To provide insight into subgrade non-uniformity and its effects on pavement performance, this study investigated the influence of non-uniform subgrade support on pavement responses (stress and deflection) that affect pavement performance. Several reconstructed PCC pavement projects in Iowa were studied to document and evaluate the influence of subgrade/subbase non-uniformity on pavement performance.
In situ field tests were performed at 12 sites to determine the subgrade/subbase engineering properties and develop a database of engineering parameter values for statistical and numerical analysis. Results of stiffness, moisture and density, strength, and soil classification were used to determine the spatial variability of a given property. Natural subgrade soils, fly ash-stabilized subgrade, reclaimed hydrated fly ash subbase, and granular subbase were studied.
The influence of the spatial variability of subgrade/subbase on pavement perform ance was then evaluated by modeling the elastic properties of the pavement and subgrade using the ISLAB2000 finite element analysis program. A major conclusion from this study is that non-uniform subgrade/subbase stiffness increases localized deflections and causes principal stress concentrations in the pavement, which can lead to fatigue cracking and other types of pavement distresses.
Field data show that hydrated fly ash, self-cementing fly ash-stabilized subgrade, and granular subbases exhibit lower variability than natural subgrade soils. Pavement life should be increased through the use of more uniform subgrade support. Subgrade/subbase construction in the future should consider uniformity as a key to long-term pavement performance.
Since the first concrete pavement was placed in Bellefontaine, Ohio in 1893, rigid pavement design and analysis have become increasingly more important. In 2001 there were approximately 59,000 miles of rigid pavement in the United States (Huang 2004). With pavement rehabilitation projects and costs continuously rising, research must investigate the influence of subgrade non-uniformity and its effects on pavement performance.
Spatial Variation of Soil Stiffness:
Soil parameters vary from point to point, even in normally homogeneous layers. Grabe (1993) shows that it is necessary to describe the spatial variation in order to predict geotechnical performance and deal with risk and reliability. Differential stiffness values lead to differential settlements. These differential settlements then cause dynamic forces that induce further settlement.
Support under PCC Pavements:
Stresses and deflection affect the perform ance of a PCC slab and depend on several support factors, including the following:
- Subgrade soil stiffness
- Base type, stiffness, and thickness
- Frictional resistance between the slab and the base
- Freeze-thaw action in the base and subgrade
- Seasonal moisture levels in the subgrade and untreated base
- Load transfer at the joints
- Erosion of base or subgrade material from traffic loading, poor drainage, or pavem
- Temperature and moisture gradients within the slab.
Case Study: Ohio SHRP Test Road, U.S. Rt. 23, Delaware, Ohio:
This project was constructed in August 1996 to study four objectives: (1) structural factors for flexible pavements, (2) structural factors for rigid pavements, (3) environmental effects in the absence of heavy traffic, and (4) asphalt program field verification.
Subgrade Models for Numerical Analysis:
In geotechnical engineering, the solution of a slab-on-grade soil-structure interaction problem has been simplified. Concrete pavements and foundations are generally treated as an elastic plate and the soil supporting the pavement or foundation is assumed to be linear, elastic, isotropic, and homogeneous. In reality, the stress-strain behavior of the soil is nonlinear, irreversible, anisotropic, and inhomogeneous.
The methods section overviews the in situ testing, numerical modeling, and statistical methods used throughout this study. Methods include (1) collection of field data, (2) finite element modeling to evaluate pavement response, and (3) statistical analysis of field and numerical results. Several tasks defined during this phase of the project are described under various project objectives.
Collection of Field Data:
Field data were generated to provide technical data for generating subgrade finite element models to evaluate pavement performance. Field data were generated using a grid system and by conducting several in situ tests at each grid point.
Finite Element Modeling to Evaluate Pavement Response:
Using the in situ test results, pavement systems were modeled using ISLAB2000, a powerful finite element analysis tool designed specifically for modeling rigid pavements. ISLAB2000 allows input parameter values for up to four layers in addition to the subgrade in an analysis.
Outputs for the ISLAB2000 software are vertical deflections and stresses (x and y directions, shear stresses, and principal stresses). Deflections are measured in inches and stress is measured in pounds per square inch (psi).
This section presents the in situ testing and laboratory analysis of the subgrade/subbase materials evaluated in this study. Field testing consisted of nuclear density gauge, GeoGauge, DCP, and Clegg Impact Hammer tests. Laboratory tests consisted of grain-size distribution and Atterberg limits test analysis of the subgrade/subbase materials.
In Situ Test Results:
- In Situ Test Results
- GeoGauge Stiffness
- GeoGauge Stiffness
- Clegg Impact Hammer
Subgrade/Subbase Index Properties:
For each subgrade/subbase sample, grain-size analysis and Atterberg limits tests were performed. The Unified Soil Classification System (USCS) group symbols and group name are provided for each sample.
This results discussion is divided into two sections: (1) numerical modeling results and (2) statistical analysis of the results. Each section details specific outcomes pertaining to that section.
This section details results obtained from the pavement modeling process outlined in the methods section above. ISLAB2000 results show decreased pavement stress and deflection with increased subgrade stiffness due to the addition of self-cementing fly ash, HFA, or granular subbase. ISLAB2000 modeling of uniform subgrade shows a decrease in average pavement stress, deflection, and standard deviation for most projects.
This section discusses results pertaining to the ISLAB2000 pavement modeling. The ISLAB2000 finite element modeling results show a few notable trends, including an overall general decrease in maximum principal stress and pavement deflection as the modulus of subgrade reaction increases.
Pavement Life Results:
The number of repetitions to failure for each project for both the non-uniform and uniform subgrade modeling conditions. Note that simulation of a uniform subgrade produced a larger number of repetitions to failure for each project tested.
This section details results obtained from statistical analysis of the generated field data and ISLAB2000 pavement modeling data. Statistical analysis generally shows that HFA, self-cementing fly ash-treated subgrade, and granular subbases perform better with a smaller standard deviation and COV.
The research confirm that a link exists between pavement performance and subgrade non-uniformity. Finite element modeling results indicate that a uniform subgrade reduces critical pavement responses, such as stresses and deflections, leading to improved pavement life.
Statistical analysis (of mean and COV values) shows that field results for self-cementing fly ash-treated subgrade, reclaimed hydrated fly ash, and granular subbase tend to be more uniform and have higher stiffness than Iowa soil subgrades.
This study did not investigate the effect of voids underneath PCC pavement. However,corner loadings with voids could lead to higher pavement stresses and deflections, thereby reducing pavement life.
The major conclusion of this study is that pavement performance is adversely affected by non-uniform subgrade support. Pavement life can be increased through the use of more uniform subgrade support.
Providing a uniform subgrade/subbase support should be considered in the future as one of the key issues in achieving long lasting and better performing pavement systems. Achieving uniformity in pavement foundation will require improvements to be made in construction methods and field quality control testing.
Source: Iowa State University
Authors: David J. White | Dale Harrington | Halil Ceylan | Tyson Rupnow