Abstract

The injection pressure is an important design and operational parameter in rock grouting since it controls the grout penetration length and may cause harmful mechanical deformation of the rock mass, such as opening/dilation of the fractures, referred to as jacking. At present, modeling of rock grouting mainly relies on analytical models where the impact of jacking on the grout penetration in rock fractures is not considered. In this study, we present a hydro-mechanical coupled model for rock grouting in a single one-dimensional rock fracture, with consideration of jacking and two-phase flow, i.e. cement grout and groundwater. It assumes that the cement grouts are Bingham fluids and that the rock matrix is an elastic material. The fracture is simplified as a pair of smooth parallel plates. A finite element method (FEM) code is developed to iteratively solve the two-phase flow in the fracture and the elastic deformation of the rock matrix. Two cases with and without consideration of jacking are simulated and compared. The results generally show that jacking of fractures significantly affects the grout penetration in the fracture, which should be properly considered in modeling of rock grouting. This numerical model is able to describe more realistic physical processes in rock grouting, which can be used to estimate the optimal injection pressure in practice.

1.
Introduction

Cement grouting has been widely used in rock engineering to increase the tightness of fractured rock masses and limit the groundwater flow. Modeling and analysis of cement grout flow in rock fractures is important in the design, execution and monitoring of activities in ever increasing demands of underground rock engineering projects (e.g. Stille 2015).

In rock grouting practices, cement grouts are often typically non-Newtonian fluids (Hakansson et al. 1992; Håkansson 1993; Rahman et al. 2015), which are injected through boreholes with a constant pressure (e.g. Stille 2015). The grouting process in fractured rocks is commonly idealized as laminar flow between parallel plates, including both 2D radial flow and 1D channelized flow (e.g. Gustafson et al. 2013; Zou et al. 2018). The radial flow occurs in the fracture intersecting the injection borehole, while channel flow takes place through connecting fractures (Zou et al. 2018). The injection pressure is an important design and operational parameter in rock grouting because it directly controls the grout penetration lengths and affects the mechanical behaviors of the rock mass. It is similar to the hydraulic fracturing that high injection pressure may cause harmful dilation of the rock fractures, termed as jacking in rock grouting (e.g. Stille 2015), which significantly affects the stability and safety of adjacent infrastructures (Rafi and Stille 2015).

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