6. Cellar collapse, cellar damage on the surface

6.1. The appearance of the bending zone, the subsidence of the soil surface

  6.1.1. Facilities in the bending zone

  6.1.2. The appearance of subsidence of the soil surface due to the curvature of the bending zone

  6.1.3. damage to engineering facilities and roads due to subsidence of the soil surface

  6.1.4. Movement of tall structures in the bending zone

  6.1.5. Damage to buildings due to soil subsidence

6.2. Penetration of the collapse zone to the surface (rupture)

6.3. Collapse due to top-down forces (rupture)

Over the past 35 years, more than 370 Hungarian settlements have been struggling with the damage caused by cellars and caverns beneath their territory.

Abandoned and poorly maintained cellars, artificial caverns and caves are constantly being damaged. These processes date back to the 18th-19th centuries. They also appeared on the surface in the 20th century, but the damage was more frequent and more severe in the 20th century. In the second half of the 20th century, urbanization accelerated. The technical-technical reason for this is primarily the expansion of towns and settlements, the higher built-in ratio and, last but not least, the multiplication of road traffic.

The damage to artificial cavities and cellars was discussed in the previous chapter. When the support structures of the cavity are damaged to such an extent that it is no longer able to be compensated for by the rock or soil above it, the damage also appears on the surface. This can be a slow process, but most of the time it is rapid, sudden, and not infrequently a disaster for human life or material goods.

These damages have long been known to deep-water mining professionals. Their scientific knowledge and analysis has a large body of literature, as it is still a problem for the inhabitants of the areas and settlements above the former mines.

It is important to know that Hungary was at the forefront in this field as well: engineers working in the mining areas of Selmecbánya and then of Tatabánya-László were already in the 19th century. At the end of the 20th century, At the beginning of the 20th century, they dealt with this problem and devised methods which were used throughout the continent.

Significant work was also done by Hungarian engineers to investigate and repair surface damage related to tunnel construction. In the most densely built-up settlement in the country, countless buildings sank during the construction of metro lines 2 and 3 in Budapest, so this had to be dealt with. The appearance of damage caused by cellars and artificial cavities is similar to the latter activity, but the damage is eliminated by the procedures applied in mining.


6.1. The appearance of the bending zone, the subsidence of the soil surface

Damage to subterranean cavities is most often manifested by the subsidence of the soil surface. The reason for this is that the components of the soil move downwards due to gravity. This depression is greatest above the axis or center of the cavity, and decreases with distance. This gives rise to a form of turtle, commonly used in the technical language: sinking hoop.



Sinking hump over basement corridor


Sinking hump over cellar space


Over the last 100-150 years, many theories and calculation methods have been developed to investigate the subsidence of soil above artificial cavities. Their detailed description can be found in the literature.

The essence of the theories is to describe the shape of a sink (a so-called sinking dip) as a body of rotation, the dimensions of which can then be calculated mathematically.

The rate of descent is influenced by many factors. The most important of these are:

- underground depth of the cavity

- the extent of the cavity

- the boundary surfaces of the cavity, or the condition of the built-in structures

- rock quality and soil quality (fracture or boundary angle, consistency, boundary stresses, stratification, etc.) above and within the cavity,

- hydrological conditions of the cavity and its surroundings

- static and dynamic loads on the cavity and the surrounding rock environment


By default, theories only consider the extent, depth and boundary of the rock (soil) above the cavity. Some of these are:


Theories for calculating subsidence


In the case of cellars, artificial cavities, these calculation methods are sufficient to estimate what and how much subsidence is expected over a given object. Of course, when it comes to dimensioning a support structure, more serious calculations are needed, but that is a special branch of engineering science

It is important to note that the sinkhole does not only have a latitude (as shown in the diagrams of the calculations). The dent follows the shape of the underground space, the cut, and its impact zone is not only perpendicular to the axis of the cut, but also at the end of the cellar.

This effect is a problem for cellars that, although not under a building, run close to it. Such cases occur in the settlements where the individual cellars are made on steep terrain (such as a hillside) and they reach the mountain.

In the case of multiple adjacent or intersecting subterranean spaces, the subsidence dents add up, which results in a larger-than-normal subsidence in a given rock environment.

The following movement zones are formed in the soil above the cavities:

A - bending zone

B - fragmented zone    

C - collapse zone


In the bending zone, the curvature of the surface can cause damage and the ground does not always collapse - but the buildings and engineering facilities above it can cause serious damage.

If the fractured zone comes close to the surface, the magnitude and timing of any breakdown is unpredictable. The behavior of the collapse zone is clear: the cavity of the cavity cannot hold the mass above, collapses. It is important to know that the failure of zones affects each other. As the collapse zone grows, the fractured zone also gets higher, with unforeseeable consequences. But the opposite is also possible: once the fractured zone reaches the foundations of the surface structures, the load will damage the collapse zone.


6.1.1. Facilities in the bending zone

The shifting of the bending zone affects both the buildings above and the utilities there. Examining their damage is one of the most important subtasks of the emergency response activity.


It is easy to see that pipelines that fall into the bending zone are more susceptible to damage. In the event that they cannot absorb soil movement (such as old cast iron pipes, asbestos cement pipes), their breaking is almost inevitable. The joints of such utilities are also endangered: old cast iron casings, various joints of eternit pipes are brittle and prone to fracture due to their age. Building foundations and their associated fixtures (such as utilities) may also be damaged. These may be damaged or broken when the base of the building is moved or tilted.


6.1.2. The appearance of subsidence on the surface due to curvature of the bending zone

The appearance of the spatial descent must be examined at the five (+ 1) points of the sinking field.

 Elements of the sinking dip

1st dent roof

2. upper curvature field between the saddle and the slope

3. dip slope with inflection point

4. lower curvature field between the slope and the turtle

5. turtle


 The surface of the soil on the roof is unchanged, however, it is to be expected that the dents will spread, resulting in the outward displacement of the slope and the curvature fields and the expansion of the turtle.

Disaster due to unexpected appearance and widening of sinking

(Nachterstedt , Germany 2009)


In the upper curvature field, the surface of the soil undergoes rock pulling at the top and compression at the bottom. Upward opening V-shaped cracks appear. These lead to damage to the structures and buildings placed on it and to the entry of rainwater. This increases the damage.

Buildings are typically most sensitive to this type of soil surface subsidence. Typical upwardly expanding cracks appear on the walls of buildings that run in the direction of the slope.


Building in semi-saddle position

It is more difficult to trace, but the same reason is due to the sinking of the corners. This occurs when most of the building stands on unmoving ground and the sinkhole is formed only on the small width of the facade. This damage is more difficult to detect because several other damage processes to the building (such as underwater washing, roof overloading, building next to it) result in similar corner breaks.

On the slope of the ditch, the soil-forming rocks and blocks slip. These slippages can be up to several decimetres. The structures on the slope of the dip will tilt, tilt towards the turtle, or even slip.

 In the lower curvature field, the soil or rock suffers from compression at the top and pulling at the bottom. This leads to an inverted "V" opening of the rock above the soil and the cellar, further breaking the cavity ceiling. This is because the frictional force for stability between the rock blocks here is significantly reduced or eliminated.


Building in the lower curvature field

Buildings in the lower curvature field and buildings show characteristic downward cracks, which are mainly visible on the wall next to the doors and windows.

Special mention should be made of the fact that the roof, which is opposite to the sinking slope, is also sinking, which results in the so-called saddle position.

In the saddle position, the soil surface suffers from pulling at the top and compression at the bottom. It also creates upward V-shaped cracks and splits, with the consequences described earlier.


Building in saddle position

It can be seen from the above - and it has long been known to those skilled in the art - that structures and buildings on sunken soil surface suffer severe damage if their sinking is uneven, but on one side of the block containing it, and not on the other.


6.1.3. damage to civil engineering structures and roads due to subsidence

Soil deflection and sinking can result in sinking up to several decimeters. This may pose a serious problem for sheet-shaped structures on the surface. Examples are road surfaces, traffic lanes.

A further major problem is that the pavement of the roads and the concrete foundation beneath them are not, or only to a limited extent, capable of absorbing the stresses caused by bending. Larger dents cause the base to break, causing the casing to fail.

Asphalt is flexible to a certain extent, but if the foundation beneath it is damaged, it will be damaged by increased speed as a result of traffic. The surface of asphalt, which is fractured by parallel crevices, is a common sight on the roads of communities threatened by cellar danger. This indicates that a sinking trough has formed beneath the roadway, along which the pavement slipped and its surface continuity was interrupted. The cracks always indicate the direction of the cavity below, parallel to its axis.

The resulting cracks cause traffic problems, but it is even more problematic that the rainwater flows through them under the cover, soaking it (or freezing it in cold weather conditions).


Road pavement failure due to sinking above the basement


6.1.4. Movement of tall structures in the bending zone

Sinking dents are the biggest problem in tall buildings (chimneys, power-tower columns, high-rise buildings). These tend to fall - which causes the appearance of off-center forces, and it is a serious problem that the wires hang on one side or are tensioned on the opposite side, which can lead to the wire breaking.


Power line column tilt


6.1.5. Damage to buildings due to soil surface subsidence

Soil subsidence causes the most damage to buildings. In the case of buildings, the damage described above both reduces the value in use and endangers the safety of use.

The forms of the appearance of damage have been presented above in the case of the individual motion fields of the sinking dip. In addition to the cracks described there, damaged buildings may include the following:

- movement of doors and windows becomes difficult or impossible

- connections to utility lines are broken

- deformation of the space causes the walls to break apart, causing the support of the slabs to be damaged or displaced,

- the walls under the arches open, which can even lead to the arch breaking,

- in the case of slabs with lining bodies, the dents formed by the cellar running parallel to the slab beams cause the beams to rotate, causing the lining bodies to slide out

- the beams of traditional "I" iron beams are opened in the upper bending area or in the saddle position, the beams of the vault fall out,

- the walls of buildings tilt as a result of the sinking.

Tilting the wall will cause some damage: cracks will appear on the wall, and the tilting wall will not only be subjected to pressure but also to bending, which can cause it to fail quickly.


Damage to building structures due to sinking


6.2. Penetration of the collapse zone to the surface (rupture)

A rupture is called when the ceiling of a cavity falls into a cavity and the rock or soil above it and the artificial structures are no longer able to hold their own weight and therefore fall into the cavity. The rupture process can only be followed from the bottom, so it takes a lot of energy to defend against it. That is why this is the most dangerous process in the vicinity of cellars, artificial cavities. It was the cause of many major disasters. Damage to buildings or roads is difficult to prevent and repair.


6.3. Collapse by top-down forces

In Hungary, it is very common that a cavity protrudes below the surface, but it is covered by an existing spatial structure (rock, possibly road, building). This is most often the case with unknown, closed, masonry cellars.


Demolition of a building

(Eger 1970)


In this case, a disaster occurs when, for various surface effects (road traffic, dynamic loads), the bridging loses its hold and collapses.

In our historic towns and districts the accessories of the old cellars are the chimneys that lead from the cellars to the surface. Where cellars were used for storage or possibly for housing, the chimneys provided ventilation. We see this kind of thing in every cellar that has a problem.



Ventilation chimney of brewery cellar

(Budapest – 10. disrict, Kőbánya)


Cross-section of the cave with a surface well in the Buda Castle


The problem is that most of the chimneys were just obscured by our ancestors, or maybe built on them. In many cases in the Buda Castle such horns were broken. There was something after 50-60 years. This was also the reason for the bus accident at Vienna Gate Square, but in the Trinity Square (opposite the Matthias Church), for example, there was a short period of three sinking that could be attributed to this cause.


Sinking of the floor covering due to a horn leading to the surface

(Buda Castle, Holy Trinity Square 2002)


Damage to horns can be prevented by examining them from below, which requires serious knowledge. Cellar safety specialists also need to pay particular attention to this because the rock near the falling chimney is often damaged, leading to an increase in surface appearance.

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