backfill for masonary wall

[DrEvo]

Hello,

I have a basement wall (load bearing side) that has cracked 75% of it's length horizontally 5 ft from the floor and has bowed in approx. 2 inches at midspan. This has happened because of two factors, highly expansive clay backfill and freeze/thaw cycles (Buffalo, NY). Outside I dug the length of the wall down to the footing and pushed the wall back to nearly plumb with some braces. Once the wall is all the way plumb (another 1/2 inch to go) I plan on repointing with 10,000 psi tensile epoxy mortar and building five pilasters anchored to the footing.

Question: What can I use for backfill? I do not want to use the clay I excavated. I have heard all kinds of answers, none which seem very good, from concrete to compacted granite. Any ideas?

 

[Trussdoc]

Crushed stone or pea gravel might work. Add a drain pipe at the bottom, and this should keep your foundation and the clay separate and drained.

 

[DrEvo]

I guess I'm a little concerned that crushed stone might absorb rain water, freeze in the winter and then expand. The foundation already has drain tile on the inside under the basement floor. How much water will crushed stone absorb? What about crusher run? That stuff seems impervious to water.

 

[0523]

This is a typical problem in North Dakota, too, due to expansive clay soils that aren't compacted properly when backfilling foundations, allowing water to absorb into it. If this material is holding excessive moisture going into winter there's double-trouble with the frost.

The clay you took out will work, it just must be replaced with appropriate moisture and compaction, and the drainage away from the building must be positive.

 

[CarlB]

I disagree with 0523's suggestion. You should use non-frost susceptible soil (NFS) against your foundation wall. Pea gravel or crushed stone would work. If there's a drain as you mentioned the pea gravel would not get saturated, so would not expand upon freezing. NFS soil is any clean soil with less than about 5% passing a #200 siee, so a clean sand or gravel woulkd also perform adequately. You can cap the backfill, say the top 12", with a clayey or silty soil to minimize the amount of surface water that could seep through the backfill.

HTH,
Carl

 

[jheidt2543]

I agree with CarlB, but I would add one additional thing. I would install a membrane waterproofing on your block wall while you have the soil excavated. It is already cracked and if water is held against it for any time, it will leak. The free draining fill (pea gravel, crushed stone, or bank run gravel) against the wall, with the 12" clay cap at the top is definitly the way to go. The free draining fill will let any water pass straight down to the drain tile. If water is held against the wall, it will leak in the summer and freeze in the winter causing addtional stress and cracking.

Good luck.

 

[DrEvo]

A little more history... I dug this out in February, and watched it all winter. When it rains/snows heavily, I get about six inches of standing water in the ditch. It does not leak inside. It does not freeze either - maybe a little slush, but no ice. I think the heat from the basement keeps it warm enough even at 0 degrees. However, the fact that water stands in the ditch means 1. there's no real drainage and 2. gravel of any type would hold water because it can't drain. My sump pump works fine! During any kind of rain, it constantly pump out water - and the basement is dry. The clay dries out in summer - right now it's shrunk about 1" away from the basement wall. And this is all the way down to the footer!!

How about using some thick rigid insulation board on the walls, then backfilling with crusher?

 

[boo1]

I agree with the guys above you can still dig drainage ditches and install waterproofing membranes, be sure there are drains installed at footing level, sloped between 1 and 2 inches per 10 feet. Pea gravel or crushed stone should be used to back fill areound the peremiter, use a ground fabric to keep the soils seperated. Additionally you need to direct water away from the house with sloping the soils away from the house and the use of gutters.

 

[Sean2]

Ideally, the soil around your basement should be drained using gravity flow. Then, the pea gravel backfill solution is probably the best.

Have you thought of ways to avoid backfilling at all?

Beware of using "filler boards". If you backfill the clay against these, then they will just transmit the pressure to your wall. If the boards are compressible, they might do the trick, but I don't favour them in this context.

The pressures you have to deal with if you backfill are:

1) Static soil pressure.
2) Water pressure.
3) Pressure induced in the soil during compaction of backfill, and some of this may be "locked-in".
4) Swelling pressure from the soil as the soil expands in moist conditions.

The wall was probably originally designed to resist "normal" soil pressure, but it has since been weakened.
Drainage will reduce the water pressure if the water can reach the drain, for example, through a granular backfill.
If you use an appropriate granular backfill, compaction forces will be minimal.
To avoid swelling pressures from the soil, you either have to stop the soil from swelling (-very unlikely-) or separate the expanding soil from the wall. If you use a ROUNDED pea gravel, the force transmitted through will be slightly reduced.

Other ideas will occur to me and other readers after I post this, but I hope the above is useful.