Backfilling Quantity Calculator
What is a Backfilling Quantity Calculator?
What is Backfilling Quantity?
The Backfilling Quantity Calculator computes the net void space between excavated pit boundaries and completed underground concrete structures.
Formula
Net Backfill Bank Volume = Total Pit Excavation Volume - Underground Structure Volume
Required Loose Fill = Net Backfill Bank Volume × Compaction Factor (1.15 to 1.25)
Variables Explanation
- Pit Volume: Gross excavated pit volume (m³).
- Structure Volume: Concrete footings, columns, and pipe volume inside the pit.
- Compaction Factor: Extra loose soil needed to achieve specified field proctor density.
How to Calculate
1. Enter total gross pit excavation volume.
2. Sub-grade structural concrete volume.
3. Input soil compaction shrinkage factor (e.g. 1.20).
4. View net backfill volume and loose import soil required.
Worked Example
Foundation Pit: 500 m³ gross excavation.
Cast Footings & Columns: 180 m³ concrete structure inside pit.
Net Backfill Bank Volume = 500 - 180 = 320 m³.
Loose Soil Import (1.20 compaction factor) = 320 × 1.20 = 384 m³ loose soil.
Practical Construction & Project Controls Use
Determining imported soil borrow pit orders, estimating backfill roller compaction passes, and site earthwork balance audits.
Understanding the Result
Ensures sufficient loose soil is imported to fill voids completely after compaction to maximum dry density.
Limitations & Assumptions
Assumes uniform compaction density throughout backfill layers (typically 150mm to 300mm lifts).
Frequently Asked Questions
Why is compaction factor applied to backfill soil?
Loose fill contains air voids; compacting it to 95% Modified Proctor density requires ~20% more loose volume than net bank void.
What materials are suitable for foundation backfilling?
Well-graded granular soil, sand, or crushed stone aggregate free from organic matter and large boulders.
How is backfill volume calculated for pipe trenches?
Subtract the pipe cylinder volume (π × r² × L) and bedding layer from gross trench volume.
What lift thickness should be used during backfill compaction?
Soil should be placed and compacted in thin layers (lifts) of 150mm to 200mm thickness for uniform density.
What is the risk of poor backfill compaction?
Inadequate compaction leads to future ground settlement, cracking concrete pavements and damaging utility lines.