Investigation Of Pullout Capacity And Failure Mechanism Of Strip Anchor In Sand

Many geotechnical designs require knowledge of soil uplift resistance where the foundation must withstand tensile forces. Such situations are common to both onshore and offshore environments, such as wind loading on transmission towers, frozen and expansive soil ground base, wave action on offshore structures, and buoyancy forces on buried pipelines. In general designs and researches, strip anchor is often simplified as a two dimensional plane strain analysis. With the introduction of a shape factor, a general formula is proposed to calculate the uplift capacity of both square and circular anchors. The extensive use of plate anchors has motivated researchers to achieve a more thorough understanding of anchor behavior.In this study, the uplift behavior of strip plate anchor embedded in loose, medium-dense and dense sand is investigated by experimental model tests in1-G condition. The width of the model plate anchor varies from50mm to400mm and the embedded depth varies from one time of plate width to12times. The densities of the dry sands are1.65g/cm3,1.73g/cm3,1.85g/cm3(relative density, Dr, as30%,55%,80%) with the frictional angle,φ, of34°,38°and43°, respectively. In medium dense and dense sand, the load-displacement curves are different from that in loose sand, which shows an increasing rapidly to a peak value, then reducing obviously, and finally reaching a relatively steady value (except some fluctuations). The Particle Image Velocimetry (PIV) technology is employed to capture the failure mechanism of the sand. The failure mechanism in medium dense and dense sand at the peak resistance clearly shows an inverted trapezoidal shape with a projecting angle of around2ψ (dilatancy angle). During the softening stage, gradually shrinking of the mobilized sand was observed. Finally, the shear band in the soil developed nearly vertically when the resistance reached the residual value. However, a vertical slip failure is observed for shallow anchors in loose sand over the process of anchor uplifting, which extends from the edges of anchors to the soil surface. No obvious shear band was observed around the anchor when the anchor embedment ratio is large than7. The effect of relative depth (depth/width) and the width of anchor are discussed. In the range of this study, the model scale has minor effect on the normalized breakout factor. The breakout factors given by the experimental tests agree well with the predictions by the equation of Meyerhof and Adams (1968). Based on some model tests, the PFC (Particle Flow Code) method was used to simulate the uplift behavior of the strip anchors in medium dense sand. Two shapes of clump particles were formed to simulate irregular sandy particles. The particle grading was generated using the similar grading method according to Fujian standard sand used in the model tests. Numerical biaxial tests were performed using a series of micro parameters. A set of micro parameters were picked out that the equivalent friction angle and dilation angle agreed with those of the sand in the model tests. Then the selected micro parameters were used to simulate the pullout process of plate anchor in the model test. The uplift resistance from numerical simulation agreed well with that from the model tests. Comparing with the model test, PFC method can provide the distribution and evolution process of the particle contact forces, thus can further provide a micro point of view to investigate the mechanism of uplift failure. Finally, the pullout process of inclined plate anchor in sand was simulated. The soil flow mechanism and the contact forces evolution laws of the particles around anchor were discussed. The ultimate pullout capacity of plate anchor by PFC simulations was also compared with previous results.