Study On The Mechanical Characteristics And Working Mechanism Of Airbag Used For Blocking Air And Water In Tunnels

With the development and utilization of underground space,the security problems in tunnel construction and underground railway operation become increasingly prominent.Underground tunnels are prone to serious leakage and water inrush when they pass through the water-rich silty soils.The toxic and harmful gases may also appear in tunnels.Then the water or toxic gases can quickly spread in tunnels due to the connectivity of underground tunnel.These disaters can cause great harm to the tunnel and the surrounding environment if the sourse of the leakage isn’t isolated in time.Because the space of tunnel is limited and underground,the traditional wall-blocking method is difficult to prevent the water and toxic gases from spreading quickly and effectively.The large-diameter airbag can be used for a temporary plugging device for blocking,which can be inflated quickly by an air compressor.The inflated airbag will be fixed by the confining force from the tunnel and the frictional resistance between the bag skin and the tunnel walls and be able to resist the water pressure or air pressure.This method can quickly isolate the safe area from the affected area and avoid the hazards spreading and it is easy to construct.Since the airbag is a membrane structure and confined by the tunnel wall,and the force equilibrium and failure mechanism are quite different from the conventional structures.Therefore,it is necessary to study the deformation characteristics and force equilibrium conditions of the airbag.The force analysis model of the airbag in tunnel is established in this paper,and the deformation characteristics,working mechanism and control conditions are studied through the model test and finite element simulation;The working performance is tested by the field full-scale test.The main research contents and achievements are as follows:(1)The shape and tensile force of the inflated geotubes placed on flat ground are studied through theoretical analysis and model test.The change laws of the shape and material tence during the filling process are got.The shape includes two stages,partially inflated and fully inflated,which are different and unite.The relationship between shape parameters and height of the geotube is normalized curve fitting,and the fitting curves are suitable for the entire process of filling the geotube from flat shape to full shape,which can be used for airbag engineering design.Combining the boundary and the force conditions of the airbag placed in the tunnel,the initial shape of airbag under the internal pressure is analyzed.The stress values of each point of the airbag are obtained.(2)The force analysis model of the airbag in the tunnel is established,and the deformation of the airbag under the uniform pressure is studied.The changes of internal pressure and shape of the airbags with the external pressure are obtained by the theoretical derivation and observable model tests.Two failure modes and three control conditions which guarantee the airbag work well are proposed,especially the unique control condition that the external pressure cannot be greater than the internal pressure.Meanwhile,force analysis model of the tensile airbag is also established and the shape at the limit state is got and veried by model tests.The effect of the material elastic modulus on the deformation and internal pressure of the airbag at the limit state is studied.A design method for airbag air-blocking is proposed,which can provide basis for airbag design calculation in practical applications.(3)Combined with theoretical analysis and the results of water-resistance model tests of airbags,the shape change law of airbag under trapezoidal pressure is studied.Two failure modes and three control conditions are proposed,and the similarities and differences of deformation and failure modes of airbag under the trapezoidal load and the uniform load are compared.The reasons of water leakage during the water-blocking process of the airbag are explained.The design method of airbag water-blocking is proposed.(4)According to the failure modes and control conditions,measures to enhance and ensure the plugging ability of airbag are proposed: One is to add other airbag to form the “double airbag system”,which can not only increase the maximum static friction of the airbag to improve stability,but also can be easy in the construction.The deformation and failure mechanism of the double airbag under the external pressure are obtained.The second is to increase the flexural rigidity of the loaded end of airbag so that it can withstand greater external pressure and good results of model tests prove its feasibility.The third is to control the tolerance of the airbag diameter.Three groups of airbag model tests with different diameters are carried.The deformation characteristics,internal and external pressure relations and failure phenomena during water or air plugging are obtained.The allowable error of the airbag diameter is proposed,providing the basis for airbag manufacturing process.(5)Using the finite element membrane element analysis module to simulate the shape of the airbag in the tunnel against external pressure.The change laws of displacement,stress and internal pressure under uniform external pressure are calculated,which are in good agreement with the theoretical analysis and model test results.The effects of the friction coefficient and the material elastic modulus on the deformation and failure modes of the airbag are also obtained.(6)Based on research results on airbag material tensile strength,friction properties,etc.Using a large-diameter airbag airbag,a field water-filling test was carried out in a tunnel project in Tianjin,and its working performance was tested in all directions.The airbag plugging effect in the test was very good,and the change rule of the internal pressure and displacement of the airbag in the process of water plugging and the water pressure it can resist were obtained.It provides a reference for the application of airbags in practical engineering.