Unbalanced fill, also known as unbalanced backfill height, is the difference in height between the exterior finish ground level and the lower of two points: the top of the concrete footing that supports the foundation wall, or the interior finish ground level.
Understanding Unbalanced Fill
Essentially, unbalanced fill is a measure of the soil level difference between the outside and inside of a foundation wall. This difference can create lateral pressure against the foundation. Here's a more detailed breakdown:
- Exterior Finish Ground Level: This is the final level of the ground on the outside of your building after landscaping and construction are complete.
- Top of the Concrete Footing: This is the top surface of the concrete base that supports the foundation wall.
- Interior Finish Ground Level: This is the final level of the ground inside the building after construction.
How Unbalanced Fill is Calculated
The unbalanced backfill height is determined using the following process as indicated in reference [1]:
- Identify the Exterior Finish Ground Level: Determine the level of the soil on the outside of the building.
- Identify the Top of the Concrete Footing: Determine the height of the top of the concrete footing.
- Identify the Interior Finish Ground Level: Determine the level of the soil inside the building.
- Compare the Footing Top and Interior Finish Ground Levels: Determine which of these two is lower.
- Calculate the Difference: Subtract the lower value (either the top of the concrete footing or the interior finish ground level) from the exterior finish ground level.
Unbalanced Fill = Exterior Finish Ground Level - (Lower of Top of Footing or Interior Finish Ground Level)
Practical Insights
Understanding unbalanced fill is crucial in construction because it directly relates to:
- Lateral Pressure: A significant unbalanced fill creates substantial lateral pressure against foundation walls. This is because the soil on the exterior side pushes against the wall.
- Structural Integrity: Improperly designed or managed unbalanced fill can lead to foundation cracking, bowing, and even failure.
- Water Drainage: Differences in ground level can affect how water drains around the foundation. It is important to control water to avoid hydrostatic pressure.
Example
Let's say:
- The exterior finish ground level is 10 feet.
- The top of the concrete footing is 5 feet.
- The interior finish ground level is 6 feet.
In this example, the lower value between the top of the footing and the interior ground level is 5 feet (top of the footing). Therefore, the unbalanced fill would be 10 feet - 5 feet = 5 feet.
Key Considerations
- Soil Type: The type of soil used as backfill affects the amount of pressure exerted on the foundation wall. Some soils are heavier and exert more pressure.
- Water Content: Saturated soil adds significant lateral pressure to the foundation walls.
- Drainage: Proper drainage systems are essential to mitigate the effects of unbalanced fill and prevent water-related damage.
