One of the major concerns in subsurface infrastructures is to provide the sealing required during both construction and operation. Ingress of water into such projects during the construction increases the time and costs. It is sometimes accompanied by environmental issues, e.g., lowering the groundwater tables, settlement of the structures, and destruction of vegetation. It can be hazardous to human life, e.g., falling icicles in tunnels in cold climate. It reduces the life cycle of the projects and increases the maintenance costs. To provide the sealing required, a governing parameter is to obtain sufficient spread of grout in surrounding fractures. This can be achieved using cement-based and/or chemical grouts. Despite satisfactory grout spread and sealing efficiency, use of chemical grout is prohibited in many countries due to hazardous environmental issues. Cement-based grout, which is cheaper with less environmental issues, is more common in grouting industry. However, in use of cement-based grout, filtration, which is a result of arching of the cement particles at a fracture constriction, restricts the grout spread. On this basis, this study is dedicated to investigate the grout penetrability/filtration properties in fractured hard rock.
The study was begun by a review of the existing methodologies developed to measure the grout penetrability. Then, a comparison was made between three of the most common used methods in Swedish grouting industry to provide a better understanding of filtration and to figure out which one is more reliable and why. Based on the achievements, a new test apparatus, so-called varying aperture long slot (VALS), was developed in the form of an artificial fracture with four-meter long and variable apertures of 230-10 µm. In the next step, an effort was conducted to improve the grout spread using low-frequency dynamic pressure impulses with different durations of peak/rest periods. The method, which was first tested using short slot, showed significant improvement on the amount of grout take. Dissipation of the pressure impulses was then examined along a much longer artificial fracture, VALS. Finally, due to the significance of optimization of grout spread in rock grouting, the real time grouting control (RTGC) theory, as one of the most common stop criteria in Swedish grouting industry, was examined in more realistic geometry condition using VALS with variable apertures. Even though the project was a limited study based on laboratory tests, the results were positive showing the potential of the methods that if could be further developed to full scale field operations, might influence on grouting industry significantly.
This report is a summary of a doctoral study that was carried out by Ali Nejad Ghafar during 2013-2017 at the Division of Soil and Rock Mechanics, Department of Civil and Architectural Engineering, Royal Institute of Technology (KTH).
Stockholm
Per Tengborg