Skip to Main Content


The process of putting back soil that was excavated when digging a foundation or trench. This soil is more porous than untouched or virgin soil, and because of this, it absorbs more water.

Schedule Free Inspection

Problems Caused By Backfill Soil

Builders often do a lot of digging and excavation work to create clean and level grounds to lay the foundation on. After erecting the foundation walls, the gaps left are filled to prevent future water problems. Most builders use the natural soil already on the property to fill these gaps while others opt to use alternative backfill material. Here, we define backfill soil, its purpose, and the different types of backfill material. 

What Is Backfill?  

Backfill is the material used to refill an excavated hole or trench in your yard. This material can be the excavated soil, a mixture of sand and gravel, or commercial products. The process of backfilling usually takes place in layers. Different materials have different properties. Some are small while others are big. 

Methods of Backfilling  

There are a few common methods of backfilling, including water jetting, compacting trenches, and flowable fill.  

Trench Compaction:

  • Here, an excavator, a compactor, or some other mechanical means are used to compact loose material into the trench. Soil is backfilled in layers of four to six inches depending on the type of compaction equipment used and the nature of the backfill. Water may be added to help with the compaction.  

Flowable Fill:

  • A cementitious material that has a low water-to-cement ratio is used in this method. Before backfilling with this material begins, utility pipes are covered with aggregate material. The main challenge of using flowable fill for backfilling is liquidity – it must be contained to prevent it from flowing into other trench areas.  

Water Jetting:

  • This technique utilizes a probe to apply pressurized water to the bottom of the fill. The force of the jetted water is used to move backfill material around. Preventative measures must be taken to contain the sediment-filled water. This backfilling method is suitable for sandy soils or those with highly fissured bedrock. 

Water in the Clay Bowl  

Homes often experience the clay bowl effect during wet months. This phenomenon is a natural consequence of excavating and backfilling during the initial construction of the foundation.  

The backfill soil is more porous and absorbent than the compacted soil around it. When there is a snowstorm or heavy rain, the backfill gets saturated with water. With nowhere to go, the water starts pushing against the foundation walls. This can lead to wall cracks that might let water into the basement or crawl space.  

The clay bowl effect also causes shallow foundations to heave. On the other hand, if the surrounding trees and plants consume the excess water, the soil could become too dry, compact, and create a void.  

The result is a cracked and sinking foundation.  

Heaving is not likely to occur in deeper foundations because the bottom lies below the frost line. However, lateral pressure along the foundation may occur when the backfill gets soggy and the undisturbed soil swells after absorbing water. This causes the mortar joints in the masonry foundation to crack and the walls to bow inward or bulge in the middle. 

Backfill Types 

Depending on the type of structure you’re looking to build and its drainage system, you can opt for the following types of backfill. We should point out that before you choose one, it’s best to consult professional contractors. With their knowledge and experience, there will be a much lower chance of you backfilling your foundation with inadequate material. 

Coarse-Grained Soil:

  • Your first option is coarse-grained soil. This is a mixture of gravel, sandy soil, and a negligible amount of fine materials. This is a high-quality backfill since it provides fine support for the foundation and is pretty easy to compact. 

Limestone Screenings:

  • Limestone screenings are great when it comes to sewer and pipe backfilling. Like coarse-grained soil, this material also compacts pretty well. Some builders will use it even as a base for brick paving. 

CA7 Bedding Stone:

  • This third option is the most popular in construction nowadays. CA7 bedding stone is a grayish material that self-compacts and is perfect for bedding pipes, subbase work, and improving soil drainage. 

CA6 Base Stone:

  • Besides the CA7 bedding stone, its CA6 counterpart is another great option that you should consider. It’s used for both commercial and residential projects. 

Trench Backfill:

  • Similar to CA6 base stone, trench backfill comes in the form of small aggregates and it drains and compacts quite well. 

3” Coarse Stones:

  • In case you’re dealing with large holes and trenches, it’s best to use 3” coarse stones. Just like all previous options, this backfilling material has great drainage properties as well. We always suggest that you use it as the first layer and then cover it up with another kind like CA6 base stone for compaction. 

Commercial By-Products:

  • Depending on the quality of the soil on your property, you might need to use certain commercial by-products like fly ash to improve the quality of the soil on your property for backfilling. 

Compacting the Backfill  

Regardless of how well you backfill the soil along your home’s perimeter, some of it will still be loose. This isn’t good as the backfill can get saturated by rainwater or melting snow. With nowhere to go, the standing water will start pushing against your basement or crawl space walls. This can lead to cracks and bowing walls. To prevent this scenario, compact your backfill properly. You can use a roller or any suitable compaction equipment to tamp down the soil.  

If you’d like to waterproof your foundation after backfilling it or inspect your drainage systems, contact Groundworks for a free foundation repair inspection and quote. We’ll come over and assess the foundation and make recommendations that’ll keep water out.  

Publish Date:

Last Modified Date: