Written by Mike Smith
January 26, 2023
Contractors should learn about geotechnical testing so you can understand how soil conditions affect your jobsite.
Mike Smith, Principal of geotechnical firm SAECO, sits down with geotechnical engineer Justin Thomas to provide an overview of geotechnical engineering and seven common geotech tests. Here's what they have to say.
Overview of geotechnical engineering
Geotechnical engineers try to identify and evaluate characteristics of onsite soils where contractors plan to build bridges, roads, and other structures.
They go to the site and drill below the surface of the materials, sometimes as shallow as five feet or deeper than 100 feet depending on the data they need. Then, they take those samples into a laboratory for testing.
Site observations before and during testing
When a geotechnical engineer arrives on the jobsite, they log observations before they start testing.
"When we go to a site, we take a holistic approach," says Justin. "We want to know as much as we can about the site topography, any slopes, cuts, fills, or existing development because that's going to make a huge difference in our final recommendations."
Geotech engineers also want to know about any undocumented fill onsite. They can't recommend building a new structure on random soil that somebody dumped, because they have no idea how it was placed.
During the drilling phase, geotech engineers ask questions like:
- What type of soil is this?
- Does it feel gritty like sand or softer like clay?
- How does it respond to moisture?
- Is any construction debris or groundwater coming up?
- What's the soil's moisture condition?
They'll also look for regional soil issues. For example, Arizona has caliche soil. "That stuff is really hard to drill through and excavate, so that's important for the contractor to know,"Justin notes.
The drill rigs have giant hammer attachments (about 130 pounds), so engineers can keep blow counts to study how the soil responds to a certain number of strikes from the hammer. That simulates the compaction process.
The geotech engineer puts all that information into their report for the project owner and contractor after the testing is done.
Types of geotechnical tests and their uses
Geotechnical engineers do all types of tests—you'd be reading all day if Mike and Justin covered every one! Fortunately, they're keeping it to these seven:
- Plasticity index
- Sieve analysis
- R-value
- Chloride and sulfate
- Consolidation
- Swell
- Expansion index
Let's dig deeper into those tests.
Plasticity index test
What is the plasticity index?
Soil can be in a liquid, plastic, semi-solid, or solid state. Liquid soils are wet, runny, and not good for building. They're mud. Solid soils are hard and have very low moisture content.
During construction, you need soil in a plastic state. That's when it contains enough moisture to be molded and hold a shape. The plasticity index, or PI, shows the maximum and minimum moisture content a specific soil can have while remaining in its plastic state.
Soils with high PI values tend to contain more clay and hold more water in their plastic state. Soils with low PI values have less clay and need less water to become plastic.
Justin says, "Soil changes its behavior based upon how much water is added and how much silt and clay are in the material. As you add more water, it moves from its plastic state to a liquid state. That's when you hit its liquid limit."
You hit the plastic limit if you remove so much water that the soil becomes semi-solid. The soil's PI is between the plastic and liquid limits.
How PI tests work
According to Justin, "We take the soil sample into the lab, and we add a little water to it. We roll it out [by hand], or we have another apparatus that we use to tamp it down."
Geotechs can determine how much clay and silt the soil contains based on how it behaves during the test. With that info, engineers can help project owners and contractors understand how the soil will behave onsite when you start trying to build on it.
"As a designer, that determines how much bearing capacity you'll get and what the shear strength of that soil will be," Justin notes. He adds that the test is so easy and inexpensive that it's a no-brainer for owners and contractors to request.
Why PI test results differ from each other
MIke says, "When you try to get different technicians or different companies to come up with the same value, it's very difficult to get the exact same numbers [because] the testing method leaves you some wiggle room."
Justin agrees,"There's an art to it. Different technicians have a different feel for it, so they might define the plastic limit differently than another technician. So that's why their numbers might be just a little off from each other."
Mike doesn't let just anybody do PI tests at his labs. He wants to make sure the engineer can repeat the exact same thing every time to get accurate results.
Accuracy is important because you need a PI test during both the design and construction phases—especially for earthwork or embankment projects where the structure's long-term performance hinges on that PI result.
However, Mike adds that the numbers don't have to match up exactly. In most cases, you just need all the PI test results to be within a close range.
PI test prep methods
Geotech engineers can use either wet or dry preparation for the PI sample.
Justin calls dry prep "a quick and dirty method."It's also the standard most people use unless the project specs say otherwise. "You see dry prep more often in the construction phase, usually when . . . the contractor or owner wants results as fast as possible," he says.
With wet prep, the engineer evenly conditions the soil with a little bit of moisture before the test. It's a slower but more uniform method.
Justin says dry prep is less accurate because, “when you start adding moisture during the test, you're not ensuring that all of the soil is evenly moisture conditioned. So you get a lot of variance between tests. If you adequately wet prep, it's a lot more consistent.â€
Sieve analysis test
Sieve analyses examine the range of particle sizes within a soil sample. These tests are crucial to construction and especially design. They help geotechnical engineers understand how the soil will compact and bear the weight of the structure you plan to build. The engineer's recommendations will depend on the spectrum of soil sizes.
"If you just have one soil size that's really uniform, it won't compact very well. You'll have issues during construction, and it potentially won't be able to support a structure. If you have a gradation curve, that's good. When you have some smaller and some larger sizes, that soil compacts well. It's easy to work with during construction," Justin explains.
To analyze the soil's particle sizes, the engineer sifts those particles through a screen, or sieve. They study how much of the soil passes through the screen.
For example, a 200 screen is really small. Justin says, "If you get 60-80% of the material passing through the 200 screen, that's a big red flag. That tells me there's a lot of silt and clay that could potentially cause settlement or expansion issues down the line."
At that point, engineers must do further testing to determine if the project can be built as planned or if it needs adjustments.
Want to learn more about sieve analysis testing? Watch Kiel Krieg, SAECO’s Construction Materials Engineering and Testing Department Manager, perform a sieve analysis.
R-value test
One test that's unique to roadway projects is the r-value test. It measures how the soil responds to some type of bearing pressure, and it helps make sure the roadway will be able to withstand the long-term load of vehicles driving over it.
"The r-value is a number from zero to 100. The higher the number, the better that material is at supporting that bearing pressure," says Justin.
Mike adds, "We use that [test] to determine what the in-situ materials are capable of bearing and make a recommendation for exactly what subgrade preparation the roadway base should get."
Chloride and sulfate tests
Chloride and sulfate tests are common, and they're typically part of a suite of corrosion testing. That panel also includes pH and resistivity testing.
"Corrosion tests are really important because you're placing concrete, rebar, metal pipes, and all types of material like that against these soils," Justin notes. That can potentially expose those building materials to high levels of sulfates and chlorides that can corrode them and weaken the structure.
Many contractors need a corrosion testing panel to know whether or not they should take any measures to counteract corrosive elements. However, corrosion doesn't affect all projects.
Mike emphasizes that it's important for project owners and contractors to help geotech engineers understand how the site will be used. "We wouldn't recommend running all these tests on every site, because a lot of them just aren't needed. We're trying to provide the best value for what we do," he says.
Consolidation tests
Consolidation tests involve sampling material and applying an increasing load to it over time. The test typically takes a few days to complete. The engineer measures how much the material consolidates, or decreases in volume, under a given load.
Mike says consolidation tests are unique: "A lot of the tests we've talked about are done on samples that have been prepared in the laboratory, but the consol test is done on an undisturbed sample."That's because geotech engineers want to know how the undisturbed material onsite will behave.
To extract the material from the site, engineers use a ring sampler that's about one inch by two and a quarter inches. They then trim that sample and put it straight into the test.
Types of consolidation tests
Full cycle consolidation tests are highly accurate and academic. They're best when you need to know as much as you possibly can about the soil. They can help you learn about soil settlement and whether the material is over or under consolidated.
However, a response to wetting is a more common consolidation test.
Justin explains how it works: "We start to apply a load, and when we get to roughly the anticipated bearing pressure [of the structure you plan to build], we add water to saturate that material. Then we add a little bit of load and measure how the material responds. That can tell you if it's going to collapse or potentially swell when it gets wet."
Swell tests
The goal of swell tests is to mimic the real world scenario the soil will be in during and after construction. "You prepare the specimen as if it's going to be underneath a slab or perhaps a foundation," Justin says.
Before swell testing can start, geotech engineers need to know the laboratory proctor values for maximum density and optimum moisture content.
Once they're ready to begin testing, the engineer applies a load—typically 100 pounds per cubic foot at SAECO, which is roughly what a slab would be—on grade. Then they add water and see if the soil swells or collapses a little. That gives the geotech firm a good idea if the soil will swell and crack the slab.
"Usually it swells a little bit. You don't want the pressure [of the simulated load] to be so much that the soil doesn't do anything, so that's why it's lightly loaded,"Justin adds.
Expansion index tests
Expansion index, or EI, tests are very similar to the swell test. That can cause some confusion, especially since technicians prepare them in similar ways.
However, engineers can change some swell testing parameters. With EI tests, the American Society for Testing and Materials (ASTM) tells them exactly what to do.
"It tells us we have to do the test at 144 pounds per cubic foot. It tells us exactly how we are supposed to prepare that specimen. It tells us that we have to do it on all of the material passing the number four. So there is no leeway in how we prepare that specimen or how we run that test," explains Justin.
A lot of agencies order EI tests because they're more standardized than swell tests.
Engineers compare the EI results against other samples that have been run using the same tests to get an idea of the material's range for potential swelling. That's why it's called an index test. With a swell test, the result comes back as a percentage instead of a range.
What makes for effective geotechnical testing and consulting?
Hopefully now you have a little better understanding of common geotechnical tests and how they work.
Keep in mind, some of these tests are regional. The swell and EI tests Mike and Justin use in Arizona might not apply in other parts of the country. However, even if you use different tests in your location, the soil characteristics these tests try to identify are important for contractors all over the world.
It's important to educate yourself on the tests in your region so you can be better prepared to recognize effective versus ineffective geotechnical consulting.
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