Study on Initial Fracture Pressure of Grouting Based on the Elastic-plastic Theory of Cavity Expansion

Fracture grouting technology has been successfully applied in many geotechnical engineering projects, however, the mechanism of grouting fracture is not very clear so far and the theory of initial grouting fracturing pressure is far behind engineering practice. To reveal the mechanism of initial grouting fracture pressure in clay, the soil is assumed to obey isotropic undrained conditions. Based on elastoplastic theory of the spherical and cylindrical expansion, coupled with boundary condition of the partial differential equations, the numerical solution of the cavity expansion problem is obtained with the stress conversion method. Simplifying the law of soil unit stress on the edge of the cavity, the elastic-plastic equations of spherical and cylindrical expansion are established. Combined with tensile failure and shear failure and the critical state model, three corresponding analytical solutions of initial grouting fracture pressure are proposed. The results show that the expanding pressure of the analytical solution is consistent with that of the numerical solution when the over consolidation ratio (OCR) is equal to 2, and the expanding pressure of the analytical solution is slightly higher than the one of the numerical solution when OCR is less than 2 or vice versa, which verify the rationality and practicability of the elastic-plastic analytical solution under various OCRs. The grouting model test and parameter analysis show that the process of grouting fracture of clay can be divided into three stages: compaction, initial fracturing in the direction of vertical principal stress and crack propagation. With the increase of grouting pressure, the soil surrounding the grouting body undergoes shear failure first, then the slurry fills the shear failure surface, finally tensile failure happens. The analytical solution of cylindrical expansion due to tensile failure is the most accurate method among three theoretical value of the initial grouting fracture pressure. © 2020, Editorial Department of China Journal of Highway and Transport. All right reserved.