Feb 04, 2022
Every building sits upon a foundation, which is built into the earth and holds up the structure sitting on top of it. Foundation walls, which are typically made of concrete, brick, or cinder blocks, form the support system for an entire structure. When a building has a basement, that basement sits at the lowest point of the foundation.
Whether your home has a full basement or just a crawl space under the floorboards, laying the foundation can be one of the most expensive parts of home construction. It’s also a crucial process to get right. Without a proper foundation built to code, a building will lack structural integrity and be unsafe to inhabit.
In this article
What is basement excavation?
Basement excavation is the process of removing dirt, rock, and other debris to create space for a basement. Typically it is the first step in building work, as excavated material gets displaced, leaving a hole in the ground where a home’s basement will be built.
Basement excavation can also take place after a home has already been built. This type of work is significantly harder and requires a seasoned excavation contractor who can dig into the earth and haul away excavated materials without damaging your building’s foundation, footings, or floor joists. To significantly reduce excavating costs, basements should ideally be dug out before any part of the building goes up.
5 common methods for basement excavation
Depending upon the nature of a project, there are five notable construction methods for excavating a basement.
There are two ways to execute open-cut basement excavation . There is the slope method, which has some of the lowest excavating costs of any technique. It creates an excavated area with walls that are on a downward slope. (Excess dirt is later layered on top of this slope). The open-cut slope method does not require any retaining walls to hold back the earth from crumbling onto your building’s foundation.
A more complicated and expensive variant is called the cantilever open-cut method, which does require retaining walls to keep the surrounding terrain from crushing your foundation. The advantage to a cantilever method is that you can dig much deeper basements.
This excavation method is often used for constructing high-rise buildings in urban areas. It starts with constructing load-bearing foundation walls and laying a concrete ground floor on top of those walls. Then, a large (often complex basement) gets excavated beneath that ground floor. This method is expensive, but it is actually more nimble than an open-cut excavation because it allows a building’s upper floors to be built at the same time that a basement is getting excavated.
The bracing method is pricier than most open-cut excavations but more economical than the top-down method. This method offloads weight from the retaining wall and transfers it to horizontal struts, which are installed in front of the wall and run from one side of the foundation to another. If you properly space these horizontal struts, they can serve as the framing for a basement in your building. This method does a good job of reducing strain on your retaining walls, although horizontal struts are not as strong and foolproof as a full concrete floor bolted to a foundation, which is what you would install in the top-down method. As such, it is safer to dig in the top-down method, although by the end of the building process, both structures will be equally sturdy. Cost factors and the lower complexity of the bracing method make it an appealing choice for homeowners and builders priced out of the top-down method.
This method of excavation involves driving steel anchors into the soil, which also run through the retaining wall. The anchoring force of the earth provides stability to the overall structure and allows excavation to proceed swiftly and safely. The anchored method will only work if you drive the anchors into bedrock or extremely dense clay. Soft clay or sandy soil cannot provide the strength needed to brace anchors.
The island method involves excavating a basement from the inside out. It is one of the safest ways a person can clear out an excavated area. This method starts by excavating an area near the center of the basement. The excavated materials are then laid into a slope near the structure’s retaining walls. The slope will continue all the way to the outer retaining walls, and eventually the center structure will be braced to the outer walls. Ultimately the island method combines elements of the open-cut slope method and the bracing method to create a remarkably sturdy basement and a safe work environment.
How to determine the best basement excavation method
The choice of how to excavate a basement will be made by the project owner and the excavating contractor on the job. When choosing the best excavation method for your project, consider the following factors.
- Type of construction. The easiest way to excavate a basement is to have a fresh plot of land to build on. It will also make for a faster, more affordable excavation. If you’re digging out a basement space beneath an existing building, you will have fewer options. Consulting with an excavating contractor and a structural engineer is a must for any excavation project. Building foundations are no joke, and you don’t want to do any construction that will compromise your structure’s integrity.
- Soil type. Building excavation hinges on the quality and density of soil beneath your structure. If you are building on sandy soil, you will need to dig deep into the earth until you reach bedrock to properly secure your structure. This means you will need a method that permits deep excavation, such as an open-cut cantilever. If you are building on dense clay soil, you may not need to go very far down and could use a slope method for excavation.
- Topography. If you are building on a hillside, your retaining walls will be pressed into extra duty. It may require the anchored method of excavation, in which the earth plays a role in keeping your walls stable as you excavate. A steep slope will eliminate the idea of a traditional basement dug into the earth. When you build on a flat piece of land, you have more options available. You will also need to consider the water table—if the water table is close to the surface, you won’t be able to go too deep without having to pump water.
- Size and scope of your building. Tall skyscrapers and office buildings usually excavate via the top-down method. This allows a builder to work quickly and ensure more early stability than they might get from the bracing method or certain open-cut methods. The drawback is that top-down excavation is often the most expensive option. If your ambitions are simpler, look into other options such as the island method which provides impressive stability for less money.
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