*3.3. Comparison between Measured and Estimated Pore-Water Pressure at Sliding Surface Depth*

Figure 9 shows the comparison between measured and modelled pore-water pressure trends at the typical sliding surface depth (1.0–1.2 m from ground level) in the representative test-site slope for different rainfall events reported in Table 4. The selected events represented typical rainfall scenarios occurring in the study area during the analyzed time span and were characterized by initial pore-water pressure conditions similar to the ones chosen for the reconstruction of the physicallybased thresholds.

**Figure 9.** Comparison of measured and estimated by TRIGRS pore-water pressure trends at the typical depth of shallow-landslide sliding surface (1.0–1.2m from ground) for the selected rainfall events at the monitoring station in the representative testsite: (**a**) 1 May 2012; (**b**) 4–5 April 2013; (**c**) 18–20 January 2014; (**d**) 28 February–2 March 2014; (**e**) 18–19 March 2018; (**f**) and 1–2 May 2018.

**Table 4.** Measured initial and final pore-water pressure values versus those computed by TRIGRS at the typical depth of shallow-landslide sliding surface for the selected rainfall events at the monitoring station in the representative testsite. The number related to each rainfall event corresponds to the following: (1) event of 1 May 2012; (2) event of 4–5 April 2013; (3) event of 18–20 January 2014; (4) event of 28 February–2 March 2014; (5) event of 18–19 March 2018; and (6) event of 1–2 May 2018.


Despite the different features of the tested events, the trend of the pore-water pressure modeled through the physicallybased method (TRIGRS model) seems to simulate in a reliable way the field measurements during each analyzed rainfall event. Differences between measured and estimated values are always lower than 2 kPa at the analyzed soil depth. RMSE values of 0.1–1.2 kPa confirmed the reliability of these simulations. The highest pore-water pressure value at the end of each rainfall event was generally attained through the physicallybased method, unless for the event occurred on 28 February–2 March 2014. Although the model results were in very good agreement with the real measured values, modeling errors in pore-water pressure trends could be linked to the simplification provided by the TRIGRS model with regard to soil hydrological features. In particular, TRIGRS model does not consider a layered soil profile, thus forcing to assume average values of the required soil parameters across the analyzed soil profile.
