Tunnel grouting is a system is primarily designed for the back grouting of ring segments in the tunnels. The main grout pump can be set to keep a pressure or flow, and the colorant pump will routinely maintain the correct proportional flow. One of the major challenges during tunnel construction is to seal the potential water inflow. To deal with these unfavourable geological conditions, tunnel grouting was adopted as an important measure for ground treatment. By using this technique the tunnelling safety of the tunnel was guaranteed. Many factors such as the grout material, grouting pressure, soil type, and stress state of the ground can affect the eventual grouting mode. Given below are some tunnel grouting techniques that can be used in unfavourable conditions.
 Backfill and permeation grouting
A grouting pipe is first filled with pressurized grout, and the grout enters the ground from holes pre-set along the pipe. Due to the effects of backfill grouting and permeation grouting, the ground permeability is reduced and the ground is thus strengthened and stiffened. At this stage, the grouting pressure and the grouting rate can be reduced gradually. Generally, the duration of this stage is very short, lasting for only tens of seconds.
 Compaction grouting
The continuously injected grout expands the soil outwards and forms a grout bulb around the pipe. With the increase in the grouting pressure, the size of the grout bulb increases until the grouting pressure accumulates up to the fracturing pressure. The formation of the grout bulb displaces and compacts the surrounding soil. At this stage, the grouting pressure increases to a peak value and the grouting rate drops gradually.
 Primary fracture grouting
When the grouting pressure reaches the first peak value, a plane of fracture is formed in the soil by hydraulic fracturing. Although the grouting pressure is reduced after fracture initiation, the fracture develops very fast due to the stress concentration at the tip of the fracture. Thus, the surrounding ground suffers continuous splitting until the pressure around the fracture tip is not high enough to split the ground any further. Meanwhile, with the development of the fracture, additional grout is required to fill the initiated fracture, leading to an incremental increase in the grouting rate.
 Secondary compaction and fracture grouting
When the grout flow is confined, the grouting pressure starts to increase. With the increase in the grouting pressure, the injected grout can enlarge the width of the primary fracture. In a tunnel grouting process, multiple boreholes are drilled for grouting in unfavourable geological conditions. In this case, the grouting serves as a method to strengthen the ground as well as to reduce the ground permeability.