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Soil compaction issue

ddubx6 | Posted in General Questions on

I recently had my foundation sealed. The dirt was removed approximately 3 to 4 feet away and 6 to 7 feet down to the bottom of the footing.After the sealing, drain tile, stone, etc. was done the area was refilled with a sandy fill with few small stones. My question is that I saw no compaction during back filling and I am concerned about settling. After¬† a few heavy rains the level dropped a few inches. The level of fill was left 12″ low on purpose because I am planning on putting 8″ of compacted stone and then a 4″ concrete walkway around the house. Am I screwed? Is there a way to compact this now with say a small car driving over it multiple times or would it have to be dug out and redone in small compacted lifts? The walkway can be postponed, will waiting til next year help? Am I overthinking this, will the relatively light walkway be okay with a good compacted stone base? I am in the northeast so freezing temps too. Any thoughts or ideas are welcome.¬†
Thanks
Darren Williams
P.S. on the upside my basement is dry now!

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Replies

  1. onslow | | #1

    Darren,

    The settling you have seen so far would be due to the soil particles snuggling up to each other as the air spaces collapse. Heavy rains will cause the soil particles to collapse the air spaces, but the down side comes if the soil is not being wetted equally across the full depth. The compaction process will not be even or complete throughout. Dampening the soil and compacting in lifts of 8 -12" is best practice for a lot of reasons. The main one being it allows for things like walkways to be placed immediately.

    Sorry to ask this question, but was fabric put down over the stone around the drain pipe? I ask because, if not, the potential settling you could experience over time may cause you issues. It also inexcusable. If no fabric was used over the stone/drain pipe, there can be long term problems with the soil infiltrating the stone bed in successive rains and seasons ahead. The migration of soil into the stone spaces will eventually choke out the flow path the stone provides to collect water for the pipe. Given enough rain and time, this can also potentially put enough sediment into the pipe to clog it. The surface of the soil will also collapse a bit as well. This is a worst case scenario, but one that is possible.

    Improper compaction of foundation back fill might not be a big deal if you intend to have plantings around the house. If the edge of your concrete walkway actually starts about 3-4 feet away from the foundation the plants will be more forgiving of soil collapse than a walkway. At 3-4 feet out, the soil under the walkway won't have been disturbed.

    However, you seem to have described putting the walk way tight to the foundation. You will most definitely want to wait til next year and if rain is not frequent in your area, I would cautiously suggest watering the fill, especially on the sides to ensure saturation. Driving anything over it or using a plate compactor would be largely pointless as the compacting effect diminishes with soil depth, hence the "lifts" part of doing compaction. The coming freeze thaw cycling of a winter will further help tighten up the fill. I think digging things up and compacting in lifts would be expensive and frustrating. That I leave to your own judgment.

    If pressed to get the walkway in next year rather than two years, I would suggest washed stone under the walkway with poly over that, just like a basement slab. Future cycling of the soil moisture content will then be buffered by the stone gaps and help keep frost heaving down. If you can also set the walkway to not need absolute precision relative to steps or the belt line of the house, you will have built in a bit of wiggle room for the walk to drift. Consider long rebar runs and more of it than a regular walk would require to keep the sections rigid. Tie the sections by letting the rebar extend past the form and into the next section.

    I have seen walks and driveways tip toward and away from foundations when soil compaction was not done or done poorly. Mud jacking can fix some of these sins, but best not to provide conditions that cause them.

    The most depressing thing I have to share is that despite having my own foundation back fill compacted in lifts, I still had some settling after four years. Not much and at least it was even across the fill area, but still enough to have thrown off a cast step next to a threshold. Being lazy about finishing my outside tasks tossed me a break on this one occasion.

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