Continuous Discrete Coupled Numerical Analysis Of Axial Bearing Deformation Characteristics Of Screw Piles Considering Screw In Process

A triaxial model and a direct shear model are established using discrete element software to calibrate the mesoscopic contact parameters between particles.Using three-dimensional discrete element software PFC3 D coupling finite difference software FLAC3 D simulation model to simulate the screw pile composite foundation and the screwing in the installation process,the spiral steel shaft screwing in numerical model of deformation characteristics of sand particle sphere and spiral steel pile to study the microscopic stress,etc,in order to reveal the screwing process disturbance.Then,the influence of screw installation effect on pile bearing capacity under different compactness is analyzed,and the distribution of pile axial force is extracted and compared with the existing bearing capacity formula.At the same time,the screw pile composite foundation model and its cyclic loading process are simulated,and the deformation characteristics of sand and axial force of screw pile are studied in the dynamic load numerical model,and the following conclusions are obtained:(1)Calibration of mesoscopic parameters of soilThe HST95 discrete element triaxial numerical model was established,and the Hertz contact model parameters were calibrated as shear modulus = 9 (6,friction coefficient1)= 0.264,and Poisson’s ratio = 0.3.In terms of mechanical properties,the Hertz nonlinear contact discrete element model can well restore the triaxial loading stress path.At the peak stress corresponding to about 4% strain,the peak stress of 31% compactness test is 160.06 kpa,which is 102.6% of the test data.The peak stress of 73% compactness test is 236.92 kpa,100.5% of the test data.At the same time,the critical friction angle calculated by the discrete element direct shear model is 31°,which is close to the experimental result of 32°.In terms of deformation trend,the Hertz nonlinear contact discrete element model well reflects the shear deformation law of "loose sand shear shrinkage,dense sand shear dilation".(2)Simulation and analysis of installation processThe continuous-discrete CPT penetration model and the discrete-continuous screw pile model were established to simulate installation process effectively.The force and torque of the installation process were in good agreement with the experimental data of centrifuge,which verified the rationality and validity of the numerical model.The influence of pile parameters on mounting force and torque of screw pile is analyzed,and the comparison between the simulation results and theoretical calculation results shows that the traditional formula is insufficient to predict the mounting force and torque.Therefore,the traditional mounting force theory is optimized and the prediction result is more conservative and safe.(3)The influence of installation effect on bearing characteristicsBy comparing the bearing characteristics of screw piles with those of embedded steel piles,it is found that in the dense sand with a density of 73%,the compressive capacity and the tensile capacity are both weakened by the impact of the compaction disturbance,and the impact of the tensile capacity is much greater than that of the compressive capacity.In the loose sand with a density of 31%,the compressive and pull-out bearing capacity is slightly strengthened due to the influence of screw extrusion,but its bearing capacity is still far less than that of the dense sand under the same working conditions.Among them,the tensile capacity is slightly more affected than the compressive capacity,and the bearing capacity of multi-blade pile is also larger than that of single-blade pile.Based on the existing theoretical calculation methods,the judgment of compressive capacity in dense sand is conservative.Due to the insufficient consideration of the perturbation effect of screwing,there is a deviation in the prediction of uplift capacity by the current theoretical calculation methods.(4)Dynamic analysis of cyclic load pileThe numerical model results of the spiral steel piles with different pile sizes of dynamic and static load loading methods are analyzed,combined with the relevant indoor dynamic load model tests to verify the pile bearing capacity characteristic curves and dynamic effects obtained by the model calculation.And the parameter analysis is carried out for the numerical model of dynamic cyclic load of 60 mm diameter spiral steel pile in 80% density sand.There are great differences in the dynamic bearing characteristics of the piles after screwing in under different densities.Cyclic loading has no obvious effect on the compressive bearing capacity in dense sand,and the uplift bearing capacity is enhanced to a certain extent.The compressive and pull-out bearing capacities of the loose sand are obviously enhanced,indicating the densification effect of the cyclic load on the disturbed sand.