The Use of Bentonite Mud in the MTBM of Rock Areas

When MTBM is used to construct a tunnel in cohesive soil area, the main force need to resist is adhesive force generated by soil or clay around the machine, which can be reduced by the lubrication. In this case, the most commonly used lubrication is bentonite mud, which is injected through ports in the jacking head and filled outside of the pipes. In rock area, the formation is hard enough to support the pipes, if both entrance and exit are sealed well enough to prevent the lubrication lose, and also make sure the mud be injected continuously and around the pipe all the time which makes the pipes "float", frictions between pipes and rocks will be nearly zero under this "perfect" status. So we can conclude that in rock area, the lubrication system is not only to reduce the resistance force coming from the soil, but also to lower the frictional force coming from rocks. In some sedimentary rock areas, clean water might be the best lubrication. This theory has been successfully used and proved in different crossings of MTBM site in China's second West-East Gas pipeline project.