Compacting Wet Clay before backfill

[MiMoCA]

Dear all,
We are in running into problems backfilling around a residential foundation. Its an older house that needed repairs and waterproofing. In order to access areas to patch, the contractor has excavated about 8" below the basement top of slab (no footing - 100 year old house). The subsoil conditions are clayey and a deluge of rain and lack of trench protection has brought water into the trench and made it impossible to fully compact the clay as it is so moist.

The first question is how crtical is it to have a good compaction on that thin (8") layer; second, how does one go about it (so far we've been leaning to filling that layer with granular B).

Your suggestions are welcome, thanks in advance!

 

[oldestguy]

Hello:

It is a little difficult to understand your situation.

Is excavation outside or inside?

Did the excavation actually undermine the existing foundation, or just go below it 8", alongside the supporting soil?

Normally one might expect the clay (supporting the foundation) to take on water and become softer, possibly resulting in future settlement. Dig far along the wall like this and an immediate slip may happen.

For backfill:

Granular backfill is preferred in wet conditions.

However, remember that saturated clay, being weaker than moist clay, will apply more pressure to the wall than what was there before. Saturated backfill is the worst you can do, since it may apply pressures as great as three times what was there before. You probably are aware you can't do much improvement to the clay backfill soil by compaction. Better no compaction.

So, please clarify the situation present.