Study On Uplift Bearing Characteristics Of Anchored Pile Foundation With Overlying Soil

The geological conditions in the mountainous and hilly areas of China are dominated by weathered rocks,which are often covered with a certain thickness of soil layer,ranging from 2 to 5 m in thickness.When building towering structures such as transmission towers on sites with such geological conditions,the uplift load needs to be considered in the design.The foundation needs to have a high uplift capacity.For the selection of the foundation,the traditional pile foundation,which is only buried in the soil layer,cannot meet the requirements of uplift capacity.It needs to be embedded in rock to meet the requirements.However,due to the limited construction conditions in mountainous areas,the embedded rock construction is difficult,the project cost is high,and the construction risk is high.If rock anchor foundation is chosen,the overburden layer needs to be removed,leading to problems such as large excavation and substandard environmental requirements.For the special geological condition of"soil on top and rock underneath"in mountainous area,an anchored pile foundation with pile in soil layer and anchor foundation in rock layer has been proposed by domestic and foreign scholars in recent years,which is a kind of combined foundation suitable for this geological condition.The pile bears the pressure and horizontal load,and the pile and anchor jointly bear the uplift load.Anchored piles have the characteristics of reasonable load transmission,convenient construction and good economy,and have been applied in some transmission tower foundation projects in China.At present,there is a lack of in-depth understanding of the uplift bearing mechanism of anchored piles.There are research gaps for the load sharing effect of the two parts,the play of uplift capacity,and the design uplift capacity calculation method of the anchored pile.The design method of the foundation for the uplift load is still unknown.The relevant theoretical research has lagged behind the practice and needs to be supplemented and improved.For these reasons,monotonic uplift and cyclic uplift field tests of anchored piles were conducted and the results were analyzed.The study of influence factors was carried out by means of numerical simulation.On this basis,the uplift bearing mechanism and failure mode of anchored piles were systematically investigated.The theoretical calculation methods of uplift mobilization coefficients and the design uplift capacity were proposed.The relevant design methods in the current code were improved.The main research works and conclusions are as follows:1.Three anchored pile foundation monotonic uplift field tests were carried out to study the characteristics of uplift load-displacement curve,axial force distribution,load sharing effect and failure mode.It was found that the uplift load-displacement curve（P-s curve）is mainly slow-varying,and the curve can be divided into the elastic linear section,the transition section and the damage section.The part that reaches the ultimate uplift state first is the control factor of the design uplift state of the anchored pile.The design uplift capacity can be calculated by the uplift mobilization coefficient k and the ultimate uplift capacity of each part.With the increase of the uplift load,the uplift resistance is gradually exerted from top to bottom along the depth direction,and the uplift capacity is composed of the frictional resistance,the shearing effect of the expanded bottom section on the soil and the anchorage force of the rock anchor.The failure mode of the anchored pile is a combination of pile and anchor group failure modes.The pile occurred inverted conical integral shear failure（shallow foundation）,and the anchor group occurred rock shear and interface debonding composite failure.2.Three anchored pile cyclic uplift field tests were carried out to study the characteristics of cyclic uplift load-displacement curve,axial force distribution,load sharing effect and failure mode.It was found that the hysteresis circle of the cyclic uplift load-displacement curve gradually shifted in the direction of increasing displacement as the number of cycles increased.The variation of the hysteresis circle was characterized by sparse-tight-sparse,which reflected the degradation of the uplift deformation stiffness and the energy consumption of the soil and the rock.The proportion of load carried by the pile and the anchor group decreased with the number of cycles.The pile were the first to experience capacity degradation,and the load is gradually transferred downward to the anchor after several cycles.At unloading to 0 k N after cycling,tensile stresses appear inside the foundation due to the difference in cyclic bearing characteristics and deformation.In the early stage of cyclic loading（less load）,the load was mainly borne by the pile.The proportion of load carried by pile for the three test foundations at the respective first stage of cyclic loading were 98%,86%,and 95%,respectively,while this proportion at the peak load P_max during the final cycle was 36%,44%,and 38%,respectively.The failure mode of the ultimate cyclic uplift state of the anchored pile was basically the same as that of the ultimate monotonic uplift state.3.The numerical simulation of monotonic uplift of anchored pile was carried out by using the finite element software PLAXIS 3D.The influence of strength parameters and foundation structure parameters on load sharing and uplift mobilization coefficient k was investigated and analyzed.The results of the study show that the numerical calculation results are in better agreement with the measured results of P-s curves,internal forces and failure modes.The cohesion of soil and rock has an influence on the uplift mobilization coefficient k.The displacement corresponding to the ultimate uplift state of each part of the foundation under different cohesions is different,which makes the uplift mobilization coefficient differently.The elastic moduli of soil and rock also have an effect on the uplift mobilization coefficient k.The displacement of each part of the foundation under different elastic modulus when subjected to the same load is different,which makes the uplift mobilization coefficient differently.The increase in the depth of rock embedded,the anchor length and the number of anchors（up to 8 anchors）can improve the ultimate uplift capacity of the anchored pile.The design uplift capacity of the anchored pile is controlled by the part reaches the ultimate uplift state first.Therefore,the uplift capacity of each part should be brought into full play and coordinated to improve the design uplift capacity of the anchored pile.4.Based on the theory of fitting the uplift P-s curve by hyperbolic model,the calculation method of the uplift mobilization coefficient k is proposed.The design requirements for the cyclic uplift bearing of the anchored pile are proposed according to the cyclic stability diagram.Theoretical research results show that the hyperbolic fitting function can fit the uplift P-s curve of the anchored pile and its parts.The uplift mobilization coefficient of piles and anchors should be taken as k₁≤1 and k₂≤1.By solving the differential equations of load transfer of piles and anchors,the expression for the initial uplift stiffness can be obtained,which can then be substituted into the hyperbolic model to obtain the expression for the uplift mobilization coefficient.The theoretical calculation results are in good agreement with the measurement results.According to the cyclic stability diagram,the simplified cyclic state determination method is proposed.Based on the above theoretical method,the design method of the anchored pile under uplift load is proposed.Two practical cases are shown,for which the design analysis is carried out to provide theoretical support and design reference for the promotion and application of the anchored pile.