Abstract:

The reason for this study is the deformations that occurred due to the collapse settlement of the loess base on Nikolaevska Street, Ruse. The reason for this was a cast-iron water pipe (Ø 250 mm) that ruptured on March 4th, 2009. The deformations affected 5 two-story residential buildings on the western side of the street and one on the opposite side – the Said Pasha Mosque.
The cracks formed on the terrain (asphalt, sidewalks on the street and courtyards) have openings ranging from 1 to 2-3 cm. They outline an ellipse with diameters of 25 and 40 m, with the long axis oriented along the street. The resulting over-wetting caused the loess massif to collapse under its own weight. In the center of the ellipse, at the accident area, the measured settlement for a period of 2 months were 32 mm, without taking into account the one that occurred on 04.03.2009. The total settlement around the accident area should be about 20 cm. As a result, the roadway has undergone visible settlement within the extent of the elliptical subsidence zone. In the lowest depressed part, a suffosion cavity developed and was filled with 8 m³ of concrete.
The buildings affected by deformations were built at the beginning of the last century. Their foundations are strip, made of stone masonry and are laid at a depth of 2.50 m below the ground surface. Two of the residential buildings are built with reinforced-concrete load-bearing structures. Deformations are observed in the stairwells. The other three buildings and the mosque have load-bearing brick walls and steel-wooden floor structures. Here the deformations are concentrated on the facade walls facing Nikolaevska Street. The cracks are 1-2 cm wide. Their orientation indicates settlement of the buildings toward the site of the pipeline failure.
In this part of the city, the terrain belongs to the so-called “three-loess terrace” T3. Here, the loess-soil complex is 20 m thick. Down to a depth of 16 m, the loess mass is macroporous with a fragile structure. It consists of yellowish silty-sandy to sandy loess horizons (L1, L2), separated by fossil soil Fs1 – dark brown loess-like clay. From 16 to 20 m, the loess mass is without macropores and has a denser structure. It consists of the loess-like clay Fs2 and the clay loess L3. At greater depth beneath L3 lies the alluvium of terrace T3 (gravel and sand), which overlies Aptian limestone.
The collapsible loess zone extends to a depth of 16 m. Within its range, the loess is collapsible, both under additional load from buildings and structures and under its own weight when wet. Two subzones are distinguished in it:
• The upper one is up to a depth of 8.0 m. It exhibits more pronounced collapsibility characteristics – relative collapse coefficient col.0,2=4.7% at p=0.2 MPa. At natural water content w=18.4-20.6% it has a compression modulus M0.2=6.2 MPa at p=0.2 MPa, deformation modulus E0=12.0 MPa and design load R0=0.15 MPa;
• The lower collapsible subzone is at a depth of 8-16 m. The collapsibility characteristics have lower values col.0,2=3.3%. With an established natural water content of w=16.4%, it has M0.2=8.0 MPa at p=0.2 MPa, E0=16.0 MPa and R0=0.18 MPa.
The total expected collapse and settlement due to wetting from geological load for the entire zone is about 30-40 cm. A similar collapse settlement was recorded in the garden opposite the library due to a failure of the water supply network.
During construction and operation, the buildings were in good condition. They were founded in the upper subzone of the collapsible loess with natural water content and the above-mentioned indicators. The water-main failure caused waterlogging to 27-32%. This waterlogging probably began several months before the failure. These values significantly exceed the critical value w=23% for the onset of settlement/collapse. Down to a depth of 5 m, the loess transitioned into a very soft consistency state, whereas in the lower part of the collapsible zone it became soft consistency state. As a result, the strength-deformation indicators decreased by about two times – E0=6-8 MPa, R0=0.06-0.09 MPa, which led to the collapse settlement of the terrain, together with the buildings.