In-situ visualization experiments on the microscopic process of particle filtration of cement grouts within a rock fracture

In this study, a microscale visualization test system for rock fracture grouting was developed to perform in-situ, dynamic, and visual observational studies of the microscopic behaviour of grout particle filtration during grouting. A series of laboratory grouting tests of ordinary cement (OC) grout and microfine cement (MC) grout were carried out under various fracture aperture conditions using this testing system. The curves of the macroscopic grouting pressure and grout weight changes during grouting, as well as visual images of the microscopic filtration of the grout particles, were obtained. For a large fracture aperture, the grouting processes of both the OC and MC grouts can be divided into three stages, namely, the pressurization stage (stage I), the constant pressure stage (stage II), and the depressurization stage (stage III), whereas the grouting processes under the medium and small fracture aperture conditions consisted of only stages I and IL Almost no filtration of either the OC or MC grouts occurred at the fracture inlet under the large fracture aperture condition. By contrast, under the medium and small fracture aperture conditions, the OC and MC grout particles began to accumulate at the fracture inlet in the pressurization stage (stage I), rapidly forming a semi-circular arch-shaped filter cake and increasing in thickness. The filter cake formed by the OC grout was more stable than that formed by the MC grout. Quantitative measurement and analysis of the microscopic images of the grout particle filtration showed that the thicknesses of the filter cakes formed by the OC and MC grouts increased rapidly with time, and with an increase in the fracture aperture, the filter cakes formed by both grout types initiated later, and the rate of increase in the filter cake thickness gradually decreased. Finally, a new method for evaluating the penetrability and filtration characteristics (Le., b(min) and b(crit) values) of grout based on microscopic images of grout particle filtration was proposed.