Friction Rock Stabilizers, trade name Split Sets, have been under research and development by the author and the Ingersoll-Rand Company for the past 4 ½ years. This patented rock anchor system has been extensively tested in both hard rock and soft rock and has been reported upon by the author in other publications. (References 1 through 6) This paper deals with results obtained from field testing under soft rock conditions in uranium mines of the western United States.
THE SCOTT SPLIT SET SYSTEM
The Split Set system of ground support is a full friction, mechanical anchor system which preloads the rock in compression as the device is installed. (Figure 1) The Split Set is essentially a split pipe, nominally 1½ inches in diameter, which is forced into a hole in rock, normally 1 3/8" diameter, which is substantially smaller than the manufactured Split Set. The device is impact driven or hydraulically thrust into the borehole. The borehole thus serves as a die and the Split Set is force-fit to the borehole not unlike a skin piling. The forces of insertion are normally 3/4 to 1½ tons per foot of length of the Split Set and, after insertion, the force necessary to dislodge the Split Set is approximately equal to the insertion force. Over a period of time, the anchorage of the Split Set becomes greater due to continued lock-up of the device from ground movement, corrosion, and other factors effecting friction on the contact surface. The anchorage of the Split Set in the rock can be controlled by three variables, namely; the wall thickness of the Split Set (the thicker the wall the greater the anchorage), the yield point of the steel in the Split Set (the higher the yield the greater the anchorage) and, finally, the diameter of the hole into which the Split Set is inserted (the smaller the hole the greater the anchorage). The variable under the control of the miner is borehole diameter and he controls this by the selection of the proper diameter bit and the care with which he drills the hole.
GEOLOGIC CONDITIONS IN WESTERN URANIUM MINING
Uranium deposits are often found in formations which are relatively unstable and which, consequently, present difficult mining problems. Sandstones can vary from being soft and friable to relatively strong and cemented. Shales and siltstones of varying hardness are often present and, in many cases, sands and siltstones are intermixed. Where these formations are above the water table, they may stand quite well; but where they are located under saturated water conditions, mine openings can be extremely unstable with massive deformation of the back and ribs often accompanied by heaving of the floor. It is under these conditions that Split Sets were tested in uranium mining.
ARTIFICIAL SUPPORT METHODS
Uranium mines have used standard roof bolts with mesh or matting as their most common method of ground control. Timber, steel sets and lagging are used where standard roof bolt systems will not control conditions.