Abstract：A new type umbrella-shaped anti-float device，being composed of stretching anchor cables and an anchor head that can stretch in soil，is designed to perfect the traditional anchors on the basis of fully integrating with the advantages of under-reamed piles and anchors. There are two groups of pullout tests for umbrella-shaped anchor with and without grouting at anchor head，and a group of vertical pile pullout test is done for comparison. The results indicate that the anti-float performance of the new type anchors is better than traditional vertical piles. Moreover，the pullout behaviour of the umbrella-shaped anchor with grouting in anchor head is better. According to the test，the force mechanism of the umbrella-shaped anchor is further analyzed. It is pointed out that the uplift bearing capacity depends on the weight of soil pole wrapped by the anchor head and the shear friction along the interface of soil pole. Therefore the ultimate bearing capacity estimation formula of the umbrella-shaped anchor can be presented，which establishes the foundation for the engineering application of umbrella-shaped anchors.
