Bulking & compaction relate to the effect on the volume of materials when excavated or filled. You may think you’re excavating 1m³ of soil, or filling a 1m³ hole full of Type 1. However, once you’ve excavated the material, or filled the hole, you realise that more material than expected was excavated, and more material is needed to fill the hole.
In this article, we’re going to explore why this happens, go through the compaction and bulking factors of different materials, and take you through the calculations in some real-world examples.
Why does the volume of filled or excavated material change?
Well, it’s all to do with air pockets and voids between the particles. Before excavation, soil or rock is in its original state where particles are tightly packed together. When excavated, the material is disturbed and broken into different sized pieces. These pieces do not fit together as easily as they did in their consolidated form, resulting in increased void spaces, reduced density, and an increase in volume. When filling and compacting material into the ground, the inverse effect occurs.
What are the compaction and bulking factors?
For clay, you have a bulking factor of 1.3 and a compaction factor of 0.8. For soil, a bulking factor of 1.25 and a compaction factor of 0.9.
How do you use these bulking factors in your calculations?
Well, if we assume you are excavating 15m³ of clay, you would multiply this figure by the bulking factor of 1.3 to give you the volume of excavated material, in this case 19.5m³. Conversely, let’s say you were filling a 15m³ hole full of clay. You’d need to divide the in-situ volume of 15m³ by the compaction factor of 0.8 to give you the volume required to fill the hole, which is 18.75m³.
You’ll notice that the volume required to fill the hole after compaction is different to the volume of material excavated. That is because bulking and compaction are not the inverse of each other, a mistake some people make. Natural ground has geological consolidation, long-term loading, and lower void ratios than site-compacted material. Therefore, even well-compacted material rarely returns exactly to its original in-situ density.
