| Geomembranes are widely used as impermeable materials in water conservancy,agriculture,environmental and other projects.When geomembranes are laid over a large area and it is difficult to discharge water and gas under the membrane quickly and effectively,the problem of geomembrane swelling can occur.Based on the location of the geomembrane in the engineering structure,the geomembrane bulging problem involves the reservoir water layer,the impermeable membrane layer,the unsaturated soil layer,the water table layer,and their respective coupling effects.The essence of the problem lies in the multi-axial stretching and deformation of the geomembrane,and the key is to clarify the pressure on and under the membrane,the bulging strength of the geomembrane material itself and its coupling with each other.However,the existing research on geomembrane bulging deformation characteristics was difficult to combine with the actual engineering application conditions,and the existing research on air pressure in the unsaturated soil under the membrane failed to consider the change of pressure under the membrane after the geomembrane bulging,and there is very few research to calculate the size,shape,and pressure of the geomembrane after the bulging deformation.Under the given reservoir operating conditions,it is still difficult to get a systematic and comprehensive answer to the questions of whether the geomembrane will bulge,how much bulge pressure there is,whether damage can occur,what impact damage has on the reservoir,and how to take measures to prevent damage.In this paper,based on the analysis of reservoir water pressure on the membrane,the air pressure under the membrane,the mechanical properties of bulging deformation of membrane layer materials and the coupling effects of the three aspects,the bulging deformation and failure of(composite)geomembranes were studied using field investigation,theoretical analysis,experimental research,numerical simulation and application research.Firstly,consider that the current paucity of field information on geomembrane bulging has led many people to question the existence and severity of the problem.This paper collects30 typical engineering examples of bulging problems in domestic and foreign engineering sites and affirm the existence and importance of the bulging problem by means of on-site examples.Meanwhile,based on the analysis,summary and statistics of these bulging problem examples,the main causes,effects,and prevention/treatment measures of most of the bulging problems are obtained,which clarify the basic scope and entry angle for further in-depth study of the bulging problem.Secondly,based on the study of example data,a comprehensive theoretical,experimental and simulation study is conducted on the bulging problem involving the reservoir water pressure on the membrane,the bulging deformation characteristics of the membrane layer,the sub-membrane air pressure,and their mutual coupling effect.(1)The effect of non-uniform pressure distribution on the membrane was studied.Compared with the uniform pressure action on the membrane,the non-uniform pressure action on the membrane,the height around the geomembrane is small while the middle height is large,and the non-uniform distribution of pressure load is recommended.(2)The formula for calculating the air pressure under the membrane due to the rise of the water table,the air trapped under the membrane during the installation,the change of pressure on the membrane,and the leakage of the geomembrane were derived.The formula is based on the three-phase dispersion system of unsaturated soil and the ideal gas law,Boyle’s law and Henry’s law,taking into account the influence of the air trapped under the membrane and the air in the space under the membrane after the bulging deformation generated during the geomembrane laying construction on the air pressure under the membrane,and is verified by using the indoor test method.(3)The mechanical properties of(composite)geomembrane bulging deformation and failure were studied by using the circumferential constrained spherical bulging deformation model.The geomembrane bulging deformation and failure process is divided into six stages,including the initial stage,spherical bulging deformation stage,crack generation stage,crack area bulging deformation stage,failure,and post-failure stage.The crack and its expansion are similar to the"thin neck"and its expansion in uniaxial stretching.The orientation of molecules and molecular chains along the stretching direction,the twisting and untwisting of amorphous regions,the slippage between molecular chains in crystalline regions and the pulling disorder in crystalline regions are the main reasons for the changes in tensile properties.The lowermost geotextile layer in the separated composite geomembrane in contact with the pressure medium is the ineffective layer of the bulge deformation,and the other structures are the effective layers,whose bulge pressure is the sum of the bulge pressure at the same bulge height of the effective layer in the structure,and whose stress is the sum of the product of the stress at the same strain of the effective layer in the structure and the percentage of the thickness of the constituent materials.Based on the principle of curve intersection method,membrane material selection methods,including simplified curve intersection method,curve intersection method based on multiaxial tension characteristics,and curve intersection method based on mix rules of geomembrane and geotextile were proposed.(4)The calculation formula of the coupled bulge deformation problem of geomembrane was derived.The"double arc bulge deformation model"was established,and the equations for the geomembrane coupled bulge deformation problem were derived in four deformation conditions((1)H_w>h,θ<90°;(2)H_w<h,θ<90°;(3)H_w>h,θ>90°;(4)H_w<h,θ>90°),coupling geometry conditions,material properties,mechanical equilibrium,and parameter determination,etc.The calculation formula of the deformation problem was derived.The basic properties of bulge deformation shape,bulge pressure,bulge height and bulge width were obtained by using trial calculation method.Indoor tests were conducted to verify the derived formulations and their results.Thirdly,the possible leakage volume caused by bulging failure was clarified.Geomembrane bulge failure produces cracks,holes or fissure-type damage,whose sizes are about 1/2 to 3/4,1/10 to 1/4,and 1/3 to 4/5 of the total diameter,respectively.With the increase of failure port size,the seepage volume gradually increases.Based on the equivalent pore size(equivalent circular pore size of different cracks and fractures at the same seepage volume),the seepage volume of different failures after geomembrane fails were calculated by using the Bernoulli formula or linear fitting formula when permeability coefficient of the support layer is high/low.Finally,the bulging deformation and failure of geomembrane in the reservoir project of Tailan River depression in Wensu County were calculated based on the above research content.Under the existing calculation conditions,rising water table,leakage of geomembrane defects and air trapped under the membrane may cause bulging and even cracking of the geomembrane,but basically no bulging failure will occur.The air pressure under the membrane caused by the compression of reservoir water is very small,and the geomembrane basically will not produce bulging deformation and failure.In addition,this paper also gives basic recommendations for prevention/treatment measures from the design and selection of materials,laying construction,project operation to the emergence of bulging problems and other stages.The research results provide a reference for solving the problem of geomembrane bulging and have certain theoretical value for establishing the theory of calculating the air bulging of the plain reservoir impermeable layer,which is of great practical significance for improving the laying technology of the plain reservoir impermeable layer and ensuring the safety and reliability of the reservoir impermeable layer. |
PDF Full Text Request