Study On Mechanism Of Pressured Under-reamed Anchor Under Horizontal Load

With the increasing shortage of urban land resources in China,the development of underground space has become one of the important ways of urban expansion.Pile-anchor support system is widely used during the process because it's efficient and economical.In the soft soil area of southeast coast,in order to improve the ultimate uplift capacity of the bolt in the pile-anchor support system,engineers developed the pressure-type under-reamed anchor.The research and analysis of its operating principles,however,are relatively scarce at present,for it differs from the traditional bolt on operation and it has been applied for a limited period.Based on the theoretical research of domestic and foreign scholars as well as the experimental data of several practical projects,this paper derives the calculation method of ultimate uplift capacity of pressure-type under-reamed anchor under horizontal load based on the principle of limit equilibrium.Besides,it studies the mechanism of expanding the head bolt through the self-developed model test device.The deformation development mechanism and displacement field distribution characteristics of the soil surrounding the anchorage section of the under-reamed anchor are obtained by PIV technology,and the theoretical calculation formula is further modified according to the model test results.Finally,the results of the above studies were verified by field tests.The main contents and innovative results of this paper are as follows:(1)According to the stress characteristics of the pressure-type under-reamed anchor,the shear failure of surrounding rock is assumed.Based on the principle of limit equilibrium,a theoretical calculation model considering the strength of surrounding rock is established,and the formula for calculating the ultimate pull-out bearing capacity of the pressure-type under-reamed anchor under horizontal load is derived.The formula also takes into consideration the influence of parameters such as size,depth,overburden weight,internal friction angle and cohesion of rock and soil mass around the anchorage section on the ultimate pulling force of the bolt.The calculated results show that the active earth pressure at the far end of the anchorage segment has a significant weakening effect on the bearing capacity of the anchor when used in the clay soil with small internal friction angle.(2)Through a self-developed model test device and PIV technology,the author studies mechanical characteristics of pressure-type under-reamed anchor under horizontal load and the development mechanism of surrounding soil.Test results show that the stress process of the pressure-type under-reamed anchor can be divided into lateral friction stage,transition stage and end resistance stage,and there will be an "inflection point" in the load-displacement curve between the transitions of each stage.At the same time,according to the image analysis,the influence range of critical displacement field of soil around the under-reamed anchorage segment of pressure-type bolt is "bulb-shaped",the radial influence range of soil near the enlarged head anchorage segment is about 3D,and the axial influence range is about4.5D.When the length-diameter ratio of the enlarged head anchoring section exceeds3:1,the displacement field at the proximal end of the under-reamed anchoring section is basically unaffected with the continuous increase of length-diameter ratio.Therefore,the proximal soil pressure of it can be fully exerted when it is over 3:1.(3)Combined with the practical engineering,the field anti-pulling test of the pressure-type under-reamed anchor is carried out,which verifies the reliability and rationality of the theoretical calculation formula and model test results.The experimental results show that the calculated values of the theoretical method and the model test results are in good agreement with the field test results.Apart from this,it shows that the excessive length of the anchor body will lead to greater elastic deformation in the process of stress,which is not conducive to the anchor displacement control.