Fundamental investigation of compensation grouting using transparent soil

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This proposed research will experimentally investigate compensation grouting through the real-time visualization of the transport of grout and hydraulic fracturing by using transparent soil. The findings from this research program are expected to directly benefit design and construction of compensation grouting in practice. Compensation grouting is a relatively new technique which injects a low viscosity grout at high pressure to compensate for ground loss due to adjacent underground construction. It has been successfully applied in many urban areas, particularly during tunnelling. However, the mechanisms of compensation grouting are not well understood as it involves a set of complex parameters due to different soil conditions as well as grouting variables. The injection of grout inside soil involves complex interactions between the soil skeleton, fluid flow, and grout transport. It is generally understood that compensation grouting consists of two grouting modes: compaction grouting and fracture grouting. The application of compensation grouting heavily relies on practical experience and empiricism. The development of an efficient and economical grouting program requires a thorough understanding of the grouting mechanisms and its controlling parameters. A few laboratory studies have been conducted that have obtained a basic understanding of grouting. However, there is limited research on the initialization and geometry of fractures, and the grout transport in soils. There are also no soil displacement measurements made during injection. This proposed research will use transparent soil to address these issues and provide a full understanding of grout transport and fracturing inside a soil mass. In order to achieve these goals, a state-of-the-art optical measurement system will be developed to be able to conduct real-time data acquisition of the grouting parameters and image capture. The optical system consists of a laser to illuminate a targeted cross section inside the transparent soil and two well-configured cameras to simultaneously capture the images of the illuminated section. Three-dimensional soil deformation will be calculated by using a stereo particle image velocity technique. The information on soil deformation during grout injection will help to address the issue of grout efficiency in compensation grouting for efficient settlement control. Digital image edge detection will be used to delineate the geometry of the grout body, identify the initialization of fractures, and observe the direction and geometries of fractures. A series of scaled model tests will be carried out in this study, mainly under 1-g conditions in the laboratory. The influence of the controlling parameters will be examined, including soil conditions, overburden pressure, grout composition, injection rate, and injection pressure. A few centrifugal modeling tests will be conducted to complement the 1-g model tests to better simulate stress gradient conditions in the field. Numerical simulation will also be conducted to provide a systematic investigation of the influence of various influencing factors. This proposed research will be the first of its kind to visualize the propagation of grout in a porous medium and the initialization of hydraulic fractures during grout injection in real time. This research program has significant implications of changing or improving many existing technologies used in civil, environmental, and petroleum engineering fields. The proposed research is also vital in training HPQs for their career development in Canada.