Long-term investigations on the pore pressure regime in saturated and unsaturated sloping soils

• Investigation on the pore water pressure regime in saturated and unsaturated slopes
• Monitoring of the pressure head in clayey slope
• Monitoring of the suction in pyroclastic slope
• Recognition of the Wet Steady Condition in pore water pressure regime during rainy period
• Modelling of the groundwater field in the wet steady condition

Pore pressure changes play a major role in the stability of natural slopes, whose safety factor value is usually much smaller than that generally accepted in any other geotechnical problem. Correct assessment of potential seasonal pore pressure distributions is therefore a crucial issue in slope stability analysis. Based on long-term monitoring of some natural slopes, this paper compares the pore pressure distribution recorded in saturated fine-grained deposits and in shallow unsaturated granular covers, both monitored in southern Italy. In spite of the expected differences, the soils display some common features. In particular, from December to mid-May (wet period) in all examined cases the pore water pressures are quite constant over a monthly scale. Thus an essentially steady-state pore pressure condition establishes. It is in this phase, which is characterised by an essentially downward flux, that the safety factor drops to its lowest yearly value. Finally, a simplified method is adopted to model the steady-state pore pressure during the wet period and to calculate the average rain infiltration in equilibrium with the steady-state condition; the results thereby obtained are compared with the findings of an advanced approach, based on the hydrological balance.