In-ground swimming pools. Underground utilities. Brand new landscaping. All these jobs have one thing in common: they require excavation.
Known as the process of removing earth to create space for construction, excavation is crucial for projects that involve significant groundwork and foundation preparation.
Done right, you ensure project efficiency, safety, and cost-effectiveness, but if done incorrectly, then costly delays and risks can arise. And that's why it's so important to conduct accurate excavation quantity takeoffs.
This guide will fill you in on the fundamentals of excavation quantity takeoffs. It'll cover how to conduct them, what calculations to use, and tools to help you be more accurate and efficient.
Let's "dig" in.
Table of contents:
An excavation quantity takeoff involves calculating the volume of soil or material that needs to be removed or added during the earthwork phase of a construction project.
By accurately calculating the volume of earth to be moved, you ensure that you have just the right amount of soil and pave the way for a safe and optimal jobsite.
A successful excavation quantity takeoff requires you to manage several components. These include the following.
As with any task, the first step to getting it right is understanding the project's detailed needs and constraints. When it comes to construction excavation, you first need to assess the project plans and specifications. Doing so enables you to hone in on the scope of excavation required.
Specifically, you need to understand:
The areas to be excavated. Review the project plans to determine all areas that need excavation. This includes marking boundaries and noting any specific zones that require more extensive digging.
The depth of excavation. Take time to understand the varying depths of excavation the project requires. Detailed knowledge of depth requirements ensures proper support structures are in place and that excavation does not exceed necessary levels, which in turn prevents unnecessary costs and safety hazards.
Any special considerations such as soil type and site conditions. Certain soil types and site conditions may call for different approaches to excavation. Rocky soil may require specialized equipment, while sandy soil might need additional support to prevent collapse. In any case, being aware of these factors will help you strategize so you can ensure that you excavate the site safely and effectively.
The information you gather from the project plan and specs will enable you to determine the type(s) of excavation necessary for the project. Here are the different ways you can excavate the jobsite.
This involves removing the uppermost layer of soil, which typically contains organic matter and vegetation. Topsoil excavation clears the site and prepares a stable foundation for construction activities. Proper topsoil handling is crucial for maintaining soil health and facilitating later landscaping efforts. Just bear in mind that a challenge that topsoil excavation presents is in preserving fertile soil for future use and preventing erosion during the excavation process.
This type of excavation entails digging beneath the topsoil to remove the underlying soil layer. Mainly used when establishing foundations, trenches, and embankments, earth excavation ensures the stability of the structure and prevents future settling or shifting of the foundation. The challenges to remember when implementing earth excavation include managing different soil consistencies and moisture levels, which can affect excavation efficiency and stability.
Rock excavation is required when the project site has solid rock formations that need to be removed. This process often involves specialized equipment and techniques like blasting or drilling. Proper rock excavation is critical for keeping the integrity of the construction and avoiding potential hazards associated with unstable rock formations. Rock excavation may require more complex (and expensive) equipment, as well as the need for precise control to avoid damage to surrounding areas.
This involves removing a mixture of water and soil. Teams typically use muck excavation in wetlands or areas with poor drainage, and it's used to create a stable and dry construction site. Properly managing muck excavation helps prevent water-related issues and supports the overall stability of the project. Note that a jobsite with high water content can be challenging to excavate and will likely require additional drainage solutions.
For obvious reasons, successfully excavating a jobsite requires knowing how much soil you need to move. Getting this right is a must, as your calculations can significantly impact project costs, timelines, and safety.
Here are the common methods for calculating excavation volumes for both regular and irregular shapes.
Volume = Length x Width x Depth
Let's say you need to excavate a rectangular trench that's 10 meters long, 2 meters wide, and 1.5 meters deep. In this instance, the calculation would be:
Volume = 10m x 2m x 1.5m = 30m3
Now, if you’re dealing with trapezoidal excavation, use the following formula:
Volume = ½ x (Base1 + Base2) x Height x Depth
If the top of the excavation (Base1) is 8 meters wide, the bottom (Base2) is 5 meters wide, the height (distance between bases) is 4 meters, and the depth is 2 meters, the calculation would be:
Volume = ½ x (8m + 5m) x 4m x 2m = 52m3
If you're dealing with irregular shapes, you need to do more complex calculations like the grid method or cross-section method.
Here’s how.
1. Divide the area: Overlay a grid on the excavation site, dividing it into smaller, manageable sections (usually squares or rectangles).
2. Measure elevations: Record the elevation at each grid point.
3. Calculate the volume: For each grid cell, average the elevations of the four corners to find the average depth. Multiply the average depth by the area of the grid cell to find the volume of each cell. Sum the volumes of all cells to get the total volume.
For example, if a grid cell is 1 meter by 1 meter and the corner elevations are 1.2m, 1.4m, 1.3m, and 1.1m, the average depth is:
1.2 + 1.4 + 1.3 + 1.1 / 4 = 1.25m
The volume for this cell is:
Volume = 1m x 1m x 1.25m = 1.25m3
1. Create cross-sections: Draw cross-sections of the excavation area at regular intervals.
2. Measure areas: Calculate the area of each cross-section.
3. Calculate the volume: Multiply the area of each cross-section by the distance between them and sum the volumes.
Example:
If the area of a cross-section is 15m² and the distance between cross-sections is 5 meters, the volume is:
Volume = 15m2 x 5m = 75m3
When soil is excavated, it often increases in volume—a phenomenon known as swell. Conversely, when soil is compacted back into place, it decreases in volume, and this is known as shrinkage.
You need to factor in swell and shrinkage to ensure accurate volume calculations. For example, if the swell factor is 20%, you'll need to adjust the volume by that much. Ditto when adjusting your calculations for shrinkage
Excavation quantity takeoffs can get decently complex, but the good news is there are tools to make this process easier. Consider the following.
Digitize your excavation quantity takeoffs using software. We recommend choosing a quantity takeoff solution with digital measurements, automated calculations, and 3D modeling capabilities. Depending on the project, you can input customized formulas and generate quantities with high accuracy and efficiency. If you work with other teams, having a cloud-based platform can facilitate a more collaborative takeoff process by ensuring all documents and data live in one place and everyone is on the same page.
Have your drones fly over the construction site to collect high-resolution images and generate precise 3D models of the terrain. This data allows for more accurate assessments of excavation needs, which then reduces the risk of errors and improves project planning.
BIM allows for the visualization of the entire construction process, including excavation phases. Project teams can use BIM to anticipate potential issues and ensure all aspects of the excavation are accurately planned and executed.
Excavation quantity takeoffs don't have to be an uphill battle. By fully understanding the project needs and running the numbers, you can (pardon the pun) build a solid foundation for success.
For best results, leverage software to ensure your calculations are on point and teams stay on the same page.
Looking to conduct more efficient and accurate excavation quantity takeoffs? Check out Autodesk Takeoff. Our advanced construction takeoff software simplifies the process, ensuring precise earthwork estimates for your construction projects. Learn more about Autodesk Takeoff today!
