A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
11. Which of the following cause much damage to bituminous
pavement?
(a) Low usage by heavy vehicles or frequent usage of light
vehicles;
(b) Low speed traffic or high speed traffic
The relationship between axle weight and the associated pavement
damage is not linear but exponential. The pavement damage caused by
one passage of a fully loaded tractor-semi trailer (80kN) is more than 3,000
passages of private cars (9KN). Hence, heavy trucks and buses are
responsible for a majority of pavement damage.
Slow-moving traffic imposes greater damage than fast-moving traffic. Past
studies showed that when the speed is increased from 2km/hr to 24km/hr,
the stress and pavement deflection is reduced by 40%.
12. In the backcalculation of moduli in Falling Weight Deflectometer,
why are non-unique solutions generated? (RS2)
Falling Weight Deflectometer is a non-destructive test applied to pavement
for structural evaluation. The loads applied in Falling Weight Deflectometer
are of impulse type (quasi-static load in Benkelman Beam) and it serves to
simulate actual truck wheel load. Surface deflections induced by the
impulse load are measured by some sensors located at the pavement and
the pavement surface’s deflections form a deflection basin. The measured
deflections can then be adopted to estimate the elastic moduli of pavement
structural layers by backcalculation.
In backcalculation process, a pavement model is firstly established to
represent the exiting pavement structure. Trial values of moduli of
structural layers are used to calculate the theoretical deflections of
pavement model and these values are compared with the measured
deflections in Falling Weight Deflectometer. The trial moduli are then
adjusted iteratively until both values agree closely with one another.
However, it is observed that the backcalculated moduli based on the
analysis of Falling Weight Deflectometer is dependent on software and
user. Different computer programmes can generate different values of
backcalculated moduli from the same deflection basin. In fact, for a given
deflection basin, there are numerous combinations of trial modulus which
can satisfy the deflection envelope with a certain accuracy level and hence
it may result in non-unique solution developed.
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