Study On Bearing Capacity Of Rigid Strip Foundations Near Soil Slopes

With the economic development,due to the limitation of site and economic conditions,there are more and more construction projects near slopes.The bearing capacity of foundation near slopes has become an important problem in engineering.However,there is no sufficient theory for calculating the bearing capacity of foundation near slopes in the current code,the correlation analysis method is not perfect,and in practice,relevant theoretical and technical problems need to be solved urgently.In this paper,the bearing capacity of strip rigid foundation near slopes is studied by using theoretical analysis,numerical simulation and laboratory test.The possible failure modes of foundation near slope,theoretical calculation method of bearing capacity near slope without retaining structure or with retaining structure such as retaining wall and stabilizing pile are analyzed,and the main research results are list as follows:(1)Based on the composite characteristics of foundation failure and slope failure that may exist in foundation near slope,and the allowable velocity condition of plastic mechanism,five possible failure modes(failure mechanism)composed of plane and logarithmic spiral surface are constructed.That is single sliding surface on the adjacent slope side,composite sliding surface on the adjacent slope side,generalized composite sliding surface of flat foundation,slope failure of generalized composite surface adjacent to slope and foundation failure of generalized composite surface adjacent to slope.(2)Based on the upper bound method of limit analysis,the failure mode with a smooth and continuous logarithmic spiral on the adjacent slope side,a logarithmic spiral and a translational triangular block on the side away from slope is established.And the calculation formula of bearing capacity of strip foundation near slope is derived.Compared with the existing test results,the relative errors are mostly within 5%,The location of the failure surface is consistent with the numerical simulation results of FLAC^3D.(3)The influence of internal friction angle,cohesion,slope angle,distance ratio between foundation and slope,safety factor of slope,foundation depth on failure mode and ultimate bearing capacity of foundation near slope are revealed.Bearing capacity of foundation near slope is nonlinear and approximately linear positive correlations to internal friction angle and cohesion respectively.However,the slope angle and the safety factor of slope have nonlinear negative correlation effects on the ultimate bearing capacity of foundation near slope.When bearing capacity of foundation under a certain value it increases nonlinearly with the increase of distance ratio between foundation and slope.At that time,the bearing capacity of foundation near slope becomes the bearing capacity of flat foundation,and the failure mode of foundation gradually changes from unilateral failure to bilateral failure.And the bearing capacity of foundation near slope is almost a linear positive correlation between the depth of foundation.(4)Based on the upper bound method of limit analysis,the composite mode of logarithmic spiral and straight line on the adjacent slope side and the logarithmic spiral area with translational triangle block on the far slope side are constructed.And the upper bound solution of bearing capacity of strip foundation near slope is derived.The example analysis shows that the results of this method are highly consistent with the existing theoretical methods,and the errors are mostly within 10%.(5)On the basis of the associated flow rule,the velocity field of the traditional log-spiral shear mechanism is optimized.The positive coefficient k,which can be determined by programming,is introduced.Then generalized logarithmic spiral shear mechanism which is more general is proposed.The formulas for calculating the internal energy dissipation and gravity power are derived.Based on the generalized logarithmic spiral shear mechanism,the upper bound solution of bearing capacity of flat foundation is derived.Compared with the existing upper limit analysis results,the calculated bearing capacity is about 10%smaller.The positive coefficient k has a significant effect on the bearing capacity,and the increase of internal friction angle and cohesion will lead to the increase of its value.(6)The improved strip method for slope stability analysis is proposed to calculate and determine the inter strip force coefficient by introducing the condition that the shear force between bars does not exceed the shear strength.The case analysis shows that the value of dilation angle has little influence on the slope stability,and the assumption of the angle of resultant force between slices by the unbalanced thrust method is more reasonable than other simplified slice methods.At the same time,the upper bound method for slope stability analysis based on generalized logarithmic spiral shear mechanism is proposed,and the difference between the calculation results of factor of safety and that of strict slice method is less than 10%.(7)The upper bound solution of bearing capacity of strip foundation near slope based on generalized logarithmic spiral shear mechanism with bilateral composite sliding surface is derived.The error between this method and the existing theoretical method is less than 10%,and the result of this method is smaller than that of the composite sliding surface on the adjacent slope side.A comprehensive bearing capacity coefficient N_s,which reflects the weakening of foundation bearing capacity by the geometric dimensions of adjacent slope foundation system,is introduced to simplify the relationship between the bearing capacity of foundation near slope and the bearing capacity of flat foundation.The calculation table of bearing capacity near slope when slope surface failure occur is compiled.And it includes the critical height of the slope where the slope surface failure and the below-toe failure.(8)The model test results show that,from the beginning of loading to the complete failure,the deformation of adjacent slope foundation roughly goes through three stages:elastic deformation,plastic deformation accumulation and failure.The failure mode obtained from the test is consistent with the asymmetric form assumed by the theoretical method in this paper.(9)For the slope-foundation system supporting by retaining wall,the reinforcement effect of retaining wall is simplified as an external force of slope support,and the calculation method of the supporting force is determined based on the requirement of anti sliding and anti overturning stability of retaining wall.Furthermore,the upper bound solution formula of bearing capacity of foundation near slope which supported by retaining wall is derived.(10)Based on the limit equilibrium method,the failure mode of horizontal soil arch between piles reaches the limit state under the action of pile back thrust is proposed.Based on the horizontal soil arching effect between piles,the calculation method of maximum thrust behind piles is proposed.The maximum pile spacing determined by this method is in good agreement with the results of existing methods.(11)For the slope-foundation system supporting by stabilizing pile,the reinforcement effect of stabilizing pile is simplified as an external force of slope support,and the supporting force is determined by the failure conditions of pile shear,soil sliding out between piles and instability of embedded section.Furthermore,the upper bound solution formula of bearing capacity of foundation near slope which supported by stabilizing pile is derived.The case analysis shows that,the difference between the results of this method and the numerical simulation is less than 25%,and the failure mode of the system is in good agreement with the numerical simulation results.(12)For calculation methods of internal force and displacement of stabilizing pile,the constraint conditions assumed at the pile bottom are removed.According to the principle of minimum potential energy,a more rigorous calculation method for the analysis of stabilizing pile is proposed.(13)In the calculation methods of bearing capacity of foundation near slope without supporting structure established in this paper,the upper bound solution of bearing capacity of strip foundation near slope based on generalized logarithmic spiral shear mechanism is relatively optimal.