The Centrifugal Model Study And Numerical Analysis On The Wetting Collapsible Deformation Of Loess Foundation

Most buildings in northwest China are built on loess foundation.Collapsible loess foundation is a special engineering term.Under the combined action of water immersion and load,this type of foundation often produces a large subsidence deformation,which seriously threatens the safety and normal use of buildings and even causes many extremely serious engineering accidents.There are many sources of water for loess foundation collapse,mainly from the upper and lower directions.Two important reasons for loess foundation collapse deformation are submerge in the upper part of the foundation and the rising of underground water level in the lower part of the foundation.The water content rising in loess greatly reduces the compressive modulus and the strength of foundation soil.Therefore,the settlement of building foundation increases rapidly and the deformation of collapse occurs.In this paper,the typical collapsible loess in Lanzhou is taken as the research object,and the Air-fall method is used to reconstruct the loess foundation model in laboratory for centrifugal model test.The collapse deformation characteristic of loess foundation under two conditions of subgrade flooding and underground water level rising is studied respectively,then the prediction method of collapse deformation under different working conditions is given.On the basis of further analysis of centrifugal model test data,the estimated coefficient of hyperbolic method for prediction of loess foundation settlement is revised,so that it can adapt to the variation of different water level rising height,different base pressure and different positions.Finally,in order to make the research results more comprehensive and detailed,the settlement deformation law and deformation characteristics of loess foundation under the influence of rising groundwater level are emphatically analysed with the numerical analysis method,and the influence of different initial water level,different rising heights and different rising speeds are analysed.Finally,the sedimentation prediction formulas are given.The research results of this paper provide important data support and reference basis for more comprehensive,scientific and detailed analysis of the loess foundation’s collapse deformation.The results of the collapsibility test and compression test on undisturbed loess and Air-fall remoulded loess show that the pressure corresponding to the peak point of p-δ_s relation curve of the two are all around150kPa.The ratio of the coefficients of collapsibilityδ_{s（Air-fall）}/δ_{s（Undisturbed）}is about 1.4,which can provide a certain basis for the experimental data to infer the actual data in field.There is a certain conversion relation between the deformation of deadweight loess foundation and the deformation of non-deadweight loess foundation.For the study of water level rising in loess foundation,if one of the values can be tested and the other corresponding value can be calculated.Then,the preliminary classification of loess subgrade can be carried out.In the study of collapse settlement caused by loess foundation wetting,the collapse deformation of loess foundation with four different base pressures of p equal to 50,80,100 and 200kPa,respectively,are analysed.The relationship between ultimate settlement and base pressure of loess foundation is approximately linear.The base pressure of loess foundation will produce collapse deformation in the centre and the horizontal direction of foundation.As the upper load pressure increases,the settlement deformation at the corresponding position will increase accordingly.For points outside the foundation,when the distance from the foundation edge is less than the foundation width of 2.83b（b is the foundation width）,the settlement decreases rapidly with the increase of the distance,and the attenuation speed increases with the increase of the base pressure.Based on the experimental results of centrifugal model test,the hyperbolic estimation method of foundation settlement is modified.The correlation coefficient between measured value and estimated value remains between 0.96-0.99,which indicates that the modified hyperbolic estimation method is reliable in estimating the influence of water on the settlement of loess foundation.In the study of the collapse deformation of loess foundation caused by the rising of underground water level,the centrifugal model test results show that,the deviation between the settlement amount of foundation under the action of base pressure p and the settlement amount under base pressure of 0 increases gradually with the rising height of water level.Two factors?_s andRF_W were proposed to consider the influence of base pressure and water table rising height,respectively.On this basis,the modified calculating formula of the final settlement caused by the water level rising in the loess foundation is put forward.The good consistency between the calculation results and the measured results suggest a reliable calculation method of the ultimate settlement of loess foundation.According to the centrifugal model test results,the hyperbolic estimation coefficient of foundation settlement of loess under the influence of different rising height and different positions outside the foundation is revised.For the settlement of foundation centre,under the same base pressure,the coefficient/₀ gradually decreases with the rise of water level.For different base pressures,the coefficient/₀ increases with the increase of base pressure.At the same base pressure,(6/(6₀ decreases as the water level increases.For different base pressures,the higher the base pressure is,the greater(6/(6₀ is.As the water level target value gradually approaches the foundation surface,(6/(6₀gradually decreases with the increase of base pressure.For settlement of points outside the foundation,under the same base pressure,S and a both increase with the increase of water level rise height.The point S outside the foundation gradually decreases as it is far away from the centre of the foundation.The point outside the foundation differs little at different positions on the foundation surface.When the base pressure gradually increases.For the same water level rising height,the parameters S and a increase with the increase of base pressure.The modified parameters from different base pressure,different water level rising height and different distance outside the base were summarised to a 3D coordinates.The obtained scatter diagram is fitted,and S is a surface under different base pressures,a is a plane,the fitting formulas of S and a are given respectively.The variation law and upper limit under different initial water level,different rising height and are also given through numerical analysis.For the loess foundation that has formed a stable distribution of pore water pressure,the initial water level is within 8m below the surface,the foundation belongs to non-weight collapse.When the initial groundwater depth is greater than 8m,the foundation belongs to weight collapse.In addition,the settlement characteristic of different initial water level,different rising height and different rising speeds were analysed,and the corresponding upper limit is given.Therefore,based on the content of each part,the collapse deformation of loess foundation in the rising process of underground water level can be preliminarily estimated under different initial water level,different rising height and different rising speed.