Study On Reinforcement Mechanism And Design Method Of Pneumatic-Vibratory Probe Compaction Method Treated Collapsible Loess Foundation

The reinforcement of the collapsible loess foundation is one of the key technical problems in the development and construction of western China.The collapsible loess has a large coverage area,and there are many foundation reinforcement projects under construction,with some expensive ways.Pneumatic-vibratory probe compaction method is a new type of environment-friendly and resource-saving foundation treatment technology for collapsible loess.This method requires no additional filler and has the advantages of operation simplicity,remarkable improvement effect,and good economic and environmental efficiency.This dissertation,supported by the National Natural Science Foundation of China Project(No.41977241),aims to reveal the reinforcement mechanism and design method of the pneumatic-vibratory probe compaction method for treating collapsible loess.Theoretical analysis,indoor model tests,field tests,and numerical simulations were adopted to systematically study the method for the treatment of collapsible loess.The main works and conclusions can be given as follows.(1)The principle and method of reinforcement of collapsible loess foundation by the pneumatic-vibratory probe compaction method were first proposed.The compaction construction equipment was independently developed with the intelligent construction control and remote monitoring system,and the construction technology was put forward.The construction cases of the vibratory probe compaction method at home and abroad are summarized,and the applicable soil types of the method are studied.An assessment method for vibrating compacted soil based on the Chinese standard soil classification using the piezocone penetration test(CPTU)is proposed and the distribution range of vibrating compacted soil on the plasticity chart is given.(2)Based on the theory of air j et,the relationship between control air pressure and axial dynamic pressure at a certain horizontal distance from the outlet is established.The laboratory model test device was developed and the model test was carried out.The results showed that the air jet had a circular shearing effect on the soil,which could significantly increase the horizontal soil pressure and thus improved the compactness of the surrounding soil layer.At the same time,the vertical vibration velocity of the surrounding soil particles increased obviously,and thus promoted the rearrangement of soil particles.The air j et undergoes a process from destroying to compacting the soil in the horizontal direction,and then the pressure gradually decays to zero along the expansion direction of the dominant channel.(3)Based on the pressure-controlled circular hole expansion theory,the pressureshear compacted area generated by the air jet is obtained to be about three times of the initial radius.Based on the theory of seepage mechanics,the seepage pressure distribution model of air j et inside unsaturated loess is established,and the relationship between seepage pressure,seepage distance,and seepage time is analyzed.(4)The pneumatic-vibratory compaction model device developed by ourselves was used to carry out the comparative tests under two conditions of air-excitation force and single excitation force.The density of the soil layer at all depths increased significantly after treatment,and the collapsible coefficient decreased significantly.With the addition of an air jet,the density of the soil layer below 100 mm depth was further increased,and the maximum horizontal soil pressure was increased by 20.4%.At the same time,the vibration response of soil caused by the method was amplified,then accelerated the arrangement of soil particles and promoted the reorganization and densification of soil structure.(5)The field test of the pneumatic-vibratory compaction method for collapsible loess foundation was carried out.It was found that the additional air jet improved the penetration efficiency of the probe on the one hand,and promoted the increase of soil compactness around the probe on the other hand.The physical and mechanical indexes of the soil were significantly improved and the collapsible coefficient was eliminated after treatment.The strength and compactness of the soil layer were increased by more than one time.The results of the surface wave test showed that the reinforced foundation had good uniformity.The safety distance of ground vibration caused by site construction is 12 m.The soil strength with 1.4 m spacing is the same as that of the soilcement pile with 1.0 m spacing,indicating that the pneumatic-vibratory compaction method has a significant economy.(6)Through the systematic study of the soil stress condition,the microstructure,the energy transfer,and the particle finite element simulation,the reinforcement mechanism of collapsible loess by the pneumatic-vibratory compaction method is revealed.Under the synergistic effect of the excitation force and the air j et,the vibration energy is transferred to the surrounding areas in the form of shear wave,which causes the shear failure of the initial granular structure system of undisturbed loess.Shear dislocation and rearrangement of soil particles occurred,and the soil particles shifted to the position of low potential energy.Meanwhile,the horizontal stress of soil increased due to lateral extrusion,and the soil structure gradually tended to be compact until it reached a new dynamic equilibrium state.(7)Considering the influence of strain effect on vibration energy propagation,the single-point reinforcement range of collapsible loess foundation during the treatment is calculated based on the variation law of soil dynamic shear strain.It is verified by the numerical simulation of the vibration penetration in the structural soil with the particle finite element method.The result shows that the range of reinforcement zone produced by the method under the condition of j et pressure 0.8 MPa is calculated to be about 4.7 times of the probe radius,which is 1.2 times of that under the condition of a single excitation force.(8)The design method and design process of the pneumatic-vibratory probe compaction method for treating collapsible loess are presented.The key parameters of the method are given,and the calculation methods of bearing capacity and settlement and the methods of quality inspection and effect evaluation are established.