Cement Stabilization of Soil for Longer Lasting Roadways

Cement stabilization is the process of incorporating cement into soil to improve its strength, durability, and resistance to moisture. The cement reacts with water and soil minerals to form a hardened matrix, reducing plasticity and increasing load-bearing capacity.

Benefits of Cement Stabilization

• Increased Strength: Improves the soil’s load-bearing capacity.
• Reduced Plasticity: Makes expansive clays more stable.
• Moisture Resistance: Reduces permeability and water absorption.
• Improved Durability: Better performance under traffic loads.
• Cost-Effective: Reduces the need for importing higher-quality fill material.

Cement stabilization for soil involves many of the same steps that are used for cement stabilization for road base or Full-depth reclamation (FDR). The major difference is the cement-soil matrix is designed with a lower strength in mind because fewer strong aggregates are present to match the strength of added cement.

Prior to construction, a soil investigation should be conducted to determine the soil type, the optimum moisture content, and the soil properties. A cement series should be prepared to determine the amount of cement that should be used in the soil. A typical 7-day unconfined compression test target for the series is generally set at 100-200 psi.

The steps for construction are the following:

• Initial Pulverization
• Preliminary Grading
• Cement Application
• Mixing
• Compaction
• Final Grading
• Curing 

Initial pulverization

For the best results, a pavement reclaimer is recommended. Other types of equipment could be used, such as tiller-type mixers or motor graders, but they are less efficient in getting the necessary gradation of the soil for a successful project. Typically, the required gradation can be achieved in one pass. More passes with a reclaimer may be required if the soil is more difficult to break up. Reclaimers can typically pulverize materials to a depth of one foot. If deeper lifts are required for your project, a top layer may need to be pushed aside in windrows in order to pulverize and treat the lower layer first.

Preliminary Grading

Before adding cement needed to strengthen the soil, a motor grader is used to shape the subgrade surface and cross-slope.  

Many times, a minor edge or shoulder widening is part of the project. If it is determined that adequate support is found in the existing subgrade, then a motor grader or road widener can be used to get the additional width required for the project. To build a much wider road, it will typically be necessary to grade and build a supportive subgrade and re-establish impacted roadway ditches before adding new or reclaimed materials for this part of the project.

Spreading Cement

After the grades, cross-slope, and width for the subgrade have been established, it’s time to spread the cement (typically dry) on the re-established subgrade template. Cement should be spread evenly across the land established for the project site. The rate of cement is determined by the pavement engineer for the project, and this rate can be calculated or one can use an industry rate table to determine the cement needed to be spread.

Mixing

The reclaimer used for initial pulverization of the soil is then used to mix in the cement subgrade. Sometimes a water truck is used to add water to the subgrade prior to mixing, but the best results are found when the reclaimer has a water line directly feeding into the mixing drum, where the rate of water can be more accurately controlled. It is important to achieve near-optimum moisture for the base-cement mixture. Testing of the laboratory determined optimum moisture; in the field with nuclear density gauges can be used for precision. As a rule of thumb, a ball test, if one can make a cohesive ball of the soil without being overly saturated, then the material is near optimum moisture.

Compaction and Final Grading

Proper compaction is important for soil-cement to achieve its full strength and durability. The cement-treated soil should be uniformly compacted to 96 percent of the maximum dry density. The type of equipment for compaction is dependent on the material properties but the types that can be used are tamping (sheep’s foot), steel drum vibratory, and pneumatic rollers. Avoid overcompaction as it could lead to loosening and drying of the surface.

The typical time to allow for mixing and final compaction is 2 hours. On cooler days, more time could be allowed. Final Grading is important to ensure the proper roadway template is achieved. The goal is to achieve a cohesive, uniform layer free of cracks and loose material. Final grading should be completed within 3-4 hours after beginning mixing.

Curing

To speed construction along, curing of the subgrade is done with an asphalt emulsion spread on the subgrade to seal in the moisture for the cement-treated subgrade, which cures while subsequent pavement layers are built. Without using emulsion, one should allow 3-7 days of keeping the surface moist for hydration.

Cement performs well in silty and sandy soil. It locks in the soil matrix and will make a stronger permanent modification that will not leach out like other soil stabilization methods. Many engineers are discovering that with modern reclamation equipment cement can be thoroughly mixed with clay soils as well. The benefits of using cement stabilization in clay soils is that strength through cement hydration occurs instantaneously, where other stabilization methods take considerable time to gain their strength in the soil. In fat clays with higher plasticity indexes, more and more engineers are specifying the combination of lime and cement to achieve a quicker, economical method to achieve the desired subgrade strength.

Longer-lasting roads start with a good foundation. A strong and economical cement-treated subgrade is a great solution that will be a solid foundation for the pavement.