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Proper backfilling of a basement is an important and essential step in the basement construction as it largely contributes to its structural stability. Moreover, properly compacted backfill provides support to the basement walls as well as acts as a barrier for external forces. Selection of the right backfill material, appropriate compaction of the backfill and the period of backfilling are some factors that influence the backfill in any basement.
In this article we discuss the steps involved in the proper backfilling of basement.
The Procedure for Proper Backfill in Basement
The procedure of backfilling a basement involves four essential steps: 1) Protection of basement structure, 2) Selection of backfill material, 3) Compaction of backfill, 4) Surface finish of backfill.
1. Protection of Basement Structure
Basement structures such as foundation, basement wall or retaining wall provide a permanent formwork for the backfilling materials. But the protection of these structures from the backfill loads is an important part of the backfill.
Backfilling is not recommended right after the casting of basement retaining structures. Once the basement structures have achieved their final strength, the backfilling process can be taken up.
Usually, contractors start the backfilling process after the casting of a slab of the basement or even before the first-floor deck is placed being negligent in regards to the safety of the structure. Backfilling shouldn't be initiated until it is ensured that the supporting structures have reached their maximum strength.
In certain conditions, where backfill is to be done right after the construction of the basement structure, bracings are provided to hold the structure and provide the support for the backfill loads coming upon the structure.
Corners, offsets, and pilasters add structural stiffness to the foundation wall. But we figure as a rule of thumb that the stiffening effect is negligible more than 12 feet away from the corner. Hence only straight spans longer than 24 feet need bracing.
Bracing is done either vertically or horizontally with the use of steel braces as per the site requirement. All the building codes suggest the use of bracing in all basement backfilling works, even if the structure is made of concrete.
2. Selection of Backfill Material
Using granular material for the backfill of the basement is never recommended as the voids between the granules attract water towards the basement wall. And if the drain tile system gets plugged, it may cause some severe problems.
It is recommended to use the same excavated soil as the backfill with proper compaction only if the soil excavated is of good quality.
If heavy, moisture-laden, or expansive clays are removed during excavation, granular backfill with an exterior drainage system would be the best choice.
Backfilling with clay, clumps, roots and tree branches do not compact to the full extent and retains waters even if the backfill has a proper drainage system
3. Compaction of Backfill in Basement
If due to inacessiblity or the remoteness of the site, one has to make-do with loose excavated soil or granular soil as the backfill, special care should be taken to achieve the desired compaction levels. With time, the natural abraders like water and tectonic movements may breach into the foundation and cause superfluous settlement which could prove perilous to the superstructure.
To mitigate this problem, proper compaction procedures are undertaken. There are many types of compaction procedures that can be used to compact the backfill in the basement. The selection of the compaction methods depends upon the site conditions.
Using heavy compactor machines is not recommended in the backfill of basement as the vibrations caused by the machine can damage the basement structure. Hand driven vibrators or manual compaction is strongly recommended. Vibrating plate compactors can also be used for granular backfill without damaging the wall.
Manual compaction is done by applying 6 to 9 inches of soil bed and compacting it by hand rammer. This is a lengthy and time-consuming process, but the method provides insurance against wall damage and against callbacks related to drainage problems.
4. Surface Finish of Backfill in Basment
The compacted backfill must have a hard, smooth and sloping surface to drain off the water. Usually, for building plinth, protection is provided. It drains off the roof water and protects the foundation backfill from the water.
The minimum slope for a finished grade should be 1/2 inch per foot. Avoiding basement leakage is a lot like preventing a roof from leaking. It must be taken care that the is water moving and prevent it from collecting in one spot. Correct backfilling methods help to accomplish this goal.
Read More : Backfilling in Foundation : Types and Procedure