Abstract:

This paper presents an engineering geological and geotechnical investigation of the ground foundation and structural deformations of the 12-story residential building Block “Russia” in Ruse, Bulgaria. Built in the period 1967–1968 based on a standardized design template, the building is situated on the fifth loess terrace of the Danube River, under highly complex engineering geological conditions. These conditions are characterized by the presence of Type II collapsible loess with a collapsible zone thickness of up to 14.5 m and intense anthropogenic wetting, further compounded by high regional seismicity (Intensity VIII on the MSK-64 scale and a magnitude of M=7.4). The foundation was executed without any anti-collapse ground improvement, using isolated pad footings at a depth of −6.10 m. This resulted in a contact pressure of p=0.3 MPa, which is double the allowable design bearing capacity for this loess foundation (R0=0.15 MPa).
The primary causes of the severe structural deformations are defined as a combination of structural design deficiencies (incorrect foundation type and excessive loading) and complex engineering geological conditions. The dynamics of the moisture regime within the loess massif were thoroughly traced. Following a major water utility network failure in 1992, localized wetting was recorded beneath the eastern section of the building down to a depth of 17 m (with water content reaching wn=27–28 %). This led to a drastic reduction in the deformation modulus beneath this part of the structure, dropping to 5–6 MPa. Investigation data from 2011 and 2019 indicate a steady trend toward gradual drainage and normalization of the collapsible layer’s properties, returning to average values of E0=13.4 MPa.
Long-term precision geodetic monitoring (1967–2019) registered highly non-uniform settlement of the building, which was further accelerated by strong regional seismic events (the earthquakes of 1977, 1986, and 1991). By 2019, the cumulative settlement at the western facade amounted to 305 mm, whereas at the eastern facade, it reached 802 mm. This differential settlement of nearly 500 mm caused a severe tilting of the building block, resulting in a total vertical axis deviation of 697 mm toward the southeast. Analysis of the time-settlement curve reveals that after 2004, the deformation process entered a sub-horizontal phase of slow attenuation.
Using the layer-by-layer summation method, a theoretical model was calculated for the maximum vertical deformation (combined settlement and collapse) under a hypothetical full water saturation of the active zone (22 m), amounting to 496 mm beneath the central section and 441 mm beneath the eastern section of the building. Based on this geotechnical diagnosis, radical structural recommendations have been proposed. These include an urgent overhaul of the water supply and sewerage infrastructure, the continuation of annual monitoring, and the execution of a deep ground improvement project for the loess foundation via area-wide grouting using cement-ash mixtures (root-like micropiles down to a depth of 15 m).