The effects of size and embedment depth of underreamed ground anchor under vertical pullout on its bearing capacity characteristics were researched through 25 model tests. The test results exhibited two different modes of underreamed ground anchor depending on depth/diameter ratio

namely shallow and deep underreamed ground anchor. During pullout

the two type underreamed ground anchors experienced several phases

such as soil elastic deformation

shear failure on normal anchor section-soil interface

soil elastoplastic deformation phase and shear failure as well. And they manifested general shear failure and local shear failure. In comparison between underreamed ground anchor and normal tension-type anchor

the test results indicated that in ultimate bearing capacity

bearing capacity ratio and safety

underreamed ground anchor had substantial superiority in comparison with normal tension-type anchor. In order to improve the bearing capacity

expanding diameter of underreamed section had more obvious advantages. Furthermore

there was an optimal diameter of underreamed section by the bearing capacity ratio analysis. So it is highly recommended to seek an optimal dimension of underreamed ground anchor to get more economic benefits in practice.