What is Backfilling in Construction?

Backfilling is the process of using soil to support and strengthen a structure. The soil used for backfilling can be the soil removed during the excavation of foundations, ground bearing slabs or during any groundworks. It can be new imported soils, rocks and stone based on the structural requirements.  

Fig.1. Backfilling with Excavated soil material after the installation of underground pipe

Backfilling is performed to protect foundations, and is a part of roadways, walkways, substructure of slabs, and other groundwork elements. Ground investigations are conducted by the engineers to formulate the quantity and need for backfilling. Backfilling is quantified in cubic meters.

Backfilling With Excavated Material

Certain construction works like foundation or concrete works require an excavation process. During the process, the excavated soil is set aside and stockpiled based on the class and type of the material. Once the construction is complete, the excavated soil is used to backfill the foundation or any other structural purpose. Any excess amount of excavated soil is either taken out from the site or saved as stockpiles for any future use.

Fig.2. Soil Excavation

All excavated soil material may not be suitable for backfill. It is dependent on the site parameters, the type of soil and design.

Backfilling With Imported Soil

In some construction scenarios, new soil is necessary as backfill to strengthen the construction. Sometimes, if the existing excavated material is of bad quality, new soil must be imported.

Fig.3. Backfilling With New Material

Grade of Backfill Soil

Backfill soil can be generally graded as:

1. Coarse-grained soils which include gravelly and sandy soils which range from clay to gravel.
2. Fine-grained soils with low to medium plasticity. This includes inorganic clays of low to medium plasticity, very fine sands of low plasticity, and inorganic silts.
3. Rock is suitable based on the hardness and gradation. Hard rock might be specified for crushing.
4. Shale is used for backfilling. They are rock-like material that breaks down under compaction.
5. Marginal materials like fine-grained soils of higher plasticity and expansive clays are used for backfilling.
6. Commercial by-products like fly ash or furnace slag are also used for backfilling.

The backfilling material used for construction purposes must be uniform, and homogeneous with consistent characteristics. It must be free from organic matters and pockets. Proper supervision must be conducted to inspect that the only suitable backfill material is employed.

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