Ground-mounted Steel Lined Reinforced Concrete Pipes And Cancel Expansion Joints

The steel lined-reinforced concrete penstock on the downstream surface of concrete dams is widely used in Dongjiang, Jinshuitan, Wuqiangxi, Lijiaxia, Sanxia projects. However, the steel lined-reinforced concrete penstock on ground has not been used ever before. Accordance with a hydropower project, a study on the steel lined-reinforced concrete penstock on ground under the action of internal water pressure and thermal load has been carried out, in which the stress and strain distribution, development of cracking and the influence of canceling expansion joints are included.It has been shown that the steel lined-reinforced concrete penstock on ground is absolutely applicable in this project. Moreover, canceling the expansion joints at the upper bending section and the settlement joint between main and auxiliary power plant is possible as long as some measurements are adopted, such as using a steel pipe with soft cushion outside. From the computation, some conclusions are obtained as follow:(1) Under the action of water pressure and structural dead weight, the circumferential stress of concrete is gradually enlarged along the pipe line from the intake to the power house. Most of the value of the circumferential stress has exceeded the tensile strength of C25 concrete, so the surrounded concrete will be cracked along the radial direction and therefore the internal water pressure will be borne by steel liner and reinforcement.(2) Computation result shows that the thermal load is the most important factor to the stress of the penstock. T he b iggest v alue o f a xial t ensile s tress i n c oncrete u nder t he a ction o f w ater p ressure a nd thermal load could be as high as 4.23MPa, which will lead the occurrence of circumferential cracks. So reinforcement along the axial direction must be laid.(3) Because of the restrain of lower rock foundation to the steel lined-reinforced concrete penstock, the expansion joint between the main and auxiliary power plant can not be used to lessen the thermal stress of the penstock and therefore is not necessary for this kind of penstock.(4) By using a pipe surrounded with soft cushion to replace the expansion joint, the displacement difference between the main and auxiliary power plant is quite small under both water pressure and thermal 1 oad. The stress of steel liner will not exceed the permissive stress of steel and can meet the requirement of design if a 10mm-thick cushion with 3.75MPa modulus is used. Meanwhile, the normal and shear stress on the foundation surface is also quite small and can meet the requirement of less than bearing capacity of the foundation.