Backfilling is an essential step in any construction work. Backfilling may be defined as the process of reusing or replacing the soil that is removed during construction of the structure to strengthen the foundation or other structural members. In general, backfilling of foundation can be understood as the process of putting back the soil in the foundation trench. Foundation is an important component of any structure and thus must be strong enough to withstand the imposed load and to maintain the overall stability of the structure. Thus, backfilling is done in the foundation to aid its strength and stability.
PURPOSE OF BACKFILLING
The main purpose of backfilling in the foundation can be listed as follows:
1. To increase the strength of the foundation to resist all the load coming from the superstructure.
2. To provide support to the foundation of the structure.
3. To increase the overall stability and performance of the entire structure.
FACTORS AFFECTING BACKFILLING IN FOUNDATION
The three important factors that affect the backfilling in the foundation are as follows:
1. Selection of the suitable backfill material.
2. Compaction of the backfill material.
3. Selection of period of backfilling.
TYPES OF BACKFILL MATERIALS
Some of the backfill materials that can be used in the backfill of the foundation are briefly explained as follows:
1. Coarse-Grained Soil:
Course-grained soils are one of the commonly used backfill material. Course-grained soil includes gravel mixed soil and sandy soil. It mostly consists of a mixture of gravel and sand mixtures with a negligible amount of fine materials.
2. Fine Graded Soil:
Finely graded soil with low to medium plasticity can be used as backfills. This includes silty or clayey fine sands, lean clays and gravelly clay.
3. Commercial By-Products:
Several commercial by-products are also available in the market which can be used as backfill materials. Commercial by-products include the fly ash and furnace slag. These products can be used if locally available to minimize the overall cost of backfilling.
4. Controlled Low Strength Material:
Controlled low strength material abbreviated as CLSM is a cementitious material primarily used for the process of backfilling. Such material is self-compacting, low strength and flowable.
PROCEDURE OF BACKFILLING IN FOUNDATION
The procedure of backfilling in the foundation includes the following series of steps:
1. Initially, the foundation has to be cured for a period of at least five to seven days before commencing the backfill to prevent the cracking.
2. The ground over which the backfilling process is to be carried out must be thoroughly cleaned. If water is present it must be pumped out or bailed out.
3. Depending upon the engineering properties and site condition, the suitable backfill material must be selected. A mixture of backfill materials can also be used.
4. If possible, the refilling material can be used from the excavated earth. This can be useful in the reduction of the overall cost. But, it must be taken care that the excavated material is well cleaned and free from rubbish.
5. The backfilling is then started from the corners. The fill material must be spread evenly.
6. An excavator can be used to fill the materials up to 12 inches on the sides of the area. The fill is duly compacted employing compaction roller or other suitable compaction equipment.
7. The filling must be done in subsequent layers with a thickness of 15cm to 20cm each.
8. Then, each layer is watered and further compacted using wooden logs rammers or steel rammers.