Design Of Permeable Lining And Load-bearing Characteristics Of High Pressure Tunnel

Traditional high-head hydropower stations and pumped storage power stations have been continually developing.Consequently,the construction of the high pressure tunnel ushers in its peak.The application of steel lining is limited to some extent in engineering,due to the stability problem under external water pressure and the restriction of construction sites.Reinforced concrete lining can avoid difficulties above,while on the other hand,it also faces the challenge that it cracks inevitably under high internal water pressure.Therefore,the permeable lining theory is often adopted to design reinforced concrete lining,and the control of seepage inflow by limiting cracks’ width becomes the core of design and research of lining.Based on the permeable lining theory as well as the conditional combined load-bearing mechanism between lining and surrounding rock,related researches of the permeable lining structure is carried on,with both the analytical method and the finite element method.The main research content is stated as follow:(1)According to the permeable lining theory,the double-layer thick-walled cylinder model consisting of lining and surrounding rock is built with the analytical method.The adhesive force between lining and rock is considered to analyze the conditional combined load-bearing characteristics of lining and rock.With a hydropower diversion tunnel as the application,the specific lining design method corresponding to the double-layer model is proposed.Furthermore,the conditional combined load-bearing mechanism during the process of water-filling is researched when the adhesive force varies.Some conclusions are drawn as follows.Under internal water pressure,it is not completely positive when lining shares the same deformation with surrounding rock.Thus the adhesive force can be controlled in case of too much reinforcement.For the first time of filling water,the maximum crack width and steel stress will reach their maximal values once lining is going to separate from surrounding rock.So the safety of lining under this critical condition is suggested to be assessed in lining design.Before lining cracking or after its separation from rock,the adhesive force doesn’t affect the stress state of lining.(2)Based on the study of the double-layer thick-walled cylinder model,the triple-layer thick-walled cylinder model consisting of lining,loose rock zone and intact rock is established with the analytical method.The lining design method relating to the triple-layer model is put forward.The influence of loose rock zone on the structural calculation of lining is discussed as well.The results indicate that it is much safer for lining design when taking loose rock zone into account.If the adhesive force is zero,loose rock zone has little effect on the structural calculation of lining.While the large adhesive force results in entirely combined load-bearing between lining and rock.Then the maximum crack width and steel stresses will be greater with the increase of thickness,deformation modulus and permeability coefficient of loose rock zone,on account of the tensile force between lining and surrounding rock.(3)On the foundation of the seepage-stress coupling theory and the conditional combined load-bearing mechanism,the double-layer thick-walled model is built with the finite element method.The impact of the seepage field on the stress-field is reflected by the equivalent node load.The influence of the stress field on the seepage field,in turn,is reflected via a formula between permeability coefficient and damage of concrete.What’s more,the contact element is used to show the bonding state between lining and surrounding rock.Finally,the concrete damage evolution and the seepage field variation during water-filling is studied.The results show that before concrete cracking,the concrete damage and steel stresses distribute uniformly;lining and surrounding rock bear water pressure cooperatively.When concrete cracks,there is only one crack going through lining;nearby the crack,the steel stress upsurges and the concrete permeability coefficient increases by orders of magnitude;lining separates from surrounding rock within a certain range near the crack.After concrete cracking,the concrete damage,steel stresses and the maximum crack width increase slowly.