Stability problems in sublevel stoping caused by excessive 30 – 70 MPa horizontal stresses, have been largely eliminated at the Outokumpu Oy Pyhasalmi mine by pre-reinforcement with cable bolting, yielding pillars and stope shaping, rock destressing by sequential stoping technique and "slot stopes" and fast stoping from opening to backfilling. The basic idea in cable bolt pre-reinforcement is to help natural rock arching around stopes and thus prevent caving. Forty km of cable bolting is done annually mainly by a mechanized jumbo, Robolt H 690–5. "Hedgehog" bolting done from sublevel drifts has proved very effective. Parallel "mandolin" bolts along the ore contact are preferred to conventional "outside bolts" because progressive slabbing can be prevented by the use of mandolin bolts.
Les problèmas de stabilite à l'exploitation en sous-etages cauaes par les contraintes horizontales de 30–70 MPa ont ete elimines dans une grande mesure à la mine de Pyhasalmi de Outokumpu Oy au-moyen du pre-renforcement par boulonnage au cable, des piliers coulissants et de la formation des abattages, du relachement des contraintes par l'abattage consecutif et les "gradins en fente" et de l'abattage rapide à partir de l'orifice jusqu'au remblayage. L'effet principal du boulonnage au cable est à aider la voussure naturelle de l a roche autour des grad ins et empêcher l'eboulement. Le boulonnage au cable est fait 40 km par an, principalement par un jumbo mecanique Robolt H 690–5. Le boulonnage en "herisson" execute par les galeries de tranches s'est montre très efficace. Les boulons parallèles en "mandoline" Ie long du contact de minerai sont preferes aux boulons conventionels parce que le decollement progress if de roche peut être empêche en utilisant les boulons en "mandoline".
Stabilitatsprobleme im Weitungsbau verursacht von sehr hohen (30–70 MPa) Horizontalspannungen sind bei der Pyhasalmi Grube der Outokumpu Ag im grossen eliminiert worden. Die wichtigsten Mitteln dazu waren Vorverstarkung mit Kabelankern, nachlassigen Pfeilern und Formengebung der Strossen, Felsentspannung mit sequentialem Abbautechnik und "Aufbruchstrossen", und schneller Abbau von Aufbruch bis Versatz. Der Grundidee der Vorverstarkung mit Kabelankern ist die natuerliche Felsgewölbung um die Strossen zu hilfen und damit Einstuerze zu verhindern. Kabelankerungen werden 40 km jahrlich gemacht, hauptsachlich mit einem mechanizierten Bohrwagen, Robolt H 690–5. Sog. Igelankerung, aus Teilsohlen ausgefuehrt, ist sich als sehr effizient erwiesen worden. Sog. Mando1 ineankerung, Parallelankerung den Erzkontakt entlang, ist traditionaler Kabelankerung vorgezogen worden, weil die progressive Ablösung des Gebirges mit Handolineankerung verhindert werden kann.
The Pyhasalmi mine is situated in the province of Oulu, about 4 km from the village of Pyhasalmi.
The ore lens of Pyhasalmi is 650 m long at the outcrop a 75 m wide, carrot-like formation. To date the ore is known to descend to 800 m. It is estimated that the mine will operate into the next century at its existing capacity of 900,000 tonnes of ore annually.
The mine produces annually over 500,000 tonnes of Pyrite concentrate, 40,000 tonnes of zinc concentrate 25,000 tonnes of copper concentrate, and 10,000 tonnes of barite. Some silver and a little gold is recovered into concentrates.
The most important rock mechanical factors affecting the mining methods applied at the Pyhasalmi mine are: the weak country rock enveloping the stiff, strong ore and high horizontal stress field. The maximum horizontal stresses below 400 m are between 40–70 MPa, which is about three times the vertical gravity load.
Since,1985 a new design for cable bolting has been Used in three stopes at the Pyhisalmi mine. The new design is a combination of "hedgehog" and "mandolin" cable bolting.
Hedgehog bolting: Cable bolts are installed in fan shape from sublevel drifts. The drifts are reinforced with wire mesh and the cable bolts are tied with bearing plates to the mesh. Finally the drifts are shotcreted. The cable bolts are installed with a fully mechanized electro-hydraulic cable bolting rig, Tamrock Cable Bolter H690–5, jointly developed by Tamrock and Outokumpu (Fig 1.)
Mandolin bolting: Cable bolts are installed sub-parallel to the stope walls (Fig.5). The spacing of the 0 90 mm holes has been 1.0–1.5 m. Usually four o 15 mm steel strands are grouted into a hole, giving a total tensile strength of 100 tonnes. The downhand holes are drilled with a Solo 890 RF hydraulic longhole drill and the cables are installed manually.
The measured initial stress was moderate, horizontal maximum 32 MPa. The shape of the stope was dictated by the main fracture directons. BEH-calculations did not show any tensile stresses in the hanging wall, but the roof of the stope was loaded by excessive compressive stresses. In Figure 2 the roman numerals refer to excavation sequence. Reinforcement with cable bolting is presented schematically. The excavation sequence was:
