Abstract
Observation of micro cracks occurred in a rock anchor, which was recovered from the underground after in-situ pullout test was performed to make clear the failure mechanizm of rock anchors. A clear distinction between micro cracks and other component is usually difficult because micro cracks don't have very distinctive feature optically. To attempt to solve this major difficulty, micro cracks were coated with cyanoacrylate mixed with fluorescent dye, and then visualized under ultraviolet light. Various patterns of micro cracks, which were not detected under natural light, were identified clearly under ultraviolet light in the specimen collected from over 2 meters under the ground.
Micro cracks were also represented realistically by several methods of image processing, and then length and direction of micro cracks were measured. Based on the micro cracks direction, the angle from the direction of pull-out load become greater gradually from about 45 deg. to 90 deg. when the depth of rock anchor increased. Moreover, many micro cracks parallel to the direction of pull-out load were identified at the maximum depth in rock anchors.
Observational results indicate that the failure of rock anchors may progress over the depth which was considered previously by the pull-out test.
