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clay layer at shallow depth and near the water table, as in the case of
the SR 26A site, would not only contribute to a loss in foundation
support, but would also channel water to the upper layers by means of
capillary rise. Pore pressures could also be generated in the submerged
layers which would also contribute to a reduction in foundation support.
In general, a weak subgrade would result in high deflections. However,
it will be shown in Chapter 8 that high deflections do not necessarily
mean that the pavement is structurally deficient. Depending on the
relative stiffnesses of the upper layers (AC, base and subbase), a pave
ment with a weak subgrade (or high deflections) could yield moderate
stresses in the various layers. But when the subbase and base course
layers also become weak as a result of moisture intrusion, the entire
pavement could deteriorate structurally.
The variation of the subgrade stiffness with depth is also impor
tant for the design of new pavements. Table 7.9 suggests that predicted
deflections could differ significantly from field measured values
depending on the subgrade modulus. In most design procedures, the
thicknesses of the upper layers are selected based on the subgrade
modulus. Therefore, if samples of the subgrade from immediately below
the subbase layer are tested for modulus, use of this modulus could
result in a completely wrong design. Knowledge of the stratigraphy
would help to arrive at an optimum and efficient design. It is believed
that the CPT has the capability of providing such important informa
tion. Also, the correlations presented in Table 7.5 could be used to
estimate the subgrade modulus. This could avoid the problems of deter
mining subgrade modulus in the laboratory.