Determination of Bedrock Depth from Falling Weight Deflectometer Data

Abstract

The accuracy of falling weight deflectometer (FWD) back-calculated moduli depends heavily on knowledge of bedrock depth. The bedrock depth can be obtained from coring, boring, and seismic refraction tests, but these operations are not practical. Determining the bedrock depth from FWD data is the easiest and least expensive way. This paper documents the effects of bedrock depth on FWD responses and makes comparisons between predicted bedrock depth and field measurements. Modified algorithms that better fit the field measurements are proposed. It was found that bedrock depth has a great influence on outer FWD sensor (r3 to r6) readings. Deeper bedrock resulted in a higher percentage of increase in r3 to r6 readings in response to increased load. When the r4 reading is less than 25.4 μm for any drop height, it is very likely that the bedrock is within 1.52 m of the surface. The time-deflection history patterns were drastically different between shallow and deep bedrock. When there is no free oscillation after the first deflection peak in a time-deflection history, shallow bedrock (within 1.52 m) is indicated. A modified equation (Equation 2) using the peak deflections provides an accuracy within 30 percent. It is recommended that these equations be used on a pavement where there is no severe surface distress. Another modified equation (Equation 6) using time-deflection history data yields an accuracy within 20 percent but cannot be used when there is no clear indication of free oscillation.