Repairing a raise in Nicaragua

It was obviously the right thing to do when Triton Minera a Nicaragua decided to increase the underground ventilation volume at the 170 level of the Talavera gold mine in El Limon, Nicaragua. Triton is a division of Toronto-based Black Hawk Mining Inc.

The existing ventilation was no longer sufficient due to the mine’s isolated location, the rising rock temperature and the increasing access of heavy equipment.

A new ventilation raise, completed the end of February 2001, linked the 170 level (130 m below surface) to surface. However, the situation quickly turned bad when the upper ground layers began to cave due to an unpredictable rock fault in the proximity of the collar. The stability of the raise and the surface surrounding areas were both endangered. To avoid a total collapse and to gain some time before finding a solution, work was suspended and the raise completely backfilled.

At this critical moment, the author was called in as a consultant for Ross- Finlay 2000 Inc., the Val d’Or, Que.-based mining contractor. Because of previous successful collaboration with Triton Minera, Ross-Finlay was invited to take over the raise repair and complete the job before the rainy season arrived.

All the parameters were very tight–money, schedule, materials, and equipment available. We proposed reinforcing the damaged area with a 30-m concrete collar built in three main sections, deep enough to reach better ground conditions at a lower elevation. The novelty of this project came from the unique manner of the raise rehabilitation, with some similarities to caisson deep foundation, diaphragm walls and bored piling technology.

There were a number of problems to be addressed, like how to install formworks one underneath the other and hold them in place safely, without any supporting ring underneath, not to mention the freshly poured concrete.

Three 10-m-long formwork pipes made of mild steel were designed in a telescoping arrangement of pipe diameters decreasing from 3.6 m to 3.2 m to 2.8 m, this last one being specially dimensioned to fit the raise standard square opening. These formworks were positioned one inside the other: the largest diameter at the surface and the smallest at the bottom. They overlapped by 30 cm at the contact area in-between two successive shells, to ensure liner continuity and to allow formworks to resist lateral pressure.

The pipes could not be installed simultaneously but only one at a time, by laying the pipe’s bottom end on the muck embankment. Following the installation of each pipe, concrete was poured to fill up the void between the formwork and the surrounding fractured rock walls. With the top end firmly anchored against lateral displacements and the bottom end held in place in the muck embankment by the pressure of its own weight, we created each time a 10-m deep, stiff formwork, able to stand against the pressure of the freshly poured concrete.

To cast in place higher volumes of ready mix concrete and at the same time cover the whole depth of the collar we employed sufficient lengths of extensional steel pipes. These 16-cm-diamater pipes with concrete funnels on their top ends, were supported at the surface only, and installed vertically against the inner side of the collar.

Upon completion of each concrete pour, the backfill embankment was dropped 10 m by mucking the raise bottom opening underground, creating enough room underneath for the installation of the next pipe.

The circular shape of the formwork together with the wedge effect from the raise decreasing diameter downwards, transformed the ground pressure into favourable compressive stresses inside the concrete and held everything tight without any other special measures. Freshly poured concrete was normally left in place to cure and attain its safe resistance before lowering the bottom-supporting muck.

alexarhire@hotmail.com