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Posted: Tue Oct 04, 2011 4:41 pm
by DNgroundworks
I must admit that ive never seen any impermeable membrane on the back of a retaining wall, but that said retaining walls are not something most lads on here do regularly anyway, but i have seen the "black jack" used for a number of things, ie painted on to the bottom of structural timber posts, some blockwork somewhere, a bridge deck - thats if its all the same stuff.
Posted: Tue Oct 04, 2011 4:46 pm
by DNgroundworks
Also i did a lift pit with a civils firm last year, that had obvious pressure from water from the ground surrounding it, no membrane used, no black jack, just back filled. I wouldnt bother with the membrane in your case, just full fill behind with clean 20mm gravel, all wrapped in terram or similar, 80mm land drain, in your case i probably wouldnt bother with the weepholes. The drainage will take care of the water, no ugly weep holes to render round and no green slime trails down the nice paint on your render 
Posted: Tue Oct 04, 2011 5:18 pm
by henpecked
I wouldn't recommend a DPC in a retaining wall as ts a 'slip joint' we had a long discussion on the merits of solid blues v's DPC and the blues came out on top, but DCP wins in the 'cheapness' stakes.
You would have to remember the wall it holding back the soil to a certain extent and a DPC joint will definitely be a weak spot in the build.
Posted: Tue Oct 04, 2011 7:29 pm
by glenne_n
Thanks guys, you've been bricks [I think thats how you say it 
], here's the deal so far:out goes the DPC and rebar, in stays the back fill gravel, land drainage, geo textile. The membrane behind the wall, black jack and weep holes are still with the jury.
Just one more question, I have some Expamet from another project, any mileage in using this with the render on the wall front.
Posted: Tue Oct 04, 2011 9:02 pm
by lutonlagerlout
expamet is a trade name
you mean expanded metal lath
and yes this will help on the front of the wall,if fixed correctly
DNgroundworks weepholes are very important dude
if water fills up behind a wall the same pressure is exerted by 50 mm of water as a mile (S.engineer told me this) creating a rotational force
this is another reason why the wall should be tied into the foundation,and also why a foundation is better 1000mm wide and 300mm deep than visa versa
to stop rotation
cheers LLL
Posted: Wed Oct 05, 2011 4:52 pm
by DNgroundworks
How would water "collect" though? Surely the land drain and shingle wrapped in terram will take care of the water pressure, its not as if the land drain will block?
Just my line of thinking.......
Posted: Wed Oct 05, 2011 7:19 pm
by glenne_n
I guess it's belt and braces to the land drainage. I'll install the weep holes anyway using 10-15mm pvc or copper tubing.It's easy to install and vented low at the front of the wall below the rendering should do the trick.
Posted: Wed Oct 05, 2011 9:07 pm
by GB_Groundworks
as a mile? a mile of what? you can have weep holes jus start your render above them, ive used 1200 as a membrane on tanking jobs with the ebb box stuff infront of it(earth side) then clean stone and drains. if you are rendering the retaining wall then you need to keep the damp of it.
Posted: Wed Oct 05, 2011 9:22 pm
by lutonlagerlout
if for any reason the land drain is blocked and the water can fill up behind the wall (quite easy with clay and engineering bricks" then the force exerted is the same regardless of the depth of water its the height of water that exerts the pressure
this is what the engineer told me and i have seen lots of retaining walls fail in luton because of no weepholes
LLL
Posted: Wed Oct 05, 2011 9:46 pm
by GB_Groundworks
yeah on walls always have low level weep holes obviously not on tanking jobs. even terram wrapped drains can block and as lll says water kills more walls than anything. hydraulic pressure
Posted: Wed Oct 05, 2011 9:59 pm
by glenne_n
Don't know what an ebb box stuff is [I'm just a DIYer]. So according to LLL theory, it must be best to have the weep holes and land drainage as low is possible [to keep the height of the standing water to a minimum]. Also having trouble finding anyone that stocks 40mm land drain [does it exist]. 60mm to 100mm seems well over the top for my little project [just 20M of retaining wall/raised bed planters].
Posted: Wed Oct 05, 2011 10:10 pm
by glenne_n
If you have a look at this site http://www.pavingexpert.com/featur03.htm it shows the model of wall I was working from. you can see that the weeps holes pass through the tanking. My question is what is best to seal the breach? bitumen?