Backfill quantity starts with the volume of space left after foundation work is placed. The basic calculation is simple: calculate the excavation volume, subtract the volume occupied by the permanent foundation work, then account for the material and compaction requirements.
For a foundation trench, this means measuring the excavated space and subtracting the concrete, masonry or other permanent material occupying the trench. The remaining volume represents the basic backfill space.
Basic backfill formula:
The actual material required for compacted backfill depends on the specified fill material, compaction requirements and project conditions.
Table of Contents
- What Backfill Quantity Means
- The Basic Backfill Formula
- Calculate the Excavated Volume First
- How to Calculate Foundation Backfill
- Subtract Concrete, Masonry and Other Permanent Work
- Worked Foundation Backfill Example
- Backfill for Foundation Trenches
- Backfill Around Isolated Footings
- Backfill With Different Depths
- Compacted Fill vs Loose Soil
- How to Calculate Imported Backfill
- How Much Excavated Soil Can Be Reused?
- Backfill Allowances and Soil Loss
- Common Backfill Calculation Mistakes
- Backfill Calculator
- Frequently Asked Questions
What Backfill Quantity Means
Backfill is material placed into an excavated area after the required below-ground construction has been completed. Depending on the project, backfill might consist of suitable excavated soil, imported soil, selected fill, granular material or another specified material.
The quantity describes the space that needs to be filled, not automatically the amount of loose soil delivered to the site.
Three different quantities often appear during earthwork calculations:
| Quantity | Meaning |
|---|---|
| Excavation volume | Volume of ground removed from the original position. |
| Backfill space | Space remaining after permanent below-ground work occupies part of the excavation. |
| Loose fill quantity | Amount of loose material required before placement and compaction. |
Keeping these quantities separate helps prevent errors when estimating soil, imported fill, transport and disposal.
The Basic Backfill Formula
For a simple excavation with known permanent work dimensions, use:
For several excavation sections:
The formula works when the excavation and permanent work are measured on the same basis and occupy the same section.
Do not subtract the foundation volume from the wrong excavation dimensions. Calculate matching sections separately when trenches, footings or foundation levels have different dimensions.
Calculate the Excavated Volume First
Backfill calculations begin with the excavation quantity.
For a rectangular excavation:
For example, a foundation trench measuring 20 m long, 0.8 m wide and 1.2 m deep has:
The excavation volume is 19.2 m³.
If the foundation concrete later occupies 7.2 m³ of the trench, the remaining space becomes the basic backfill volume.
The basic backfill space is therefore 12 m³.
How to Calculate Foundation Backfill
Foundation backfill usually involves several steps:
- Determine the excavation dimensions.
- Calculate the excavation volume.
- Determine the volume occupied by the permanent foundation work.
- Subtract the permanent volume from the excavation volume.
- Separate reusable excavated soil from imported fill.
- Account for the specified placement and compaction requirements.
For a simple rectangular trench, the process looks like this:
Backfill = Excavation − Foundation Work
When the foundation contains several different sections, calculate each section separately.
Subtract Concrete, Masonry and Other Permanent Work
The excavated space becomes smaller after permanent construction is placed inside the excavation.
Typical permanent materials occupying excavation space include:
- Foundation concrete
- Strip footings
- Foundation walls
- Concrete blocks
- Reinforced concrete elements
- Drainage structures
- Other permanent below-ground construction
Calculate the volume of each permanent element and subtract the appropriate quantities from the excavation.
Example
Suppose an excavation has a volume of 30 m³. Permanent foundation work occupies 18 m³.
The remaining backfill space is 12 m³.
Use matching boundaries. If a concrete footing occupies only part of an excavation, subtract the footing volume from the corresponding excavation section rather than from an unrelated area.
Worked Foundation Backfill Example
Consider a foundation trench with these dimensions:
- Length: 30 m
- Excavation width: 0.9 m
- Excavation depth: 1.2 m
Step 1: Calculate excavation volume
Step 2: Calculate the permanent foundation volume
Suppose the concrete foundation occupies 16.2 m³.
Step 3: Subtract the permanent work
The basic backfill space is 16.2 m³.
Step 4: Determine the material source
The next question is whether suitable excavated soil is available for reuse or whether imported fill is required. This is a site and specification decision rather than a mathematical assumption.
Result: The foundation leaves 16.2 m³ of basic backfill space based on the stated excavation and permanent work volumes.
Backfill for Foundation Trenches
Long foundation trenches are often easier to calculate by dividing the work into sections.
For a constant rectangular trench:
Then subtract the volume occupied by the foundation.
Example
A trench is 40 m long, 0.7 m wide and 1 m deep.
Suppose the foundation construction inside the trench occupies 11.2 m³.
The basic backfill space is 16.8 m³.
If the trench contains different foundation sections, calculate each section separately before combining the results.
Backfill Around Isolated Footings
Isolated footing pits often have open space around the finished footing. Once the footing and column starter or pedestal are placed, the remaining space requires backfill where specified.
For a rectangular footing pit:
Then subtract the volume of the permanent work inside the pit.
Example
Assume one footing pit measures 2.5 m × 2.5 m × 1.5 m.
If the footing and pedestal occupy 3.375 m³:
Each identical pit therefore leaves 6 m³ of basic backfill space.
For eight identical pits:
The total backfill space is 48 m³.
Backfill With Different Depths
Foundation layouts often contain different excavation depths. A stepped foundation is one example.
Do not use one average depth unless the project calculation specifically calls for an average section.
Instead, divide the excavation into sections.
| Section | Excavation | Permanent Work | Backfill |
|---|---|---|---|
| A | 12.0 m³ | 7.0 m³ | 5.0 m³ |
| B | 18.0 m³ | 11.0 m³ | 7.0 m³ |
| C | 9.5 m³ | 6.5 m³ | 3.0 m³ |
Add the backfill quantities:
This section-by-section approach provides a clearer quantity when foundation levels differ.
Compacted Fill vs Loose Soil
A major source of confusion in backfill estimates is the difference between compacted volume and loose material volume.
When soil is excavated, the original soil structure is disturbed. When fill is placed and compacted, the material occupies a different volume again.
| Term | Meaning |
|---|---|
| In-situ soil | Soil in its original ground condition. |
| Loose soil | Excavated soil after disturbance and before compaction. |
| Compacted fill | Placed material after achieving the specified compaction condition. |
Therefore, a requirement for 10 m³ of compacted backfill does not automatically mean ordering exactly 10 m³ of loose material.
Use project specifications for compaction and material requirements. Soil type, moisture, placement method and required compaction affect the quantity of loose material needed.
How to Calculate Imported Backfill
Imported backfill is required when suitable excavated material is insufficient or when the project specification requires a particular fill material.
Start with the required backfill space:
Then determine how much suitable excavated soil is available for reuse.
For example, suppose the required backfill space is 20 m³ and 12 m³ of excavated soil is approved for reuse.
The basic imported fill requirement is therefore 8 m³ before considering the difference between loose delivered material and compacted placed material.
How Much Excavated Soil Can Be Reused?
Not every excavated material is suitable for backfill. Reuse depends on the project specification and soil characteristics.
Factors affecting suitability include:
- Soil type
- Organic content
- Moisture condition
- Particle size and grading
- Presence of debris
- Required compaction
- Location of the backfill
- Drainage requirements
- Engineering or geotechnical requirements
For example, material suitable for general landscaping fill might not satisfy a specified structural backfill requirement.
Do not assume all excavated soil becomes backfill. Separate the excavation quantity from the quantity approved for reuse.
Backfill Allowances and Soil Loss
Backfill calculations should reflect the actual construction method. An automatic percentage does not apply to every project.
An allowance might be relevant where the project has known handling losses, uneven excavation, material degradation or other documented conditions.
If a specified allowance is required, use:
For example, a calculated requirement of 15 m³ with a specified 5% allowance gives:
The allowance should come from the project's estimating method, site conditions or specification rather than an arbitrary rule.
Common Backfill Calculation Mistakes
Using excavation volume as the backfill quantity
The entire excavation is not normally refilled with soil. Permanent concrete, masonry and other below-ground work occupy part of the excavated space.
Forgetting foundation thickness
Concrete footings and foundation walls reduce the remaining backfill space. Include their actual volume.
Using one calculation for different foundation levels
Separate sections with different depths or widths should receive separate calculations.
Confusing loose soil with compacted fill
Loose material and compacted material have different volume relationships. Follow the specified compaction requirement.
Assuming all excavated soil is reusable
Only suitable material should be counted as reusable backfill.
Ignoring imported fill
If reusable excavated material is insufficient, calculate the remaining imported fill separately.
Applying an arbitrary percentage
Backfill allowances depend on the project. Use the stated estimating or construction requirement rather than adding an automatic percentage.
Mixing cubic metres and tonnes
Volume and weight are different measurements. A density value is required when converting between them.
Backfill Calculator
A useful backfill calculation follows a simple sequence:
− Permanent Work Volume
= Basic Backfill Volume
For more detailed estimates, separate reusable excavated soil, imported fill and compacted placement quantities.
Calculate Construction Quantities
Use BuildersMeasures tools for common construction measurements and material estimates.
Explore BuildersMeasuresFrequently Asked Questions
How do you calculate backfill quantity?
Calculate the excavation volume first, calculate the volume occupied by permanent below-ground work, then subtract the permanent work volume from the excavation volume.
What is the basic formula for backfill?
The basic formula is Backfill Volume = Excavation Volume − Permanent Work Volume.
How do you calculate foundation trench backfill?
Calculate the trench excavation using length × width × depth. Then subtract the volume occupied by the footing, foundation wall, concrete or other permanent work inside the trench.
Does backfill equal excavation volume?
No. Permanent foundation work occupies part of the excavation, so the remaining backfill space is normally smaller than the original excavation volume.
How do you calculate backfill around a footing?
Calculate the footing pit volume, then subtract the volume of the footing and other permanent elements occupying the pit. Repeat the calculation for each different footing type.
Is excavated soil suitable for backfill?
Suitability depends on soil properties and project requirements. Soil might require testing or approval before reuse, especially where specified compaction or drainage performance is required.
What is the difference between loose and compacted backfill?
Loose backfill refers to material before placement and compaction. Compacted backfill refers to material after placement and compaction to the specified condition. The two quantities do not necessarily have the same volume.
How do you calculate imported backfill?
Subtract the quantity of suitable excavated soil approved for reuse from the required backfill volume. The remaining quantity forms the basic imported fill requirement before accounting for placement and compaction differences.
Post a Comment