Pressure balance method vs. compressed air method in pipe jacking
Pressure balance method generally requires the use of mechanically operated tunnel-boring machine at its cutting head in pipe jacking. Slurry or steel bulkhead is usually adopted to provide the balance of earth pressure and groundwater in front of boring machine. Slurry used in balancing earth pressure and ground water pressure is constantly supplied to the face of cutting wheel through slurry pipes. Excavated materials drop into a crusher for reduction in material size. Afterwards the debris and spoils would enter the spoil removal chamber near the invert of shield and would be transported to ground level via slurry discharge pipes. This method of construction is generally adopted in gravel andsand. Though, it suffers from the disadvantage that it is quite tough to remove large rock boulders at the time of advancement of the machine. It is quite time-consuming for workers to go inside the comparatively small airlock chamber and remove large bounders by hand tools.
The other kind of pressure balance technique is known as earth pressure balance method that is commonly used in clay and silty soils. It makes use of principle of sustaining the pressure of excavation chamber the same as the pressure in ground. Excavated materials are transported through screw conveyor to jacking pit.
Compressed air method in pipe jacking is generally adopted in locations where groundwater table is high. An air pressure of less than 1 bar is normally sustained to provide the face support and to avoid water ingress. Depressurization and Pressurization has to be conducted for workers leaving and entering the pipe-jacked tunnels. In the case of porous ground, certain ground treatment such as grouting has to be carried out. Removal of boulders by this method is convenient however it hasthe demerits of slow progress and significant noise problem produced by generators and compressors.