I have a walkout basement as the house is built into a slope. I need to retain the earth between one side of the basement and an existing retaining wall on the boundary, approx 2.5m height of earth.

My structural engineer has come up with a design which extends the basement slab and one of the basement retaining walls to the boundary. It just doesn't look quite right to me.

My concerns are mainly:

• rain water splash back from the extended slab, particularly as there is a door threshold and external insulation/render with no ventilation cavity.
• thermal bridging (not too concerned as just one corner)

This is the plan view:

This is the section view

I don't want to just say "I don't like it" because maybe i'm wrong and this looks fine to others. I will also get the view of the builder. 

I'm just wondering what the general thoughts are as to whether it is a detail that will cause issues in the longer term and whether there are any ideas for improvement that I could take back to my SE?

For example, the cowboy developer next door is retaining the same earth with sheet piles bashed in using a digger bucket. Not an approach I will take but does show that there may be other approaches.

 

 

Like my foundation slab, that on three sides has retaining walls, your slab is part of the strength of the retaining wall (although not shown i suspect there is rebar and mesh that connects around the bottom  corner of the new wall and into the slab).

So, the slab has to be there AFAI can see. Your final finish will need to have a gradient on it anyway and you can make that gradient be away from the door opening.

Render should not go all the way to the ground to avoid splash back

There is rebar yes. I don't doubt all that is necessary for this concept where the 2m section of retaining wall beyond the edge of the basment is structurally integrated, and it may be the best solution, but it can also be done by freestanding sheet piles for example, so I just want to get a feel for any other options.

 

For example I could have a separate retaining wall for that 2m wide section using something like this: