In-situ and analytical investigations of rock mass with low strength properties have been carried out. It was shown that the model of a medium with post-peak characteristics should be chosen. Calculation based on the brittle failure model have demonstrated a good correspondence with in-situ observations, and allowed the choose of support systems for different conditions characteristic of the deposit under consideration to be substantiated.


on a effectue les recherches analytiques et sur le terrain pour le massif à faibles caracteristiques de resistances. On a montre la necessite de la reception du modèle du milieu à caracteristiques hors limite. Les calculs d'après le modèle du milieu à destruction fragile ont montre leur bonne correspondance aux donnees des observations sur le terrain et ont permis d'argumenter le choix de types de supports utilises dans de diverses conditions du gisement en question.


Es wurden Gelande- und analytische forachungen fuer ein Massiv mit niedrigen Festigkeitscharakteristiken durchgefuehrt. Es ist die Notwendigkeit von Annahme eines Mediummodells mit Kennwerten im post-pick-Gebiet gezeigt worden. Die Berechnungen nach dem Modell der gebrachigen Bodenverhaltnisse haben gute Übereinstimmung mit den.. Angaben der Gelangeforschungen gezeigt und die wahl des Ausbautyps fuer verschiedene Verhaltnisse der gegeberen Lagerstatte ermöglicht.


Investigations carried out at many ore deposits with hard rock masses have shown that the linear deformable medium model, even if adapted to relative weak rock masses, describes adequately the stress strain state of rocks during underground mining within error range of rock mass geo-mechanical characteristics being determined. As a rule, rock strength of a specimen by exceeded the initial stresses by factors of 3–4 or more. At the Donskoj Mining-and-Processing Integrated Works' deposits, the above relation was almost the same but there emerged some problems concerning stability of workings due to both weakness of fracture filler and dividing rock blocks by secondary fractures into smaller blocks, with presence of the filler too. The common practice of support choosing has entailed, in particular, a large accident in the vertical shaft of the second mine (being under construction) where rock mass is even weaker than that investigated by us. The first observations have shown that at a 50 m distance from the mined-out area having an over 100 m extension across the strike, deformations approach 0.01 and more, causing fractures to be opened. Apparently, the zone of inelastic deformations (ZID) was formed near the mined-out area and, therefore, it was necessary that a rock mass model should be adopted and substantiated to obtain a quantitative estimation. For reasons of shortening of the presented paper volume, not all the following data are supported by corresponding grounds but they give, on our suggestion, a rather complete picture of the investigation procedure and results obtained.


Investigations were conducted at the Molodjozhnaja mine of the Donskoj Mining-and-Processing Integrated Works (the Donskoj MPIW) in Northern Kazakhstan, the largest chromite producer in the world. Mining operations are being carried out at a 500 m depth. Host rocks are presented mainly by dunite with compressive strength in specimen of up to 80 MPa and Young's modulus of up to 4·10 4MPa. Chromite ore has compressive strength in specimen of up to 40 MPa and Young's modulus of up to 6·10 4MPa. Linear coefficient of fracturing for dunite is 0.7–1 and more while that for ore equals 0.3–0.5; fractures are filled with a weak filler such as serpentinite, soapy to the touch. Fracture width is 1–2 mm, rare - up to 4 mm.

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