Axial Mechanical Characteristics Of Backfill Steel Pipe Considering Temperature Effect

In recent years,with the increasing shortage of water resources and the increasing restriction of water resources on China’s economy,the construction of water conservancy and hydropower projects and water diversion and transfer projects is imminent.As the main water conveyance structure,the bearing capacity of penstock is the key to the safe operation of hydropower projects.Among them,the backfill steel pipe has the advantages of simple structure,ecological protection and convenient and fast construction,and its application opportunities are gradually increasing.In hydropower projects or water diversion projects,if extreme weather occurs occasionally during construction or operation,the effect of temperature will become a key factor affecting the stress of steel pipe and safe and stable operation.In addition,with the development of giant thin-walled penstock in hydropower projects,the problem of external pressure stability of penstock is becoming more and more obvious.And because the backfill steel pipe belongs to a relatively new layout form in the hydropower industry,Therefore,it is very important to study the axial stress characteristics of backfill steel pipe under temperature and the adaptability to temperature after setting anti external pressure stability measures.Combined with a practical hydropower project and relying on the finite element software ANSYS,this paper studies the adaptability of flexible backfill steel pipe,backfill steel pipe wrapped with concrete and backfill steel pipe with stiffening ring to temperature,as well as the influence of expansion joint on the axial stress characteristics of backfill steel pipe.The main research contents and results are as follows:(1)Study on axial stress characteristics of flexible backfill steel pipe under temperature.The results show that when there is no bellows expansion joint,the steel pipe is "flattened" in the vertical direction,which is similar to the deformation of the beam fixed at both ends.The overall axial displacement is small,and the axial stress of the backfill steel pipe is significantly affected by temperature.After setting the bellows expansion joint,the steel pipe is similar to the deformation of the cantilever beam fixed at one end.Although the setting of the expansion joint greatly improves the axial stress of the steel pipe and the axial push(pull)force on the anchor block,due to the increase of the flexibility of the steel pipe,the decrease of the pipe soil friction coefficient and the increase of the internal water pressure,the separation of the pipe soil contact surface near the expansion joint will be exacerbated,resulting in a large axial displacement and slip of the steel pipe.Therefore,for the backfill steel pipe whose axial stress exceeds the strength range,the setting of expansion joint is necessary.On the contrary,from the perspective of construction and economy,the setting of expansion joint is not necessary.(2)Study on axial stress characteristics of backfill steel pipe wrapped with concrete under temperature.The results show that the ability to resist external pressure is increased,the vertical deformation and stress are greatly reduced,and the wrapped concrete produces large circumferential stress because it bears part of the internal water pressure;Because there is no joint between the anchor block and the outsourcing concrete,there is a large axial tension at the connection between the two,and its value has exceeded the tensile strength range of the concrete.After the expansion joint is set between the anchor block and the outsourcing concrete section and the joint is split at the downstream side of the expansion joint chamber,the axial stress of the steel pipe can always be maintained at a low stress level,and the concentrated stress phenomenon of the outsourcing concrete and the anchor block can be greatly improved.Therefore,for the backfill steel pipe with outsourcing concrete,whether to set the expansion joint should also comprehensively consider the axial stress of the outsourcing concrete and the anchor block,However,it also shows that for the steel pipe wrapped with mass concrete structure,it is recommended to take joint separation measures at the places with large changes in section size.(3)Study on the stress characteristics of concrete filled steel pipe with soft cushion under the action of temperature.The setting of cushion layer can give full play to the bearing capacity of steel pipe and greatly improve the circumferential stress of wrapped concrete;The larger the wrapping angle of the cushion layer,the more the internal water pressure borne by the steel pipe,and the smaller the circumferential stress of the outsourcing concrete,which is more beneficial to the stress of the outsourcing concrete;However,when the wrap angle is the full wrap angle(360 °),because there is no rigid support at the bottom of the steel pipe,the pipe top bends vertically downward under the action of external load,and both sides of the pipe waist bulge outward,resulting in the separation of the pipe cushion contact surface,resulting in large deformation,slip and concentrated circumferential stress at the deformation of the pipe top and pipe waist,but the increase of the elastic modulus of the cushion can improve the slip and stress of the steel pipe at the deformation.Therefore,for the backfilled steel pipe with the full wrap cushion,The elastic modulus of cushion can be appropriately increased;However,considering the convenience of steel pipe construction,stable operation and economy,it is suggested that the cushion wrap angle should be 220 °.(4)By applying stiffening ring to the backfilled steel pipe,although the anti external pressure ability of the steel pipe can be improved,the stiffening ring will limit the sliding of the steel pipe in the soil,so that the temperature stress cannot be released,and there will be large stress concentration at the connection between the steel pipe and the stiffening ring,which is obviously affected by the temperature;In addition,the setting of bellows expansion joint will cause large deformation of steel pipe and stiffening ring near it,and the concentrated stress of steel pipe will increase greatly.For steel pipes that can maintain stability against external pressure,it is recommended not to set stiffening rings.For example,if the steel pipe has poor stability against external pressure and has to set stiffening rings along the pipeline,it is not recommended to set expansion joints at this time.