Footings on chalk - Footings on chalk

[buk]

I'm building a patio (4.2M x 3M), that will be surrounded on 3 sides by a crushed slate over geotextile "succulents" area.

To form an edge between the patio and the slate, I'm going to cast a concrete footing around the 3 sides. With a view to the future, and the possibility that I might afford to cover the new patio with a conservatory at some point, I am considering making this footing suitable to carry a conservatory without futher modification.

The general advice regarding footings is for them to be 450mm deep, and 450mm wide (for double skinned cavity wall). However, the Building regs on this page (table 12) indicate that on "rock" subsoil, where the condition of the subsoil is "Not inferior to sandstone, limestone or firm chalk.", the width of the footing need only be the same as the wall itself (300mm) and the depth need only be 150mm.

Another test described elsewhere for deciding how deep to go is whether it is possible to drive a 50x50 stake into the subsoil more than 150mm. On my subsoil this is impossible. I am having to obtain some short lengths of rebar to use as marker stakes. Another indication of the solidity of the ground is that I had to move a 450mm long rotary washing line stake when starting work and it took me nearly 3 hours to extract it using a 10lb sledgehammer.

As there is no way to get a mechanical digger to the rear of my property, I'm considering casting 300mm wide by 150mm deep footings directly on top of the subsoil once I've chipped out enough of it with a pick axe to level it.

Is this going to be sufficient to support a uPVC/miniwall conservatory if I ever get around to building one?

Sorry for the long buildup to the question. Buk. :p

[buk]

I guess I asked the wrong question? So, I'll try for one that requires a less definitive answer.

Have, or would, any of the professionals hereabouts consider using the narrow, shallow 'on rock' footings specification as described on the linked building regs document under the specific circumstances of the need for hand dug trenches in extremely hard, virgin chalk?

I chased out a 4 meter channel today, 1 shovel wide, 50mm deep at the house wall (300 below dpc) tapering to 200mm at 4m from the house. It took 4 hours of constant chipping with a english pattern mattock specially bought for the purpose. It killed my back, my hands (especially when you catch a buried flint the size of a large spud or bigger!) and elbows.

The prospect of going down another 300mm in this stuff for the total of 27 meters of footing (3+3+4 for the patio & 4+4+9 for a retaining wall around the succulents area), I need to cast is daunting to say the least. Any positive indication that it is unnecessary would be be great. Conversely, if the concensus is that deeper is really necessary would be equally valuable information.

Cheers, buk.

[Tony McC]

Not the wrong question - you were just unlucky enough to post while I was on me travels.

I can't see owt wrong with what you describe. It's fairly standard practice where there is a shallow bedrock.

[buk]

That's a relief. Thankyou.

Another question. Where (as in what type of company) do I go to get soap oil?

Thanks again.

[buk]

Can I re-use the chalk chips I'm digging out of the footings as hardcore for compacting under the slate?

Seems crazy to pay to have the chlk chips taken away by the same guy that would sell me hardcore

Cheers, Buk.

[Tony McC]

Soap oil, or Mould Release Oil as it is more correctly known, is available via the better BMs. Don't be surprised if the useless twonks at TP/Jewson et al. have no idea what you're on about ask to speak to the manager and you should get some sense out of him/her. A civils merchant, such as CoooperClarke, UGS, Burdens, etc., is a better bet, as they are used to dealing with formwork.

Not sure what you mean about using chalk chips under the slate. Why would you want chalk underneath slate?

[buk]

The overall area at the back of the house that is to be slated is 9m x 4m. whilst the grade is level adjacent to the house wall, it slopes up (~200mm) over the 4m at one end, and down (~250mm) over the 4m at the other.

Once I've cast the footings and built the retaining walls, I want to level the whole area before laying the terram & slate. I'm considering re-using the chalk I chip out for the footings, and from the high part, to infill the low part, in place of scalpings or similar.

I just wanted reassurance that once compacted, the chalk chips aren't going to turn into chalk soup during the first rain?

Cheers, Buk.

[Tony McC]

Aaah - so you want to use slate as a patio material. I wasn't thinking straight and just assumed you were talking about roofing slate.

Now that I understand you plan to use slate for a patio, the next question would be whether you mean slate mulch or slate flagstones.

Regardless, as it's only a patio, there shouldn't be any great problem re-using the excavated chalk as a fill material, as long as it is well and truly compacted.