Experimental Analysis Of Soil Deformation Around Uplifting Screw Anchors And Theoretical Research Of Uplifting Capacity Models

Screw anchor, with the advantages of fast construction, environment-friendly, economical efficiency, is one of the most popular anchor foundations which is widely used in buildings, retaining walls, transmission towers and transportations. Because of its complexity of soil-anchor interaction properties, the uplifting mechanism and capacity are not well understood by the researchers. Most of proposed theories are based on assumed failure plain.The actual failure mechanism of uplifting screw anchor is very difficult to obtain due to the limitation of experimental technique. The predicting results by theories based on assumed failure mechanism are varied widely with the tests results. At present, the design of the screw anchors are heavily depend on experiences.The undisturbed deformation measurement technology - Digital Image Correlation (DIC) is used to study the soil deformation fields around screw single and group anchors when uplifting in sand. The displacement fields,shearing strain fields and volume strain fields are obtained to analyze the deformation mechanism of soil around screw anchors. The soil parameters which control the uplifting capacity of single screw anchors is analyzed by deformation fields. The main factors which have influence on failure shape are obtained. The theoretical uplifting model is developed based on the main parameters. For group screw anchors the group efficiency coefficient influence factors are well understood by deformation analysis. A theoretical method which determine the group efficiency coefficient is proposed. Main conclusions are summarized below.(1)Due to the complexity research on deformation of soil around the anchor, digital image correlation method is used to measure the soil deformation around an uplifting anchor. A test set-up is developed to realize the effective combination of force loading system and StrainMaster image acquisition system. The precision test is conducted to verify the precision of StrainMaster. The results show that the high precision can be obtained by this system.The method has the advantages of simple operation, real-time measurement, high precision, interference free.(2) The soil deformation around an uplifting screw anchor is a complicated process which influnced by many factors.The uplifting tests in sand with different density at different depth are conducted to study the control factors of uplifting capacity.The deformation mechanism around screw anchors are analized systematicaly.(3) Based on the soil deformation analysis around uplifting screw single anchor, the uplifting failure mechanism is studied. The failure plain is determined based on dilatancy thoery of sand. The relationship beteween critical embedment ratio and ralative density is proposed.A prediction model is developed according to the analysis of single anchor deformation field,which can calculate the uplift capccity of shallow and deep anchor. The comparion between theoretical calculation and test results show that the prediction result of the theory and experimental results have high consistency.(4) Tests for group screw anchors in sand with different density are conducted to study the failure mechanism and force-displacement relationships. The results show that:The group anchor uplift capacity is not a simple superposition of two single anchors, but with the superposition effect. The sand density, embedment depth and distance have significant effects on the uplifting capacity of group screw anchors.The relationships between group anchor effecienchy,sand density,distance and embedment depth are developed.(5) The theoretical calculation models are proposed for different group anchor configurations. The theoretical results give a good agreement with tests results.