| Group of combined stabilizing micropile is a lightweight retaining structure with several micropiles connected by a reinforced concrete plate at their top.The structure can be constructed conveniently,quickly and safely,and be helpful to the environmental protection,which is very suitable for moderate or small landslide or engineering slope.In particular,the structure is very suitable for dealing with project emergencies.However,the mechanism and analytical theory of the structure is still not established,which restricts its extensive application to the engineering practice.Therefore,this dissertation aims at the question so as to establish the calculation theories for such structure.Based on the project of National Natural Science Foundation of China,and aimed at the two common types of slope,namely homogeneous soil slope and bedrock-overburden slope,research on the reinforcement mechanism,the calculation theory of structure and the stability of the reinforced slope for this two slopes are carried out in detail by multiple methods including elastoplastic theoretical analysis method,3D numerical simulation method and laboratory model test.The main research results obtained are as follows:(1)The mechanism of combined stabilizing micropile reinforcing slope is revealed.The slope is effectively reinforced mainly through the composite reinforcement,bending and shear resistance action,tension and compression resistance and roof combination.In particular,the deformation and internal forces of single micropiles can be effectively coordinated and controlled by the rigid roof on their top,so that they can be connected as a whole to resist the thrust.It makes the combined stabilizing micropile play the mechanical efficiency of "pile group is greater than the sum of individual piles.(2)The calculation method of combined stabilizing micropile for reinforcing homogeneous soil slope is established.The calculation formula of landslide thrust acting on a combining micropiles structure considering the resistance is derived by kinematical limit analysis.The part of a combining micropiles structure above slip surface may be regarded as the plane rigid frame under landslide thrust,and the rest can be analyzed by m-method in terms of beam-on-elastic-foundation model.The internal forces and deformation of the whole pile are analyzed by using their coupling conditions at the sliding surface.At the same time,the calculation method of combined stabilizing micropile for reinforcing bedrock-overburden layer slope is also given.(3)Based on the research results of internal forces calculation method for a combining micropiles structure,the influence of weakened strength of slip surface on the internal forces of a combining micropiles structure is studied for homogeneous soil slope and bedrock-overburden layer slope,respectively.The results show that has the shape and location of slip surface and the internal forces of a combined stabilizing micropile are greatly affected by the weakened strength.The distribution shape of micropile internal forces is almost unaffected by the weakened strength,however,its magnitude is greatly affected by the weakened strength.Compared with homogeneous slope,the thrust force acted on the composite structure in bedrock-overburden layer slope is more influenced by the weakened strength.In the case of the same reduction of the weakened strength,the average variation of internal forces of composite structure for the bedrock-overburden slope is larger than that of homogeneous soil slope.(4)The influence characteristics of main factors on internal forces of the combined stabilizing micropile are determined.The internal forces of the combined stabilizing micropile increases nonlinearly with increasing the single pile stiffness,and decreases nonlinearly with the increase of pile inclination angle and pile number in one group.According to the principle that the internal forces of micropile is small and the magnitude of each row of micropiles is relatively close,the reasonable combined stabilizing micropile types for homogeneous soil slope are obtained as follows:the spacing between piles in/out plane is 5d and 3d,respectively;the diameter of the thread steel is 28 to 32 mm;the embedded ratio is 0.44 and 0.53.The reasonable combined stabilizing micropile types for bedrock-overburden layer slope are as follows:the spacing between piles in/out plane is 5d and 4d,respectively;the diameter of the thread steel is 28 mm;the embedded ratio is 0.50 and 0.58.The reasonable dip angle of micropiles for the two type's slopes is 20?25 degrees,and the number of the micropiles in one group is 9(3 rows×3 columns).(5)A fast convergence optimization algorithm based on strength reduction technique is proposed.The shear strength reduction coefficient(safety factor)when the slope in the critical state is searched by binary method.In the calculation of stability coefficient,the maximum number of steps for each reduction calculation are reduced to 50%of the traditional strength reduction method.Besides,ratio of imbalance force is assumed as 1.0×10-5,which is one of the termination conditions during each step of strength reduction calculation.These measures can greatly reduce the machine time used in the computation.(6)Based on the kinematically admissible failure mechanism,a limit analysis approach is introduced,which considers two potential slip surfaces that individually lie in the upslope and downslope of the combined stabilizing micropile.Considering the mode of coordinated action between piles and soil,the upslope thrust force and downslope resistance on a combined stabilizing micropile are derived by plastic limit analysis method,and the relationship between the difference between the two(thrust force and resistance)and the safety factor and sliding surface depth is established,and then the minimum safety factor of the stabilized slope is determined by the maximum principle of the difference value.(7)Energy method for slope stability evaluation based on deformation energy and extremum principle is given.The safety factor of slope can be defined by the square root of the ratio of ultimate energy of the sliding zone soil over accumulated " elastic" energy.The Fs is then reduced to a simple expression of the power to shear stress and shear strength,by taking soil as an elastic-plastic material obeying the Mohr-Coulomb failure criterion.The safety factor of the reinforced slope can be determined under the condition that the natural stress field of the reinforced slope is obtained.(8)The advantages and disadvantages of three methods for slope stability analysis are presented.The optimization reduction method overcomes the shortcomings of traditional strength reduction method,such as long time to solve the problem and the breakpoint discontinuation,but the calculation efficiency is low.The double sliding surface limit analysis method overcomes the shortcoming of the traditional limit analysis method,which assumes that the front and back sliding surfaces of piles are the same logarithmic spiral surface.It has high calculation efficiency,but it cannot consider the deformation of rock and soil,and is not suitable for heterogeneous slopes.The energy method can take into account the deformation of rock and soil,and it has a very high computational efficiency compared with the shear strength reduction method of numerical simulation.The calculation efficiency of the three methods is ranked from high to low:energy method,kinematical limit analysis method based on double slip surface,and optimization reduction method.(9)The case analysis shows that the safety factor obtained by the kinematical limit analysis method based on double slip surface for homogeneous soil slope is the largest and the slip surface is relatively deep.The safety factor obtained by the energy method is between that by the limit analysis and optimization reduction method,but the slip surface is the shallowest.The deviation of the safety factor obtained by the three methods is no more than 6%.The computational time of the first two methods is about 90%less than that of the optimization reduction method.For the bedrock-overburden layer slope,the safety factor obtained by the energy method is about 4%larger than that obtained by the optimization reduction method,but the rate of computational time-saving is about 95%.In this paper,the research results of the mechanism of combined stabilizing micropile reinforcing slope,the calculation and analysis method of combined stabilizing micropile,and the stability analysis method of the slope reinforced by the stabilizing micropile groups can provide scientific basis and guidance for practical engineering,and have extremely important theoretical significance and application value. |
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