| Buried steel pipe(BSP)is one of the common types of penstock in the water conservancy and hydropower industry,especially suitable for long-distance pipeline projects with large diameter,high internal pressure,and across various topography and geology.With the vigorous development of water diversion and hydropower engineering in China,large-diameter BSPs referred to as pipe diameters no less than 1.2 m have been increasingly applied in major water diversion and hydropower engineering,embracing a broad prospect.Large-diameter BSPs have some inherent characteristics,such as enormous pipe diameter,small stiffness,various working conditions inside the pipe,and complex pipe-soil interaction.As lifelines of major water diversion and hydropower projects,the mechanical properties of large-diameter BSPs are related to the whole project’s safety and economy.However,the current force-deformation mechanism research is immature and their calculation methods in specifications need further improvement.Therefore,this thesis takes a large-diameter BSP as the research object and adopts methods of theoretical derivation,numerical simulation,and prototype testing to systematically investigate the pipe-soil interaction,structural calculation method,and abnormally deformed BSP assessment and rehabilitation.The main research contents and results are as follows:(1)To further understand the mechanical mechanism of large-diameter BSPs,numerical simulation is applied to study the characteristics of pipe-soil interaction under the working conditions of empty pipe,water filling,and normal,and compare the differences of force and deformation between large-diameter BSPs and small-diameter BSPs.Furthermore,sensitivity studies are performed on 13 parameters based on the orthogonal test method.The results show that the working condition has a significant influence on the pipe-soil interaction.The water weight inside the pipe will greatly increase the pipe deformation,while the internal water pressure is the opposite.Soil pressure around the pipe has a similar distribution at the empty pipe and water filling conditions,different from the normal condition,and all of them are not exactly consistent with the Spangler model.Meanwhile,the larger the pipe diameter,the more influence of the pleural soil and water weight,and the lower the safety margin of the prism load.With the increase of pipe diameter,distribution patterns of soil pressure at the top of the pipe are parabola shape,"inverted basin " shape,and "M" shape in order.Additionally,parameters of backfill and trench have significant influences on the structural behaviors of BSPs.(2)To improve the poor applicability of structural calculation methods in existing specifications for large-diameter BSPs,the calculation formula is deduced by theoretical analysis based on the Spangler model firstly;then,design methods of multiple countries have been compared and a simplified design method of the maximum allowable cover depth of pipe is proposed to quickly determine the cover depth,which finds the diameter-to-thickness ratio’control boundary about 190 between the stiffness and stability against external pressure;finally,a set of calculation methods for large-diameter BSPs under combined loading are developed to improve the calculation accuracy by taking into account the pipe weight,water weight,and internal water pressure.By comparing new methods with the Iowa formula,the pipe weight and water weight can not be ignored for the BSPs with large pipe diameter and shallow cover depth and internal water pressure can significantly decrease pipe deformation.Moreover,using water pressure at the pipe axis,ignoring water weight,is recommended for calculation,when the pressure head is much larger than the pipe diameter.(3)To reduce the great deviation between existing models of soil pressure at the top of the pipe and the actual soil pressure of large-diameter BSP,the distribution and size of soil pressure under various influencing factors are studied by the numerical simulation and theoretical analysis methods.Furthermore,two new models of soil pressure at the top of the pipe are proposed,i.e.basin model and parabolic model,deriving their corresponding calculation methods subsequently.For the basin model,the soil pressure distribution adopts the "straight line + parabola" form,and the maximum soil pressure adopts the "Marston + pleura" load.For the parabola model,the soil pressure distribution adopts the parabola form,and the peak soil pressure adopts the 1.2 times prism load.After comprehensively comparing the results consistency between the analytical method and finite element method,the basin model method using "prism + pleura" load is recommended using in the empty pipe and water filling conditions,and the combined method of parabola and basin models is suggested to adopt in the normal condition.(4)As little research on the mechanical mechanism and rehabilitation methods of largediameter BSPs subjected to extreme deformation,methods of numerical simulation and prototype testing are performed to reveal the mechanical behavior,evolutionary mechanism,and bearing capacity;then,a new water-pressure rehabilitation method is proposed and compared with the traditional internal-bracing rehabilitation method.The results show that abnormally deformed BSPs typically have high stress and insufficient soil support,and the pipe deformation and pipe stress will increase significantly after suffering adverse loading.Meanwhile,a stiffening ring is detrimental to the pipe stress and for the abnormallydeformed pipe,the stress concentrations and plastic zones are mainly near the stiffening ring,pipe crown,pipe springline,and pipe invert.Besides,the BSP with ring deformation of 8.9% can be rehabilitated by the water-pressure rehabilitation method,which employs internal water pressure to make pipe rounding and grout for the surrounding soil.The new water-pressure rehabilitation method can significantly reduce the pipe stress and makes it uniformly,which applies conveniently and is very suitable for long-distance pipelines. |
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