Study On Bearing Capacity Of Foundation Soil About Power Transmission Project Under Heavy Snow Conditions In Southern China

The extreme snow and ice disaster which is hundred-year return period occurred in most parts of southern China in early 2008, the long duration,he scope extensive and the deep damage are all rarely seen in history. Large-scale, high-strength snow and ice melt caused hundreds of geological disasters, which tremendously affected the safe operation of the power transmission project in southern regional. According to incomplete statistics, more than 20 power transmission lines of 500kV destroyed, and more than 100 base towers were broke down, the power grid which grade under 500kV were affected broader, and another more substations can not function properly, this disaster directly caused more than 200 billions economic losses. Because of the preliminary engineering design standards was not considered the impact of this extreme snow disaster, and the current lack of a comprehensive response of the integrated control system to the extreme snow and ice disaster, and lack of an effective engineering control measures and emergency measures, coupled with the extreme snow and ice disaster is paroxysmal, as a result, the power engineering facilities suffered heavy losses in this extreme snow and ice disaster. At present, research results on bearing capacity of foundation soil about power transmission project under extreme snow and ice disaster conditions in Southern China are relatively few, and the results in northern areas are not suitable for the extreme snow and ice conditions in southern areas, therefore, it's an urgent need for research on bearing capacity of foundation soil about power transmission project under extreme snow and ice disaster conditions in Southern China.Based on experimental study of freezing and thawing soil, variations of shearing strength parameters are analyzed under freezing and thawing process and freeze-thaw action, impacts of water content, temperature and cycles of freezing and thawing on shearing strength parameters of soil are selectively analyzed. And then introduced factors of water content, temperature and cycles of freezing into the formulas of bearing capacity of foundation soil, which are from the code for design of building foundation and based on the limit equilibrium theory and the elastic and plastic theory, in the formulas the bearing capacity coefficient were revised, and the revised bearing capacity formulas were gained under freezing and thawing process and freeze-thaw action. Through the process of freezing ultimate bearing capacity of strip footings of the numerical analysis, analysis of strip footings at different temperatures and different loads of degeneration failure law, and has been the numerical solution of ultimate bearing capacity; The deformation and force characteristics of power transmission tower base at different temperatures and ice loads were analyzed by technique of numerical simulation. The main study results are as follows:(1) Analyzed the variations of shearing strength parameters of under freezing and thawing process and freeze-thaw action by experimental study on freezing and thawing soil:①Under the normal temperature, the cohesion and friction angle values of remolded soil decreased along with the increasing of water content; the cohesion and friction angle values of undisturbed soil are higher than remolded soil which with the same water content;②During the freezing process, the inherent freezing temperature of soil was found; both undisturbed and remolded soil samples, the cohesion and friction angle values increased along with the decreasing of temperature, when temperature was above or below the freezing temperature of soil, variations amplitude of the cohesion and friction angle values were different;③For remolded soil on Xian-Meng line, along with increases of water content, the cohesion and friction angle values are reduced during the freezing progress. Influence to cohesion by water content is obvious, which can be divided into two phases by boundary of the middle water content:when water content is lower than the middle water content, cohesion value increases with the decreases of temperature; when water content is higher than the middle, cohesion value decreases as the temperature decreases. For remolded soil on Qu-Jiang line, along with the water content increases, the cohesion and friction angle values under each temperature are first increase and then decrease; but at the same water content, as the temperature decreases, the cohesion and friction angle values are all increased;④During the thawing process, cohesion value of both undisturbed and remolded soil is decreased, and friction angle value is slowly increased. As the freezing temperature of soil is exist, when temperature is lower than the freezing temperature of soil, with variations of temperature, the cohesion and friction angle values changed little; but when temperature is higher than the freezing temperature, the phase transition of frozen ice in soil is intense, unfrozen water content increased, caused changes of cohesion and friction angle values observably;⑤Feeze and thaw action strongly changed the structure of soil, thus changed its mechanical properties. After freeze-thaw action, soil structure became loose, soil particle shape became broken, tend to irregular, caused the soil structural transferred and changed the stress system. After cycles of freezing and thawing, cohesion value decreases and friction angle value increases, the whole shear strength of soil decreases.(2) Based on research on the cohesion and friction angle values of foundation soil under freezing and thawing process and freeze-thaw action and based on reaserch on influencing factor of foundation soil bearing capacity, methods of analysis of foundation soil bearing capacity under freezing and thawing conditions were proposed:①revised the cohesion and friction angle values which were received during freezing and thawing process by introducing the impact factor temperature, the table of corrected bearing capacity corrections which were used during freezing and thawing process were gained, the corrected formula of bearing capacity in code for design of building foundation and the corrected Terzaghi bearing capacity formula were summarized;②introduced the impact factor of cycles of freezing and thawing, the cohesion and friction angle values were revised under freeze-thaw action, and the corrected bearing capacity corrections were gained, thus brought forward the modified formula of foundation bearing capacity based on elastic and plastic theory;③In accordance with the modified formula of foundation bearing capacity, the bearing capacity of foundation soil with different freezing temperatures and thawing temperatures were calculated, and the bearing capacity of foundation soil with normal temperature was also calculated, thus the proportional relationship table between the two bearing capacity of foundation soil with different temperatures were gained④Obtained the mathematical relationship between water content and bearing capacity of foundation soil with different temperature;⑤Also obtained the relationship between cycles of freezing and thawing and bearing capacity of foundation soil.(3) By numerical analysis of bearing capacity of strip shallow foundation in freezing process, deformation and damage law of the strip shallow foundation were analyzed, and the number of ultimate bearing capacity was solved by numerical solution:①the influence of temperature on deformation of the strip shallow foundation was little, and the main factor was the upper load. At the same temperature, along with the increases of upper load, the maximum displacement of base soil and the maximum shear stress were all increased, when the load increased to a critical value, the maximum displacement of base soil suddenly increased, the maximum shear stress decreased, and the greatest depth of plastic area surpasses 1/3 of foundation width, can be considered shear failure occurred in the foundation soil;②by numerical simulation could obtained the P-S curves between settlement and load of base soil, the values of ultimate bearing capacity of foundation soil could obtain at different temperatures through numerical solution,and the use of freeze-thaw conditions, the modified formula and modified bearing capacity norms Terzaghi calculated ultimate bearing capacity of foundation soil bearing capacity of comparative analysis, numerical simulation shows the foundation of the ultimate load obtained force has a certain reliability, and also further illustrated the freezing and thawing conditions in the revised formula has a high bearing capacity is reasonable and reliable.(4) By numerical simulation on power transmission tower foundation in extreme snow and ice conditions, from displacement, shear stress and strain, the deformation and force characters of the power transmission tower base affected by temperature and ice load were analyzed:①Ice load had a certain influence to deformation of tower base; under precondition of the bottom boundary temperature in the model was certain, the surface temperature affected the stress and deformation of the rock and soils of tower basis obviously, but when the surface temperature down to a specific value, the deformation and stress became stable;②The tower base received the vertical load and horizontal wind load that flow to left, horizontal displacement of tower base was very clear, loads on the two tower bases were unbalance, the maximum displacement appeared on the left of tower base, and the minimum displacement appeared on the right side, and can see the potential slip surface of destroyable deformation of ground; the maximum vertical subsidence point was in the middle of the two tower bases;③Effect of the wind load and the gravity load of tower, stress concentration appeared at the tower base and foundation contacted point. In addition, near the bottom of the tower base and silty clay with weathered sandstone contact surfaces also appeared different degrees of stress concentration, and with the temperature decreased, stress concentration expanded with a significant trend;④As the stiffness of tower foundation is big, resulting in less strain, the maximum deformation of soil appeared near the basis and the maximum strain appeared near the top of the base. Because of clay and silty clay have lower mechanical strength and smaller deformation modulus, so these two layers have large deformation, they were the main deformation layer to foundation.The results of this study provide scientific theory basis for the analysis and calculation on bearing capacity of foundation soil in extreme snow and ice condition in southern China. Research results enrich the manage technology and design theory of geological disaster in extreme snow and ice condition, and it can directly guide the engineering construction in extreme snow and ice condition in southern region.