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Best Way To Fill Voids Below A Concrete Floor

Determine cause, then formulate a plan.

“What’ the best way to fill voids below a concrete floor?” is one of the most common questions we’re asked at CJGeo. 95% of the time, the answer CJGrout geotechnical polyurethanes. The remaining, occasional projects use chemical grouting or CJFill cellular concrete. All of these materials are alternatives to flowable fill, which is often the first thought because it’s familiar to a lot of contractors and engineers, but it’s rarely the best material.

Step 1: Determine the Cause

There are lots of potential reasons for under concrete floors. But, they usually fit within three main reasons:

  • erosion
  • settlement
  • intentional


Erosion under slabs is most commonly the result of a utility failure. Stormwater pipes, roof leaders, and sanitary piping below floors can erode significant quantities of backfill over time. The deeper the pipe, the bigger the potential problems. One of the most common causes of floor voids below concrete floors in commercial buildings is shallow gravity sewer failures.

Sometimes, the erosion is a result of poor exterior drainage, which channels water towards a structure. If the perimeter walls aren’t deep, such as with turned down slabs, water can start to run under the slab. This can erode significant quantities of backfill, at times very quickly.


Consolidation, or settlement, of underlying backfills is very common. Poor control or oversight of the backfilling process can result in significant amounts of settlement. Settlement can also start erosion, by redirecting exterior water flow towards a building. Settlement can also cause breaks in utilities below a slab that then cause erosion. Since erosion involves water under the slab, it can also reduce bearing capacity, accentuating settlement.

On this project near Pittsburgh, 35 feet of poorly controlled backfill material settled. This caused plumbing lines to break, which then washed out voids up to three feet deep under an industrial kitchen. This erosion was in addition to the six inches of slab settlement.

Underlying compressible soils are another cause of settlement that can result in voids under concrete floors. As soils compress with time, the ground level goes down. If the foundation is pile supported, or if the settlement is isolated to a specific area, the building itself may not move, but the ground effectively pulls away from it, leaving voids.


Sometimes there are intentional voids below slabs. They can be due to structural slabs designed to tolerate voids that were anticipated from settlement. They’re also commonly used to address expansive soils. It’s very important to determine the original designer’s intent behind intentional voids before consider filling them. This usually involves a structural engineer reviewing construction documents if they’re available, or thoroughly inspecting the structure. A geotechnical engineer may also need to investigate the soils as part of this process.

Step 2: Formulate A Plan

Formulating a plan for the best way to fill voids below a concrete floor is driven by the cause, and then economics. The best people to help make this call are specialty grouting contractors working in conjunction with your engineer. The independent engineer can help evaluate alternatives, and the specialty contractor can provide constructibility input, which is the best way to ensure the most cost effective repair that appropriately addresses the problem.

For voids resulting from settlement, using a lightweight grout is usually most important. This reduces the likelihood of inducing additional settlement during the void filling process. The lightest grouts are geotechnical polyurethanes, such as CJGrout 20SDB. If voids are exceptionally large, low density controlled low strength material (also called cellular concrete) is heavier than polyurethane, but is generally more economical at scale.

Speak With An Expert

Need help figuring out the best way to fill voids below a concrete floor? Give us a shout or shoot us a text. Click the state marker for the location of the project for contact info for the appropriate rep.

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