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To improve the capacity of ground anchors, scholars and engineers worldwide have developed various types of underreamed anchors with expanded anchor parts. Underreamed anchors have a completely different mechanism from traditional shaft anchors. The expanded section of an underreamed anchor induces an end bearing force to endure the uplift force similar to a reversed pile. Therefore, the total resistance of an underreamed anchor includes friction and end bearing force. To clarify the bearing mechanism of underreamed anchors, a series of model tests were performed using fiber Bragg grating (FBG) sensors and the photogrammetry measuring method. Based on the tests, the distribution and development of the friction and end bearing force of the underreamed anchor model were acquired by the FBG sensors when being pulled out. Moreover, the deformation state of the soil around the anchor model was observed by the digital photogrammetry measuring method. Finally, the interaction mechanism between an underreamed anchor and surrounding sand was obtained, which was identified as the inherent reason for the distribution and development law of the resistance of the underreamed anchor in sand.

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