| Most of the early constructed municipal water supply and drainage pipelines have reached the designed service life nowadays,in which there have been a large number of damages and failure.There is urgent need for the rehabilitation and renovation of damaged and failed buried pipelines.Trenchless rehabilitation technologies as efficient methods,rehabilitating and renovating buried municipal pipelines with little or partial excavation of the ground,have been widely applicated around the world.The thin-wall stainless steel liner is an emerging approach for trenchless rehabilitation of damaged pipelines.Stainless steel liners can be installed into the host pipe through the the manual welding or fold-and-form method,form a new structural liner inside,improve the overall structur strength of pipelines,and provide good sealing and anticorrosion properties.Generally,applied stainless steel liner is a thin shell structure,which tends to fail due to buckling under the external pressure or inner vacuum pressure.There have been buckling failures of stainless steel liners used in pipeline rehabilitation projects.That is mainly due to the lack of research on this new renovation technology,especially on the buckling strength of the liner inserted into the host pipe.Many investigations have proven that liners' dimension,elastic modulus and geometrical imperfections due to different installation methods have great impact on buckling strength of the liners used for the trenchless rehabilitation of pipelines.The thin-wall stainless steel liner is made from metal material,which has higher elastic modulus compared to the commonly used chemical material liners,such as Cured-in-Place-Pipe(CIPP),polyethylene(PE)and polyvinyl chloride(PVC).In addition,the genearelly used dimension and installation methods are markedly different from those chemical material liners.ASTM standards have been applied in buckling strength design of currently used CIPP,PE and PVC liners.However,the applicability and rationality is questionable when employing ASTM standards in buckling design of stainless steel liners.This innovative trenchless rehabilitation method of thin-wall stainless steel liner has been listed in China's national industrial standard Technical Specification for Trenchless Rehabilitation and Renewal of Urban Water Mains CJJ/T 244-2016.However,the related buckling design theory in the specification is still referencing ASTM standards,which is not appropriate or feasible.At present,there is little related investigation on the buckling performance of the stainless steel liner,which is not conducive to the popularization and application of this new rehabilitation method.In view of this,this paper researched the buckling strength of stainless steel liners through theoretical and experimental methods.The main research contents and conclusions are as follows:(1)The origin of the design theory on liners' buckling strength in ASTM F 1216 have been introduced,and related experimental data have been analysed in details.It was found that the annular gaps have great impact on bucking strength of liners,in addition the experimental design and chosed calculation parameters were irrational.The conclusion can be made that the enhancement factor K=7 in ASTM standard seems to be inaccurate.(2)Several primary buckling calculation models used for liners have been introduced,and the critical buckling strength based on each model has been compared with each other.The research results indicate that the application condition for each buckling model is different.We need to choose the appropriate model according to the actual situation to calculate the buckling strength of liners.(3)The buckling strength of thin-wall stainless steel liner with different dimensions was obtained by carrying out buckling tests of stainless steel liners.The results based on currently used buckling models are compared with the experimental data to verify their applicability in stainless steel liner.The results show that the annular gaps of stainless steel liner are the main factor affecting the buckling strength.Comparing to the manully welding method,the fold-and-form process,which is currently applied to stainless steel liners less than 800 mm in diameter,is likely to lead to significant installation defects,which will dramatically decrease the buckling strength.This currently applied installation process should not be used for pipe rehabilitation.Current models of predicting buckling strength of liners inserted into host pipes cannot be applied to stainless steel liners.However,using the strength curves provided by El Sawy and Moore with the consideration of gaps,the results fit reasonably well with the test data.This method has possibility for determining the bucking strength for stainless steel liners in renewal projects.(4)A new method of multi-segment spliced thin-wall stainless steel liner was proposed,which eliminates the impact of annular gaps and improves the buckling strength without increasing the wall thickness of stainless steel liner.Based on the structural form and the mechanics characteristic,the buckling model of the arch liner was established,and the analytic formula of buckling strength was deduced based on the assumption of nonlinear deformation and minimum energy principle.(5)Based on the thin-wall stainless steel properties and contact relationships,the buckling numerical model of stainless steel liner under rigid restrain was established by finite element software ABAQUS.The model introduced a micro radial displacement by applying a concentraed force,as initial imperfection.The arc length method was used to abtain the load-displacement curve and the critical buckling strength of liners.Results of critical buckling strength and radial displacement of the reference point from the derived analytical formula and the finite element model were contrasted.It was found that both results were highly consistent with each method.The conclusion can be made that the analytical formula is conrrect and can be used to predict the critical buckling strength of this new liner structure. |
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