How to restrain hardcore in a garage foundation?

[i_am_fubar]

From the NHBC website, this is their guidance for garage foundations:

I plan on using this plus some mesh as my new workshop base. Likely omitting any DPM as I'll be paining a liquid DPM over floor and up to a high DPC due to retaining walls.

My question, is how are you supposed to compact the hardcore under the slab without it spilling into or collapsing the perimeter trenches?

Also, what purpose does it serve, just reduces heave risk?

Cheers,
Fubar.

 

[Mrs Ten Names]

Is this for an ordinary domestic garage?
If so, pour your foundation concrete, build up to DPC level, hardcore for the floor, and concrete.

For the usual domestic garage, the type of foundation illustrated is over-kill, unless you have unusual ground conditions.

If you want to include mesh in the concrete slab, just do so with the above process.

 

[i_am_fubar]

It's for a workshop with a few heavy machine tools. Two walls will be holding back 600mm grade increase of highly loaded land. As such, dpc will be 600mm up on one side and one end. Then one block up the rest of it.

I am having to dig this by hand due to access and budget, so trenching is highly undesirable. Single slab pour, then walls with DPC. Then paint-on epoxy DPM & floor paint upto DPC.

However, my question remains, how to constrain the hardcore. And is it even needed. Ground is rock filled clay that takes a pickaxe to dig, even when wet.

 

[Mrs Ten Names]

I'd suggest specialist advice as to whether any hardcore is needed, given your specific circumstances.
I appreciate your budget constraints. Maybe someone with more experience and knowledge might offer their advice.

Is the build subject to Building Regs?

 

[i_am_fubar]

No regs as below 15m3. Single story permitted development. No plumbing. Insulated roof. Within a meter of house and boundary though.

 

[Mrs Ten Names]

That foundation design is for hazardous ground.
Is your ground conditions 'hazardous'?

Other wise normal strip trench-fill foundations and brickwork up to slab level, then hardcore and concrete slab.

Or are you avoiding diffing deeper foundations due to access problems?

Otherwise I don't know the answer to your question of retaining the hardcore. It's a conundrum.

 

[i_am_fubar]

Cool. Thanks the the input Mrs Ten.

However.... Constraining hardcore?

 

[Notch7]

Also, what purpose does it serve, just reduces heave risk?

Its because you need to dig out about 200mm to get to subsoil generally

I don’t think your proposed foundation design is suitable for a workshop, it is effectively a flat slab on top and it’s like to let water in where the walls are built on the slab edge.

If you really want a raft foundation, it would be best to build it with a rebate around the outside edge, so the outer skin of block or brickwork sits down below the slab upper face and thus preventing water ingress.

Something like this:

Two walls will be holding back 600mm grade increase of highly loaded land. As such, dpc will be 600mm up on one side and one end.

Your raft design as drawn has no damp arrangement to prevent ingress from a soil bank.

Likely omitting any DPM as I'll be paining a liquid DPM over floor and up to a high DPC due to retaining walls.

Unfortunately that’s not a robust construction detail.

Are you building a cavity wall?

 

[i_am_fubar]

So, the patch of land I've already mostly excavated is already well below grade. I've spend the last 12 inches chipping through rock, stones and clay. So I would likely be good just compacting hardcore on top and adding some trenches.

Single skin. Blockwork rendered outside and painted.

The retaining elements will have a waterproof layer, draining and gravel backfill to deal with any hydraulic pressure. Was entrusting the internal DPM for final dryness.

I'm not against adding additional DPM. And the 'simple' wall is fine with continuity as you show. But I would need to extend the DPM over 600mm up the retaining wall to hit its DPC (as it transitions from 215's to 100's. Only way to cover that is to pour the screed 600mm thick surely

But then I notice your diagram shows the red DPM line as visible inside the building. Surely it would get damaged and compromised. Or is there a trick to that?

What do you have against the liquid DPMs?

 

[Notch7]

So, the patch of land I've already mostly excavated is already well below grade. I've spend the last 12 inches chipping through rock, stones and clay. So I would likely be good just compacting hardcore on top and adding some trenches.

Single skin. Blockwork rendered outside and painted.

The retaining elements will have a waterproof layer, draining and gravel backfill to deal with any hydraulic pressure. Was entrusting the internal DPM for final dryness.

I'm not against adding additional DPM. And the 'simple' wall is fine with continuity as you show. But I would need to extend the DPM over 600mm up the retaining wall to hit its DPC (as it transitions from 215's to 100's. Only way to cover that is to pour the screed 600mm thick surely

But then I notice your diagram shows the red DPM line as visible inside the building. Surely it would get damaged and compromised. Or is there a trick to that?

What do you have against the liquid DPMs?

I’ve nothing against liquid DPMs, the problem is there is no way to make it continuous from slab and up wall.

Yes the drawing I included would need DPM going up the wall, so it would need covering

Hopefully an architect or surveyor will be along to provide some additional ideas….personally I think you are creating risk of damp inside.

I've spend the last 12 inches chipping through rock, stones and clay

It sounds like you are down to load bearing, stable ground…probably no need for additional hardcore

Single skin. Blockwork rendered outside and painted

As an option you could screw on vertical timber battens outside and then clad with cement board / rendaboard - which can then be painted, or if you like skimmed and painted if you want the joints invisible.

The benefit is you create an air cavity and thus reducing risk of damp ingress….I know render is considered waterproof but any cracks etc and you will start to get damp coming in. Using that construction the DPM could go up the outside to above the start of the battens and then the soil backfilled but you would have to be careful of stones

 

[i_am_fubar]

Unfortunately, I'm going down from an existing 3m wide workshop to a 2.64m wide workshop due to available pace so I'm desperate to preserve every cm I can. I'm hard up against the fence posts (minus 20mm to get a finger in) and 850mm from the house wall (needed for access). So a thin slurry and paint layer has it's appeal. No internal lining, so a wee bit of damp on the walls isn't the end of the world. And no worse than my current one.

Not understanding why you think a liquid DPM wouldn't be continuous up the wall? I would lay slab, build walls with a DPC, then paint DPM upto it. Hydrostatics would be minimum with drainage behind the wall.

The appeal of DPMing the whole mass has its appeal, but surely exposed DPM is still an issue on the non-retaining side?

 

[Notch7]

Not understanding why you think a liquid DPM wouldn't be continuous up the wall?

It won’t be continuous because you would have a flat slab with blockwork on top and that junction is a leak point.

 

[i_am_fubar]

I agree the joint is not the strongest. However with the mass above, I can't see it moving. With the DPC painted on slab and up wall in one go, it would be a continuous membrane? I've nabbed the below off (dare I say it) B&Q liquid DPM as an example:

If your concern is with water tracking under the block, the step rebate would work, but would still require a painted floor-wall transition, just halfway up a block instead of on a mortar joint.

How would one go about forming the rebate? Drop a sacrificial timber in and peel it out once he slab has cured?

(Also, appreciate the input Notch )

 

[i_am_fubar]

Research done. Gotta get fancy with my form work. And not make the mistake of underestimating how heavy concrete is again.

Will update the design and pop a few render up. Ta.

 

[^woody^]

Oh yes, the phenomenon of straight pencil lines in the real world.

You scrape the surface, import the hardcore and whack it, then dig the trench and batter the side.

And a two page thread to boot!