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Backfill is used to fill the gaps between the foundation and the earth. Find out which materials are used for it and why it must be compacted.

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Every time a structure is being built, the soil needs to be excavated to make room for the foundation. Naturally, the hole is always bigger than the foundation, so when the construction is done, there are gaps and holes around the foundation walls. After the concrete has completely cured, these holes need to be filled within five to seven days. Otherwise, water damage might occur. Most contractors use the backfill soil to place in the holes, while others use alternative refill materials. 

Read on as we take a closer look at the backfill, its types, and its purposes. 

foundation backfill

What Is Backfill? 

Any material that is used to refill an excavated hole is called backfill. It can be the soil that has been dug out to make that hole, a mixture of gravel and sand, or some commercial product. The backfilling process is done in layers instead of all at once. After the foundation is made and the floor joists are built above it, contractors begin to fill the holes.  

So, why is backfilling important? By filling the gaps between the foundation and the earth, contractors are reinforcing the foundation and enabling it to bear the load from your home or building. Backfilling also increases the structural stability of the building and promotes exterior water drainage. During this process, it is important to choose the most suitable backfill material and the best way to compact it. 

Types of Backfill 

Depending on the project, drainage requirements, and other factors, your contractor will choose the most suitable type of backfill. Here are the materials that are often used for backfilling. 

Coarse-grained soil: This combination of gravel, sandy soil, and a trivial amount of fine materials provides good foundation support. It is also very easy to compact. 

Limestone screenings: Another type of backfill that is easy to compact is limestone screenings. This backfill is usually used for sewer and pipe backfill, as well as a base for brick paving. 

Trench backfill: This type of backfill is made of small aggregate that drains and compacts well. 

CA7 bedding stone: One of the backfills that contractors mostly use is the CA7, otherwise known as ¾” limestone. This angular white/gray stone compacts on its own, which is why builders love to use it. It is commonly referred to as “bedding stone” and is used for pipe bedding, subbase, and improving soil drainage. 

CA6 base stone: This type of backfill is also known as subbase granular backfill because it is mostly used along roadway shoulders. However, some contractors use it for residential projects. 

3” coarse stones: When it comes to projects that require larger amounts of material, the 3” stones are ideal for the first layer of material. These stones can easily fill in large areas while leaving some voids, which are great drainage properties. Once the trench is filled, a top layer of CA6 base stone can provide additional fill and compaction. 

Commercial by-products: Depending on the condition of the construction site and engineering properties, the contractor will choose the most suitable commercial backfill. This type of backfill is used where natural backfills do not exist and contractors are forced to use fly ash, furnace slag, or some other by-products as a substitute. 

How to Backfill Your Foundation 

Backfilling the foundation is a process that involves several steps. First, the area that will be backfilled is cleaned after the foundation has completely cured. If after the construction and before backfilling rain fell and flooded the trench, the water will be pumped out.  

Based on several factors, the contractors will choose the most suitable material for backfilling. The backfill material is not placed in the hole all at once. Instead, the contractor will refill the trenches in layers. These layers are usually about seven inches thick.  

The contractor first fills the corners and then the middle. After the backfill material is placed in the trench it is compacted with a roller or some other machine that has the same capabilities. Each of the layers is compressed before the next layer is refilled. In the end, all the layers are watered and compacted with steel or wooden log rammers. 

Compacting the Backfill 

If the soil along your home’s perimeter is not backfilled properly, it can become loose. It mustn’t stay that way, otherwise it will easily become saturated with rainwater or melting snow. Hydrostatic pressure will rise and eventually the water will damage your foundation and infiltrate your basement walls. This can cause cracks and bowing walls. 

Compacting the backfill properly is extremely important. Unfortunately, this soil can never be as dense as it was, no matter which machines are used. Regardless of how hard the builders try to compact it, it will never be as it was. Because the backfill soil is more porous than the soil that has not been tampered with, it creates a” clay bowl” around your home. Therefore, it is possible that when rain falls or there is a flood, water will collect in this artificial water table. Luckily, with an interior drainage system that will collect the excess water and direct it to the sump pump, wall anchors that will prevent the walls from bowing, and professional foundation waterproofing, you can avert potential damage. 

If you want to waterproof your foundation after backfilling or you wish to see if anything is wrong with your drainage, contact professionals at AFS. Schedule a free inspection appointment and one of our experts will evaluate your foundation, determine whether there are any signs of damage, and recommend suitable waterproofing solutions. Contact us today and be one step closer to having a waterproofed home. 

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