Dragline Digging Techniques

Choosing the Right Method

• Picking the best method for excavating using a dragline depends on controlled and uncontrolled variables. Geological conditions remain a big variable in spite of all the technological advancements due to the depth of mines increasing as coal deposits are becoming ever scarce. Other factors include the dragline or bucket size and characteristics including the operational limits of the machine. The experience and skill of the operator is also very important when determining which dragline method is appropriate.

Standard Digging Techniques

• The simple side cast involves throwing the overburden, or the geology above the material intended to be extracted, sideways on to a spoil pile or to make a bench. It is a technique used commonly as part of other methods of digging. The standard extended bench is used when the variables involved are centered around the geology and the design of the pit. The standard bench is implemented where the thickness of the earth being excavated is less than 45 meters in thickness. The technique known as pulling back is exactly what its name implies. The Dragline proceeds by pulling material towards the low-end side of the pit so that it can easily be deposited it in the spoil pile.

Skilled Digging Techniques

• The extended key cut is a technique that accomplishes two things at once. First a pre stripping operation in implemented using a shovel loader and a dump truck, minimizing the amount of overburden the dragline will have to remove, effectively minimizing rehandling of the material. Second, it also shifts the bench further towards the low-wall end of the pit. A key cut is sometimes utilized as part of other digging techniques. The Split bench (or deep stripping) technique is implemented when the pit reaches a depth greater than the height of the dragline boom meaning the dragline needs two shifts to bring the excavated material out of the pit. The split bench method involves the bucket being dropped down onto an angled highwall and brought up to a temporary pile. The dragline is then moved to another, higher bench, to remove the temporary pile. The last technique is the block pass, which moves the most material at one time. However, this technique involves an initial key cut to access the material beneath the overburden. Once the key cut is completed the intended material will be lifted up to a spoil pile or to an elevated bench level so that it can be dug out further, at a later pass.

Complex Digging Techniques

• Single and double highwall cast can be identified by the number of times a dragline must pass over a pit to extract the materials. It typically involves a combination of the techniques already described, but on the highwall side of the pit. For example, After the pit is broken up by a mining blast, the dragline makes its first pass, removing the top layer of overburden by way of a key cut. The second pass is implemented to remove the rest of the broken up earth by executing the split bench method. Similarly the single and double low wall cast is implemented one or two times, but on the lowwall side of the pit. For example, after the pit is broken up by a mining blast, a crew enters the pit and reduces the depth from up to 20 to 25 meters. The dragline is then implemented to remove the rest of the broken up earth using a simple side cast.