This paper presents an experimental investigation on the injection of chemicals into rock fractures by using a transparent replica to visualize grout propagation. The fractured rock mass is simulated by using fused silica blocks, fused silica grains and mineral oils. Fused silica blocks in a cubic form are used to form rock mass and the fused silica grains are used to fill the space between these blocks to simulate fractures with a certain aperture. In addition, a mixed mineral oil that has the same refractive index as that of the fused silica blocks and grains is used to simulate fluid and groundwater. The experimental setup consists of a Plexiglas container, loading frame, grout injection system, and three black and white charge-coupled device (CCD) cameras. A modified urea-formaldehyde resin and an acid solution are mixed together as the grout. The point source of the grouting is simulated by using an injection needle inserted into the model. As the silica blocks are impermeable, the grout instead propagates into the fractures that are filled with silica sand. Propagation images during the grouting are captured by using the CCD cameras. Digital image processing is then applied to analyze the grayscale pictures to detect the edge of the grout front and calculate the grout penetration length at different times. The shape of the boundary of the grout–groundwater interface is found to be spherical when the fractures are filled with porous transparent materials. The results indicate that the flow velocity calculated by using a modified cubic law is in good agreement with the measured penetration length in the images. The results also verify the possibility of the use of a transparent replica to visualize grout propagation inside a fractured rock mass so as to understand grout propagation during grouting.