What footing changes needed for sand?
I’m new to the experience of additions and foundations, so what we hit
while digging for the new footings for our addition surprised me.
We’re adding on an addition which will have 9′ deep basement to our existing house, built in the early ’50’s, which has a standard depth basement.
So we’re digging pretty deep, past the existing home’s footings, about 4′ deeper, maybe 12′ deep total.
Clay, clay, more clay. Pure clay. Such pure clay, there aren’t even any stones/rocks/pebbles in it, just pure, pure, clay. Clay everywhere.
Until we reach the corner where the addition will join the existing house. Sand. Black sand. Looks like an ancient bank of a stream.
Dry sand, but the pocket seems deep. Just a pocket of black sand in the corner of the excavation, sort of rising up in the corner about
two feet above the floor of the excavation covering a triangular area in the corner of about 10′ along the new side wall, 6′ along the existing wall.
So again, I’m new to this. Is this anything to worry about? Are we going to have future settling problems in that corner? What happens when the drain tiles around the footings bring moisture to this currently dry pocket of sand? It is dry, just a pocket of sand capped by 10+ feet of pure clay. What would you do in this situation? And again, it’s in the corner of the excavation, where the new basement wall will butt up against the existing basement’s wall and footing. The rest of the excavation’s floor seems to be solid clay (it’s holding up under the weight of a good-sized backhoe anyway).
Our contractor is saying wider footings with rebar (and that’s the first time I heard footings aren’t regularly poured with rebar around here…I thought that was a given). I’ve asked him to add rebar to all of the footings, not just in the affected area. But will that do it? Anything we should do to the pour above the footings? To the ‘joint’ between the house and the addition? Any special concrete mix/blend?
This is in the Chicago area, so we are subject to freezing and get plenty of rain.
Thanks in advance for any help you can offer.
In a different vein: anyone have any theories on the geology of the situation? How does a black sand beach get capped by 10+ feet of clay?