Research On Ultimate Load And Failure Modes Of Soft Interlayer In Foundation

Soft interlayer is a vital stratum in foundation engineering,and its ultimate load is related to the core issue about bearing stability of foundation.Different from the ordinary layered soil structure,the soft interlayer has a wide range of sources and its ultimate state is affected by many factors.At present,there is still a lack of systematic research in this filed.In this paper,the ultimate load and the failure mode of the soft interlayer are analyzed by the combination of theoretical analysis,numerical calculation and laboratory experiment.The formula for calculating the ultimate load of soft interlayer are deduced respectively for flow plastic material and general geotechnical material under the condition of plane strain and axial symmetry.The results compared with the existing theories and industry specifications are also analyzed.The relationship between failure mode and ultimate load of soft interlayer is studied,and the influence law of relative factors on them is also discussed.Finally,the related occurrence conditions for different failure modes are explored.First of all,based on the Prandtl plastic extrusion model,the stress component of the soft interlayer and the lower bound solution of the extrusion force on the plane of interlayer are obtained through classical plasticity theory.The stress boundary condition of the original Prandtl solution is improved.And the formula of the ultimate load of soft interlayer is obtained accordingly,which can reflect the influence of roughness conditions on the surface of interlayer.Secondly,on the basis of the upper bound method of limit analysis,the ultimate load of the soft interlayer of plane problem,axisymmetric problem and with lateral extrusion effect are analyzed separately.According to the research,the factors such as the thickness and width ratio,the roughness of the surface layer,the weight of the interlayer material and the overload are all have an impact on the ultimate load of the soft interlayer.When the surface is completely rough,the upper and lower solutions of this text are relatively close,which could well define the real solution.Correspondingly,standard solutions are partial to insecurity,and Prandtl solutions are partial to conservatism.The relationship between roughness,thickness,slip mode on the interlayer surface and extrusion force is analyzed by extrusion experiment.The various slip laws of planes in rigid parallel plate extrusion of plastic materials are verified.On this basis,according to the static equilibrium principle and the upper bound limit principle respectively,ultimate load calculation formula of soft interlayer composed with general geotechnical material under plane and axisymmetric condition are derived.The analysis of related factors is conducted.Research shows that,the slip mode is greatly affected by the roughness of surface and the strength parameters of interlayer.When the surface friction coefficient is close to the tangent value of the internal friction angle of the interlayer,the mixed slip mode is easy to form.While the larger the difference between them,the surface of interlayer is easier to reach the single adhesion or Coulomb sliding slip mode.The static equilibrium solution and the upper bound solution of this text can consider the effect of the plane slip mode on the ultimate load,and the ultimate load are different obviously under different slip modes.The ultimate loads obtained by Mandel-Salencon theory and industry specification are both higher under certain conditions.It is necessary to pay attention to the proper safety reserve in practical application.Influence factors such as the thickness,strength of soft interlayer and surface roughness condition as well as the mechanical properties of the lower layers on the failure modes and ultimate loads of the soft interlayer are explorered.The evolution law of the failure mode is analyzed.According to the results,when the thickness width ratio is small,the ultimate load of the soft interlayer is greatly influenced by its change.And there is a critical scale of thickness to width ratio which could cause the change of the ultimate load and the failure mode.The range of the scale is related to the interlayer strength,the surface roughness conditions and the overload.As the thickness to width ratio is lower than the critical scale,the lateral plastic extrusion failure mode will occur in soft interlayer,which is basically not affected by other related factors.Meanwhile,the ultimate load of soft interlayer can be conformed to the calculation theory derived in this text.While the thickness to width ratio is higher than the critical scale,the overall shear failure or the transitional failure mode occurs in the soft interlayer.And the variation of the corresponding ultimate load is not obvious with the ratio of thickness to width.When the other conditions are constant,the ultimate load of soft interlayer will remain invariably as the strength ratio of lower stratum and interlayer increases to a critical value.This process is accompanied by a change trend of failure mode from the overall shear failure to the lateral plastic extrusion.