  The void underneath semi-rigid base is a common defect in roads. There are
some difficulties in the detection and repair for this kind of hidden damage,
as well as in the evaluation of the effects of grouting treatment. For the
detection and maintenance of roads, it is essential to study the detection and
judging for voids underneath base and the evaluation of the spread of grout.
Through theoretical analysis, numerical simulation and analysis of real data,
this research generated the characteristics of under-base voids of different
types and dimensions on GPR images, proposed the detecting and
dimension-measuring methods for under-base voids, and studied the process and
effects of data analysis techniques.
  (1) The characteristics of under-base voids of different types (air-filled,
water-filled or grout-treated) and dimensions (height and horizontal
dimensions), on A-scan and B-scan GPR image respectively, were analyzed
theoretically.
  (2) Approaches for detecting voids and for estimating its height were
studied, focusing on voids with a height ranging from 0.01m to 0.3m.
  (3) The approach for estimating the horizontal dimension of voids was
studied, focusing on voids with a length ranging from 0.04m to 0.52m.
  (4) The data processing process was discussed. Also, the effects of different
data processing techniques were studied in terms of noise filtering and
attenuation compensation, and their influence on the image characteristics was
also discussed.
