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th
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The surface modulus obtained from testing with an LWD
will however be significantly less than 100MPa due to material
non-linearity. Stage 1 of the PUMA test generated a stiffness of
just 50MPa for the gravel. For the subgrade, had it been tested,
Stage 2 would be appropriate due to the increased level of
confinement at depth which, if Stage 4 had given 60MPa, would
be likely to have given a value of around 40MPa. Turning to
multi-layer linear elastic analysis once more, with 260mm of
material of 50MPa stiffness overlying a subgrade of 40MPa
stiffness, the equivalent surface modulus is calculated to be
46MPa. This value would therefore represent the direct LWD
test equivalent of a Class 2 foundation.
4.3
Discussion
The design approach described in Highways Agency (2009)
represents a considerable step forward in that it puts pavement
analysis onto a sound basis. Nevertheless, implementation in the
UK has given rise to questions as to the appropriate in-situ
measured surface modulus requirement for a given foundation
class. For example, the limits currently included in Highways
Agency (2009) for a Class 2 foundation are an absolute
minimum LWD modulus of 50MPa and a mean value (from
groups of 5 tests) of 80MPa which, from the foregoing example,
would appear to be an unnecessarily conservative requirement.
It is understood that a similar point has been made on several
occasions by contractors who have found the 80MPa mean
value difficult to achieve with standard unbound crushed rock
aggregates. Clearly this is an issue that would benefit from
further comparative study, including tests carried out on
completed pavements using a Falling Weight Deflectometer.
Nevertheless, judgments have to be made in order to move
forward and testing in a PUMA or similar confined compression
test (Springbox, K-Mould) can provide suitable information on
which to base such judgments. The evidence put forward in this
paper suggests that the LWD-measured stiffness modulus on a
granular pavement foundation should typically be less than half
the value that would apply in the finished pavement, although it
is clearly important to ensure that limits placed in specifications
are suitably robust.
Once appropriate limits are agreed, including suitable factors
of safety, the PUMA or similar tests can then be very useful in
allowing a designer to evaluate alternative foundation material
combinations in order to achieve a desired foundation class. The
challenge for any highway authority is to allow road
construction and design organizations to innovate and, thereby,
save resources and costs while maintaining an appropriate
degree of conservatism.
5 CONCLUSIONS
This paper represents a snapshot of developments in the UK that
are still ongoing. It is believed that the generic test type, one
version of which has been described, could be widely used in
pavement foundation design. This is, however, reliant on there
being a specification that commands the trust of the industry
and that ensures the designed long-term in-service foundation
stiffness modulus is achieved subject to matters that are outside
the remit of direct testing, notably the provision of appropriate
drainage.
6 ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of Cooper
Research Technology in obtaining data from the PUMA
equipment.
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