deflection only, it is impossible or difficult to determine the shape

and size of the deflection basin.

 In spite of their shortcomings, the large amount of data developed

using static deflection techniques makes such procedures an important

part of structural pavement evaluation. For this reason, several inves-

tigators have attempted deflection comparison and correlations from the

static devices with those measured by the dynamic devices. The

following is a list of concepts developed from the deflection response

of a pavement using static NDT (79):

1. For adequately designed pavements, the deflections during the same

 season of the year remain approximately constant for the life of the

 pavement.

2. There is a tolerable level of deflection that is a function of

 traffic type, volume and the structural capacity of the pavement as

 determined by the pavement's structural section. This tolerable

 level of deflection can be established through the use of fatigue

 characteristics of the pavement structure.

3. Overlaying of a pavement will reduce its deflection. The thickness

 required to reduce the deflection to a tolerable level can be esta-

 blished.

4. The deflection history of a well-designed pavement undergoes three

 phases in its behavior (71). A typical curve representing these

 phases is shown in Figure 2.1.

 a. In the initial phase, immediately after construction, the pave-

 ment structure consolidates and the deflection shows a slight

 decrease.