110
extremely low or high, a simple power law equation can be used to
predict E from the modified Dynaflect sensor 10 deflection.
4.3.3 Development of FWD Prediction Equations
4.3.3.1 Prediction Equations for E,. It was observed from the
sensitivity analysis that the magnitude of E, did not produce much
change in the FWD deflections. The maximum change occurred at D, with
reduced sensitivity for deflections D2 through Ds. Beyond Dg there was
essentially no response effect. Therefore, initial effort was expended
in developing simple power law equations between E and D1 for fixed
values of tI, E2, E3, and E4. The resulting power law relationships
provided excellent prediction reliability with R2 values greater than
0.95. However, efforts to achieve a generalized E, prediction equation
with Di as the main independent variable was complicated by the inter-
action of thickness and layer moduli values. Therefore, the difference
in deflections between D1 and D2 through D were analyzed to determine
if any of these values could be used to characterize E A multiple
linear regression analysis (39) was used to obtain three equations for
different ranges of t1 values. These El prediction equations are
presented below and hold for deflections obtained with a 9-kip FWD load.
Case 1. For 1.5 < t < 8.0 in.,
1
log (E ) = 1.87689 0.41689(t ) 23.8735 log (D D )
1 1 1 2
+ 43.582 log (D D ) 29.7179 log (D D )
1 3 1 4
8.80 log (D D ) Eqn. 4.18
(N = 400 and R2 = 0.932)