We are building a one story addition on our house in Northern California. It will have a crawl space with 16" wide perimeter footings and an 8" stem wall. The bottom of the footings are approx 24" below grade.
Everything was going well digging for the footings until we hit an old abandoned redwood septic tank about 2' below grade. (The house was converted to sewer in the early 60's.) The tank is approx 4' x 11' and 5' deep. Unfortunately, one of the exterior walls (and footings) runs right over the length of the tank.
Any suggestions on how to fill the tank/hole to provide good support for the foundation? One thought would be to fill in the hole with native soil (clayey) and compact in 6" layers. Or should we use base rock?
I'm not an excavation expert, but I would think you'd want to use base rock compacted in layers to fill in the area in this area anyway it would be more economical than clean fill dirt. Check with local concrete contractors as to what the code is there in your area. California has a more strict code than most other areas of the country due to the earthquakes you folks are prone to out there.
All you need is ignorance and confidence and success is assured. - Mark Twain
Call a ready-mix supplier and see if they sell "flow-able fill". It's a recipe of fly-ash, portland cement and various other ingredients. It'll completely fill all voids, doesn't shrink and will develop compressive strengths on par with undisturbed soils.