Soil Stabilization

Portland Cement: Application as Soil Stabilization for Construction Activities

SIGNIFICANCE, USE AND BENEFITS

Soil stabilization is a procedure utilizing Portland cement as a drying and stabilizing agent to achieve specific construction compaction as well as to provide additional subgrade soil strength, when properly mixed with over-optimum, moisture-sensitive soils. This procedure, when used in conjunction with the correct product handling Best Management Practices (BMPs) provides a legitimate use in the earthwork and utility phase of many construction projects with over-optimum wet soils. The results of testing indicate that the use of a cement additive will reduce the moisture of wet un-compactable soil and improve its consistency to the point where it can be compacted to produce a stable bearing subgrade.

The amount of additive added to the untreated subgrade is dependent upon three factors: soil type, the moisture content of the soil, and the construction application. A qualified soils engineer who is experienced with soil stabilization procedures should determine the correct mix design, or ratio of cement additive to soil. At a minimum, the soils particle size distribution, plastic characteristics, and laboratory moisture-density relationships need to be measured for successful stabilization results. Tests to determine these characteristics are commonly performed on fill soils for most earthwork projects.

The economic benefits of stabilizing an overly-saturated subgrade are readily apparent. Cost savings are realized as a result of eliminating the cost of excavating and replacing the unstable subgrade with additional structural material. Cost savings are also realized since the improved characteristics of the treated subgrade can often result in a reduction in base course thickness and may even eliminate the failure of new pavement during ongoing construction traffic.

MIXING PROCEDURES

Portland cement is delivered to the jobsite in pneumatic trucks and pumped directly into an enclosed spreader truck equipped with dust filters. Once the spreader truck is loaded, the material can be applied to the subgrade per the mix design and mixed in 6” to 16” lifts using a cross-shaft rotary mixer. After the cement and the wet soil are mixed, this amended product can be compacted and gradcd in place or it can be trucked to other areas for application as well. Machinery then places the amended material to the established grades. With moderate lifts, compaction is achieved with a smooth drum vibratory roller; with deeper lifts, compaction may require a sheepsfoot roller.

ENVIRONMENTAL CONSIDERATIONS

DUST: Due to the fine powdery properties of bulk cement, additional precautions are required to prevent any amount of airborne release. All personnel involved in the offloading procedure should wear NIOSH-approved respirators. Once spreading is completed, the mixing procedure should commence immediately to ensure against any unpredictable high wind activity. Spreading of cement should not take place during high wind events.

METALS: No known release of heavy metals has been reported in either lab tests or field studies conducted for stabilization with Portland cement.

pH: The chemical nature of cement can lend to the significant rise of the pH of any storm water runoff traveling over the amended soils. Per the WSDOE standard, any water leaving the site may not elevate the receiving water offsite more than 0.5 pH units over background, and in some instances, no more than 0.2 pH units depending on the waterway. The most practical measures for controlling pH are to maintain an erosion control program that reduces exposed soils and diverts storm water runoff away from treated areas. Due to the controlled application of the cement with a spreader truck and mixing by a cross-shaft rotary mixer, the material is blended much better and less cement per cubic yard is needed, which reduces the possibility of a significant pH rise in storm water runoff.

In the event that storm water runoff pH levels in are affected, runoff can be treated with pelletized dry ice very simply and effectively. The application of one pound of pelletized dry ice per 2,000 gallons per point of pH rise is adequate to neutralize the pH level. The dry ice is spread evenly over the top of the body of water and no mechanical agitation is required. A check of pH levels two hours after treatment will verify proper application of dry ice.

CONCLUSION

Although the application of cement products for soil stabilization has been ongoing for many years, it has become more widely used in recent years and consequently more scrutinized by various regulatory agencies. Experienced and responsible contractors who follow BMPs for soil stabilization projects can provide the best reassurance to both developers and regulatory agencies of the effectiveness and viability of this outstanding product and application.