Deflection basins from any nondestructive testing device can be characterized by parameters that describe the structural characteristics of an existing pavement. An exponential curve of the form y equals a times e to the bx power, where y is the deflection in mils andx is the radial distance in inches, approximates the deflection basins simulated from elastic layer theory and measured by the falling weight deflectometer (fwd). The coefficients a and b describe the structural characteristics of the pavement. Usually, a pavement with a stiffer upper layer or layers is indicated by a lower a value, whereas a stiff subgrade or the presence of a rigid bottom at a shallowdepth (or both) is indicated by a higher b value. The value of the coefficient of determination for the exponential fit, r-squared, wasfound useful for judging the suitability of an fwd-measured basin for backcalculation of layer moduli in a deflection-matching technique. Generally, a low value of r-squared for an exponential curve fitted to an fwd-measured basin indicates that there will be a high error in the backcalculation of layer moduli using elastic layer theory.Guidelines are presented for using the value of r-squared to indicate the error between measured and computed deflections that can be expected during a backcalculation analysis. This paper appears in transportation research record no. 1293, Backcalculation of pavement moduli 1991 .
Abstract