Study On Initial Fracture Grouting Based On Small Cavity Expansion Theory

The small cavity expansion theory is widely used to explain some field test results and actual engineering phenomena,such as cone penetration and self-boring pressuremeter tests,analysis of stress and deformation caused by static pressure sinking piles,and simulation of tunnel excavation and shrinkage.In the application of grouting,its theoretical research lags behind engineering practice.Therefore,in this paper,through theoretical analysis,laboratory model test verification,and application of engineering examples,the elasto-plastic theoretical solution and application of undrained small cavity expansion in overconsolidated clay are studied,and the small cavity expansion theory is studied in grouting.The main achievements are as follows:(1)A new spherical expansion theoretical model is proposed,which overcomes the phenomenon that the ’dry surface’ yield stress is overestimated and the failure stress is too large.The yield surface with two state parameters was used,and a spherical expansion theory model considering strain softening of overconsolidated clay was established based on the non-associated flow rule.The theoretical solutions of stress and displacement are derived.(2)Based on the CASM yield criterion,the classic stress-dilatancy relationship is used,the three-dimensional characteristics and the different consolidation degrees of soil elements around the small cavity expansion are considered.By transforming the stress space method and based on the non-associated flow rule,the relationship between the expansion stress and displacement of the cylindrical cavity expansion is established.The Lagrangian method is used to transform the problem into solving the first-order partial differential equations.Drainage conditions and volume invariance principle are used to solve the displacement change of the elastoplastic boundary of the small cavity expansion.Finally,an accurate solution of the cylindrical small cavity expansion is obtained through numerical solution.(3)Combined with CPTU field test data analysis and verification.The results show that the theoretical model can better explain the mechanism of CPTU penetration test in soft soil.At the same time,the theory can be applied to pile driving and self-boring pressuremeter tests analysis,which is of great significance in practical engineering.(4)Based on the theory of cavity expansion,the mechanical mechanism of the soil around the grouting body during the fracture grouting of normal consolidated clay is explained.And theoretically compare and analyze the initial fracture pressure of spherical and cylindrical determined by shear failure,tensile failure,and ultimate expansion pressure.It is found that the theoretical method of cylindrical expansion controlled by tensile failure predicts the initial fracture pressure more accurately.(5)Based on the fracture grouting model test,a research scheme for initial fracture of grouting was designed,and the initial fracture pressure under the ultimate pressure of shear failure control,tensile failure control and pinhole expansion was compared and analyzed Theoretical and experimental values.The grouting fracture law was analyzed after excavating grouting body,and the relationship between initial fracture pressure and confining pressure was studied.Combined with laboratory tests,the accuracy of different fracture models in predicting the initial fracture pressure was analyzed.