The 9400 Level Access Decline project was constructed to access the existing Bulldog Mine. Previously, the mine was accessed through a 10-foot by 8-foot drift which is located at an elevation of 9,360 feet. This original drift suffered a cave-in after the mine was closed in 1985. While in operation, between 1969 and 1985, the Bulldog Mine produced over 20 million ounces of silver.
Initially the project was designed using steel sets and lagging for ground support. This method of ground support was cost prohibitive from the owner’s standpoint and therefore they changed the RFP to a design-build in order to allow for a more creative approach to the ground support problem. Traylor Mining approached the design from the standpoint of soft ground tunneling and drew from Traylor Underground’s experience dealing with soft ground to propose a support system that utilized a combination of friction bolts, shotcrete and lattice girders. The final detailed design was provided by Hecla Mining’s Ground Support Specialist. Traylor Mining was awarded the project due to its unique approach to the soft ground excavation and support.
An additional challenge to the original design was the crossing over of the original 9,360 foot decline in the area that had previously caved in. During the final negotiations it was decided that the access decline alignment could be changed in order to avoid crossing over the existing decline, greatly reducing the risks associated with the project.
The access decline cross section was originally specified as a 14-foot by 14-foot modified horseshoe to accommodate the steel sets and provide adequate clearance for mining equipment during mine operation. Since the steel sets were too expensive to be used, the cross section was changed to a more stable14-foot by 14-foot parabola cross section, since lattice girders could be manufactured in virtually any cross section configuration. This also reduced the risk associated with mining through the soft ground formation.
Originally. the construction of the decline was to pass through soft ground (Creede Formation), approximately 1,600 feet, and hard rock (Campbell Formation), approximately 1,100 feet. Actual ground conditions were Creede formation, along with a mixed face that at times required blasting as well as mechanical mining. Mechanical mining in the Creede formation was completed by utilizing a roadheader attachment on an excavator, coupled with the soft ground support system. The mixed face formation was excavated by a combination of drill and blast/mechanical mining coupled with friction bolts, lattice girders and shotcrete for support.
The Bulldog Decline Project, all told, excavated approximately 1,900 linear feet utilizing roadheader, hydraulic picks, drill and blast, and SEM techniques. The initial length was to be 2,700 linear feet, but due to ground condition and other factors the decision was made to break into the existing drift at 1,900 feet.