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What is Backfill in Construction? A Guide to Types and Uses

Pile of soil.

Backfill is an essential part of construction and landscaping projects. But what exactly is backfill, and why is it important?

In this guide, we’ll explore what backfill is, the different types available, and their various uses. We’ll also discuss an important concept called the clay bowl effect, which can lead to foundation issues in some cases.

What Is Backfill? 

Backfill refers to the material used to refill an excavated area. Its most often used during the construction of a new building. Backfill may be placed around a foundation, in a trench, or behind a retaining wall. The purpose of backfill is to provide support and stability to structures, prevent water accumulation, and ensure the ground is level and safe.

Uses of Backfill

Backfill is used in various construction and landscaping projects, including:

Dirt under a concrete slab.
Exposed backfill soil under a concrete slab foundation.
  • Foundations: Backfill supports foundation walls and prevents water from pooling around the base.
  • Retaining Walls: It provides stability to retaining walls and helps manage water drainage.
  • Trenches: Backfill fills in trenches after installing pipes or cables, ensuring they are secure and protected.
  • Landscaping: It levels the ground and creates a stable base for lawns, gardens, and other landscape features.

Types of Backfill 

Depending on the project, drainage requirements, and other factors, contractors will choose the most suitable material. Here are the most common ones. 

Coarse-Grained Soil

  • Description: This mixture of gravel, sandy soil, and a small amount of fine materials provides excellent foundation support and is easy to compact.
  • Uses: Ideal for general foundation support and areas needing easy compaction.

Limestone Screenings

  • Description: Limestone screenings are fine particles of limestone that compact easily.
  • Uses: Commonly used for sewer and pipe backfill, and as a base for brick paving.

Trench Backfill

  • Description: Made of small aggregates, trench backfill drains well and is easy to compact.
  • Uses: Perfect for filling trenches after pipe or cable installation, ensuring secure and stable fill.

CA7 Bedding Stone

  • Description: Also known as ¾” limestone, this angular white/gray stone compacts naturally. It’s widely referred to as “bedding stone.”
  • Uses: Ideal for pipe bedding, subbase applications, and improving soil drainage.

CA6 Base Stone

  • Description: Known as subbase granular backfill, CA6 base stone is often used along roadway shoulders but can also be used in residential projects.
  • Uses: Provides a stable base for roads and driveways, and is useful for various residential applications.

3” Coarse Stones

  • Description: Large 3” stones are ideal for filling large areas. They leave voids which provide excellent drainage properties.
  • Uses: Suitable for the first layer of material in large projects, with a top layer of CA6 base stone added for further fill and compaction.

Why is Backfill Important?

Backfill assists in several important functions:

Water in a crawl space.
What could happen without proper backfill soil.
  • Structural Support: Backfill provides the necessary support to foundations, retaining walls, and other structures. It helps distribute the load evenly to mitigate foundation settlement.
  • Water Drainage: Proper backfill materials, such as sand and gravel, ensure adequate drainage around structures. This helps prevent water from building up under your home and reduces water-related damage, such as foundation cracks and leaks.
  • Erosion Control: Backfill helps prevent soil erosion by stabilizing the ground around structures. This is particularly important in areas prone to heavy rainfall or flooding.
  • Aesthetic Appeal: In landscaping projects, backfill is used to create a level and stable base for lawns, gardens, and other features. It ensures a neat and aesthetically pleasing appearance.

How the Backfilling Process Works

Backfilling the foundation involves several steps to ensure stability and proper drainage. Once the foundation has completely cured, the area is cleaned, and any accumulated rainwater is pumped out.

Excavator machine moving soil from a pile.

Contractors then select the most suitable backfill material based on factors like soil type and project requirements. Using construction machinery, the backfill is added in layers, each about seven inches thick, ensuring even compaction and stability.

The process starts by filling the corners of the excavated area, and then the middle, with each layer being compacted before adding the next.

After all layers are placed, they are watered and compacted again using steel or wooden log rammers to achieve optimal stability and density. This methodical approach prevents settlement and supports the foundation, ensuring long-term durability and preventing issues such as water accumulation or structural instability.

By following these steps, contractors ensure that the backfill supports the structure’s foundation.

Why Soil Compaction is Important

Properly compacting backfill after construction is done to prevent structural issues. If not done correctly, the soil can become loose and easily saturated with rainwater or melting snow, increasing hydrostatic pressure.

Unfortunately, backfill soil can never be as compacted as densely as undisturbed soil, making it more porous and prone to water saturation. For many homes, this leads to foundation settlement and water damage, such as wall cracks or bowing walls.

These problems often arise from the “clay bowl effect”, where newly backfilled soil forms a “bowl” that easily traps water. Contractors can try their best to mitigate this situation, but if it does happen, homeowners will have to invest in foundation repair solutions (and AFS can help).

Work With AFS if Your Home’s Backfill Causes Trouble

AFS crewman posing for camera in front of home.

Backfill is a crucial component in construction and landscaping, providing stability, support, and proper drainage. While it is an important part of your property, it isn’t always perfect.

If you have more questions about your soil or you’re noticing foundation issues and are wondering if it has to do with your home’s backfill, contact AFS today. We provide free home inspections and can determine if soil is affecting your home!

Backfill FAQs

Common equipment for compacting backfill includes rollers, plate compactors, and rammers. These machines ensure that each layer of backfill is properly compressed to prevent future settling.

Properly compacted backfill will feel firm underfoot and show minimal settling over time. Hiring a professional to test the soil density can ensure it meets the necessary compaction standards.

Good drainage in backfilling prevents water from accumulating around the foundation, which can lead to increased hydrostatic pressure and potential damage. Proper drainage practices (like gutters and proper yard grading) help direct water away from the structure, protecting it from moisture-related issues.

Related Resources

Ted Dryce

Ted Dryce

Content Writer

Ted is an SEO Content Writer who has been with Groundworks since 2021. He’s covered home repair topics ranging from crawl space encapsulation to regional soil conditions. When he’s not working, Ted is performing improv comedy and working on his own creative projects.

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