By default, one can think of RS2 as modeling the liner as being free draining, where water pressure does not build up behind the liner.
Applying a water pressure to a liner with groundwater is a bit complicated. You actually need to use a composite liner with a joint element or structural element between the soil and the liner. You can apply the water pressure through the joint.
Two sample files are attached to this article, which include a composite liner with a joint that gets pressurized by the groundwater pore pressure. The axial force increases by about 0.2 MN in Stage 3, which is about right for a thin walled cylinder subjected to a hydrostatic pressure of 0.1 MPa (10m*10kN/m3). N=PR, where P is the water pressure and R is the tunnel radius (2m).
The first file (Pressurized Liner.fez) uses a groundwater analysis to compute the pore pressure. The second file (Pressurized Final Liner piezos.fez) uses piezo lines.
Composite liners are also discussed in Tutorial 15.
Pressurized liner examples using groundwater analysis (left) and piezo lines (right)