What Is a Swell in Construction and Why It Matters

June 12, 2024

Understanding soil volume changes during construction is crucial for effective project planning and cost management. Mismanaging these shifts can lead to unexpected project costs and significant delays. Let's explore the intricacies of soil volume changes, their impacts on construction, and how they can be managed effectively.

Soil Composition and Units of Measure

Soil is a composite material consisting of air, water, and solid grains. The void or pore space between these grains is filled with a mix of air and water, visually represented through a phase diagram. In the industry, soil is typically measured in cubic yards (CY), with three primary conditions for soil states: Bank (BCY), Loose (LCY), and Compacted (CCY).

Soil States

Bank Cubic Yard (BCY): This is soil in its natural, undisturbed state prior to excavation.

Loose Cubic Yard (LCY): This refers to soil that has been excavated and transported, which tends to fluff up and increase in volume.

Compacted Cubic Yard (CCY): This is soil that has been placed in a fill area and compacted to reduce its volume.

Shrink and Swell Behavior

Understanding the concepts of soil swelling and shrinking is fundamental for managing construction projects.

"Construction Swell Surf Camp" by Swell Surf Camp is licensed under CC BY-SA 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/2.0/.


When soil is excavated and moved, it increases its void space and swells. This means that Bank Cubic Yards (BCY) transition into Loose Cubic Yards (LCY). For instance, 1 BCY might swell into 1.25 LCY.


Conversely, when soil is compacted, its void space is reduced, leading to shrinkage. Soil shrinks from both BCY and LCY to CCY, often resulting in a 10% reduction from BCY to CCY and up to 28% from LCY to CCY.

Management and Planning Implications

Accurately accounting for soil volume changes is essential to avoid project surprises. Here are three typical scenarios:

Balanced Site

In this scenario, the total BCYs excavated match the required CCYs post-compaction, reducing the need for external soil movement.

"Construction Swell Surf Camp" by Swell Surf Camp is licensed under CC BY-SA 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/2.0/.

Export or Waste Project

If a site has excess soil, it requires transporting the surplus offsite. Soil swells into LCY during transportation, increasing the volume and the cost contractors pay for removal.

Import or Borrow Job

This scenario occurs when a site lacks enough soil, necessitating the import of additional material. Contractors must purchase more LCY than required CCY due to shrinkage during compaction.

Challenges and Limitations

Predicting soil volume changes is not straightforward, as there is no standard amount for soil shrinkage or swelling. Factors influencing these changes include:

  • Variability in soil types
  • Differences in soil moisture content
  • The type of compaction equipment and energy used

An experienced professional must evaluate soil conditions individually for each project to estimate accurate shrink and swell factors.

Shrink–Swell Soils: Causes and Effects

Shrink–swell soils, also known as expansive soils, exhibit volume changes due to moisture variations, leading to significant geotechnical challenges. In the UK, these soils are mainly found in the south-east, whereas in the USA, they affect one in four homes, causing extensive damage and economic costs.

"Orange Oak Cup" by David D. Gilbaugh is licensed under CC BY-NC-ND 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-nd/2.0/.

Heave and Subsidence

Heave: Uneven swelling pressures lift parts of structures, often damaging corners or centers of buildings.

Subsidence: The lowering or collapse of ground due to factors such as artificial disturbance or changes in drainage, which can lead to foundation damage evident as diagonal wall cracks.

Influence of Climate and Urban Development

Dry weather and high temperatures can increase the risk of subsidence in clay soils. Additionally, urban factors such as tree root interactions and impermeable paving surfaces can exacerbate soil shrinkage and subsidence issues.

Modern Solutions and Recommendations

Technological advances like the SHRINKiT system and the 3D Swell-Strain apparatus have improved the measurement of soil properties. Predictive maps combining geological data and climate projections help determine areas at risk, particularly in south-east England. Practical recommendations include using permeable paving surfaces and being mindful of the impact of landscaping near buildings.

In conclusion, understanding and managing soil volume changes are pivotal for construction project success. Engineers must conduct thorough soil analyses and apply strategic planning to mitigate potential risks and ensure project efficiency and cost-effectiveness.

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