Backfilling Around Foundations 2

[usaedy]

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.

 

[GeoPaveTraffic]

In good construction practice you would always compact backfill around a foundation,no matter how deep or wide. In reality, at least in this area, in residential construction this compaction is often not done. This practice leads to problems but continues.

 

[cvg]

I think an important issue is to keep surface water out of the excavation by backsloping or creating a berm along the edge of your foundation excavation.

 

[Zambo]

I am surprised it is standard practice not to compact the backfill. I do not work on residential projects, always civil engineering. In a civil engineering project compaction is always required, how this is carried out is the only thing to be decided. You could excavate further to give space for a 1 tonne vibrating roller (the jumping jacks that you mention are also ok for this type of work or trenches). You could use sand and water for the lower layers with less space and then a graded fill for the upper layers where there is space for compaction.

 

[looksatstars]

There should be a parging compound on the concrete foundation to keep the water from penetrating into the concrete. There will also be weeping tile around the footing perimeter of the building to take away excess water.

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.

 

[DRC1]

Foundation excavations should be compated generally with plate tampers, although other methods are used, tamping with a bucket may not give satifactory resuts. Backhoe mountable tampers are available. The sides should be sloped to be sufficently stable. (someone had to form pour and strip the footings and the walls, so it should be safe to work in)Pogo sticks and jumping jacks do not give good coverage or provide a lot of compactive effort. The backfill should be free drainig with a footing drain at the bottom. the footing drain should be wraped in filter fabric to match the backfill and the drain should be adequately pitched and outlet to daylight or a drainge system. Infiltratin can play havoc in karst terain. It is important to be sure the drainage is done properly. Many times it is not well understood and just thrown in and does not function. The little extra attention and cost to be sure it works is nothing compared to the problems of a sinkhole near the structure.

 

[cvg]

at 8 feet deep, placing a drain at the bottom will be difficult to daylight using gravity flow. you can drain to a sump and pump it out, however another option is to place the drain higher and near ground surface to prevent surface infiltration away from the wall. roof drainage should be discharged away from the foundation walls and grading away from the building as well.

 

[Oson]

Compacting backfill will also give more supporting capacity to the foundation developing cohesive forces, specially in clays.
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.

 

[mgrez]

Let me ask you something...before you decide in compacting the backfill around the foundations, Wouldn't you need to compact material under your footing if needed? would your excavator leave enought safe space to let the concrete contractor install their forms and work regarding the OSHA regulations?
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.

 

[usaedy]

No compaction is done under the footing on virgin soil. No need to do that. In a structural fill situation, we compact under the guidance of a geotechnical engineer. The entire foundation pad gets rolled with a minimum 5 ton vibratory roller before footings are formed.

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'.

 

[JustinJones]

I showed this topic to my dad and asked for his 2 cents. He told me that you should never use mechanical packing devices like a jumping jack or even worse a backhoe bucket in proximity of residential structures, it's fine with a civil structure with 16" concrete walls for example, but when you're working with cinder block basements or <8" concrete basements for example it's a very bad idea and can cause the walls to crack very easily. Very risky business.

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...

 

[Ron]

Yes it should be compacted, even though it is a residential structure. The foundation wall is a retaining wall and should be designed to accommodate the stresses of compacted material. If not, it is a negligent design.

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.

 

[JoeinCal]

usaedy: iNo compaction is done under the footing on virgin soil. No need to do that. In a structural fill situation, we compact under the guidance of a geotechnical engineer. The entire foundation pad gets rolled with a minimum 5 ton vibratory roller before footings are formed.

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.

 

[navin25]

In residential projects the shape of the foundation is generally spread footing living the base rectangular & prone to less compaction. As very rightly said the vibratory plate compactors can be used in layers less then specified. well compacted Gravel fill or lean concrete is highly recommended.