Building Safe in 2017

Builders and constructors can take four steps to better understand the makeup of the soil and design a stronger, more cost-effective foundation

By Gary Seider

Foundation design can be a risky business. Because of the high variability of soil, the ground that surrounds a foundation can have a profound effect on its load-bearing abilities. Where one type of soil might require a deeper foundation, another may work perfectly with a shallower — and less expensive — base.

While it’s possible to design a foundation without knowing the makeup of the surrounding soil, that lack of information comes with potential side effects. For example, not properly identifying soil’s composition significantly increases the chances that a building will shift or settle after the fact, causing damage to a structure’s walls, doors, or windows.

Clay tends to shrink or swell during wet and dry seasons, leading shallow foundations to shift. In other situations, soil that is made up of incorrectly packed gravel, sand, or fill can settle over time, causing the foundation to move down along with it.

There could also be organic matter in the soil that breaks down over time, or there could be drainage issues that could potentially erode the soil. Depending on its makeup, both less-than-ideal options can cause soil to expand.

The possibilities of soil complicating and compromising a residential foundation are numerous and varied. But as long as you know what you’re dealing with, the vast majority of them are also preventable.

That’s why it’s critical that a foundation designer and installation contractor analyze the soil and work together to manage this risk. With that in mind, here are four steps builders and constructors can take to better understand the makeup of the soil surrounding the foundation and design a stronger, more cost-effective foundation:

1. Conduct at least one soil boring test per project. A soil boring test provides construction teams with valuable information about the dirt’s profile. Soil boring can tell you a sample’s type, strength, description, and ground water location, if any is present. When designing a stable foundation, these are all relevant, helpful pieces of data to have on hand.

2. Conduct a soil sampling using ASTM D-1586 standards. One of the most important factors when it comes to choosing the proper foundation for your building is a soil’s shear strength. The shear strength is a measure of how much stress soil can take before it falls apart. 

By testing a sample of the soil to ASTM D-1586 standards, you can assess the shear strength of your soil and have a better idea of what steps need to be taken to make sure your foundation can compensate for a soil’s potential weaknesses.

3. Document the soil profile in accordance with the USCS. The Unified Soil Classification System categorizes soil based on particle size, type, and distribution. It provides a simple method for a contractor, geologist, or engineer to quickly understand the physical characteristics of the soil on a project.

In other words, it gives you a shorthand by which to identify the type of soil you’re dealing with and makes it easier to find relevant solutions to any potential problems it may have.

4. If there’s groundwater present, figure out where it is. Groundwater can have a significant effect on a foundation’s performance. Its location — and possible movement — should be considered in the design of a foundation and with the use of any anchors or helical piles. Luckily, you can determine the moisture content of the soil from the samples taken during boring.

This is done through a simple laboratory process. The soil sample is weighed, placed in an oven to dry out for a few hours, and then weighed again. The moisture content is the relative difference in the weight. By undergoing this method, you can pinpoint where the groundwater is in your soil and design your drainage accordingly.

The difference in each soil’s makeup isn’t usually visible to the naked eye, but this invisible variability can have major effects on a foundation’s stability. By taking the proper steps to analyze and classify the soil surrounding your building site, you have a better chance at designing a foundation that can truly stand the test of time.

Gary L. Seider, P.E., is engineering manager of Hubbell/CHANCE® civil and utility helical products. With four U.S. patents and more than 40 years of industry experience,
Seider oversees the company’s civil construction and utility application/project
engineering staff. Contact Seider at
glseider@hubbell.com.

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