I'm GCing my own new residential build later this year. I'm going to hire a geotechnical firm to do soil analysis before construction starts but for now, I want to get a feel for what is considered best practice for a footing-level drain tile. The residential construction world seems to just prescribe the usual gravel and filter fabric burrito-wrapped PVC pipe. But I've seen a lot of information from the geo/civil engineering world that the filter fabric can end up plugging up with fines and just ends delaying the failure of the system by a few years rather than providing a true long-term solution. Ideally, I'd like a water management system that lasts indefinitely, or at least 50 years so that when it needs to be replaced, I'll be dead and it's someone else's problem.
Here's the essential info:
- SE Michigan build, frost line is roughly 3 feet down
- water table currently unknown but almost certainly above the footings
- ICF basement wall that goes approx 10.5 feet below grade. Bottom of the footings will be somewhere between 11' and 11.5' below grade.
- soil analysis not yet done but the USDA soil survey indicates that the build site is likely a loam/expansive clay mix
- chloride ion concentrations in the soil are likely to be high, so I definitely want to minimize water exposure to the footings, slab and ICF wall core to protect the concrete and rebar
- I am planning to implement best practices here including silica fines in the below-ground concrete mix, waterproofing membrane and drainage mat around the wall, putting several layers of PE sheet film under/around the footing and floor slab, drain tile on the outside and inside of the footings going to a sump pump, ~4' wide at grade concrete shingle around the house to divert precipitation away from the basement wall and a surface level french drain that intercepts surface flow to the storm drain.
- I am also planning to run water supply PEX line out to the drain tile drainpipe so that I can flush fines out of it periodically. While that doesn't address my primary concern of blockage out at the fabric layer, hopefully it does allow some flexibility in the drain tile design to include versions that allow a low rate of fine migration into the drainpipe.
I'm nervous about the idea of clay fines clogging up the filter fabric and have seen that a lot of current practice in civil engineering projects is to skip the fabric entirely and to simply use the proper fill around the drainpipe. What is the current best practice for this in clay-heavy soils? I was thinking of coarse gravel around the drainpipe that transitions to fine rock and then sand above and below the drainpipe but have no idea if that is a reasonable solution. I also need to be mindful that an inspector may require the use of some sort of filter membrane, so the relative placement of such a membrane in the drainage assembly is also important.
- Will the placement of graded diameter fill around the drainpipe keep clay fine migration down to the level that a layer of filter fabric embedded somewhere in the assembly will last for several decades before clogging up? Or is it better to just forgo the fabric and to occasionally flush out the fines that make it into the drainpipe?
- Does it make sense to pour the footings on a layer of crushed rock to help keep water level below the bottom of the footings?
- Does it make sense to place the drain tile slightly below the bottom of the footings so that the water level is always kept below them? (keeping the drain tile outside of the load-bearing soil, of course) While there's the obvious advantage of always keeping the water level below the footings, I worry that drying out part of the soil that the footings rest on could potentially destabilize it and cause some sort of foundation settling.
- As a related question, are there any chemical additives that can be added to chloride-rich soils to bind up some of the chloride in them in a chemically inert form or to at least lower the Cl ion migration rate through the soil?
Thanks in advance!
Here's the essential info:
- SE Michigan build, frost line is roughly 3 feet down
- water table currently unknown but almost certainly above the footings
- ICF basement wall that goes approx 10.5 feet below grade. Bottom of the footings will be somewhere between 11' and 11.5' below grade.
- soil analysis not yet done but the USDA soil survey indicates that the build site is likely a loam/expansive clay mix
- chloride ion concentrations in the soil are likely to be high, so I definitely want to minimize water exposure to the footings, slab and ICF wall core to protect the concrete and rebar
- I am planning to implement best practices here including silica fines in the below-ground concrete mix, waterproofing membrane and drainage mat around the wall, putting several layers of PE sheet film under/around the footing and floor slab, drain tile on the outside and inside of the footings going to a sump pump, ~4' wide at grade concrete shingle around the house to divert precipitation away from the basement wall and a surface level french drain that intercepts surface flow to the storm drain.
- I am also planning to run water supply PEX line out to the drain tile drainpipe so that I can flush fines out of it periodically. While that doesn't address my primary concern of blockage out at the fabric layer, hopefully it does allow some flexibility in the drain tile design to include versions that allow a low rate of fine migration into the drainpipe.
I'm nervous about the idea of clay fines clogging up the filter fabric and have seen that a lot of current practice in civil engineering projects is to skip the fabric entirely and to simply use the proper fill around the drainpipe. What is the current best practice for this in clay-heavy soils? I was thinking of coarse gravel around the drainpipe that transitions to fine rock and then sand above and below the drainpipe but have no idea if that is a reasonable solution. I also need to be mindful that an inspector may require the use of some sort of filter membrane, so the relative placement of such a membrane in the drainage assembly is also important.
- Will the placement of graded diameter fill around the drainpipe keep clay fine migration down to the level that a layer of filter fabric embedded somewhere in the assembly will last for several decades before clogging up? Or is it better to just forgo the fabric and to occasionally flush out the fines that make it into the drainpipe?
- Does it make sense to pour the footings on a layer of crushed rock to help keep water level below the bottom of the footings?
- Does it make sense to place the drain tile slightly below the bottom of the footings so that the water level is always kept below them? (keeping the drain tile outside of the load-bearing soil, of course) While there's the obvious advantage of always keeping the water level below the footings, I worry that drying out part of the soil that the footings rest on could potentially destabilize it and cause some sort of foundation settling.
- As a related question, are there any chemical additives that can be added to chloride-rich soils to bind up some of the chloride in them in a chemically inert form or to at least lower the Cl ion migration rate through the soil?
Thanks in advance!
