Basement and Garage Backfill: Gravel vs. Sandy Fill
Hi All,
I’ve run into an issue with my excavators that has me puzzled. Until this point they have undoubtedly been accommodating and done a fantastic job with the complex build that we have going on. I am at the point where the basement and garage are backfilled and compacted. I arrived to the site to find that they backfilled the basement and garage to grade, and did not put the gravel that I expected to put under my poly and insulation. Given that we are in a unique situation I am inclined to ask for your opinions before a costly and annoying request to removed compacted fill and replace with gravel.
Our home is built in central Massachusetts (Zone 5) in an area with a high water table (1 ½’ down) and on a slope with a walkout basement. To make the lot buildable, we have brought in 3,000 yards of well draining sandy fill (yes three thousand). Plus the backfill and the septic. I have included a diagram of what is happening. My excavator states that since we are built on well-draining fill with a full perimeter drain that discharges twice and with all gutters run into the perimeter drain he does not see a reason to bring in gravel at a higher cost when the fill is already well draining and we are above the water and have gone to such extremes to avoid water issues.
My question is, Should I make them remove the fill and put in the gravel or are they correct not to sweat the added expense of the gravel and making them remove it?
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Replies
How much distance from water to slab? Sand won't capillary lift more than about 1 meter (but this is highly composition dependent - might be worth a quick test).
Is radon a concern? Mitigation usually requires the under-slab airflow of coarse clean gravel.
I'd have enough concerns about building on fill without adding lots of water to the soil just below the footings. Dry soil is strong and stable. Mud isn't.
https://www.jlconline.com/how-to/foundations/connecting-gutter-downspouts-to-footing-drains_o
When I was a builder on Nantucket, which is a sand dune 30 miles off the coast, and on other sandy sites, I have not worried much about water infiltration because sand drains freely, just like gravel. Sand and gravel do have different structural bearing capacities, so you should be sure that the sand or sandy fill can support your house. But it is not likely to be a problem. I would add a passive radon mitigation system.
This is sounding like good news. We now have several feet (3 or 4?) between where our footings land and where the water table lies. The fill has all been compacted in 4-6" lifts. We have had a week of good heavy rain AFTER the watering they already did to the basement and garage. Compaction and watering is most definitely no longer an issue. My concern was long term water mitigation and whether or not the gravel was absolutely necessary. With lumber and sheet good going the way they did we are already stretching the budget and having to remove that fill to replace with gravel would have been a question of this year or next year framing and completing the house.
Charley,
I'm hoping that it's just a misunderstanding of your description and your sketch, but what worried me more than the absence of the gravel layer is the lack of a separate solid pipe for the downspouts, and that your slab appears not to be well above the level your footings on the high side.
A course granular layer (no fines) is used as both a capillary break and as part of a radon depressurization layer (if required by code). Has your inspector seen this detail yet?
You mentioned that rain gutters discharge into the perimeter drains. Hopefully the rain leaders go into solid pipe that joins into the storm drain DOWN STREAM of where your perimeter drains join to the storm.
During a heavy rain the last thing you want is your rain leaders adding a surcharge of water to your footings.
Malcolm,
Fortunately I believe that is a misunderstanding of my poor artwork. There are solid pipes that come down from and terminate into the perforated perimeter drain. The perimeter drain is perforated 4" PVC covered with around 6" crushed stone and all wrapped in filter fabric. The perimeter drains are sent out to the back of the property at both of the back corners of the house to daylight. The drainage pipe wraps the house and garage 360 degrees.
Additionally, the slab will be poured to a level above the footing. 4" slab with 2" within footings and 2" over footing I believe. I could be wrong on that. I do know that it will be above the footings.
Looks like it would be easy to fix the "dump extra water on the footings" issue. Just add more solid piping to get the roof water further downstream.
"There are solid pipes that come down from and terminate into the perforated perimeter drain."
Just to be clear, in case it isn't, this is not what you want IF those vertical pipes are for gutters. If they're just clean-outs then that's fine. You don't want to send water from the roof straight to that bed of rock surrounding your foundation.
But as Jon says, you can just add more SOLID pipe to connect to, or use a more surficial system.
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Well, that is not great news. We put the gutters into those perimeter drains. Am I understanding correctly that the problem becomes the gutters filling the perimeter drain with such volume that the perimeter drain is no longer able to function to properly remove the water that meets the footings? I assumed that they would evacuate the water from the gutters as well as carry the necessary groundwater without a problem. If this is not the case, I will have to work on a solution that does not include sending the gutters directly into the perimeter drain.
Charly, while sandy sites can be so forgiving that even dumping roof runoff directly into the footing drains might be ok, it's definitely not ideal. I would change it as others have noted while you still have the opportunity.
unfortunately the site has been backfilled already.
"gutters filling the perimeter drain with such volume that the perimeter drain is no longer able to function to properly remove the water that meets the footings?"
Sort of/sometimes. The pipes are full of holes so the 'first exit' if you will is down into the crush bed. If the crush bed becomes 'filled' with water up to the pipe, it will drain as intended, but the goal is not to needlessly 'fill' the crush bed up with water from the roof. The footer drains work to keep rising water from rising above the pipe level.
So arguably they will function fine in that once the water rises to that point they evacuate the water, but ultimately it's not where you want the water to be--so the best practice is to get it out of there directly with solid pipe. Another issue could be clogging of the perf pipe from roof debris.
This detail won't necessarily kill your house, it's just not the ideal as Michael says.
edit: seeing your last post, I would suggest at least putting a screen filter on the gutter water downspouts, or better yet, implement a surficial system of sorts if doable.