A question about ramp function in the "deep excavation" example

Hi all,

In the ramp function in this example, the slope is defined as S_strut/U_max which has the unit of N/m^2, and the location is -U_max. However, when it is used as the force on struts, it has the form of -rm1(-u[1/m])*Depth<Stage_1,2,3 and the unit of N. Why are the units not compatible?!

Thanks, Ramin