There are three categories of TBMs： Namely （1）hard rock TBM； （2）soft ground TBM； （3）mixed ground TBM； The strata in Hong Kong are mixed ground in nature and hence the six machines to be used in the Express Rail Link (XRL) project are mixed ground TBMs. There are two types of mixed ground TBMs: Slurry Mixed Shield TBM and Earth Pressure Balance TBM.

Disc cutters mounted on TBM

Slurry Mixed Shield TBM：

There is a large rotating steel cutter head mounted with disc cutters in the Slurry Mixed Shield TBM. The disc cutters will break the rock and large stones into pieces, mixing with gravel, mud and sand to the pressurized slurry media to be removed via slurry pipes. The mechanism is similar to that of a vacuum cleaner using air as the medium to carry the dirt away.

The tunnel sections to be built by Slurry Mixed Shield TBM in the XRL project include：

Earth Pressure Balanced (EPB) TBM：

Earth Pressure Balanced TBM also has a large rotating steel cutter head mounted with disc cutters. There will be chemical liquid mixing with rocks and sediments to form a paste for delivery by the conveyor belt for disposal.

The tunnel sections to be built by Earth Pressure Balanced TBM in the XRL project include:

The Alignment of the Express Rail Link

Legend: ： Tunnel sections to be constructed by tunnel boring machines