Using fly ash to stabilize non-uniform subgrade soils
Two facts about non-uniform subgrade soils are clear. First, PCC pavements laid over non-uniform subgrades experience long-term performance problems. Second, conditioned fly ash (CFA) or hydrated fly ash (HFA) can help stabilize subgrade soils. Neither fact, though, has been well understood.
So David White, assistant professor of civil engineering at ISU, led a research team to look into the matter. In a two-part project sponsored by the Iowa Highway Research Board (TR-461) and the Federal Highway Administration (Project 4), White’ team combined laboratory evaluations, analytical models, and in situ field tests to study two aspects of fly ash-soil stabilization:
- the engineering properties of soil-fly ash mixtures
- the influence of non-uniform subgrades on long-term pavement performance
The results suggest not only that subgrade uniformity is a key issue for predicting long-term pavement performance, but that fly ash effectively stabilizes Iowa soils’ engineering properties. With these findings, White and his team proposed three specifications for using self-cementing fly ash, HFA, and CFA.
Engineering properties of soil-fly ash mixtures
For the first part of the research, White mixed CFA or HFA, reclaimed from six coal-fired power plants in Iowa, with five different soil types. Generally, he found, fly ash effectively stabilizes the engineering properties of Iowa soils in the following ways:
- increases dry density
- reduces optimum moisture content
- increases freeze/thaw durability
- increases strength
- raises the California Bearing Ratio of fine-grained soils
In addition to increased soil stability, fly ash has a few advantages over other select fill materials:
- environmental incentives, because the material used is not wasted
- cost savings, because fly ash is typically cheaper than cement and lime
- availability, because fly ash sources are distributed geographically across Iowa
From the results of this study, White proposed three specifications for using self-cementing fly ash, HFA, and CFA. These specifications describe laboratory evaluation, field placement, moisture conditioning, compaction, quality control testing procedures, and basis of payment.
Influence of non-uniform subgrade support on PCC pavements
In the second part of the research, White examined the impact of non-uniform sub-grades on 12 reconstructed PCC pavement projects in Iowa. He studied four subgrade types:
- natural subgrade soils
- fly ash-stabilized subgrade
- reclaimed HFA subbase
- granular subbase
He then entered the data gathered on stiffness, moisture and density, strength, and soil classification into the ISLAB2000 finite element model program. This procedure modeled the elastic properties of the pavement structure and foundation.
White’s data and analysis revealed some important features of subgrade soils:
- Non-uniform subgrade support leads to premature failures, fatigue cracking, faulting, rutting, and other long-life problems in the pavement.
- Natural Iowa subgrade soils vary in moisture content, density, stiffness, and strength more than the other three types of subgrades studied.
- Stabilizing the subgrade using fly ashes or a granular subbase will improve uniformity and pavement performance.
- Without an effective drainage layer, natural Iowa subgrade soils saturate and become unstable, intensifying pumping in the subgrade layer.
- Pavement foundations analytically modeled with a uniform subgrade experience lower maximum principal stresses and deflections, increasing these pavements’ fatigue life.
- ISLAB2000 pavement modeling shows that maximum bending/principal stresses and pavement deflection decrease as the modulus of subgrade reaction increases.
Because a uniform subgrade is crucial for reliable pavement performance, White suggests that subgrade uniformity should be a key issue for determining long-term pavement performance. However, consistent subgrade uniformity will require better construction methods and field quality control testing.
For more information
The two project reports for Fly Ash Soil Stabilization for Non-Uniform Subgrade Soils include Volume I: Engineering Properties and Construction Guidelines and Volume II: Influence of Subgrade Non-Uniformity on PCC Pavement Performance, as well as two technology transfer summaries. These are available at CTRE’s website, www.ctre.iastate.edu.
If you have specific questions, contact David White, 515-294-1463, djwhite@iastate.edu.
