Filling a well below a foundation

[mjr6550]

I was recently hired to remedy a foundation failure. An approximately 2-foot wide by 10 foot long foundation wall (about 6-7 feet high) settled suddenly about 1-foot at one end and about 3 feet at the other end. The soil below the FW was weakened by the recent rains and sheared off into an adjacent hand dug well. The well is located in the basement of an old farmhouse. The foundation wall supported one side of a floor slab for a sunporch and one end supported a beam supporting floor joists. Temporary shoring is in place. The well is about 4 1/2 feet in diameter and is 26 feet deep from the basement floor level (about 1/2 of the circumference of the well wall extends up to the floor slab level for support of the slab and the soil behind the well wall. The well is dry. The area of disturbed soil is approximately 8 feet by 8 feet with the well located at one corner of this area. Sorry for the long post, but I want to accurately describe conditions.
My thinking at this point is to fill the well with flowable fill up to approximately two feet below the floor slab level to stabilize conditions and to allow the contractor to safety demolish the stone FW and an adjacent stone wall that was installed to limit access to the well (that wall is hanging partially above the well). The contractor would like to do some demo work and dump the stone into the well. Besides safety issues I have concerns about very large pieces of the wall falling into the well and blocking the flowable fill. I don't see a problem with dropping chunks of stone into the well (say up to about 1 c.f.), other that safety issues. Above the flowable fill I would have the contractor excavate loose soil adjacent to the well and replace with compacted stone. Then place a reinforced concrete slab over the stone. Several stud walls would be constructed on the slab to support two edges of the sunporch floor slab and the floor joists.
I don't think there is a problem dropping the flowable fill into the well from 26 feet high, but I have never worked with flowable fill. Also, there is good access to the well from nearby bulkhead doors, but getting a concrete truck close to the doors is a problem. I certainly do not want to get involved with compacting any other type of fill. My only other thought was whether the well could be filled with large uniformly graded stone, but I suspect some consolidation could occur. My other thought is that since the well walls are very intact from approximately two feet below the basement floor to the bottom they should provide good support for the reinforced floor slab so any uncompacted granular fill should be fine to assure lateral stability of the walls. Any input would be appreciated.

 

[ishvaaag]

Normally most would think of filling it with one or another kind of concrete. Stands the issue of the new weight holding from the cylindrical walls of the well since one might think of the bottom be somewhat disturbed.

The ongoing failure from rains is quite big and one should look also away from the well to see why that has happened. You seem at it, but insist on the thing.

A moderate compaction of the bottom could be benefitial but may be not very feasible, nor maybe necessary. Ramming with something long enough hanging from something or whatever, or some ramming machine; more thinking in investigating the bottom than compaction if doesn't seem necessary.

Make tests if possible, this will give you clue of what at hand.

 

[msquared48]

Why don't you fill it up with soil and cast a slab over the top?

Mike McCann
MMC Engineering

 

[aeoliantexan]

I would fill it with flowable fill as you have proposed. I would not be concerned about the condition of the bottom, as there should be adequate skin friction between the FF and the well walls and between the walls and the soil to support the FF itself and the light foundation loads. I would not fill the well with big stones and expect the FF to find its way through the gaps, but you could drop isolated stones into the FF as you place it. You need a continuous column of FF to exert fluid pressure against the walls as it sets.