Soil Stabilzation

Lime can be used to treat soils in order to improve their workability and load-bearing characteristics in a number of situations. Quicklime is frequently used to dry wet soils at construction sites and elsewhere, reducing downtime and providing an improved working surface. An even more significant use of lime is in the modification and stabilization of soil beneath road and similar construction projects. Lime can substantially increase the stability, impermeability, and load-bearing capacity of the subgrade. Both quicklime and hydrated lime may be used for this purpose. Application of lime to subgrades can provide significantly improved engineering properties.

Soil Modification:

Lime is an excellent choice for short-term modification of soil properties. Lime can modify almost all fine-grained soils, but the most dramatic improvement occurs in clay soils of moderate to high plasticity. Modification occurs because calcium cations ( KAT -eye-əns ) supplied by hydrated lime replace the cations normally present on the surface of the clay mineral, promoted by the high pH environment of the lime-water system. Thus, the clay surface mineralogy is altered, producing the following benefits:

Soil Stabilization:

Soil stabilization occurs when lime is added to a reactive soil to generate long-term strength gain through a pozzolanic reaction. This reaction produces stable calcium silicate hydrates and calcium aluminate hydrates as the calcium from the lime reacts with the aluminates and silicates solubilized from the clay. The full-term pozzolanic reaction can continue for a very long period of time, even decades -- as long as enough lime is present and the pH remains high (above 10). As a result, lime treatment can produce high and long-lasting strength gains. The key to pozzolanic reactivity and stabilization is a reactive soil, a good mix design protocol, and reliable construction practices.

Benefits of Soil Stabilization

Lime substantially increases soil resilient modulus values (by a factor of 10 or more in many cases). In addition, when lime is added to soil, users see substantial improvements in shear strength (by a factor of 20 or more in some cases), continued strength over time, even after periods of environmental or load damage (autogenous healing), and long-term durability over decades of service even under severe environmental conditions.

Short- and Long-Term Economic Benefits

In the short-term, the structural contribution of lime-stabilized layers in pavement design can create more cost-effective design alternatives. A recent interstate project in Pennsylvania, for example, began with a $29.3 million traditional design approach. An alternate design using lime stabilization, consistent with AASHTO mechanistic-empirical designs, cost only $21.6 million—more than 25 percent savings. The savings came from treating the existing subgrade material with lime, rather than removing the material and replacing it with granular material; and thinner layers of flexible pavement for the lime stabilized alternate due to the increased strength of the lime stabilized sub base. In the longer term, lime stabilization provides performance benefits that reduce maintenance costs. To illustrate, stabilizing an 8-inch native clay subgrade with lime as part of an asphalt pavement project can reduce 30-year life cycle costs from $24.49 to $22.47 per square yard. In addition to stabilization of new materials, lime is an excellent choice for reclamation of roadbases. As more and more governmental entities are choosing to reclaim existing roadbases rather than replace them, this use of lime will become even more important.

How to Use Lime for Soil Stabilization

Lime stabilization is not difficult to carry out. After proper mix design and testing is performed, in-place mixing is usually used to add the appropriate amount of lime to soil, mixed to an appropriate depth. Pulverization and mixing is used to thoroughly combine the lime and soil. For heavy clays, preliminary mixing may be followed by 24 to 48 hours (or more) of moist curing, followed by final mixing. For maximum development of strength and durability, proper compaction is necessary. Correct curing is also important. If sulfates are present at levels greater than 0.3 percent, special procedures are required.

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