The creep behaviour of large slow landslides may pose threats to interacting infrastructures, thus reducing their service life. A case study is here described where a large landslide is threatening the safety of a motorway. To reduce the displacement rate of the landslide, an extensive drainage intervention has been designed, comprising wells and drains. This paper illustrates the evaluation of the intervention’s effectiveness through Finite Difference numerical modelling. To this extent, a time-dependent constitutive model has to be selected for the simulation of the landslide movement. First, a realistic hydro-geological and geotechnical model for the site is defined. Then, the drainage system’s effectiveness is evaluated by running transient flow analyses. Lastly, the time-dependent mechanical behaviour of the landslide is obtained before and after the intervention. To build representative and reliable scenarios of future evolution, it is fundamental to validate the numerical model against observations and the available monitoring data. In this case, satellite (InSAR) detected surface displacements and inclinometer measurements were used to compare and calibrate the viscous parameters. Numerical results are discussed by underlying the role of the geotechnical parameters and appropriate modelling to get reliable predictions.