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The calculation of the number of passes for compactors and lift (soil layer) thickness for soil is critical to attain the required degree of compaction. Generally, the lift thickness varies between 15-30cm based on soil type, and the majority of compaction is achieved through the first five passes.
One can also use field tests and expertise to determine lift thickness and the number of passes for a specific soil type and condition.
Strip test is one such test that enables engineers to specify the number of passes and machinery type. Additionally, various lift-thickness versus achieved soil density graphs can be used to find the optimum lift thickness and number of passes.
The safety and reliability of buildings and roads are heavily dependent upon the strength of compacted soil below. Improper compaction (non-uniform compaction) may cause building settlements and lead to potholes in roadways.
How to Determine the Number of Passes and Lift Thickness for The Compaction of Soil?
- The thickness of lifts is an important factor in soil compaction because no matter how advanced a compactor is, thick lifts cannot be adequately compacted.
- Suitable lift thickness decreases the number of passes and hence improves production rates.
- One can plot a graph between lift thickness and density to determine lift thickness. Begin with a lift of 15cm and add 7.5 cm until you find the optimum lift thickness and number of passes for the given condition.
- One can use strip test to determine the number of passes and equipment types to achieve the required degree of compaction for the given soil type and condition. Fig.1 shows the performance of different compactors at a certain number of passes.
- By and large, lift thickness varies between 15cm and 30cm based on soil conditions.
- As a rule of thumb, lift thickness equals the maximum aggregate size of soil times four.
- Sometimes, compactor manufacturers provide ideal maximum lift thickness for the machine. However, you should consider 75% of the ideal lift depth in the field.
- In the case of large lift thickness, bridging may occur at the bottom of lifts; hence the project may suffer problems in the future.
- The number of passes required to obtain the required compaction depends on the lift thickness, contact pressure, and soil moisture content. However, tables in the literature provide the number of passes of a specific compaction machine for a certain type of soil.
- Table-1, based on field experience, presents a number of passes of various compactors for each soil type.
- Most of the compaction (high density) is achieved within five passes based on soil conditions. Additional passes would slightly increase soil density and sometimes can produce adverse effects.
- Determination of lift thickness and number of passes may not be enough to get desirable compaction because soil properties and moisture content may vary. Therefore, supervise the compaction process and make suitable changes to achieve uniform compaction throughout the entire project.
- Carelessness during pass counts and coverage can lead to non-uniform compaction.
- Energy returns to the compactor driver as the soil gets compacted. The energy returned gets more prominent as the compaction degree increases, which may be used as a sign for reaching required compaction.
- When the soil starts to crack, it means the number of passes has exceeded the required limit, and the soil is over-compacted.
- Use the same soil material for each lift; otherwise, you cannot obtain uniform compaction. Non-uniform compaction can be problematic for roads and structures.
- Placement of uniform lift yields uniform compaction if materials are the same with optimum moisture content.
| Type of compactor | Types of soil | Lift thickness, cm | Number of passes |
| Sheepsfoot roller | Fine-grained soil | 15 | 4-6 |
| Sheepsfoot roller | Dirty coarse-grained soil with more than 20% passing sieve No. 200 | 15 | 6-8 |
| Rubber tire roller | Clean coarse-grained soil with 4-8% passing sieve No. 200 | 25 | 3-5 |
| Rubber tire roller | Fine-grained soil or well-graded dirty coarse-grained soil with more than 8% passing sieve No. 200 | 15-20 | 4-6 |
| Smooth wheel roller | Well-graded sand-gravel mixtures | 20-30 | 4 |
| Smooth wheel roller | Fine-grained soils except in earth dams | 15-20 | 6 |
| Vibrating baseplate compactor | coarse-grained soil with less than 12% passing sieve No. 200, Material with 4-8% passing sieve No. 200 placed thoroughly wet | 20-25 | 3 |
| Crawler tractor | Coarse-grained soils with less than 4-8% passing sieve No. 200 placed thoroughly wet | 25-30 | 3-4 |
| Power tamper or rammer | Silt or clay | 10-15 | 2 |
| Power tamper or rammer | Coarse-grained soils | 15 | 2 |
FAQs on number of passes and lift thickness for soil compaction
The number of passes can be found by strip test and field experience.
The soil layer that is being compacted is termed as lift.
It is a soil layer thickness that can be properly compacted by compaction machinery. The lift thickness ranges from 15-30cm based on soil type and compaction equipment.
No, over compaction creates cracks in soil and decrease its density.
It means that in-situ soil would be compacted to 95% of the maximum dry density. Commonly, the proctor compaction test is used to determine the maximum dry density of soil and its optimum moisture content.
Read More:
Different Types of Soil Compaction Equipments -Types of Rollers
Proctor Soil Compaction Test – Procedures, Tools and Results
Compaction of Soil – Test Methods of Soil Compaction and their Uses
How to Select Compaction Machine Based on Soil Type? [PDF]
