Rock mechanics analyses and tests have been carried out to assess the feasibility of underground long room and pillar mining of potash in Sergipe, Brazil. Special attention was given to the influence of a thick layer of tachyhydrite (CaMg2C16 l2H20) immediately below the potash. Tachyhydrite is a low strength, very soluble evaporite, not previously reported as occurring in potash mining. Calculations suggest that:
closure rates in rooms will be increased due to the tachyhydrite, but it should still be possible to mine the overlying potash.
although potash and other evaporites do exhibit strain-hardening, room closure at approximately constant velocity is predicted when creep of the roof and floor are considered together with creep of the pillar in the analysis.
Des analyses et tests de mecanique des roches ont ete conduits afin d'evaluer la possibilite d'une exploitation par longues chambres et piliers de la mine de potasse de SERGIPE, BRESIL. L'etude de 1 'influence d'une epaisse couche de tachyhydrite (CaMg2C16 • 12H20) qui sousincombe directement la potasse requiera une attention speciale. La tachyhydrite est une evaporite tendre et très soluble dont la presence n'a jamais ete reportee dans une mine de potasse. Les etudes suggèrent que la presence de la tachyhydrite aura pour effet d'augmenter la convergence dans les chambres mais il serait cependant possible d'exploiter la couche de potasse surincombante. Bien que la potasse et d'autres evaporites soient caracterisees par un durcissement, on peut prevoir une convergence des chambres à vitesse a peu près constante lorsque le fluage du toit et du mur ainsi que celui des piliers est pris en consideration dans 1 'analyse.
Analysen und Experimente auf dem Gebiete von Gebirgsmechanik sind gemacht worden um die Moeglichkeit zu untersuchen von Raum-und Pfeiler-Untertagbau von Kali in Sergipe, Brasilien. Dem Einfluss einer dicken Schicht von Tachyhydrit, welche sich direkt unter dem Kali befindet ist besondere Aufmerksamkeit gewidmet worden. Tachyhydrit ist ein schwacher aufloesbarer Evaporit, von welchem das Vorkommen bei Untertagbau von Kali bisher noch nicht gemeldet worden ist. Berechnungen geben den Enidruck dasz die Verformungsgeschwindigkeit der Raeume vergroessert wird durch das Vorkommen des Tachyhydrit, aber es muesste trotzdem moglich sein um die ueberliegende Kalischicht abzubauen. Obwohl Kali und andere Evaporite Verformungserhaertung zeigen, werden verhaeltnismaessig constanten Verformungsgeschwindigkeiten vorausgesagt, wenn das Kriechen von Dach und Liegendes zusammen mit dem Kriechen des Pfeilers ln der Analyse betrachtet wird.
Potash was discovered in the State of Sergipe, Northeastern Brazil, during drilling for oil and occurs over the same general area as the Carmopolis-Riachuelo oil fields. With all potash now imported and a growing demand for fertilizers there is considerable incentive for Brazil to develop domestic ore bodies such as those in Sergipe.
The Sergipe cretaceous basin is limited to the northwest by faults and has a continental extent of approximately 40 km. It is composed of small sub basins which are, in fact, grabens resulting from continental plate extension, that are filled with thick evaporites (soluble salts) of average thickness approximately 450 m, with depths to the top of the evaporites varying from less than 400 m to more than 1300 m. The main sub-basins are those of Taquari-Vassouras, Santa Rosa de Lima and Aguilhadas which cover an area of almost 900 km2, of which 380 km2 consists of evaporites. The largest deposit is in the Taquari-Vassouras basin comprised of the Muribeca formation and Ibura member, where the thick evaporites contain halite, sylvinite, carnallite, and tachyhydrite intercalated with dark shales dipping at about 5°. The "centers" of Taquari-Vassouras and Santa Rosa de Lima are approximately 18 km apart, and approach with 6–8 km of each other. The evaporitic section consists of 10 cycles overlaying the oil fields and has suffered intense structural deformation. Eight cycles are of soluble salts and two contain anhydrite, limestone, and shale. The overlying sediments are principally limestones and shales that are essentially free of water. In the Santa Rosa de Lima area the sylvinite occupies about 28 km2 and has an average thickness of 3 m, tachyhydrite about 25 km2 and average thickness 20 m, and carnallite 25 km2 and thickness averaging 15 m. The overlying sediments includes limestones and shales, but also the major Ria Angica sandstone aquifer which is of substantial thickness. The lithology of the evaporites varied appreciably both in purity and grain size within and through the cycles. Cores also revealed substantial small scale folding within the various layers. From the point of view of characterizing the rheological behaviour of the evaporites, nine basic lithologies were defined, as follows:
coarse-grained sylvinite
fine-grained sylvinite
fine-grained carnallite with small amount of halite
fine-grained carnallite with large amount of halite
coarse-grained carnallite with small amount of water
coarse-grained carnallite with large amount of water
pure tachyhydrite
impure tachyhydrite
halite
