There are a number of different methods in use for grouting cablebolts. The selection of the best method depends upon the orientation of the borehole, the type of cablebolt, the grout flow characteristics and the grouting equipment available. A brief description of the different installation methods is given in the following pages. Installation methods are discussed in more detail in Section 2.10 and in Chapter 3.
The cablebolt installation methods most commonly used are:
- Breather Tube Method. This method is used for upholes only and with grout of 0.375 - 0.45 water:cement ratio. The optimum grout for this installation method is 0.4 W:C. (The consideration of other important items such as cablebolt types, breather tube diameter etc. may alter the range of optimum grout water:cement ratio specified in design). This method should be used with caution in areas with open fractures in the back which may cause grout loss with thinner grouts and may prevent complete filling of the hole. In loose, thinly laminated ground, caution is required to avoid over pressurizing fractures causing the laminations to separate and rupture.
- Grout Tube Method. This method can be used for any hole orientation and with grout of 0.30 to 0.375 water:cement ratio. The optimum grout for this installation method is 0.35. These thicker grouts may cause pumping difficulties with less powerful pumps and long holes. The use of modified geometry cablebolt elements may require slightly wetter grout at W:C of 0.37.
In both of these methods, the tube(s) are attached to the cablebolt strand prior to the placement of the cablebolt in the borehole. The grout front flows along the entire length of the borehole in these methods.
Variations on the grout tube method in use at some mines are:
- Retracted Grout Tube Method. The grout tube and cablebolt are placed in the borehole and then the reusable grout tube is withdrawn from the borehole as the grout is being pumped. The grout is placed in position in the borehole and does not flow over appreciable distances. Care and experience is required to prevent void formation in the grout column. A highly skilled operator is required.
- Grout and Insert Method. In this method the borehole is grouted using the Retracted grout tube method and then the cablebolt is pushed into the grout filled borehole. The diameter of the borehole can be reduced in this method, and the grout tube is reusable. This method is generally used with automated cablebolting equipment. Overly rapid tube withdrawal or cable insertion will result in a poorly coupled system.
Each of these installation methods is described in the following pages. The method most likely to completely grout the hole should be selected.
In this method, the breather tube extends to the toe of the hole, while only a short length of grout tube is used at the collar of the hole. A cablebolt hanger and borehole collar plug are required.
- Grout of 0.4 water:cement ratio is optimum for this method.
- The grout is pumped through the short grout tube into the borehole. The grout flows upward against gravity in the hole. Air and then grout are expelled from the hole through the breather tube. Return of good quality grout through the breather tube is essential to indicate that the borehole is full of grout.
- A piston pump or progressing cavity pump can be used.
- Problems encountered with this method include: leaking or blown out collar plugs, caused by poorly plugged collars or undersized breather tubes, grout much wetter than design consistency; and no grout flow from the breather tube due to loss of grout into a badly fractured rockmass, an undersized breather tube for the design grout consistency, or inadequate pumping time.
- The grout tube extends to the toe of the hole. A cablebolt hanger at the toe and/or a wooden wedge inserted at the collar secure the bolt in upholes.
- Grout of 0.37 water:cement ratio should be used for upholes.
- In upholes, the grout is pumped to the toe of the hole through the grout tube. The grout then flows downward with gravity inside the borehole. The grout must be thick enough so that at the instant the pump is stopped, the position of the grout flow front will freeze in the hole. A thick consistency "donut" of grout appearing at the collar indicates complete grouting of the hole. Obstructions, such as the wires of a modified cablebolt strand or spacers, may divide the grout front, leaving voids in the grout column.
- A continuous stream of grout is required, so a progressing cavity pump is usually used.
- Voids can easily be created in upholes: too thin grout will slump or spiral down the hole, and thick grout may hang up in the hole preventing complete grouting.
- The grout tube extends to the toe of the hole, but is retracted and can be reused.
- A cablebolt hanger is required to secure the cablebolt in upholes.
- Grout of 0.37 water:cement ratio should be used for upholes.
- The grout is pumped to the end of the grout tube, which is withdrawn slowly from the borehole. In this method, the grout is placed at the required position and flows only a short distance within the borehole. If the grout tube is withdrawn too quickly, voids will be created in the grout column. The grout must be thick enough so that it will hang up in an uphole. This method is the most reliant of the four on good crew skills and training.
-The pump must have enough power to pump thick grout into the longest hole.
-Voids are easily created: too thin grout will slump down upholes, and too thick grout may freeze in the grout tube. If the grout tube is withdrawn from the hole too quickly, voids will also be left in the grout column.
- This method is generally used for cablebolting machines only, since a lot of force is required to push a cablebolt through the column of grout.
- In this method the reusable grout tube is pushed to the end of the hole, then is retracted during grouting. The cablebolt is inserted into the grout filled hole.
- Grout of 0.37 to 0.35 water:cement ratio should be used for upholes.
- The grout is pumped to the end of the grout tube, which is withdrawn slowly from the borehole. In this method, the grout is tremmied into place so that it flows only a short distance within the borehole. The grout must be thick enough so that it will not slump down in upholes (W:C 0.37), but not so thick that it will not fully encapsulate the cablebolt strand.
- The pump must have enough power to pump thick grout into the longest hole.
- Either a piston or progressing cavity pump can be used.
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