The traditional method of designing concrete pavements is through the assignment of a single modulus of subgrade reaction (k-value) to the soil for the section under consideration. It is well known that soil under pavement is not a homogenous, elastic, and isotropic half-space but varies spatially due to variations in the soil geological properties, environmental factors, and construction methods. Few studies have attempted to characterize this heterogeneous behavior as non-uniform subgrade support, theoretically analyze its effect on slab responses, or its effect on concrete pavement performance.
This research has collected geotechnical data from two roadway sections in Michigan, MI I-94 and MI I-96, to characterize the effects of the foundation layer spatial non-uniformity on tensile stress changes in a concrete slab. For both the MI I-94 and MI I-96 roadway section, k-values were correlated from field Dynamic Cone Penetrometer (DCP) tests that were either deterministically or randomly assigned to a predefined area size. These spatial plots were discretized into various uniform area sizes to compare tensile stresses from a non-uniform support under a concrete pavement to a uniform support condition. The individual area sizes varied from 0.7x0.7 m2 and 1.16x1.16 m2. A 2-D finite element program was used to analyze the critical slab tensile stresses for multiple uniform and non-uniform conditions subjected to three axle configurations, loading paths, and temperature differentials.
The results for MI I-94 stress analysis showed that the deterministic assignment of k-value from the field did not result in any significant increase in critical tensile stresses compared to the uniform support assumptions even for varying individual area sizes. However, when the k-value of the foundation layer was randomly assigned to these individual areas, using a normal distribution, for a soft subgrade (k-value = 63 psi/in and standard deviation = 25.6 psi/in), the overall peak tensile stresses along the edge loading path increased by 31% and the average peak tensile stress increased by 37%. The greatest increase in tensile stresses relative to the uniform support condition occurred for individual support areas of 1.16x1.16 m2. When the k-value was randomly assigned with a beta (B) distribution for a lower limit of 20 psi/in, there was no increase in the overall peak tensile stress in the slab relative to the uniform support condition. Although the section with stiffer soil (mean k-value = 397 psi/in), MI I-96, had a large range in measured k-values, it only increased the overall peak tensile stresses in the slab relative to uniform support conditions by 11% when randomly assigned to 81 k-value areas and increased the tensile stress by 6% when the k-values were deterministically assigned.
The field data and theoretical analysis presented in this research has shown that non-uniform support conditions can lead to significantly higher slab stresses under certain geometric, loading, and slab support conditions. Non-uniform support along the edge of the slab especially very low support values near the location of maximum tensile stress substantially increased the slab tensile stresses. These tensile stresses are further increased under daytime temperature curling. Variability in the foundation stiffness had a larger impact on slabs supported by softer soils relative to stiff soils. For the inputs analyzed in this study, the size of the individual area of uniform support defined around 1 m2 produced the greatest increase in tensile stress in the slab. Detection and treatment of areas of weak and variable support along the anticipated free edges of the slab are important to improving the performance of concrete pavements
