| Recent years,with the development of large-scale and complex water diversion projects,higher requirements have been put forward to the carrying capacity of water diversion pipelines.Underground penstock is widely applied in water diversion projects because the surrounding rock can share part of internal water pressure,which helps to reduce the thickness of steel lining and improve the technical and economic feasibility of large-scale steel penstocks.The topography and geological conditions that long-distance water pipelines pass through are complex,and it is inevitable to cross active faults in many cases.The permanent deformation caused by fault creep will pose a significant threat to the safety of underground penstock structure.In addition,stability failure of steel penstock happens a lot due to the various external pressures that underground penstock bears,such as grouting pressure and external water pressure.Therefore,it is very important to study the adaptability of underground penstock to fault creep and the stability of underground penstock under external pressure.Based on a practical project and the finite element software ABAQUS,the adaptability of underground penstock with cushion and expansion joint to fault creep,and the buckling stability of underground penstock under external pressure are studied in this paper.The main contents and results are listed below:(1)Based on a three-dimensional finite element model of underground penstock with cushion,the bearing mechanism of underground penstock under fault creep is analyzed,as well as the influences of fault dislocation,fault displacement mode,segment length of backfill concrete and cushion thickness on the deformation and stress of underground penstock.The research shows that the steel penstock bends in an āSā shape under the vertical fault creep.At the interface between fault zone and hanging wall,and the interface between fault zone and foot wall,large axial and circumferential stresses appear in both steel penstock and backfill concrete;the contact between steel penstock and cushion,and the contact of backfill concrete segments are obviously separated.As the increase of fault dislocation,the displacement and stress of steel penstock basically increase linearly,and the stress and contact opening of backfill concrete gradually increase;while segment length of backfill concrete has little effect on underground penstock.In general,underground penstock with cushion has a good adaptability to the fault lateral creep.However,the axial stress level of steel penstock is still very high under the fault axial creep,and the various stresses of steel penstock under axial compressive displacement mode are greater than those under axial tensile displacement mode.As the increase of cushion thickness,the various stresses of steel penstock and the maximum axial stress of backfill concrete decrease gradually,but the effect is obviously weakened when the cushion thickness reaches 30 mm.It is suggested that the cushion thickness should be 30ļ½50mm after comprehensive consideration.(2)To study the adaptability of underground penstock with expansion joint to fault creep,the mechanical characteristics of underground penstock with expansion joint under fault creep are analyzed,and the influences of the axial stiffness and the number of expansion joints on the structure are studied.Under fault creep along the axial direction of penstock,the expansion joints absorb part of axial displacement of steel penstock caused by fault dislocation,and the axial displacement and stress of steel penstock are obviously reduced.The stresses of steel penstock and backfill concrete also decrease obviously near expansion joints under the fault lateral creep.In general,underground penstock with expansion joint can play a good role in adapting to the axial and lateral creep of fault.As the axial stiffness of expansion joint decreases,the axial displacement change and various stresses of steel penstock decrease gradually;the more the number of expansion joints,the gentler the axial displacement curve and the lower the various stress levels,which is more conducive for steel penstock to adapt to the fault creep.(3)The finite element buckling analysis on external pressure stability of underground penstock with stiffener ring is carried out.It can be concluded that: the critical external pressure of underground penstock with stiffener ring decreases linearly with the increase of initial imperfection;and gradually decreases with the increase of initial gap,but when the initial gap reaches one thousandth of penstock radius,the critical external pressure of steel penstock is the same as exposed penstock without backfill concrete.The critical external pressure of steel penstock tends to increase with the increase of the height of stiffener ring,but it remains basically unchanged after the height of stiffener ring reaches 250 mm.When the ratio of the spacing of stiffener ring to the penstock radius is between 0.8 and 1.2 or greater than 2,the critical external pressure of steel penstock also does not change much.It can be drawn that the stiffener ring may not fail until the pipe wall is buckled through the finite element method,there is no need to separately check the stability of stiffener ring under external pressure.Therefore,if the stiffener ring is considered for underground penstock,the section size of stiffener ring can be determined directly according to the construction and installation conditions,and the spacing of stiffener ring can be designed by the critical external pressure of pipe wall between the stiffener rings. |
