The depths of surface cracks in expansive clay deposits control the depths of the active zone in many cases. Rainfall and surface runoff can fill these cracks, and the water in the cracks can travel, impelled only by gravity, wherever the crack goes. If the water travels beneath a pavement, it will remain there, soaking into the soil on each side of the crack, and cause swelling. Thus, the depth of the surface cracks determines the depth to which a vertical moisture barrier should be placed in order to control moisture beneath a pavement, and a means of determining this depth is needed. This paper describes such a method using wave propagation. A summary review of wave types is included -- their generation, propagation, and alteration at a crack, and the feasibility of their detection and positive identification. This is complemented by the selection of several trial procedures for detecting surface cracks and estimating their depth. The results of field tests using several trenches excavated to different depths and at naturally occurring shrinkage cracks are presented.The testing set-ups are illustrated and the analysis performed with the field data is described. The best approach to determine the depth of the crack is from the increase in travel time of the surface wave caused by the crack.
Samenvatting