Original Article: https://safetyculture.com/topics/backfilling/

What is Backfilling?

Backfilling is the process of filling in the excavated area around a foundation or structure. The backfill material can be anything from soil to gravel and is usually compacted to provide support and stability. Backfill can also be used to insulate foundations or improve drainage. This means that backfilling is essential in the construction process, following the foundation’s pouring and installing utility lines.

Depending on the project, backfilling can be a simple or complex operation requiring special equipment. It is vital to ensure that the backfill material is compacted correctly to provide a stable foundation.

Factors Affecting Backfilling in Foundation

Several factors need to be considered when backfilling a foundation. Below are some of the most critical factors:

Choosing the Right Backfill Material

The backfill material must be appropriate for the job. Soil, sand, gravel, and crushed stone are all common backfill materials. The type of material will vary depending on the application. As an example, crushed stone can improve drainage around foundations.

Compacting the Backfill Material

The backfill material must be appropriately compacted to provide a stable foundation. Hand tampers, plate compactors, or roller compactors can be used for this process. The equipment used for compacting differs based on the material’s size and type.

Period of Backfilling

The backfill material must be placed in the excavation layers and compacted after adding each layer. Depending on the material, each layer will have a different thickness. 

Generally, the soil should be placed in layers 6 to 12-inch-thick and compacted with a hand tamper or plate compactor. Gravel and sand can be placed in 3 to 6-inch-thick layers and compacted with a hand tamper or roller compactor. The crushed stone should be placed in 2 to 4-inch-thick layers and compacted with a hand tamper or plate compactor.

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Types of Backfilling in Foundation

Based on the type of material used, backfilling can be classified into the following types:

Coarse-Grained Soil

Coarse-grained soils include gravelly and sandy soil types and clayey sands through gravel-sand mixtures with little or no fines. These soils are relatively easy to compact.

Fine-Graded Soils Of Low To Medium Plasticity

Organic clays of low to medium plasticity (such as gravelly, sandy, or silty clays and lean clays), silts (including organic and inorganic silts), and fine sands (such as silty fine sands or clayey silts) are all examples of fine-grained soil.

Commercial By-Products

Using commercial by-products, such as furnace slag or fly ash as backfill material, may be desirable when natural materials are unavailable. Fly ash can be used as a lightweight backfill behind a 25-foot-high wall. It can also be mixed with clay to make it more plastic. 

The suitability of these materials depends on how well they meet the foundation’s needs and the products’ engineering characteristics.

Controlled Low-Strength Material (CLSM)

CLSM is a cementitious material used as backfill, void fill, and utility bedding. Fly ash, cement, and other similar products make up the mixture. The fly ash mixture can come from thermal power plants.

Tips in Backfilling

  • You must wait at least five to seven days before backfilling a foundation. Even if you backfill immediately, cracks may develop.
  • Before the fill is applied, there must be no grass, loose stones, or rubbish on the surface. If the area has water, it should be pumped or bailed out.
  • Determine the materials you’ll utilize for your foundation backfill. Mixed materials often include rocks, soil, and stones. Some dirt can retain too much moisture and is thus unsuitable for your foundation.
  • Try to use the excavated earth as a filling material.
  • Once the accepted excavated material has been stored, it must be cleaned of rubbish, significant stones, vegetation, and other items.
  • Backfill at the corners and ensure the soil distribution is uniform to give your property solid lateral support.
  • Using an excavator, fill in twelve inches of mixed materials on the sides of the area. Compress the soil and stone after that. You must continue to backfill until the entire area is filled, repeating the process until all of it is completed.
  • Fill the space in layers; each layer should be 15cm to 20cm thick.
  • Each layer is trampled and compacted with big wooden logs or steel rammers.
  • Never fill in plinths and footing pits with black cotton soil.