The use of ground penetrating radar for monitoring water movement in road structures

The Ground Penetrating Radar technique was used to monitor water movement in road structures and the subgrade in 1992-93 as a part of the Road Structure Frost and Drainage Research Project conducted by the the Finnish National Road Administration. The purpose was to observe how water infiltrates from the road shoulder into different types of road structure, especially during the critical season of the year, ie. the autumn rainy season and the period in the spring, when the frost is thawing. The survey was performed by inserting a 3 m long tube into the road shoulder at the sites where the road structures and groundwater conditions were studied carefully by drilling and sampling and by GPR and then introducing 1-3 m3 salt water with a chloride content of 4 % in autum 1992 and 8-10 % in spring 1993 into the road structure and ground through the tube.The movement of water in the ground was monitored by visual inspection and by groundwater indicators and GPR. The results showed that GPR can be used for monitoring water discharge and movement in the road structure and subgrade. The factors which affect water movement were found to be the gradient of the road and subgrade, the direction of groundwater flow and the grain size and material properties of the structural courses and subgrade. The results also showed very clearly . that water of ,high density will penetrate through the fresh groundwater layer without being entirely mixed with it. The high chloride water was also found to thaw frozen road structures.