Immersion is a phase in the installation of pipelines using trenchless technology. It covers the time between the pipe being suspended in the bore to the time it is installed or supported on the bedding. Trenchless construction methods use horizontal directional drilling (HDD), microtunneling and auger boring to bore a hole through which the pipe can pass easily.
Trenchlesspedia Explains Immersion
HDD and other trenchless methods vary in the way the pipes are installed and have different methods of pipeline immersion. HDD utilizes a bentonite lubricant or slurry in which the pipe is suspended. After the pipe is installed portland cement-bentonite grout is used to fill the annular space so that collapse of surrounding soil resulting in a depression at the surface does not occur. This also ensures that the pipe has adequate support from bending stresses once the lubricating and stabilizing fluid is withdrawn.