Study On Bearing Mechanism And Ultimate Bearing Capacity Of Underreamed Anchor

Underreamed anchors in soil obtain higher capacity with a little higher cost and a part of anchor section expanded based on normal shaft anchors.And the technical innovation on underreamed anchor has become research focus in the field of geotechnical engineering.However,the development and application of underreamed anchors are significantly limited due to the complicated failure mode,unknown bearing mechanism,and inaccurate predicted ultimate bear ing capacity.Thus the engineering project designs are always based on engineering experience,and the predicted bearing capacities of underreamed anchors are always far from their actual values.The above defects cause high risk and cost of anchorage engineering.In order to solve these problems,this dissertation invents the underreamed anchor model with fiber grating stain sensors.By using the digital photogrammetry method,model and in-situ tests,and finite difference numerical simulation,the load transfer mechanism of underreamed anchor and the failure mode are researched and revealed.The analysis method on the ultimate bearing capacity of underreamed anchor based on limit equilibrium theory is established,and the hyperbolic model for predicting the bearing capacity is presented.The major contributions of this dissertation are as follows:1.The underreamed anchor model with strain sensing function is invented.By the model,the internal force distribution of underreamed anchor in model test is measured,and the load transfer mechanism and bearing mechanism of underreamed anchor is revealed.Based on this,the digital photogrammetry method is adopted to observe the deformation state of soil around the underreamed anchor.The failure mode of underreamed anchor according to the failure of foundation soil is clarified.The gradual failure and sudden failure of underreamed anchor and their mechanical causes are discovered according to the response of foundation soil.2.In-situ test of underreamed anchor with optical fiber strain monitoring is held.The load transfer rule along the full size model of underreamed anchor is obtained.Based on the monitoring data,the load distribution and development on the unbounded anchor section,normal anchor section and underreamed section of the underreamed anchor is clarified.Besides,the comparative test between underreamed anchor and shaft anchor is carried on to state the advantages of underreamed anchor in bearing capacity and deformation-control ability quantitatively.3.The numerical model of underreamed anchor under uplift in soil is established.By the model,the influence of dimension of underreamed anchor and the shear strength parameters of foundation soil to the capacity of underreamed anchor are researched.The Quantitative assessment model of the bearing capacity of underreamed anchor is established based on the concept of bearing ratio,which provides theory basis for the optimum design.4.The critical state of underreamed anchor under uplift is prese nted based on the measured deformed zone in soil.According to the limit equilibrium theory,the calculation formulas of deformed zone size in soil and end bearing capacity of underreamed anchor are established.Based on this,the calculation method of the ultimate bearing capacity of an underreamed anchor is provided,and the calculation results are close to the measured ones.Besides,the hyperbolic model representing the relationship between load and displacement of underreamed anchor is obtained based on the measured engineering data.The bearing capacity of the underreamed anchor can be predicted according to the hyperbolic model and the real engineering test data.Hence,the mathematical model can predict the bearing capacity of the underreamed anchor,which solves the problem that many anchors can not be loaded to failure status to test in practice.