Distribution, hydrogeological features and landslide hazard of pyroclastic soils on carbonate

Abstract

The debris slides - debris flows represent both one of the most significant denudational processes in the pyroclastic soil-mantled carbonate massifs that surround the Campanian Plain (Italy), and the main risk source for the towns located at the foot of their slopes, as is well known. The ash-fall deposits, mainly derived from the explosive activity of Mount Somma-Vesuvius, were varyingly distributed among the carbonate mountain ranges that surround the Plain, according to the dispersion axes of each eruption, reaching total thickness values ranging from 4 to 7 metres onto the Sarno Mountains and values ranging around 2 metres onto the Lattari Mountains. After deposition on slopes, the pyroclastic deposits underwent denudational processes, mostly by means of mass movements, generally regulated by slope angle and by prolonged and/or heavy rainfall occurrence, which accounts for the volcaniclastic series along the slopes often being incomplete. In this work, a field survey based on test pits and laboratory tests has been carried out on two sample areas in the Sarno and Lattari Mountains, both representatives of debris slides source areas. One of the principal results put in evidence is a relationship between slope angle ranges and the stratigraphic schemes of ash-fall, which only shows more complete stratigraphic records in slope angles range generally up to about 30°, and incomplete stratigraphic records for the higher slope angle range. The complex stratigraphy consisting of ash-fall deposits alternated with the pedogenetic products developed during the intervals between consecutive eruptions (TERRIBILE et alii, 2000), results in a multilayered and very contrasting permeability surficial hydrogeological system, as it resulted from grain size analyses and estimation of hydraulic conductivity by means of empirical formulas. This physical feature, along with the varying distribution of pyroclastic series along the slopes of the source areas, caused by the diverse morphologic conditions, leads to an engineering geological model in which high hydraulic transmissivity horizons consisting of pumiceous lapilli, confined by less permeable deposits, undergo a reduction in thickness as slope angle increases, until they assume a lenticular shape with closure approximately above 35° of the slope angle value. This conceptual model seems to be validated by the more frequent slope angle value of debris slide depletion areas and by the more frequent value of the friction angle, as known in literature from these landslides. The model represents an improvement on the previously hypothesized slope stability models, giving a different comprehension of landslide susceptibility assessment due to occasional saturated throughflow within the pyroclastic cover, whose instability effects seem to be amplified in those areas where flow convergence and thickness reduction of lapilli horizons, as well as natural or artificial interruption of the soil mantle, coexist.