Risk mitigation in deep excavations affected by Lava-baked Paleosols

Summary

Abstract

Deep excavations are traditionally designed based on ground conditions, geomorphological and hydrogeological framework, geometry and construction techniques. In this paper, the authors present a risk mitigation strategy to improve safety during construction of a typical deep excavation through retaining system in a geologically unconventional context. The project is located in the proximity of Catania, in the volcanic geological district of M. Etna and, specifically, in the synthem unit of “Supersintema Stratovulcano”. The local stratigraphy is characterized by a shallow basaltic layer 12-16m thick, mainly consisting of lava flows and pyroclastic fall deposits, overlying a variable (1.5-4.0m) layer of Paleosols altered by lava flows and a sub-stratum of Quaternary marine clay deposit. The unconventional order of this lithostratigraphy lies in the significative decrease of stiffness between the upper layer, characterized by competent geomechanical properties, and the underlying layers, which present average to weak geotechnical properties. The retaining structure consists of a multi-anchored micropile wall. The project excavation height is 21.0m, intersecting all three geotechnical units. The retaining system stiffness and deformative response is sensitive to the variability of Paleosols thickness and depth. The response of the retaining wall to the several excavations stages is modelled through a numerical 2D analysis. The study investigates how the thickness of Paleosols altered by lava flows influences the behavior of retaining walls. Multiple scenarios are analysed, accounting for the variability of the thickness of Lava-Baked Paleosols due to geomorphological conditions, eruption sequence and volcanic history. The results support the definition of a predictive model which consents identifying attention and alarm thresholds for specific excavation stages. The integration of sensitivity analyses into the design of the monitoring system enables the early identification of potential risks, associated with shallower layers of Lava-Baked Paleosols, allowing for mitigative measures to prevent critical conditions.