The pit lining is one of the most important safety precautions in mining. It absorbs mountain stresses and derives them using simple means. The resulting support of the rock allows pit spaces to be kept open and the miners in them to be protected from falling rocks or collapsing tunnels.
The limestone surrounding the alpine salt deposits is largely stable and therefore does not require any complex expansion work. Marl and slate zones occurring at the transition to the Haselgebirge, as well as many areas of the Haselgebirge itself, must be kept open with mine linings.
The first extensions were simply timbered, rectangular "door frames" made of wood. They usually consist of 2 vertical uprights ("stamps") and a horizontal crossbeam ("cap") lying on top. Among the door frame extensions, a distinction is made between the "German door frame" and the "Polish door frame".
In the case of the “German door frame”, the caps are leafed with the stamps . A distinction is made as to whether ridge pressure or side pressure prevails. Due to the different design of the leafing, the door frame is designed in such a way that it can better absorb either the ridge or side pressure. The disadvantage of the "German door frame" is the complex production of the leafing.
Figure 1: German door frame extension, 1910, from Heise Herbst "Mining Science"
Figure 2: German door frame extension, sheeting, internet
Figure 3: German door frame extension, Kefer, 1836, Archive Salinen Austria
Figure 4: German door frame extension with basic timber, Kefer, 1836, Archiv Salinen Austria
Figure 5: German door frame carpentry with wood lag, Thienfeldkehr, Leopoldstollen, 1998, Kranabitl archive
With the "Polish door frame", the stamp is provided with a round indentation (“scar-out”) at the top end, the cap is not processed and is placed directly in the panel. As a result, the Polish door frame can only absorb ridge pressure and no laterally acting forces. In order to give the stamps better stability against lateral pressure effects, a wooden pole, the so-called "fox", is driven in just under the cap. Due to this head brace, the Polish door frame can also absorb lateral forces to a small extent. The advantage of the Polish door frame is its simple construction, which can also be made by inexperienced carpenters.
Figure 6: Polish door frame extension, 1903, from Köhler "Bergbaukunde"
Wood as a finish has the advantage of being cheap, readily available, easily machined and immediately resilient. It also has a warning function that should not be ignored, since scaffolding wood begins to crunch under heavy loads. However, wood has the disadvantage that, compared to other materials, it is not very strong and is susceptible to fire and rot.
As early as 1796, Oberamtrat Kner ordered that wood be soaked in brine in order to limit the consumption of firewood at the Ischler Salzberg. For this purpose, a separate brine room was built next to the mouth of the Amalia tunnel. This could accommodate 1000 stamps, which gave way in the brine for three months.
If necessary, the door frame is clad on the side with wooden boards or sheet metal (“warping”) in order to secure the side walls (“elm”). The hollow space behind the warp is filled with heaps of timber in order to be able to create a non-positive connection with the rock.
If necessary, the door frames can be reinforced by installing additional "knee stamps" or "polygon stamps".
Figure 7: Polygon lining, main shaft, Leopold tunnel, 1998, Kranabitl archive
As early as 1807, Hofrat Gigant recommended replacing the door frame carpentry, which often had to be renewed, with elliptical dry lining made of quarry stone in stretches with very billowing rock and thus correspondingly high rock pressure.
For example, the main shaft of the Johannes tunnel was in such squeezing clay that the door frame had to be replaced every year. To keep this 230 m long stretch open, 2 scaffolders were constantly required.
Figure 8: German door frame extension with quarry stone masonry, Steiner, 1820, Archiv Salinen Austria
Figure 9: Extension of the stile, description of the manipulation, 1807 - 1815, Archiv Salinen Austria
Figure 10: Tower extension, Dürrnberg salt mine, around 1950, Archiv Salinen Austria
A special form of wood construction was the so-called "Stöckelausbau". In the process, a completely closed, elliptical lining was built from precisely cut pieces of wood in particularly squeezing sections. Despite the longer service life, this expansion, which was expensive to produce, was not able to gain acceptance.
Figure 11: Timbering in stacks, Altaussee salt mine, around 1930, ÖNB archive
It took a few more years before the Ischl mountain championship could be convinced of the advantages of brick lining. The first pit walls made of quarry stone and white lime mortar were carried out in 1840 in the Ritschner conversion in the Neuberg tunnel and on the main shaft of the Ludovica tunnel.
Figure 12: Dry Quarry stone masonry, Hallstatt salt mine, Wallmann, 1846, GBA archive
Figure 13: Dry quarry stone masonry, Altaussee salt mine, around 1930, Archiv Salinen Austria
In 1845, a deposit of marl layers that could be combusted into hydraulic lime (“Roßfeld marl”) was discovered on the Ischler Salzberg. Since cement was still unknown at that time, hydraulic lime was used as an admixture to white lime mortar for lining squeezing, damp sections. The salt mine built a small quarry, crushing and stamping works, as well as a furnace for the production of this hydraulic lime, which is now referred to as "Roman cement". With a cubic cord of wood (6.82m³) 30 hundredweight of lime (1,680 kg) could be burned, the hundredweight packed cost 40 Kreuzer. In 1847, the saltworks sold 120 hundredweight (6,720 kg) of burnt hydraulic lime to customers every week.
Figure 14: Cement works, Josefstollen, site plan, around 1850, Archiv Salinen Austria
Large sections could now be lined with Roman cement mortar. Many of these old tunnel linings, such as in the Ludovica tunnel - main shaft, still serve their purpose today.
Figure 15: Quarry stone lining with Roman cement mortar, main shaft, Matthias tunnel, 2011, Kranabitl archive
Figure 16: Quarry stone lining with Roman cement mortar, main shaft, Ludovica tunnel, 1998, Kranabitl archive
Illustration 17 : Quarry stone lining, main shaft, Empress Maria Theresia tunnel, around 1930, Archive Salinen Austria
In addition to brick lining with quarry stones, prefabricated concrete segments were also used from the 1930s for lining wet stretches that were in danger of breaking.
Figure 18: Lining with concrete segments, main shaft, Empress Maria Theresia tunnel, 1998, Kranabitl archive
Today, steel door frames are installed, mainly on wet tracks. Different profile steels can be used in the creation. The easiest way to create a steel door frame is to use used railroad tracks. Due to the profile of the railroad tracks, however, it is difficult to connect the stamp and the cap. One way of connecting is to use angle brackets. For this purpose, the rail head and the rail foot must be processed accordingly.
Steel as a finishing material has the advantage that it is very strong, fireproof, reusable, durable and space-saving. A disadvantage, however, is its susceptibility to corrosion, especially on stretches of the Hasel Mountains.
Figure 19: Iron ring extension with wood lining, Kaiser Franz Josef Erbstollen, around 1925, Archiv Salinen Austria
Figure 20: Iron ring extension with wood lining, Kaiser Franz Josef Erbstollen, around 1930, Archiv Salinen Austria
Figure 21: Steel ring expansion, Hallstatt salt mine, around 1950, archive Salinen Austria
At the beginning of the 20th century, steel ring support was developed. The ring expansion has a circular or elliptical shape. Normally, it consists of 4 arc segments that are lashed together so that they overlap. A distinction is made between rigid ring support and flexible ring support. The flexibility of the expansion is achieved by sliding clamp straps. Due to the design, the lining segments can be pushed into one another in the flexible lining, which means that the lining can absorb the rock pressure within certain limits and is not destroyed. Flexible ring support is used in particularly squeezing sections, such as the Riethalerkehr in the Maria Theresia tunnel.
Figure 22: Riethalerkehr, Kaiser Maria Theresia Stollen, 1998, Kranabitl archive
From 1980, shotcrete was used in the salt mines to secure the pits. When using shotcrete, the surface of the rock is bound with concrete and thus protected from temperature, moisture or vibration. Before placing shotcrete, steel mats are attached to the side rails and ridges. They are used to absorb tensile forces and, in combination with the shotcrete, form sufficient final strength. At the Ischler Salzberg, the brittle marl layers of the Maria Theresia tunnel were completely secured with shotcrete.
Individual, slab-like spalling of the rock ("coffin lid") can be secured with rock bolts. Rock anchors are inserted into the mountain at Ulm or at Firste. They each consist of a long rod or a rope, which is anchored in the rock at the mountain-side end with an anchor base and holds the rock at the protruding end with a plate ("anchor head"). In this way, individual layers of rock can be "nailed together".
Figure 23: Anchoring with grid, Hallstatt salt mine, 2020, Kranabitl archive
In general, only 40% of the stretches in the salt mines are finished (timbering or walling). The remaining mine workings are in the stable mountains (limestone) without lining.
As a result of the absorption of moisture, the clayey Haselgebirge becomes bloated; the same also occurs through the pressure of the mountains. In order to protect the room, the same must be regularly "depressurized". In the process, the stamps and caps of the door frame are removed (“stolen”) field by field, the routes are torn down and new timbering is erected. The mine workings in the Haselgebirge grow together by a few centimeters every year because of the enormous rock pressure of the limestone floes lying on the salt deposits, such as the diaphragm wall. Therefore, in order to restore the normal cross-section for driving, they have to be regularly torn down.
The normal routes have a cross-section of around 2 m² and an incline of 1-2%.
The maintenance construction accounts for around 30 to 40% of the total work on the salt mountains.
Figure 24: Road outline in the Hasel Mountains, Hallstatt salt mine, around 1910, from Brandstätter “Salzkammergut”
Figure 25: Road map in the Haselgebirge, Kaiser Franz Josef Erbstollen, around 1925, Archiv Salinen Austria
Carl Schraml "The Upper Austrian salt works from the beginning of the 16th to the middle of the 18th century", Vienna 1932
Carl Schraml "The Upper Austrian Salt Works from 1750 to the time after the French Wars", Vienna 1934
Carl Schraml "The Upper Austrian Salt Works from 1818 to the end of the Salt Office in 1850", Vienna 1936
"Ischler Salzbergbaubetrieb", manuscript, typescript, ca. 1950 -1955
L. Janiss "Technical help book for the Austrian salt mining company", Vienna 1934