Plastic membrane behind sleeper walls

My client is convinced that I should use an impermeable membrane behind a new sleeper wall (reclaimed creosoted Oak).

I have suggested using geo-textile and gravel to prevent them touching soil and to allow for drainage but I know they will prefer polythene dpm, as where this is installed behind other softwood posts in their garden it has lasted better than areas where there is no polythene.

The height of the wall is 55cm and the length 18 metres, no easy way to install other drainage. I would be concerned about water build up behind if an impermeable membrane is used and as this is hardwood it should not rot in the near future.

Asking for advice from other landscapers if you would refuse to use polythene in this situation?

Cheers,

Gareth

Hi Gareth,

What type of soil is on site? Are the sleepers fixed via posting? In my opinion, a 55cm wall wouldn't build enough water pressure behind it to cause major problems. Water will always find the easiest route and most likely drain down through sub-soil and underneath timber. By the sounds of it, the customer would like to future-proof the timber against rot.

You can use DPM on the rear of the timber, and pea gravel backfill directly behind sleepers. The pea gravel will allow a free draining area behind the wall. You could also add a length of land drain wrapped in geotextile membrane, to keep fine particles of soil out of pipe. The land drain will create an equal void behind the wall, rather than a build up of pressure on any specific point. You can also create weep holes drilled though timber at low level (approx 2nd sleeper up) this will allow any build up to simply pass through the wall, dispelling pressure. I've used 25mm DIA PVC conduit.

I've attached an example from one of my jobs using new sawn, soft wood sleepers. Any other questions or info need, let me know.

Cheers, Ben

Replies

Ben Harrison July 23, 2013 at 11:07pm

Hi Gareth,

What type of soil is on site? Are the sleepers fixed via posting? In my opinion, a 55cm wall wouldn't build enough water pressure behind it to cause major problems. Water will always find the easiest route and most likely drain down through sub-soil and underneath timber. By the sounds of it, the customer would like to future-proof the timber against rot.

You can use DPM on the rear of the timber, and pea gravel backfill directly behind sleepers. The pea gravel will allow a free draining area behind the wall. You could also add a length of land drain wrapped in geotextile membrane, to keep fine particles of soil out of pipe. The land drain will create an equal void behind the wall, rather than a build up of pressure on any specific point. You can also create weep holes drilled though timber at low level (approx 2nd sleeper up) this will allow any build up to simply pass through the wall, dispelling pressure. I've used 25mm DIA PVC conduit.

I've attached an example from one of my jobs using new sawn, soft wood sleepers. Any other questions or info need, let me know.

Cheers, Ben
David Benson July 23, 2013 at 11:08pm

i would use geo-textile but if the client insists you use plastic make shore you put some drainage holes in it with a bit of geo and gravel over the holes
Gareth Main @ Newick Landscapes July 23, 2013 at 11:20pm

Thank you both for your responses and Ben for the photo.

The sleepers are vertical (cut to size) and concreted 60cm into the ground. Site is sloping but stable, good depth of topsoil but heavy clay below.

There's not much room for putting drainage pipes behind (replacing old softwood round posts and don't want to make more work than it's already been getting the old posts out which was a nightmare even with a mini digger.).

Drainage holes in plastic every so often might be just the trick, thanks. Hopefully being Oak it shouldn't rot for years anyway, the softwood posts only lasted 10 years hence my clients concerns