was done by the same company that did the drainage the landscaping work was largely performative. That is, they wanted to be seen to getting work done so they'd be paid but after a period of time it became clear that the work they did was of a very poor standard