Mineralogical Drivers of Ground Failure in Neogene Sediments: a Case Study from Northwest Bulgaria

The stability of critical infrastructure in Northwest Bulgaria (Western Moesian Platform) could be compromised by ground instability within Neogene sediments that cover the region. This is evidenced by the collapse of the I-1 national road near Dimovo town in 2006, which involved vertical displacements of 3–4 meters. The purpose of this study is to identify the underlying geological drivers of this failure and to evaluate the specific hazard in the area resulting from the interaction between the sediments and the local environmental conditions. We hypothesize that the instability is not merely a result of conventional failure mechanisms but is governed by an anomalous mineralogical composition, specifically by the presence of aragonite and gypsum layers, which could create a dual hazard.

To elucidate geological drivers, we employed a methodology that integrates field mapping and sampling with laboratory analyses. Samples from the Neogene sediments in the area of 2006 damage underwent mineralogical analyses using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) to determine phase composition and morphology. These analyses were coupled with X-ray fluorescence (XRF) for chemical profiling and standard geotechnical testing to determine grain size distribution, Atterberg limits, and Activity index according to Skempton’s classification.

The analysis reveals a heterogeneous sediment succession with the presence of inorganic clays with high plasticity. The XRD and SEM results identified a mineralogical anomaly where the high concentrations of metastable, acicular aragonite coexist with active swelling phyllosilicates (smectite/illite). Furthermore, various amounts of gypsum were detected in some of the samples, indicating an evaporitic paleoenvironment. Geotechnically, these materials exhibit extreme reactivity. Liquid limits range from 34.85% to 67.88%, and plasticity indices reach up to 47.39%. The Activity index peaks at 2.00, categorizing the sediments as "highly active" and prone to volume change driven by moisture variations.

The study concludes that ground failure is a direct consequence of a synergistic hydro-chemo-mechanical mechanism driven by the sediments' mineralogy. The specific aragonite fabric allows rapid water infiltration, triggering the hydration of smectites that could lead to loss of shear strength. Simultaneously, gypsum dissolution could create secondary porosity, reduce effective stress, and release sulfate ions, which could pose a potential chemical hazard to concrete foundations through sulfate attack. Furthermore, the high silt content facilitates internal erosion and possible piping through fracture networks, which could explain the sudden loss of support and large vertical displacements observed in the 2006 case. These findings imply that standard geotechnical data alone are insufficient for risk assessment in this region. Effective mitigation strategies must integrate mineralogical analysis to address both the physical swelling and the chemical durability risks.

Acknowledgements: This research was funded by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, Project No BG-RRP-2.004-0008-C01.