Semi basement options

[Clc136a]

Hello experienced folks. I’m a complete novice looking for opinions on the best construction for a highly insulated 175m2 of simple form.

Site is sloping downwards towards the front of the house from where it is pretty flat, hence semi/walkout basement to the frontage.

4-5m in front and parallel is a main sewer at 4-5m depth, so I think the front of the house (at least) should be piled.

In addition soil survey shows lias clay. 
A couple of piling contractors have suggested CFA retaining wall across the back although a temporary works co would like to hire me trench sheeting.

Once the footprint has been excavated I think the piling company could probably put in 8 piles and connect a ring beam.

Does it make sense to then use an insulated raft and ICF system like Jakon? Need to find a local (Glos) SE with experience?

Obviously TF doesn’t make sense in this location and I am persuaded by the thermal mass argument with exterior insulation.

it has been suggested to use old fashioned shuttered insitu concrete and attach EPS afterward?

Grateful for any comments, thanks

 

[tonyshouse]

I built a full basement and so recommend that you tell your engineer what you want, presumably an economic solution 

I don’t see any wisdom in asking contractors what to do, you decide what you want then tell them 

 

piling the foundations sounds ok again get a design, but then for me and I am only a builder not an engineer you can build your own retaining walls off those foundations.

 

there is no way I would build myself a home with different foundations under the front than under the back. 

 

 

[Clc136a]

Thanks for your comments.

I have spoken to a few local SEs but none appear to have experience with insulated rafts or ICF so far…

Regarding retaining wall, the soil survey did suggest slip planes are a possibility and CFA retaining wall would pin that, 

plus it eases waterproofing issues and it would mean house piles are only taking vertical loading I think?

 

[Wumpus]

We are building a half basement into a 30 degree slope. The retaining wall is shuttered waterproof concrete and nearly 4m high.  The main house is 215mm blockwork with 200mm eps ewi. This makes the air tightness easy and a continuous insulation layer, while the concrete acts as a heat store. 
 

It has worked out very well, although any retaining wall like this is big engineering, so having a sensible structural engineer is important. Also minimising the dig out and any cart away could be a big design objective? We had ours with a big foot under the block and beam deck fro minimise the dig, and just as well as it was pecking out bedrock at the back.

Edited December 20, 2021 by Wumpus

[saveasteading]

4 hours ago, Clc136a said:

Grateful for any comments

Sounds an expensive solution. A piling rig costs thousands to turn up and then so much per pile.

A sketch would help a lot.

 

4 hours ago, tonyshouse said:

I don’t see any wisdom in asking contractors what to do, you decide what you want then tell them 

 

Ouch. You will possibly pay dearly for the privilege of not listening to the people who do the work.

Not all 'contractors' use shovels you know.

[Clc136a]

Yes, piling doesn’t come cheap but once you’ve got a slope and suspect ground it’s safe and CFA is quick I’ve been told.

You can get an indication of the slope from the attached (not wedded to the layout).

@Wumpus what temporary works did you need to excavate and build retaining wall?
In our case the ground doesn’t flatten until 62m so a way above the top of our ground floor retaining wall 58m.

@tonyshouse thinking by talking to different specialist contractors you home in on best way of doing things.

 Thanks for your ideas

[saveasteading]

I recommend considering sleeper walls up to the slab level  and planks (or beam and block) over.

This is then a simple brickies' job. The blocks will probably be laid flat for strength and stability, on normal trench foundations. you can then over clad in stone or whatever.

There you are, I've saved you about £10k. 

 

(I am not guessing as have done this several times, when the original SE had gone for piling.)

 

Your SE will also consider ground heave because of the excavated area, but that is easy enough too.

[Wumpus]

5 hours ago, Clc136a said:

@Wumpus what temporary works did you need to excavate and build retaining wall?
In our case the ground doesn’t flatten until 62m so a way above the top of our ground floor retaining wall 58m.

Nothing too complicated, just a steep slope dug out behind and some heras fencing so nobody fell off the edge. We were lucky that the ground conditions were pretty much self-supporting. Our cost overrun was pecking out the bedrock to be able to dig far enough back, it was higher than the ground conditions survey suggested.

 

it’s a relief to have it all backfilled now.

[Wumpus]

2 hours ago, saveasteading said:

I recommend considering sleeper walls up to the slab level  and planks (or beam and block) over.

This is then a simple brickies' job. The blocks will probably be laid flat for strength and stability, on normal trench foundations. you can then over clad in stone or whatever.

This is what we did. It worked well for our ground conditions, just depends on whether your SE can see enough ground support for the trench foundations under the clay.

Our main contractor worked out the plan with the SE and came up with some good savings and simplification between them. The original was a lot more concrete poured as a slab.

[Clc136a]

15 hours ago, Wumpus said:

it’s a relief to have it all backfilled now

A few hundred meters away, a few years ago, a self builder (and by trade) excavated about 6m at about 80 degrees! With gardens above! 

FORTUNATELY, it was a dry summer and it stayed a looming bank whilst the triple skin wall was built and water proofed.

Professional advisors will (I think rightly) not let you do this any more.