GNGTS 2015 - Atti del 34° Convegno Nazionale

The GPR survey was joined by FWD performed on the same roads crossed by R.Ex. Due to the objective of the survey, which was not dedicated to fractures or defects location, we used an horizontal trace spacing of 1m and we fixed a mean acquisition speed of 50 km/h. In Fig. 3 we show an example; this profile is peculiar because it evidences strong lateral variations of thickness of the aggregates layers. This trend has been also confirmed by the sample cores executed by a geotechnical company, which have been used for velocity calibrations. Along the 2.3 GHz profiles we interpreted the trend of the conglomerate layers (binder and base interfaces, in green and yellow) while along the 800 MHz data we interpreted the horizons associated to the old pavement (in pink) and subgrade (in light blue). Till about trace 2000 from the south, the base of conglomerates lies at an almost constant depth; prosecuting toward the north, the thickness increases and is based on the old pavement not evident southward; the subgrade layer is also irregular. This different behavior from north to south has been observed even by the FWD data response, associated to strong inhomogeneities of the road embankment. Generally, for this survey, GPR profiles depth conversion has been executed using a vertical variable velocity field: we used a constant velocity for the conglomerate layers derived by the available drill cores, and different velocity values for the subgrade. The latter have been calculated for each road lane based on the building project specifications. Fig. 2 – example of results from CASE 1 applications of R.Ex. system; a) example of GPR profile acquired in Test Site 1 with 1.6 GHz data till 10ns and 500 MHz data from 10 to 60 ns. b) depth map of sedimentary contact horizon H4 of Test Site 1where the layer deeps toward north; c) example of GPR profile acquired in Test site 2 where in green color is depicted the weak reflection of the base of the concrete plates. 110 GNGTS 2015 S essione 3.2