Patent Publication Number: US-6984091-B2

Title: Yielding rock bolt

Description:
FIELD OF THE INVENTION 
   Seismic disturbances in underground mines are common. The forces involved in such disturbances can cause the tunnels of these underground mines to collapse and the use of rock bolts to prevent catastrophic collapse of the tunnel walls is known. The rock bolts are secured into holes drilled in tunnel walls and are designed to yield to a certain extent to prevent collapse of the tunnel walls. The movement which occurs as the rock bolt yields then provides warning to persons in the mine of possible collapse of the tunnel. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention there is provided a yielding rock bolt arranged to be inserted into a hole in a rock surface characterised by comprising: 
   a shaft having a widened portion adjacent a first end thereof, the first end being arranged, in use, adjacent an inner end of the hole; 
   an anchor member arranged to be secured within the hole in the rock surface; 
   a longitudinal bore provided in the anchor member, the longitudinal bore being arranged to receive a second end of the shaft and being of a diameter less than that of the widened portion of the shaft; and 
   a rock face engaging means adjacent a second end of the shaft to secure the second end of the shaft adjacent the rock face; wherein movement of the rock face causes the widened portion of the shaft to be pulled through the longitudinal bore in the anchor member. 

   
     DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a yielding rock bolt in accordance with the present invention; and 
       FIG. 2  is a cross sectional view of the rock bolt of  FIG. 1 . 
   

   DESCRIPTION OF THE INVENTION 
   Referring to the Figures, there is shown a yielding rock bolt  10  comprising a shaft  12  and an anchor member  14 . The shaft  12  comprises a plurality of peripheral cable strands  16  spirally wrapped around a centre wire  18  (commonly referred to as a ‘king wire’). 
   The centre wire  18  may comprise more than one sections and includes a segment having an increased diameter adjacent a first end  20  thereof. The centre wire  18  of the rock bolt  10  shown in the Figures comprises a first centre wire  22  adjacent the first end  20  and a second centre wire  24  extending from the first centre wire  22  to a second end of the shaft  12 . The second centre wire  24  is of a diameter generally equal to that of the peripheral cable strands  16 . The first centre wire  22  has a diameter greater than that of the second centre wire  24  such that the shaft has a widened portion  26  adjacent the first end  20  thereof. 
   The anchor member  14  includes a longitudinal bore such that the shaft  12  is receivable in the longitudinal bore. The diameter of the longitudinal bore is arranged to be slightly less than that of the widened portion  26 . The first centre wire  22  and internal surfaces of the longitudinal bore of the anchor member  14  may be of a material that is softer than that of the peripheral cable strands  16 . The peripheral cable strands  16 , the first and second centre wires  22  and  24  and anchor member  14  in this case will generally be made of steel with the first centre wire  22  and the anchor member  14  being of a softer steel than the second centre wire  24  and the peripheral cable strands  16 . 
   The softer material of the first centre wire  22  and internal surfaces of the longitudinal bore of the anchor member  14  allow the first centre wire  22  and internal surfaces to deform as the shaft  12  is pulled through the anchor member  14 . Alternatively, if the first centre wire  22  and internal surfaces of the longitudinal bore are not constructed of a softer material then a first centre wire  22  of a slightly smaller diameter will be required. 
   The longitudinal bore may include an outwardly tapered portion at an end arranged adjacent the widened portion  26  in use. Further, the internal surfaces of the longitudinal bore will preferably be metallurgically treated, such as nitrocarburised to prevent molecular welding to the shaft  12  as the shaft  12  is drawn through the anchor member  14 . 
   The rock bolt  10  also includes a rock-face engaging means  28  adjacent the second end of the shaft  12 . The rock face engaging means  28  as shown in the Figures is in the form of a generally conical shaped plate member. The plate member is arranged such that when the rock bolt  10  is inserted into a hole drilled into the tunnel wall with the first end  20  of the shaft  12  entering the hole first, the plate member engages with the rock face around the hole. 
   The rock bolt  10  further includes a clamp member  30  adjacent the first end  20  of the shaft  12 . The clamp member  30  holds the peripheral cable strands  16  together and the peripheral cable strands  16  are preferably splayed outwardly at the first end  20  of the shaft  12 . The splayed peripheral cable strands  16  may engage with internal surfaces of the hole when the rock bolt  10  is inserted into the hole, to hold the rock bolt  10  in the hole until it is secured with grout in a known manner. The shaft  12  is also provided with a de-bonding sheath  32  along the length of the shaft  12 . The de-bonding sheath  32  causes the shaft  12  to not bond with the grout so that the shaft  12  is slidable within the grout while the anchor member  14  is secured in place by the grout. 
   Further, the centre wire may also include an end segment adjacent the first end  20  (not shown) of diameter greater than the first centre wire  22 . The end segment is of a diameter whereby the resulting diameter of the shaft  12  around the end segment is such that the shaft  12  cannot pass through the longitudinal bore of the anchor member  14 . Thus when the shaft  12  has been pulled through the anchor member  14  until the anchor member  14  approaches the end segment, the portion of the shaft  12  around the end segment will seize against the anchor member  14  allowing the rock bolt  10  to achieve its maximum load. 
   In use, a hole is drilled into the wall of a tunnel and the first end  20  of the rock bolt  10  is inserted into the hole until the plate member adjacent the second end of the shaft  12  engages with the rock face around the hole. The anchor member  14  is located around the shaft  12  adjacent an end of the widened portion  26  remote from the first end  20  of the shaft  12 . 
   The hole around the rock bolt  10  is then filled with grout in a known manner. The anchor member  14  is secured in place by the grout while the shaft  12  may to slide within the hole due to the presence of the de-bonding sheath  32 . 
   If movement of the rock around the rock bolt  10  causes a portion of the rock face to begin to break away, the portion of the rock face is held in place by rock bolt  10  being secured at the anchor member  14 . The movement however, will cause the widened portion  26  of the shaft  12  to be pulled through the anchor member  14 . The rock bolt  10  therefore yields as the rock face moves, reducing the possibility of sudden failure of the rock face. In the case where softer steel is used for the centre wire  22  the softer steel will allow the internal surfaces of the longitudinal bore in the anchor member  14  and the first centre wire  22  to deform as the shaft  12  is pulled through the anchor member. 
   It will be appreciated that while the present invention has been described with reference to a rock bolt comprising a plurality of peripheral cable strands, the invention could also be applied to a solid type rock bolt. 
   Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.