All construction work involves some form of excavation. It may be a shallow hole for the crawl space or a deep one to accommodate a full-depth basement foundation. It could also be small holes for footings for a house to be built on a slab. Whatever the case, the hole must be filled properly once the footings are constructed and the concrete poured.
What Is Backfill?
Backfill refers to soil that is used to cover holes or trenches around the foundation. This can be native soils that are dug out from the ground or a mixture of rocks, stones, and imported soil. Because soils have different properties, each might require different backfilling techniques.
Backfilling refers to the process of replacing the soil that was removed during the excavation of slabs, foundations, and other groundwork. The process requires heavy equipment, skill, and knowledge of the soil conditions. Soil type, site parameters, and building design all determine the suitability of backfill during construction.
Methods of Backfilling
There are a few common methods of backfilling, including water jetting, compacting trenches, and flowable fill.
Trench compaction: Here, an excavator, a compactor, or some other mechanical means are used to compact loose material into the trench. Soil is backfilled in layers of four to six inches depending on the type of compaction equipment used and the nature of the backfill. Water may be added to help with the compaction.
Flowable fill: A cementitious material that has a low water-to-cement ratio is used in this method. Before backfilling with this material begins, utility pipes are covered with aggregate material. The main challenge of using flowable fill for backfilling is liquidity – it must be contained to prevent it from flowing into other trench areas.
Water jetting: This technique utilizes a probe to apply pressurized water to the bottom of the fill. The force of the jetted water is used to move backfill material around. Preventative measures must be taken to contain the sediment-filled water. This backfilling method is suitable for sandy soils or those with highly fissured bedrock.
Improper Backfill Can Cause Foundation Damage
The excavation usually takes place in undisturbed soil and creates a bowl in which the foundation sits. The problem is that backfilling, even when done well, can be problematic for the foundation. Improper backfilling almost guarantees future foundation damage.
This is because excavated soil becomes loose and must be compacted when backfilling the trench. Regardless of how well the compaction is done, it doesn’t compress the soil as well as the undisturbed soil. This creates two types of soil – loose backfill and compacted virgin soil – that react differently with water.
Water in the Clay Bowl
Homes throughout Missouri often experience the clay bowl effect in wet months. This phenomenon is a natural consequence of excavating and backfilling during the initial construction of the foundation.
The backfill soil is more porous and absorbent than the compacted soil around it. When there is a snowstorm or heavy rain, the backfill gets saturated with water. With nowhere to go, the water starts pushing against the foundation walls. This can lead to wall cracks that might let water into the basement or crawl space.
The clay bowl effect also causes shallow foundations to heave. On the other hand, if the surrounding trees and plants consume the excess water, the soil could become too dry, compact, and create a void.
The result is a cracked and sinking foundation.
Heaving is not likely to occur in deeper foundations because the bottom lies below the frost line. However, lateral pressure along the foundation may occur when the backfill gets soggy and the undisturbed soil swells after absorbing water. This causes the mortar joints in the masonry foundation to crack and the walls to bow inward or bulge in the middle.
Protect Your Foundation
If the water table on your property is high or your foundation lacks proper waterproofing, the clay bowl effect can devastate your home. For the perfect solutions tailored to your home’s repair needs, contact the experts at Foundation Recovery Systems. To get started, request a free inspection and repair quote.