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Cross Mine(M1977-410) Technical Revision No.5 <br /> 6. Engineering <br /> All work of an engineering nature will be performed under the responsible charge of a qualified <br /> Professional Engineer licensed in the State of Colorado. <br /> 6.1. Portal Rehabilitation <br /> 6.1.1.Rock Mass Characterization <br /> Once the portal collapse area has been repaired,rehabilitation of the remaining 200 ft+/-is estimated to <br /> require approximately 4 months to complete. <br /> The well-known and widely used Q-system of rock mass classification will be adopted for determining <br /> ground support requirements around the enlarged tunnel opening. This empirical system was developed <br /> to classify rock masses around an underground opening based on estimation of six rock mass parameters <br /> relative to the geometry of the opening. The resulting Q-value provides an indicator of the rock mass <br /> quality that can be related to the maximum unsupported standup span and different types of permanent <br /> ground support by means of a schematic support chart. <br /> At a distance of 250 ft into the tunnel the top of the opening is at a depth of approximately 90 feet. At <br /> this relatively shallow depth the in-situ stress levels around the opening and any water pressures will be <br /> low. The host rocks tend to weather and decompose into blocky or granular fragments and swelling or <br /> squeezing ground is not anticipated. These conditions will simplify the ground support requirements. <br /> 6.1.2.Ground Control <br /> The actual types of ground control used may be composed of one or more of the following types of <br /> supports,depending on the loading condition and the extent of localized rock failure: <br /> • Steel Sets <br /> Steel sets act like passive beams that sit loosely against a rock surface to provide support in case there's <br /> some movement. The beams are designed to carry the full rock load based on their spacing. Typically <br /> installed with lagging or shotcrete to transfer the rock load to the steel set and prevent fall of loose <br /> material. <br /> • Lagging <br /> Planks or small timbers placed between steel or timber sets along the roof(back)or sides(ribs)of a tunnel <br /> or drift to prevent loose rocks from falling,rather than to support the main weight of the overlying ground. <br /> The lagging also serves to transfer the ground load to the primary timber or steel supports. Lagging is <br /> commonly chemically treated to increase fire resistance. <br /> • Shotcrete <br /> Shotcrete is a specially designed,liquefied concrete product that is sprayed or'shot'onto rock faces under <br /> high pressure. It's sometimes known as gunite and consists of cement powder,fine aggregate and water. <br /> This pressure application forces shotcrete into rock imperfections and crevices,sealing it and preventing <br /> raveling or erosion. Often includes steel,polypropylene or fiberglass fibers or is used with mesh for added <br /> reinforcement and tensile strength. <br /> • Mesh <br /> 11 <br />