Experimental and theoretical studies on modeling of uplift bearing characteristics of screw anchors in sandy soil foundation

At present, it is necessary to introduce the concepts of the critical burial depth ratio and critical spacing ratio artificially into the theoretical researches on the uplift bearing of screw anchors with multiple blades, which divides a continuous evolution problem into two special working conditions. However, the criteria of the two critical values are inconsistent, leading to risks in the engineering application of the theory. Based on model tests and combined with the digital photographic measurement technology, the studies on the uplift bearing characteristics of screw anchors in sandy soil foundation are made, and the main conclusions are drawn as follows: (1) The variation of the uplift load displacement curves of the screw anchors with single and double blades can be divided into two stages, growth interval and oscillation decline interval. The larger the burial depth ratio, the higher the peak load, but then the enhanced soil arching effects also lead to the more significant oscillation amplitude of the curve. (2) The curve of the breakout factor N_γ with the burial depth ratio has a peak characteristic, which increases rapidly first and then decreases slowly. The bearing capacity playing coefficient η of lower blade increases rapidly and then increases slowly with the increase of spacing ratio. (3) The failure sliding surface corresponding to each layer of the screw anchor with multi-blades can be characterized by an ellipse with the decreasing axis ratio as the depth ratio increases. and the sliding surface shape of the first blade is consistent with that of the single blade with the same burial depth. The axis ratio of the ellipse of the sliding surface of the lower anchor can be divided into three stages under the influences of spacing ratio. (4) There are three sliding surface conditions of the first-layer anchor and the lower-layer anchors, respectively, corresponding to three mechanical models, which depend on the buried depth ratio and the spacing ratio. The overall mechanical model for the screw anchors is composed of them. (5) A method for calculating the uplifting capacity of the screw anchors with multiple anchor plates is developed to consider any number of anchor blades, buried depth ratio and spacing ratio. The effectiveness of the proposed method is verified by the calculation of four test cases in loose sand and medium-dense sand foundation.