Bearing Capacity Study Of Pipeline With Circumferential Cracks Under Uneven Settlement

Buried pipeline,which is kind of a special underground structure,has become the lifeblood of national economic development and an important part of human daily life.The environmental factors could easily cause the deformation and failure of pipeline.In particular,soil settlement and landslide are two main factors for failure of pipe.Moreover,pipelines usually contain unavoidable defects which are formed in the process of transportation,laying and service;surface crack is one of the most common defects.Once the cracked pipe has uneven settlement,it is easily to form a leakage at the crack sites.Thus,it is of great significance to study the bearing capacity of pipelines containing ring-cracks on uneven ground.In this paper,the finite element numerical simulation method was utilized to study the limit settlement of Circumferential cracked?different depths?pipelines.The main contents are as follows:Experimentally,uniaxial tensile test was carried out for No.20#steel pipeline to investigate its mechanical properties,and the fracture toughness J_IC of the pipeline was obtained from literatures,which would be adopted in the later finite element simulation.A combining hydrostatic pressure with bending moment loading test was carried out for a full-scale pipeline with circumferential surface cracks.The influences of internal pressure and bending moment on the pipe crack were analyzed.The results show that the axial strain on two sides of the crack tips increases with the internal pressure and bending moment.In addition,the axial strain on two sides of the crack surfaces is negative and negatively increases with the internal pressure and bending moment.Using finite element software,the plate model with surface crack was established and the J integral value at the front of the crack was calculated.The comparisons between the calculated J integral value and the value obtained from the Newman-Raju formula proved the correctness of finite element modeling method and the accuracy of simulation results.Furthermore,the full-scale pipeline finite element modeling?FEM?was established and FEM analysis was conducted for the full-scale pipeline with circumferential surface cracks undergoing combining hydrostatic pressure with bending moment loading.The simulated results could well predict the variations of experimental data,which prove the applicability and reasonability of the FEM method extending to the full-scale pipeline.On the basis of the above method,FEM models were established for pipes with circumferential cracks under different depths.Variations of the axial strain on two sides of the crack and J integral value at the deepest point under different depths with the internal pressure and bending moment were discussed.Using the fracture toughness J_IC determined above,the ultimate bearing capacity of pipes with circumferential cracks under different depths was obtained.The results show that both the axial strain on two sides of the crack and the J integral value at the deepest point increase with the internal pressure and moment.Meanwhile,the deeper crack corresponds to the larger values of the axial strain on two sides of the crack and the J integral value at the deepest point.At the internal pressure of 8MPa,the ultimate bearing bending moment of the pipeline containing circumferential cracks with the depths of 3.0mm,3.5mm,and 4.0mm are86.0kN·m,81.5kN·m,and77.0kN·m,respectively.A pipe-soil interaction model was developed based on the soil spring model.The mechanical response of pipeline on the settling sites was analyzed by simulations.The results show that when the soil occurs uneven settlement,the pipeline will settle along with the soil and the settlement of pipe is basically consistent with the value of soil.In the interaction area between the settling regions and the non-settling regions,both the bearing bending moment and axial stress-strain of the pipeline will abruptly change.In contrast,the moment and axial stress-strain away from the interaction area are almost zero.The influences of internal pressure in pipeline,settlement width,burial depth,and settlement on pipeline were quantitatively analyzed by developing a plastic-pipeline model.The force and deformation law of the pipeline were obtained from the calculated results.At last,the limit settlement of the circumferential cracked?different depths?pipelines under was determined by the limit bearing capacity of the pipeline.Correspondingly,the suggestions for preventing the uneven settlement of pipeline were proposed.