UFC 3-260-03

15 Apr 01

2

2

2

%j

(eq 4-1)

%

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