Concrete slabs perform best when they are even thickness. The worker shown here is using a 2x4 to strike flat subgrade. Drylines serve as a guide to ensure that the subgrade has proper elevation.
Joe Nasvik Concrete slabs perform best when they are even thickness. The worker shown here is using a 2x4 to strike flat subgrade. Drylines serve as a guide to ensure that the subgrade has proper elevation.

When it comes to compacting a subgrade for a residential application, consistency, quality, and good practice by the contractor are critical to prevent concrete cracking and sinking concerns for the homeowner.

A residential contractor must ensure that the site builder avoids the temptation to poorly prepare a subgrade. A low-quality subgrade results in unexpected settling and water accumulation against the foundation leading to seepage and a weakening of the foundation. Poorly compacted subgrades also can be cited for the cracking and sinking of sidewalks, driveways, and patios. Ultimately, it is the homeowner who will bear the repair expenses, especially when settlement causes slabs to move water in the direction of the house.

Subgrade Failure

According to the Portland Cement Association, cracks, slab settlement, and structural failure can result from an inadequately prepared and poorly compacted subgrade. In the case of slab on ground for residential applications, an ideal subgrade should be well drained, have a uniform bearing capacity, achieve the appropriate compaction density, have the correct slope, and be free of sod, organic matter, and frost.

What causes an uneven subgrade in a residential project? First, the presence of soft, unstable soil mixed with hard and rocky soil. Another reason addressed later in this article may be filling low areas or backfilling without adequate compaction.

Vibratory plate compactors do a good job of compacting fill. Surfaces are flat afterward, making slab shrinkage possible with little resistance from the ground.
Joe Nasvik Vibratory plate compactors do a good job of compacting fill. Surfaces are flat afterward, making slab shrinkage possible with little resistance from the ground.

In order to achieve a level, firm subgrade, soft or saturated soils and rocks must be removed during initial excavation. During excavation, dirt is pushed into low areas (fill) from other areas (cut). For compacted fill in low areas, the soil should have the same density as in cut areas.

It is important that contractors identify what is cut and what is fill when constructing a subbase. Operators of heavy machinery can unknowingly move or mix the cut and fill materials creating identity problems between the two.

Depending on the environment and geographic location of the project, builders may encounter native soils that offer the ideal strength for supporting a slab, which can then be placed directly on the soil. However in many areas, finding an onsite soil adequate for use as fill may be difficult.

Fill Alternatives

With slab-on-grade foundations, an alternative to native soil material may be used to restore grade. These imported fills may include granular materials such as sand, gravel, or crushed stone. These high-strength and permeable materials offer low compressibility, which is ideal for a slab-support system.

“Six inches of well-graded gravel or crushed stone material properly compacted will give good results,” said Mark Conrardy, a sales engineer manager for Wacker Corp., Menomonee Falls, Wis.

When properly compacted, both native soils or imported granular materials can provide good support. It must be noted, however, that using good material cannot fix poor compaction.

“Granular material placed under the slab can be open graded, which some believe is self-compacting. Sand and other material should be compacted,” says Chris Tull, owner of Concrete Consulting LLC, Fishers, Ind.

Whether it is a foundation, sidewalk, driveway, or patio, native soil or granular material cannot be simply placed in the fill area and then covered with concrete.

It is critical that the contractor require the compaction of all fill materials in proper lifts, regardless if it is a native soil or a granular material to provide a flat, dense subgrade. If the fill material is not compacted in a uniform fashion, uneven settling will occur, resulting in cracks in the concrete and destroyed work.

Compaction Necessities

It is advisable to compact the soil near optimum moisture content to achieve required density levels. This is most likely to occur when the soil is freshly excavated. Maximum compaction is possible before the material has dried out too much or been exposed to periods of rain where water can collect. If the soil is compacted during or shortly after the grading process, there are fewer pores in the material that can absorb water.

Primarily used for larger home site developments, proof rolling is a measure that allows contractors to identify whether or not the subgrade has been adequately compacted. Unstable or improperly compacted soil can be identified by driving a fully loaded dump truck or a similar piece of heavy equipment over the compacted area to reveal unusual soil movement or notably deep tire rutting.

The measured density of the subgrade material for a residential application should be 90% to 95% of the Standard Proctor Density. This density can be achieved using a combination of three to four passes with a vibratory plate compactor or rammer, in addition to good moisture content in the subbase.

Rammers or “jumping jacks” do a good job compacting dirt fill materials. Care should be taken, however, not to exceed specified soil thicknesses for each lift.
Joe Nasvik Rammers or “jumping jacks” do a good job compacting dirt fill materials. Care should be taken, however, not to exceed specified soil thicknesses for each lift.

Ideally, compactable fill should be watered or “processed” so that it contains approximately 12% moisture throughout, in order to achieve maximum compaction.

Some fill materials also provide good drainage properties and can be used to avoid moisture concentrations beneath the slab.

A proper moisture level in the subbase not only helps with consolidation, it also reduces bleeding out of the water from the base of the concrete slab.

In order to keep the concrete as close to its design strength, forms should be dampened to keep water from being drawn out of the concrete.

Backfill For Foundation Walls

Contractors often ask how to backfill foundation walls for an area that will end up under concrete flatwork.

Backfill beneath concrete flat-work along foundation walls should be accomplished with a granular material that is uniform in size. Some fill materials, such as round stones like pea gravel, tend to be self compacting. “I recommend open-graded granular material be used,” says Tull.

Gap-graded aggregates allow water that is present along the foundation to travel down to drain tiles and drain into a sump where it can be pumped to a storm sewer. This ensures that water is taken away from the foundation. The material still needs to be compacted in order to reduce settling that will occur over time. The top 6 to 12 inches of the backfill under the slab should be filled and compacted with well-graded gravel or crushed material.

“Walls are typically backfilled with the removed earth or with a granular, open-graded material,” says Tull.

According to the CFA Foundation Standard 8.3 Backfilling and Final Grading, “backfilling shall not commence on a foundation wall that is designed to be supported laterally at the top and bottom, until such lateral support is in place, or until the wall has been sufficiently braced to withstand the pressures of the backfill material, compaction process, and backfilling operation. Backfill shall be placed so that damage to the wall, waterproofing, or other work does not occur. Bracing shall not be required for walls supporting less than 4 feet of unbalanced backfill.”

In regards to final grading, a minimum slope away from all foundation walls of at least 1% to 2% or ¼ inch per foot pitch away from the building must be provided. Surface water should be diverted to a storm sewer or other point of collection. All drainage should be pitched away from the building. Lots should be graded to drain surface water away from foundation walls. Where lot lines, walls, slopes, or other physical barriers prohibit at least a 2% pitch, drains or swales should be provided to ensure drainage away from the structure. “It is important that enough slab slope exists so the site drains away from the wall,” says Tull.

Clearly, pitch plays a role in subgrade preparation as well. Generally, the best practice is to pitch the surrounding grade away from the house so water does not collect against the foundation. Water collection over time results in seepage and can undermine a foundation, resulting in wet basements.

Equipment To Use

In order to properly prepare a subbase for a residential application, plate compactors and rammers are the preferred equipment. Vibratory plate compactors, and their counterparts, the rammer or “jumping jack,” should be used in confined areas for sub-grade compaction, according to Conrardy.

Vibratory plates are good for compacting granular materials because they are fast and leave the surface flat. A flat, smooth surface is good for flatwork because the concrete can shrink as it cures without being restricted by the base material.

Rammers, which work best for compacting 6 to 12 inches of fill, are designed to handle clay soils where the impact creates shearing forces that remove air voids from the clay. For soil fill on residential applications, jumping jacks can safely compact 6 inches of soil fill.

Higher frequency vibration from a plate compactor forces movement of the particles allowing gravity to cause them to settle. In regards to fill depth, smaller vibratory plate compactors and rammers work well for 6 to 12 inches of either soil or granular fill.

Quality Is Key

All too often, builders do not adequately prepare the subgrade for concrete foundations, sidewalks, driveways, or patios. The concrete contractor is responsible for ensuring that granular backfill receives proper compaction, density, and moisture levels prior to the placement of concrete. Some contractors will specify in their contracts that they are responsible for the proper compaction of any fill materials that they place, but not materials placed by others. It becomes the builder's responsibility to ensure that fill placed prior to the arrival of the concrete contractor is properly placed and compacted. Designers should specify that fill materials be installed properly as well.

Unfortunately, once the concrete has settled and shows signs of failure—whether it is 2 years or 20 years after being placed—in the form of cracks and sinking, the homeowner is stuck footing the repair bill. More often than not, the homeowner in turn blames the concrete contractor for the subgrade failure.