Abstract: The failure modes of underreamed ground anchors in sand under vertical pullout,which were embedded in various depths,were researched by model test.The test results exhibited three different modes of failure depending on the embedment ratio.The rupture surface of shallow underreamed anchor presented like an inverted bell,inclined slightly inward as it extended to the sand surface.The failure mode of shallow underreamed anchor was known as general shear failure,shallow underreamed anchors should not be used in practice.The rupture surface of deep underreamed anchor was completely limited inside sand and presented like a ellipsoid.There was no deformation in the shallow sand layer either during or after pullout,and the failure of underreamed anchors embedded in deep sand resulted from local shear failure.It was highly recommended to apply the deep underreamed ground anchors in design.There was another type of underreamed anchor between shallow and deep underreamed anchors,namely the transitional anchor,and a combined failure surface of the anchor was appeared in the failure zone.But the failure mode of the transitional anchor was near to the shallow one,which was classified as the shallow failure mode.