posted on 2012-12-11, 12:38authored byMatthew W. Frost
There is an impetus in the UK to move away from empirical pavement foundation design
and the current method specification, towards an analytical design approach. For an
analytical design approach to be adopted, the required performance properties of stiffness
and resistance to permanent deformation of the foundation materials (sub grade and
capping) need to be measured, both in the laboratory for design and in the field in order
to ensure compliance.
This thesis studies the influence of the sub grade on the constructability and performance
of a series of full-scale pavement foundations. This has been achieved by measuring the
performance parameters of several sub grade materials in the laboratory, using repeated
load triaxial testing. These data have been compared to comparable data collected in situ
using dynamic stiffness measuring devices during the construction of trial pavement
foundations. The performance of the trial foundations has been measured during the
placement and compaction of the different foundation materials, and again after their
subsequent trafficking.
The testing demonstrates the stress dependency of the foundation materials. The
laboratory testing shows that the sub grade permanent deformation under cyclic loading
(used to simulate construction operations) becomes unstable at a deviator stress of half
the deviator stress at failure (0.5qmax)· The stiffness at this applied stress and above is
shown to be approaching a consistent value. This indicates that large changes in the
stiffness of inversely stress dependent fine grained soils occur below the deviator stress at
which the permanent deformation becomes unstable. Significant variability of data has
been found in the performance parameters measured (both in the laboratory and in the
field) for samples of subgrade collected from small areas of the same site. However
comparable patterns of stress dependency have been observed between measured
laboratory and field performance using the different apparatus.
The resistance to permanent deformation is shown to be a more critical design load case
for construction than the need for adequate stiffness of support required to compact the
foundation layers. The performance of a composite road foundation is shown to be
material and site specific, and this will have important implications for design and site
compliance testing.