This blog is a follow-up to our previous discussion on the three critical steps for successful lime stabilization:
- Applying the right amount of lime
- Mixing it to the specified depth
- Adding the correct amount of water
While all three steps are essential, getting the lime dosage right is the foundation for achieving lasting soil strength and durability.
Why the Right Amount of Lime Matters
The goal of lime stabilization is to transform expansive clay soils into a pozzolanic cement. This chemical reaction produces calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH)—compounds that strengthen the soil over time. In this reaction:
- C represents calcium which is derived from the lime (CaO)
- S represents silicate which is derived from silica in the clay soil
- A represents aluminate which is derived from alumina in the clay soil
- H represents hydrate which is derived from the water (H20)
Without sufficient calcium, the reaction cannot continue, and the soil will not achieve the desired long-term stability. Maintaining a pH of 10 or above is critical because silica and alumina become highly soluble at this level, enabling the transformation into durable pozzolanic cement. Lime is uniquely effective at maintaining this high pH throughout the life of the pavement, preventing shrink-swell issues while boosting soil strength.
Real-World Results
Lime stabilization typically increases the modulus at least 3-5x higher than the native clay soil. On projects that have been in place for many years, it is not unusual to observe modulus increases much higher than 5X such as on a recently observed lime stabilization project where we observed a modulus increase from 10 ksi to 140 ksi which is a 14-fold increase. Positive phenolphthalein testing on a number of lime stabilization projects over 50 years old has confirmed the pH remains in the 10 or above range indicating the lime is still present and serving its design purpose.
How to Ensure the Right Lime Application
Applying the correct amount of lime requires careful planning and execution. There are two key steps:
The TxDOT test procedure, Tex-121E Part 3, determines the optimal lime percentage for a particular soil. Applying less than the recommended amount only modifies the clay, reducing its plasticity or improving workability temporarily—but it won’t achieve permanent stabilization or maintain a pH above 10.
Once the design specifies the lime percentage (for example, 6%), a lime calculator can determine the precise quantity needed per square yard. The calculator requires three inputs:
- Percentage of lime
- Mixing depth
- Unit weight of native soil
Example: For 6% lime, an 8-inch mixing depth, and a soil density of 110 lbs per cubic foot, about 40 pounds of lime per square yard would be needed. This lime is then evenly spread and thoroughly mixed to the specified depth.
The Takeaway
Applying the correct amount of lime is the first and most crucial step toward a strong, durable, and stable subgrade. In future blogs, we’ll cover the other two keys: mixing to the proper depth and ensuring the correct water content.
