Study On Bearing Capacity Of Cone-Cylinder Foundations Under Multi-Fields Coupling Effect In Frozen Soil Region

The frozen soil,with unique properties,is widely distributed in northeast,northwest,and Qinghai Tibet Plateau of China.With the construction of power grid projects in these regions,more and more tower foundations need to pass through permafrost areas.At present,the researches on the bearing capacity of transmission line tower foundations are very rich,but most of them focus on the non frozen soil foundations whereas the researches on the bearing capacity of frozen soil foundations are less.In this paper,according to the special engineering characteristics of frozen soil,a kind of assembled cone column foundation suitable for frozen heaving environment is proposed.By means of reduced-scale model test indoor,numerical simulation,statistical analysis and theoretical derivation,the bearing capacity and mechanism of this foundation is systematically studied,with the emphasis on its uplift bearing capacity.The main research conclusions and achievements are as follows:(1)In order to study the influence of temperature field,seepage field,stress field and external force field coupling effect on the bearing capacity of cone-cylinder foundations in frozen soil,nine sets of indoor model tests are carried out under three loading modes at three freezing temperature(-5℃,-10℃,and-15℃).The test results show that the bearing capacity of the test foundations is good and the structural stability is high under the multi-fields coupling effect,but when the foundation is loading by composite load,the uplift bearing capacity of the foundation decreases by 13% on average due to the influence of lateral load,while the lateral bearing capacity only plays 28% of the ultimate lateral bearing capacity,which shows that the uplift bearing capacity of the foundation is the key factors of the overall bearing capacity of the cone-cylinder foundation.(2)In view of the uplift bearing capacity of cone-cylinder foundation,a series of numerical simulation are carried out based on the results of indoor model tests.The calculation parameters of numerical simulation are inverted and verified.Based on the idea of orthogonal test,three factors(the ratio of depth to width of foundation,the top diameter of the cone,and the bottom diameter of the cone)are designed for a total of nine sizes of foundations at three levels,and then the uplift loading is simulated at three freezing temperatures.The main conclusions are as follows: The results of range analysis and variance analysis show that the influence degree of foundation size on uplift bearing capacity is: the ratio of foundation depth to width λ→ the bottom diameter of cone D →the top diameter of the cone d.Whereas according to the results of F test,the inpact of these three factors are not significant;2)the failure modes of frozen soil foundation can be generally divided into three types: "T","V",and "U".When the freezing temperature is-5℃,the failure mode will be affected by the foundation size.When the freezing temperature reaches-10℃ or-15℃,the failure modes of foundation with different sizes tend to be the same,all of which are U-shaped;3)the results of indoor model tests and numerical simulations show that the coupling effect of multi-fields has a significant impact on the bearing capacity of foundation.The temperature field changes the seepage field and stress field of soil through water ice phase change,which affects the bearing capacity of foundation from three aspects: freezing force,frost heaving force,and physical and mechanical strength of soil.When the external force field is single uplift load,the bearing order of frozen soil is freezing force,side friction resistance,physical and mechanical strength of soil.When the freezing temperature is-5℃,the bearing effect of the freezing force and the side friction is significant,and the cylindrical shear failure area appears around the side of the foundation;when the freezing temperature reaches-10℃ or-15℃,the bearing ratio of the freezing force and the side friction is significantly reduced,and the physical and mechanical strength of the soil gradually bears most of the load.(3)In order to study the characteristics of uplift load-displacement curves of the cone-cylinder foundation,27 curves obtained by numerical simulation are analyzed using hyperbolic model,and a total of six prediction curves of load-displacement are obtained for three freezing temperatures under two guarantee probabilities(50% and 95%).Furthermore,the relationship between the fitting parameters a and b of the prediction curves and the freezing temperatures is analyzed,and then the linear fitting is carried out.Finally,the prediction formula of the uplift load displacement curves of the cone-cylinder foundation in the frozen soil area considering the freezing temperatures is established.(4)In order to study the calculation method of uplift bearing capacity of conecylinder foundation in frozen soil region,the similarities and differences of the soil weight method,the pressure method,and the shear method are compared.Based on the thought of shear method,the shear force integral of U-mode sliding surface obtained by numerical simulation is carried out.Finally,based on the Kotter semi infinite horizontal granular plastic balance equation and the M-C criterion,the calulation formula of ultimate uplift bearing capacity of spread foundations in frozen soil is derived.(5)In order to evaluate the practicability of the cone-cylinder foundation,the structural reliability and economic benefit are calculated and analyzed.Firstly,based on JC method,the structural reliability of the assembled cone-cylinder foundation is calculated.The results show that the structural reliability(3.36 and 3.40)of the foundation under the action of wind load and frost heaving load meet the requirements of transmission line tower foundation with voltage level of 500 kV and below(greater than3.2),whereas it does not meet the safety standard of UHV transmission line(greater than3.7).In terms of economic benefits,the cost of prefabricated cone-cylinder foundation is reduced by about 12%,the time cost is reduced by about 96% compared with cast-inplace concrete foundation,and it has more cost advantages in 220 kV single and double circuit and 500 kV single circuit lines.(6)Based on the test results of an engineering example,the prediction formula of uplift load displacement curve and the calculation method of bearing capacity proposed in this paper are verified.The results show that they are of high accuracy and suitable applied to the design of transmission line expansion foundation in frozen soil region,and also be further extended to the application of spread foundation of transmission line.