The Brew Cabin

Posted: **Thu May 27, 2021 10:39 pm**

I have just had a new soil drainage pipe installed along with a new IC inserted in the old clay drain run. By bad luck/coincidence where the trench was dug for the new soil pipe is exactly where I need to put a small retaining wall for a new raised patio.

The trench was dug down to about 600mm, with gravel around the new plastic drain and then backfilled with the clay/rock that was excavated. It hasn't been compacted since backfilling

I now need to dig the footings (plan was to dig 300mm deep) right on top of the backfilled trench (about a 2-3m run), and I imagine this is not going to be as solid/compact as it was before. Is there anything in particular I should do to address this? I was thinking maybe:

Compact ground with whacker plate before digging footings?
Compact base of footings after digging with whacker plate, before laying concrete?
Compact Type 1 in base of footings?
Dig deeper/concrete more than the standard 150mm?
Put rebar in the concrete?
All of the above?

Appreciate any advice

Posted: **Tue Jun 08, 2021 11:16 am**

If you'd used a clayware or concrete pipe, this would be much simpler to resolve, but plasticware is far trickier because it distorts when loaded, so attempting to get a good degree of compaction in the backfill by using, say, a rampactor, is probably not a good idea.

So: that leaves us with a situation where we have toe xpect there to be some settlement, and therefore we have to provide for this, by building a footing that bridges the excavation and is sufficiently strong to carry the weight of the wall.

Start by compacting the backfill *carefully*. I would even consider replacing the backfill with clean,10mm angular gravel, because that gives reasonably good settlement/compaction with minimal effort.

The foundation will need to span the drainage trench and I'd look for 300mm spread to each side, if at all possible.

Double 1200 gauge DPM benath the concrete, and T20 bars running transversely at 300mm centres, top and bottom, min 50mm cover. There's a good argument to be made to actually build a 'cage' to carry the T20s, but that may be OTT for a DIY gig.

C30 concrete, 75-100mm slump, min 20mm depth, vibrated in place.

That's about as much as you can realistically do, but it should be adequate.

The Brew Cabin

Footings on disturbed ground

Re: Footings on disturbed ground