Backfilling Around Foundations
Backfilling Around Foundations
(OP)
I am looking to determine whether the stadard practice of not compacting backfill material around the foundation perimeter is a good practice when building in geologic areas containing Karst features. I am specifically conerned with water penetration and the possibility of sinkhole formation.
Does anyone compact their foundation backfills in any way, whether by tamping with the backhoe bucket, or using a jumping jack or other tamping device? Of course I am talking about a 2' wide space between the foundation wall and the limit of excavation and at 8' deep, it is usually not practical or safe for a person to use a walk behind compaction device.
I am looking into this practice because I have just had a situation where the perimeter grading around a foundation settled and was backsloped toward the foundation
wall. A sinkhole opened up in the front yard of the lot away from the foundation and I suspect these were related issues.
Any advice would be helpful. Hopefully this will be a good discussion.
Does anyone compact their foundation backfills in any way, whether by tamping with the backhoe bucket, or using a jumping jack or other tamping device? Of course I am talking about a 2' wide space between the foundation wall and the limit of excavation and at 8' deep, it is usually not practical or safe for a person to use a walk behind compaction device.
I am looking into this practice because I have just had a situation where the perimeter grading around a foundation settled and was backsloped toward the foundation
wall. A sinkhole opened up in the front yard of the lot away from the foundation and I suspect these were related issues.
Any advice would be helpful. Hopefully this will be a good discussion.





RE: Backfilling Around Foundations
RE: Backfilling Around Foundations
RE: Backfilling Around Foundations
RE: Backfilling Around Foundations
The soil will settle/compact over time but even on my own house, I made a backslope to get water away from the basementwalls as the building settled.
RE: Backfilling Around Foundations
RE: Backfilling Around Foundations
RE: Backfilling Around Foundations
I go a little further when building on clay soils, as soon as I excavate I place a plastic liner covering all the three faces of the excavation, after that I cast and then compacted backfill leaving the plastic buried between the natural soil. This helps me keep the clay dry, which when if dry has a high bearing capacity.
RE: Backfilling Around Foundations
If compactation is required sloping, shoring or other method can be used to provide a safe work environment to the person with the walk behing compactor.
OSHA has realy good guidelines regarding the type of soils, how to reconize them and how to slope an excavation.
RE: Backfilling Around Foundations
OSHA standard overdigs are required. We have a 2' overdig at 5' high max. then a 2' high bench if the foundation excavation is greater than 5'.
RE: Backfilling Around Foundations
After his advice I would try hydraulic tamping via a 'soaker hose' running the perimeter of the structure on top of the backfill, it might be a slower process but will insure no damage/stress come to the basement. Material will probably have to be added a couple times after settlement has taken place...
RE: Backfilling Around Foundations
Light compaction equipment should be used, since the wall is likely not designed for the surcharge of large equipment.
The compaction should be done is shallow lifts, since you are using light compaction equipment and its effective depth of influence is shallow.
As for the safety of an 8-foot excavation....if it is not protected, it should be...that would be an OSHA violation otherwise.
As for the Karst topography....compacted soil is not going to save you from a sinkhole if there is prominent, active sinkhole activity in the area. Sinkholes develop from below, not from above. The amount of water getting through compacted soil versus uncompacted soil in the small areas you are discussing would be irrelevant. Further, sinkholes usually develop when water tables drop, due to lack of rainfall.
RE: Backfilling Around Foundations
Just as a warning, if you have a situation where you are placing fill for some footings and placing others in native material within the same structure, you could experience differential settlement. This is because the compacted soil and the native soil is probably going to be two different densities, and the less dense foundation soils will settle faster than the harder side, leaving you with an unlevel foundation.
Also, its usually best to place backfill mechanically (rammers or walk behinds) if you have the room and placed in 8" lifts if possible to eliminate the possibility of bridging. Most rental yards can equip walk behinds to 24" wide, they just normally come 33"wide .We usually backcut behind walls with a 3' minimum because in the long run backfill goes faster.
Be careful on using water-based compaction or "jetting" around structures. Here in California its unacceptable because of the unpredictable results.
If you do have a truly confined space, you may opt to backfill with a pea gravel material, which does not need as much compaction effort.
RE: Backfilling Around Foundations