Step by Step Guide to Calculate Excavation Quantity for Foundations

Calculating the excavation quantity for foundations is a crucial task in any construction project. Getting this right helps ensure the project stays on schedule and within budget. If the excavation volume is underestimated, it can lead to delays and extra costs. Overestimating, on the other hand, means unnecessary work and wasted resources. This guide breaks down the process into clear steps anyone involved in construction can follow.

Foundation excavation site showing soil layers and excavation depth

Understand the Foundation Type and Dimensions

Before starting any calculations, you need to know the type of foundation you are working with. Common types include strip foundations, isolated footings, raft foundations, and pile foundations. Each has different shapes and sizes, which affect the excavation volume.

Gather the following information from the project plans:

• Length, width, and depth of the foundation trenches or pits

• Number of footings or trenches

• Any additional features like steps or slopes in the excavation

  
  

For example, a strip foundation might require a trench 0.5 meters wide, 30 meters long, and 1 meter deep.

Calculate the Basic Excavation Volume

Once you have the dimensions, calculate the volume of soil to be removed. The basic formula for volume is:

Volume = Length × Width × Depth

If you have multiple footings or trenches, calculate the volume for one and multiply by the number of units.

For example, if a footing is 2 meters long, 2 meters wide, and 1.5 meters deep:

• Volume = 2 × 2 × 1.5 = 6 cubic meters

  
  

If there are 10 such footings:

• Total volume = 6 × 10 = 60 cubic meters

  
  

Account for Soil Swelling and Shrinkage

Excavated soil does not occupy the same volume once removed. It tends to swell, increasing in volume, which affects transportation and disposal. The swelling factor varies depending on soil type but typically ranges from 10% to 30%.

To adjust for swelling, multiply the calculated volume by the swelling factor. For example, if the swelling factor is 1.2 (20% increase):

• Adjusted volume = 60 × 1.2 = 72 cubic meters

  
  

This means you should plan for 72 cubic meters of excavated soil to handle.

Include Extra Volume for Working Space and Over-Excavation

Excavation rarely matches exact foundation dimensions. Workers need extra space to move, and sometimes the soil is removed beyond the planned depth or width to ensure proper foundation placement.

Add about 5% to 10% extra volume to cover these factors. Using the previous example:

• Extra volume (10%) = 72 × 0.10 = 7.2 cubic meters

• Total excavation volume = 72 + 7.2 = 79.2 cubic meters

  
  

Rounding up, plan for about 80 cubic meters of excavation.

Consider Soil Conditions and Water Table

If the site has loose soil or a high water table, excavation might require additional volume for dewatering or shoring. These conditions can increase the excavation area or depth.

Discuss with the site engineer or geotechnical expert to understand if adjustments are needed. For example, if shoring requires a 0.5-meter wider trench, recalculate the volume with the new width.

Document and Review Your Calculations

Write down each step of your calculation clearly. Include:

• Foundation dimensions

• Number of units

• Swelling factor used

• Extra volume percentage

• Any special site conditions

  
  

Review your numbers with the construction team or supervisor to catch any errors or overlooked details.