基于小孔扩张弹塑性理论的注浆起始劈裂压力研究

劈裂注浆技术已在众多岩土工程中成功应用,但劈裂注浆机理不明,起始劈裂注浆压力计算理论远远落后于工程实践的发展。为了揭示黏土注浆过程中起始劈裂注浆的力学机理,假定土体服从各向同性不排水条件,基于球形和柱形扩张弹塑性理论,采用应力转换方法,结合偏微分方程组的定解条件,得到2种小孔扩张问题的数值解,通过简化分析小孔扩张孔壁处土体单元的应力变化规律,建立了球形与柱形扩张弹塑性方程,并结合受拉破坏、剪切破坏以及临界状态极限破坏模型,提出黏土注浆3种起始劈裂压力解析解。解析解与数值解对比结果表明:当土体的超固结比(OCR)为2时,数值解与解析解计算扩孔压力一致,当OCR小于2(或大于2)时解析解计算扩孔压力略高于(或略低于)数值解,验证了不同超固结比下弹塑性解析解的合理性以及实用性。黏土劈裂注浆模型试验与参数分析表明:在黏土劈裂注浆过程中,先后经历土体压密、沿垂直小主应力方向产生起始劈裂以及裂缝扩展3个阶段;随着注浆压力的增大,注浆体周围土体先发生剪切破坏,随后浆液会填充剪切破坏面,再发生拉伸破坏;对比由剪切破坏、拉伸破坏以及临界状态极限破坏控制的球形和柱形扩张起始劈裂压力理论值与模型试验结果,柱形小孔扩张拉伸破坏理论方法和模型试验结果更吻合。

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.