Physico-mechanical and mineralogical investigations of red bed slopes (Cluj-Napoca, Romania)

The southern slope of the Cetățuia-Hoia Hill, located in the urban setting of Cluj-Napoca (Romania), consists of clayey deposits interbedded with sands and sandstones. The slope deposits belong to the Moigrad Formation and are representative of “red bed” continental deposits found across the world. Although documented as a landslide-prone area, the hillslope receives increased attention due to urban development and real estate pressure. The slope was affected by an old, deep-seated landslide; mudflows; and anthropogenic interventions. The stability of the old landslide is close to equilibrium (1.35) and its reactivation may be triggered by rainwater infiltration. The Moigrad Formation deposits are mostly active, expansive, and display medium to high plasticity. Their shearing resistance is described by effective friction angles of 6.7–33.7° and effective cohesive strengths of 7–122 kPa. With regard to mineralogy, the deposits are made up of quartz, feldspar, calcite, dolomite, and phyllosilicates (micas, chlorite, kaolinite, and minerals belonging to the smectite group), while their clay-sized fraction (< 2 µm) is made up of illite–smectite interstratifications, illite, chlorite, kaolinite, and quartz. The physical and mechanical properties of the deposits are closely related to the type and amount of clay-sized minerals, especially to the illite–smectite interstratifications and illite, which are the main components in the clay-sized fraction. Interactions between these components and water infiltrated within the hillslope are proposed as driving mechanisms for landslide reactivation and as active phenomena undermining the stability of the area.