Retaining wall- preventing water ingress

[Jason Livesey]

Hi, I’m just in the early planning stages of a self build annex and one side of the building will be built into an are that will act as a retaining wall.

what’s the normal way that the wall is built to hold back the soil and to prevent any moisture penetration.

it’s a single storey annex and the sheight of the soil/ground is around 1.2m above finished floor level.

I’ve attached a pic

[Brickie]

1.2m is a pretty substantial load to be retaining-imagine how much extra force will be exerted when it’s been raining for a couple of days. 
It’s something your architect and/or SE needs to design really. 
I’ve built houses where the wall is retaining far less (<500mm) we lapped membrane from the dpc at FFL up the back of the wall& back in to a bed joint 150mm min above ground level,with an additional dpc at that level which was lapped down to as the ground level fell away to meet with the dpc at FFL. Absolute PITA to be honest. No cavity ties in that zone too so not an ideal solution I’d say,but I didn’t spec it. 

[Mr Punter]

If you have the space you may be better building a retaining wall a metre away from the structure.

[Conor]

You'll l need a reinforced concrete wall, covered with a tanking membrane, protective sheeting, backfilled with clean stones and a pwrimoter drain at the bottom. Cavity wall isn't a suitable retaining structure.

[Wumpus]

I agree that a structural engineer should take a look at this.

There are different systems possible, waterproof concrete is a different approach to traditional tanking and can work well with the steel needed to reinforce the structure. I would definitely plan on something to take the water away such as a land drain. Some projects will use two methods to be absolutely sure that water does not penetrate - for example waterproof concrete AND a membrane on the inside out outside. You may need to think about any breaks in the concrete pours if the wall is long, and how to make the joins watertight.

[Wumpus]

Also, is 1.2m the exposed height of the wall, or including foundations? The structural design should take into account the design for the foundations as they will likely need to tie together.

[Simplysimon]

is the soil level as per drawing or on a slope? if it's level i'd suggest digging further back and putting gabions in with a french drain in front between them and the cavity wall

[Temp]

Needs proper tanking as you would for a basement in my opinion.