Gewerkschaft Sachsen-Weimar — Schacht Unterbreizbach I Unterbreizbach · Thüringen · Germany

The enterprise that would become the Gewerkschaft Sachsen-Weimar was initiated in 1897 when the Großherzoglich Sächsisch Staatsministerium granted rights to explore for salt in the Unterbreizbach area of the Amt Vacha in the Großherzogtum Sachsen-Weimar-Eisenach. The Kalibohrgesellschaft Sachsen-Weimar was founded in Eisenach in the same year. It was the first potash enterprise in what would become the Werra potash district.

Exploratory borehole Sachsen-Weimar I, sited in the Unterbreizbach field, and borehole Sachsen-Weimar II, near the Räsaer Mühle, were drilled in 1897 and 1898 respectively. Sachsen-Weimar I located the upper Hessen seam at 705 metres depth and the Thüringen seam at 775 metres. In 1899 the drilling company was converted into the Gewerkschaft Sachsen-Weimar.

The decision to sink a shaft was taken on 9 December 1899, but could not immediately be executed because 205 Kuxe had been acquired at auction by the Gewerkschaft itself and the remaining 695 levy-paying Kuxe, together with 100 Kuxe exempt from contributions held by the Weimarische Regierung, were insufficient to finance construction. Operations were temporarily suspended on 11 May 1901. At the beginning of 1905 the Gewerkschaft succeeded in selling the 205 Kuxe for 105,234 Marks, realising a book gain of 72,034 Marks, and on 22 May 1905 the decision was formally taken to proceed with shaft construction.

Preliminary works began on 4 July 1905 and shaft-sinking commenced on 21 August 1905. Substantial water inflows were encountered up to 125 metres depth, reaching 4.5 cubic metres per minute in the zone between 59 and 67 metres. By 8 July 1906 the water inflows at 125 metres had been sealed and sinking then proceeded in dry ground with simultaneous masonry lining.

The shaft reached 584 metres by the end of 1907. To manage anticipated Plattendolomit water inflows between 529 and 550 metres, ten cementation boreholes were sunk around the shaft and two Thomson pumping devices were installed between October and November 1907. The base of the Plattendolomit was reached at 540.5 metres on 1 January 1908.

The first rock salt stratum was encountered at 602 metres on 9 February 1903. The first potash seam was reached at 709.5 metres on 28 May 1909. The seam, 3.5 metres thick, was characterised as a sylvinite layer (Hartsalz, Sylvinit) and displayed an average potash content of 18 to 19 percent, rising to 45 percent in the best sections.

The shaft reached its final depth of 728.5 metres. Fitting-out work in the shaft was completed on 21 November 1909. The operation opened provisionally on 28 June 1910, was temporarily suspended for Füllort installation, and regular production resumed on 7 February 1910 once the headframe and shaft buildings were complete.

Production began at an initial rate of 500 tonnes per day. By the time the surface installations were complete, the mine had a headframe, raw salt mill, winding engine building with winding machine, power station with five turbines totalling 3,000 kW, steam distribution, ventilator of up to 6,000 cubic metres per minute, boiler house with fourteen boilers of 1,700 square metres heating surface, workshops, magazine building, water supply system with piping, administrative building, pithead baths with washhouse, a guest house, and seventeen officials' houses accommodating 45 families. The rail connection, Grubenanschlussbahn, was opened on 22 June 1909.

Unterbreizbach received rail access to the wider network in 1906 via the Ulstertalbahn connecting Vacha with Geisa and later Tann (Rhön). On 4 August 1911 the Bezirksausschuß in Dermbach granted a concession for a chloropotassium and sulphate processing facility. An efficient combined chloropotassium and sulphate factory was established and operated at the site; the sulphate factory at Unterbreizbach was opened in 1923.

In 1921 the Wintershall AG acquired the works. The years from 1914 to 1928 were characterised by repeated production interruptions. From the early 1930s production was periodically curtailed owing to fluctuating demand during the global economic downturn.

From 1938 to 1945 the mine operated continuously and the workforce was expanded. Worker housing settlements at Räsa and Sommerliete were planned and constructed in the 1930s by the Hessische Heimstätte GmbH in conjunction with Wintershall. In 1942 the existing headframe at Schacht I was replaced by the steel structure that remains standing today.

During the Second World War a prisoner of war labour camp for British servicemen, numbered 137 and subordinated to Stalag IX C in Bad Sulza, was operated in Unterbreizbach; some prisoners were compelled to work in the mine. In April 1945 American troops advanced into Unterbreizbach. Following a brief engagement in the vicinity of Unterbreizbach, in which a waffen-SS unit fired on American vehicles, a retaliatory action on 3 April resulted in a large portion of houses, barns, and ancillary buildings being set alight.

The large finished-products warehouse and loading station at the works were burned down to their foundations, along with the drying station, raw salt mill, magazine, construction workshop, and seven company-owned houses. After the war the Wintershall AG was expropriated and the works transferred to Soviet ownership as part of the SAG Kali. In 1952, after the DDR blocked the through connection via Philippsthal on the Ulstertalbahn and the Deutsche Bundesbahn closed the Philippsthal crossing in response, Unterbreizbach was temporarily cut off from the rail network.

An emergency bypass railway over DDR territory via Räsa and Sünna to Vacha was constructed in 90 days, opening on 30 November 1952. In 1952 the mine was converted into a Volkseigener Betrieb (VEB) and in 1953 renamed VEB Kaliwerk Marx-Engels. In 1958 Unterbreizbach was attached as a component enterprise to the VEB Kalikombinat Werra.

In 1959 a new sulphate factory came into operation. In 1970 the Unterbreizbach works were grouped with the Merkers and Dorndorf/Springen mines in the VEB Kalibetrieb Werra, which was later incorporated into the VEB Kombinat Kali in Sondershausen. On 23 June 1975 a blast in the mine triggered a collapse in which 1,800 pillars at 800 to 900 metres depth gave way in a chain reaction.

The subsidence affected 3.3 square kilometres and produced a seismic event of magnitude 5.2, felt as far as Cologne and Frankfurt. Significant surface damage was caused in Sünna. A second major seismic event in 1989, centred in the Westfeld of the Merkers mine field, damaged the ventilation connection between the Unterbreizbach and Merkers underground workings and caused surface destruction in Völkershausen.

Following the political changes of 1989–1990 the Mitteldeutsche Kali AG (MdK) took over the Thüringen operations in 1990. At the end of 1993 the Kali merger reunified the east and west German potash industries in Kali und Salz GmbH, and Unterbreizbach became part of the new combined enterprise. From 1997 the mine was integrated into the Verbundwerk Werra of K+S, together with the Hessian works Wintershall and Hattorf.

Schacht I remains the principal hoisting shaft; raw potash salt is raised here and processed in the adjacent factory into 60er KCl (potassium chloride) and Kornkali fertiliser products. On 1 October 2013 a CO2 outburst following a production blast at 900 metres depth killed three miners. The mine continues in active production as of 2024 as part of the K+S Verbundwerk Werra.

The 1942 headframe at Schacht I is a prominent feature of the works landscape.