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

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 thewet period and to calculate the average rain infiltration in equilibriumwith the steady-state condition; the results thereby obtained are compared with the findings of an advanced approach, based on the hydrological balance.