Other than one test site (SR 80 Section 2), the D10 values pro-

vided an excellent, highly reliable relationship (Figure 6.69). How-

ever, the slope of 0.95 suggests that predicted deflections are about 5

percent less than measured D10 values. The discrepancy occurs because

the straight line log-log relationship for predicting E4 from D10 (stan-

dard Ds) tends to be a curvilinear (hyperbolic) relationship for E4

values below 10.0 ksi or above 100.0 ksi (see Figure 2.8).

 6.7.2.2 Development of Simplified Layer Moduli Equations. Since

the tuned layer moduli provided predicted Dynaflect deflections which

correlated exceedingly well with the measured deflections, regression

analyses were performed to assess the relationship between

 a) Composite modulus of asphalt concrete and base course layers

 (E12) and D1 D4

 b) Subbase or stabilized subgrade modulus (E3) and D4 D7

 c) Subgrade modulus (E4) and Do0.

As mentioned previously, these sensor deflections were selected from the

analytical study on the basis of being related to the moduli of specific

layers. It was necessary to combine the asphalt concrete and base

course moduli because the analyses had indicated that no sensor location

or combination of sensor deflections were suitable for separation of E

and E The series of equations (Equation 4.1 through 4.10) developed

for prediction of either El or E2 from 01 D4, with a reasonable esti-
mate of E2 or E respectively, albeit their high degree of prediction

accuracy, were considered to be too complex for routine evaluation of
pavements. Therefore, it was necessary to simplify the various E1 and

E2 prediction equations. The approach used is described below.