UFC 3-260-03
15 Apr 01
2
2
2
ND
%j
ISM & ISM
DF & DF
AREA & AREA
ERROR '
(eq 4-1)
%
ISM
DF
AREA
1
where
ISM = computed ISM
DF = measured deflection
AREA = computed area
ND = number of deflection sensors
= average ISM
ISM
= average deflection
DF
= average basin area
AREA
h. The deflection basin with the least error is selected as the representative basin for evaluating the
pavement feature.
I. The representative basin determined above is used whenever the coefficient of variation of the
ISM from all basins in the feature is less than 15 percent. If the coefficient of variation is greater than
15 percent, then judgment is used to select an appropriate basin.
6. STEP 3 (PREDICT LAYER MODULUS VALUES). The deflection basin produced by applying a load
to the pavement with an NDT device gives input parameters to the system analysis that can be used to
derive the relative strength parameters of the pavement layers. To determine modulus values, the pave-
ment structure is modeled as a layered system similar to that illustrated in figure 4-4. The computer pro-
gram WESDEF was developed to determine a set of modulus values that provides the best fit between a
measured and a computed deflection basin when given an initial estimate of the elastic modulus values, a
range of modulus values, and a set of measured deflections. To summarize the modulus backcalculation
routine:
a. Consider the pavement system where:
(1)
The modulus is unknown for a number of layers (NL).
(2)
The deflection due to an NDT loading is measured at a number of deflection locations (ND).
(3)
ND is greater than NL.
The objective is to determine the set of elastic modulus (E) values that will minimize the error between the
computed deflection (CD) and the measured deflection (MD).
b. A set of E values is assumed, and the deflection is computed at the sensor location correspond-
ing to the measured deflection. Each unknown E is varied individually, and a new set of deflections is
computed for each variation. Figure 4-5 is a simplified description of how the deflection basins are
matched. This illustration is for one deflection and one layer. For multiple deflections and layers, the
solution is obtained by developing a set of equations that defines the slope and intercept for each deflec-
tion and each unknown layer modulus as follows:
4-3
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