Brown, S.F.
(1967)
Stresses and deformations in flexible layered pavement systems subjected to dynamic loads.
PhD thesis, University of Nottingham.
Abstract
Many of the proposed rational design methods for flexible pavements are concerned with the stresses and strains which occur in the various layers of the structure. The purpose of the work reported is to investigate, in the laboratory, the complete stress and strain distributions set up in the different layers under dynamic loads.
Two systems have been investigated, a single layer of clay and a two layer system consisting of a granular base on a clay subgrade. The loading in each case consisted of a single pulse having a duration of loading between 0.1 and 2 sec. The load was uniformly distributed over a circular area and of varying magnitude.
In-situ measurements of stress and strain were made using pressure and strain cells, -, at various orientations. Surface deflection was measured with a rectilinear potentiometer.
Stress and strain distributions were determined by moving the load relative to the buried transducers. By superimposing results, values of principal stresses and strains and maximum shear were derived. By combining stress and strain measurements, values of in-situ elastic modulus and Poisson's ratio were calculated.
Results were compared with elastic theory, both Boussinesq and layered system, the latter being computed using a recently developed program. Stresses showed good agreement with theory in both systems, but strains, being dependent on modulus, were less easy to predict theoretically.
In-situ values of modulus were stress dependent for both materials. For the clay, at low stress levels, the modulus increased sharply with decreasing stress, while for the granular material modulus increased with stress level.
In the two layer system results compared less favourably with theory, but the important values of tensile horizontal stress above the interface and vertical strain below the interface appear to be predicted adequately. The values of modular ratio were near to unity and hence Boussinesq theory was equally as adequate as the layered system approach for most effects.
Strains were predicted with fair accuracy when local values of modulus were used i.e., those in the neighbourhood of the points concerned. The assumption of perfect roughness at the interface, used in most theoretical solutions, was shown to be valid.
The stress dependence of modulus is thought to be one of the main problems at present in the application of layered system theory and, for the calculation of strains, in the use of the Boussinesq approach also.
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