The enclosing rocks of the risky domestic cellars are very diverse, which significantly determines the development of the dangerous state and therefore the prevention activities. Given that a general description of the various types of rock can be found in the literature, I present their properties in terms of their effect on underground cavities and cellars.
4.1. Major rock types that surround it
4.1.1. Pannonian sand and clay (Pécs, Érd-high shore, Eger) are typically low stability, water sensitive and swell. Total dehydration and repeated soaking can cause very serious damage to underground objects. Cavities created in the clay version are also exposed to the risk of so-called foot swelling, which must be taken into account when dimensioning the support structures and revising the cavities. The breakage of the cavities can be flowing or blocky. In these materials the cellars were often built with a supporting structure, an arch.
4.1.2. The air-contact surface of the cavity cut into sedimentary limestone ("lime" limestone and "sarmatian" limestone) is damaged, mossed, and the rock loses its strength. Its bending strength is low, so cracking (sometimes only a few tens of meters) of the ceiling formed during the mining is almost inevitable. Typically, large cracks and displacements occur in limestone, which at first cause slow movements. They break down in blocks. The problem is the large size of the cavities as well as the installation (and thus overload) of the surface above them. 110 km in Budafok and 32 km in Kőbánya can be found.
Properties of Sarmatian limestone:
weight: γ = 2,0 - 2.07 t / m3
Uniaxial compressive strength: σc = 3.6 - 8.0 MPa
Tensile strength: σt = 0.3 MPa
Shear strength: τ = 0.52 - 0.58 MPa
(source: KBFI-ALFA 1997, 2002)
Sarmatian limestone is mostly inhomogeneous: from lime slurry to hard limestone, it can be found in all varieties. As a result, the various layers are easily separated, leading to discrete layers. This phenomenon occurs primarily on the drawn side of the rock body (in the lower plane of the headstone, in the side walls at the point of protrusion).
4.1.3. The caverns of the freshwater limestone (Budapest I. district, Eger) are typically naturally formed, and later received various functions (protection, storage) as a result of human activity. The rock forming the hollow of the cavities is brittle and of low bending strength. It is sensitive to road traffic and vibration and transmits vibration waves far. The rupture usually takes the form of a coffin lid.
The clay, marl, which appears as an encrusting rock and is embedded between blocks, is water-sensitive and prone to swelling.
Characteristics of freshwater limestone:
weight: γ = 2.5-2.6 t / m3
Uniaxial compressive strength: σc = 6.8 - 9.0 MPa
Shear strength: τ = 0.52 - 0.58 MPa
(Source: FŐMTERV Rt.)
4.1.4. In the volcanic rock tuffs (rhyolite tuff, rhyodacite tuff) the cavities were formed primarily through the mining of building stone. The tuffs are sensitive to water, their strength in the air deteriorates greatly, and they are characterized by weathering. This phenomenon is found mainly in the surface-related entrance of the cellars. Like sedimentary rocks, tufts have low tensile strength. The strength properties of tufts vary widely, depending on their condition and location.
Properties of the rhyodacite tuff (when dry):
weight: γ = 1.33 - 1.45 t / m3
Uniaxial compressive strength: σc = 2.25 - 7.09 MPa
Tensile strength: σt = 0.52 - 0.97 MPa
Shear strength: τ = 0.35 - 0.50 MPa
(source: GEOSERVICE Gmk 1985)
Characteristics of basalt tufa (wet or air-dry):
weight:: γ = 1.9 - 2.1 t / m3
Uniaxial compressive strength: σc = 7.35 - 11.08 MPa
Tensile strength: σt = 0.25 - 1.57 MPa
(source: Réthelyi 1986)
It is characteristic of the rhyodacite tuff that the main constituents of the feldspar are converted to clay by moisture. It is important to know that it is not only groundwater or rainwater that can cause this, but also the vapors that accumulate in the air space of the cellars. As a result, their strength is significantly reduced, up to 20% of the original strength.
The rock structure of the tuffs allows the root of the vegetation to penetrate, and this factor also contributes to the degradation of the vegetation.
The cellars built in the solid rock environment described above are characterized by the fact that the supporting structures (masonry, arches, entrance barriers) are made of materials extracted from the cut. Therefore, the process and time course of their failure is almost identical to that of the parent rock - that is, it occurs with it.
Also typical of such solid cellar-driven cellars is that the headstone above them is usually of low thickness and has significant natural or artificial layers. The reason for the small thickness is that most of the cavities were made for material extraction, ie the extraction man was trying to extract as much material as possible from the site. A typical example of this, as well as the size of the natural and artificial fillings, is the cellar system located under the Old Hill Park in Budapest's X. district. The quarry, formerly used for quarrying and then clay mining, has been filled with communal rubbish and building debris since the 1960s. The 12, sometimes 18 meters thick, filling places a significant load on the headstone of the deep cellars, which are barely 0.8 to 1.2 meters thick.
The rock environment of the cellar passages with filling
Budapest, Kőbánya Cellar system in Óhegy Park
In such a thin rock body, which is loaded with headstone, bending forces cause vertical cracks and, on its lower plane, discrete deposits. This process can also be found in cellars cut into freshwater limestone and tuff.
4.1.5. More than one third of our country's territory is covered by loess soils. Our loess can be divided into three types. The so-called typical loess (which is typically found in Transdanubia, the Somogyi and Tolna hills, along the Danube, on the Balaton extension) is the most problematic soil type in terms of both cellar damage and construction. Loess bank breaks in large areas (eg Dunaföldvár, Dunaújváros, the eastern basin of Lake Balaton) are known, and each year the level requires one or two deaths due to improper construction of the loess.
The most damaging feature of a typical loess is its collapse. This process usually starts with water. The main reason for the collapse lies in the structure of the loess. It contains SiO2 as its main constituent, along with feldspar, carbonates and mica. Typical loess in our country consists of dust and sand carried and deposited by the wind in the Holocene (that is, very close to our time in geological history). Changes in the Ice Age, during the warmer periods, resulted in vegetation settling on the loess, with tubules left in place of dead bodies, making the loess macroporous. Water or a significant load causes these tubes to collapse and lime to dissolve. This causes the loess to suffer up to 55-60% volume change (collapse). In loess, collapse occurs on vertical and oblique free surfaces, which is why the entrance part of the cellars is endangered. In such cases, the cellars are dug deeper and deeper into the stomach of the mountain. Bertalan Andrásfalvy describes in his study of the Páty Cellar Mountain that a cellar carved in a loess is known to have entered its full length under a manhole, as it was originally.
Loess on the surface is characterized by blocky breaks, and cellars have coffin-shaped tears and detachments. The latter lead to a severe, often fatal disaster.
The destruction of the loess cavities is rapid. One day, there are only a few cracks that can break in a few hours or a day.
From the point of view of cellar hazard prevention, it is important to know that although some of the cellar passages cut in the old days have been destroyed today, the collapses never fill the underground space in their entirety and, therefore, sinking or cracking is not excluded.
The other two types of Hungarian loess, the good loess on the soil, and the properties of the infused loess prevalent in the Great Plain, differ from the above and do not pose the same risk as the typical loess. But the cellars established in them have the same damage as the clay cellars discussed earlier.
The entrance of the cellar carved into a loess with a broken wall
Kalazno, Tolna County
4.2. Occurrence of some major rock types (look in hungarian page)
4.3. Relationships between the surrounding rock environment of cellars and the appearance of cellar damage
The following factors should be considered when determining the appearance of cellar damage and how to treat it:
1. The maximum depth of underground cavities and cellars subject to cellar hazard prevention is approximately 25 m, therefore, in addition to rock examination in mining, the methods of engineering (civil engineering) soil mechanics and geotechnics should be applied. The reason for this is that most of the endangered cellars are made of materials (eg clay, loess, spill rocks) that deep-water mining does not encounter.
2. The shear strength of rocks surrounding cellars and cavities is significantly reduced by the cracks in them and the softer materials that fill them (typically clay).
3. Moisture content of the surrounding rocks is also a factor that reduces stability. Soaking moisture or pressurized water in the rock affects stability.
4. In Hungary, the so-called so-called Qualification for the classification of rock bodies is currently developed in engineering practice. The application of RMR is the generally accepted method. The essence of RMR is to measure, classify, and then aggregate the following six properties of boundary rocks:
- uniaxial rock compressive strength (σc)
- rock body breakdown index (RQD)
- distance of the rock body (Jn)
- state of articulation surfaces (Jr)
- water conditions, groundwater and groundwater (Jw)
- direction and location of partitions (Or)
Taking into account the above with different weights, it sets 5 classes of rocks and assigns them different types of support and freely extending cut lengths.
The method is primarily designed to drive new cuts, but can be used with some additions and modifications to test existing cellars, cavities without support structures, and to determine the work to be performed.
The author of this site has prepared a modified version of the RMR method for existing cellars.