Every successful subgrade stabilization project starts in the same place: the design. You can have the right materials and a skilled crew, but if the design is wrong, the project is already in trouble before anyone breaks ground. Getting the design right gives you the best chance at a successful outcome, and skipping steps is one of the most common reasons pavements fail.
Here’s what a thorough subgrade stabilization design actually involves.
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Do You Even Need to Stabilize?
The first question isn’t which stabilizer to use — it’s whether stabilization is needed at all. That starts with a geotechnical investigation of the in-place soil. From there, a few tools help inform the decision:
- Existing soil maps can tell you whether you’re in an area likely to have expansive soils. In Texas, we use a well-established map showing where highly expansive clays are most prevalent — the red and yellow zones that follow the I-35 corridor from Oklahoma down through Dallas, Austin, and into South Texas. But clay soils can show up anywhere in the state, so don’t rely on the map alone.
- Field testing gives you direct readings on subgrade strength and behavior. The Dynamic Cone Penetrometer (DCP) is a low-cost option — it measures how far a cone penetrates into the subgrade, giving you a quick read on support capacity. The Falling Weight Deflectometer (FWD) is more sophisticated and can be used on existing pavement or new construction sites to back-calculate subgrade stiffness values. Both are valuable tools at the design stage.
- A formalized pavement design — using TxDOT’s Flexible Pavement System (FPS) or an equivalent — will incorporate those field values and tell you what each layer of the pavement system needs to do. Performance history of similar projects in the area is also worth considering. If lime stabilization has been used consistently in a given corridor, that’s telling you something about the soils.
Know Your Soil Type
Subgrade soils generally fall into three categories: clay, sand, and silt. Each has different characteristics in terms of strength, cohesion, plasticity, and drainage, and the stabilization approach differs accordingly. Once you’ve collected soil samples, standard lab testing will determine the Atterberg limits — the key indicators of plasticity and soil behavior.
As a general rule:
A Plasticity Index (PI) above 15 typically requires stabilization.
A PI above 35 indicates a highly expansive soil — and at that level, lime is almost always the stabilizer of choice.
Soil survey maps, including the USDA Web Soil Survey, are a useful starting point and can help you characterize what you’re working with before lab results come back.
Test for Sulfates and Organics
Two tests that deserve special attention are sulfates and organics. In Texas, we recommend testing for both on every project — even if you think the site does not contain significant amounts of sulfates or organics.
Sulfates
If sulfate levels exceed 7,000 parts per million, you have potential for sulfate heave, and calcium based stabilizers such as lime or cement may not be viable options. TxDOT has specific guidelines for dealing with high-sulfate soils. In the 5,000–7,000 ppm range, lime stabilization may still be viable, but it typically requires a longer mellowing period — something that needs to be spelled out clearly in the project specifications.
Organics
If organic content is at or below 2-3%, lime stabilization can generally proceed. Above 2%, you may need to consider other options such as the use of select fill.
These aren’t edge cases. Finding sulfates or organics late in the process — or not at all — is the kind of thing that can turn into a costly failure. The key to success is testing early on to figure out if you have sulfates or organics and if so account for them in the specifications.
Choosing the Right Stabilizer
Once you know what you’re working with, TxDOT’s stabilization guidelines provide a clear framework for selecting the appropriate stabilizer based on soil type and Atterberg limits. For high-PI soils above 35, lime stabilization or the use of select fill are typically the only viable options.
How Much Lime Do You Need?
After confirming lime is the right choice, the next step is determining the correct percentage. The procedure for this is Tex-121E Part 3 — the Eades-Grimm test, also commonly called a lime series. This test determines the minimum amount of lime needed to raise the soil’s pH to 12.4, which will ensure the silica and alumina will remain soluble and the stabilization can proceed fully.
Once you have the lime percentage from that test, our calculators can convert it to pounds per square yard based on the unit weight of the soil and the intended mixing depth.
How Deep Should the Stabilized Layer Be?
Mixing depth is determined by several factors. The pavement design drives most of it — the full system of layers needs to be in balance, and the lime-stabilized subgrade layer is one input into that calculation. Practically speaking, 12 inches or less can typically be mixed in a single lift. Anything deeper requires additional lift, which adds time and cost. In some cases, it may be more efficient to increase the base layer thickness rather than going deeper on the stabilized subgrade.
Potential Vertical Rise (PVR) is another factor. PVR is a measure of how much a pavement can heave under adverse moisture conditions. Most agencies have maximum allowable PVR thresholds, and if the design exceeds those limits, the stabilized depth may need to increase to bring the PVR down to an acceptable level.
Design and Specifications Are the Same Package
One thing I want to emphasize: when I talk about the design phase, I mean both the geometric design and the specifications that govern how the work gets built. They’re the same package. For example, if you expect to get 95% in place density you need to require it in the specifications and then inspect and measure in the field to insure it gets done. As the saying goes, “what gets measured gets done.”
A thorough design gives you the best possible starting point. It doesn’t guarantee a successful project on its own — you still need the right materials and proper construction execution — but without it, you’re working against yourself from the start.
Ready to move past design? Find the other phases of successful stabilization here
To learn more about subgrade stabilization design or to reach me directly, email dalerand@limetexas.org. Follow the Lime Association of Texas on LinkedIn and subscribe to our YouTube channel for future episodes of Lime Time in Texas.
About the Author
Dale Rand, Executive Director
Dale Rand, P.E. is the Executive Director of the Lime Association of Texas. With more than 40 years in the industry — including over 25 years with TxDOT and a decade at Atlas Technical Consultants — Dale has designed more than 300 hot mix asphalt mixtures and has seen firsthand what lime can do when it’s used correctly, and what happens when it isn’t. He joined Lime Association of Texas in 2023 with a simple vision: make this a practical, trustworthy resource for engineers, contractors, and anyone working with lime in Texas.
