Abstract
Abstract. A one-dimensional hydrological model of a slope covered with pyroclastic materials is proposed. The soil cover is constituted by layers of loose volcanic ashes and pumices, with a total thickness between 1.8 m and 2.5 m, lying upon a fractured limestone bedrock. The mean inclination of the slope is around 40°, slightly larger than the friction angle of the ashes. Thus, the equilibrium of the slope, significantly affected by the cohesive contribution exerted by soil suction in unsaturated conditions, may be altered by rainfall infiltration. The model assumes a single homogeneous soil layer occupying the entire depth of the cover, and takes into account seasonally variable canopy interception of precipitation and root water uptake by vegetation, mainly constituted by deciduous chestnut woods with a dense underbrush growing during late spring and summer. The bottom boundary condition links water potential at the soil–bedrock interface with the fluctuations of the water table of the aquifer located in the fractured limestone, which is conceptually modelled as a linear reservoir. Most of the model parameters have been assigned according to literature indications or from experimental data. Soil suction and water content data measured between 1 January 2011 and 20 July 2011 at a monitoring station installed along the slope allowed the remaining parameters to be identified. The calibrated model, which reproduced very closely the data of the calibration set, has been applied to the simulation of the hydrological response of the slope to the hourly precipitation record of 1999, when a large flow-like landslide was triggered close to the monitored location. The simulation results show that the lowest soil suction ever attained occurred just at the time the landslide was triggered, indicating that the model is capable of predicting slope failure conditions.
Highlights
Large areas of the Apennines of Campania are characterised by slopes covered with few meters of loose pyroclastic deposits upon a fractured carbonate bedrock, in which karst aquifers are often located, drained by either perennial or temporary springs (Celico et al, 2006; Petrella et al, 2007, 2009)
The bottom boundary condition links water potential at the soil–bedrock interface with the fluctuations of the water table of the aquifer located in the fractured limestone, which is conceptually modelled as a linear reservoir
The equilibrium of such deposits is strongly affected by pore water pressure: in gentle slopes, with a slope angle smaller than the friction angle of the soil material (φ ) the increment of positive pore water pressure causes the reduction of the effective stress, which may lead to the failure of the slope; in steep slopes, with inclination comparable or higher than φ, the equilibrium is possible in unsaturated conditions, thanks to the cohesive action between particles exerted by soil suction, which, after soil wetting, may drop until the triggering of a landslide
Summary
Large areas of the Apennines of Campania (southern Italy) are characterised by slopes covered with few meters of loose pyroclastic deposits upon a fractured carbonate bedrock, in which karst aquifers are often located, drained by either perennial or temporary springs (Celico et al, 2006; Petrella et al, 2007, 2009) The equilibrium of such deposits is strongly affected by pore water pressure: in gentle slopes, with a slope angle smaller than the friction angle of the soil material (φ ) the increment of positive pore water pressure causes the reduction of the effective stress, which may lead to the failure of the slope; in steep slopes, with inclination comparable or higher than φ , the equilibrium is possible in unsaturated conditions, thanks to the cohesive action between particles exerted by soil suction, which, after soil wetting, may drop until the triggering of a landslide. The proposed model allows soil suction and water content to be reproduced, observed at various depths in the pyroclastic cover, during the rainy season (from autumn till early spring), as well as during the dryer and warmer season (from late spring till the end of summer)
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