Block caving

Underground mining methods compared

Block caving is an underground mining method that uses gravity to exploit massive, steeply dipping ore bodies located at depth. These ore bodies are generally low grade in nature and are too deep to be extracted through open pit mining. The technique is commonly used to extract low grade copper, copper-gold, iron and molybdenum ores although there are other applications. Due to the low cost and high productivity of this cave type mining interest is growing and new deposits that have the ability for block caving techniques are in high demand. A comparison table with various other mining methods can be seen below.

In order to better understand this mining method, a link to a video has been provided here.

Development

The time taken to establish caving operations varies, and it is not uncommon to take a period of at least 10 years from the time of study commencement to achieving project approval. For some large-scale planned caving operations, studies have been under way for more than 20 years with a final decision still yet to be made. The early stages of a block cave development involve a high level of construction activity, as the undercut and extraction infrastructure is created to last for the life of the ore body. During the construction period, the total number of personnel can easily reach 3,000 for a small block cave and up to 10,000 for a large block cave. Once steady state production is achieved, the ongoing demand on logistics is drastically reduced, and is low compared with other underground mining methods. Once approved, establishing a cave mine can be very complex. Speed is of the essence, and it is not uncommon for development in the order of tens or even hundreds of kilometres to be required on some of the larger operations. This can be challenging, particularly if ground conditions are weak or wet in areas.

A generic development sequence for block caving is described below. This technique is illustrated in the figure below.

Block caving mining sequence

1.Mining begins by developing access to the orebody, ventilation raises and the like
2.When access is deep enough a drift is extended towards the orebody and is then developed so that it circles the orebody. Several other drifts are developed for workers and equipment.
3.A grid of crosscuts is developed so that every part of the orebody can be reached on the original level. These crosscuts are where the ore will be collected from. Jumbos are used to blast the drift. Broken rock is removed.
4.A new level is developed just above the first. This is where mining begins. Mining begins by blasting a slice of ore across the orebody.
5.Funnels are built connecting the top level and the bottom level so that broken ore falls to the crosscuts below so that it may be collected. A slice of ore is cut out in many small sections on the top level. First the slice is drilled using jumbos. The drill holes are pumped full of explosives and the rock is blasted. Holes are drilled again up in to the rock from the lower level and more slices are blasted. After blasting the same way many times, a funnel of broken rock is created.
6.Many funnels are created and ore can be extracted from many drawpoints at once. Eventually funnels will be created across the whole orebody.
7.As each load is taken out from the drawpoints, the solid rock above the funnel begins to crack and break until it also falls down the funnels.
8.Bit by bit the entire orebody will gently fall through the funnels. Mining continues until all the ore has been removed.

Selection Considerations

Caving is a non-selective, bulk mining method. It requires large-scale mineralisation in all three dimensions (length, width and height), in rock conditions that are sufficiently weak to allow caving and have suitably fine fragmentation, yet strong enough to ensure that the excavations will last the 10 to 50 year life usually associated with caving operations, although preconditioning can be used to weaken rocks that would otherwise be unsuitable. It cannot be stressed enough that the size and orientation of the the orebody is an important determinant for suitability for caving. The size of the undercut must be sufficient in length and width to generate stresses that can initiate and sustain caving. A large number of mines have used and are using the Laubscher caving chart that can be seen below.

Laubscher stability graph

The Laubscher chart gives estimates of the undercut dimensions required to induce continuous caving using the shape parameter known as hydraulic radius. The hydraulic radius compares the area of the cave undercut to its perimeter.

Geological/Geotechnical Requirements

Block caving mines require large horizontal extents that automatically cave when the orebody is undercut. Large vertical extents, ranging from 60-180m, justify the use of level developments. Grades should also be uniform. In order for the rock mass to break, there must be at least one horizontal and two vertical joint sets. Exhaustive mechanical characterization studies should be completed to confirm that the material will indeed cave by gravity.

Joint Orientation

Well-defined, vertical to steeply dipping joints govern the direction of cave propagation and the mechanism of near surface rock mass mobilization. The shallower the dip of these joints the more inclined from vertical the cave propagation direction is and the more asymmetrical the surface deformation with respect to the block centre vertical axis. In cases where multiple well defined and persistent steeply dipping joint sets are present, the steepest set will generally have the predominant influence. Significant subsidence asymmetry is observed in the dip direction of the sub-vertical/steeply dipping set. Where joints are inclined towards the cave, the rock mass fails through a combination of block-flexural and block toppling and the detachment and sliding of major rock segments. Where a sub-vertical joint set is dipping into the cave, the surface deformation direction is controlled by the dip of the sub-vertical joint set. In this case the rock mass fails predominantly through block toppling and sliding along the sub-vertical joints. The orientation of well defined, gently dipping joints influences the extent of the rock mass mobilized by the failure and the degree of subsidence asymmetry.

Faults Inclination and Location

Unequivocally, the inclination of the fault partially intersecting the caving area controls the extent of surface subsidence deformations. Low dipping faults will extend and steeply dipping faults will decrease the area of surface subsidence. For faults daylighting into the cave, failure of the hanging wall is likely inevitable. For the assumed hard rock mass conditions in the current modelling, the stability of the exposed footwall is dependent on its slope, the amount of passive support provided by the muck pile and the orientation and persistence of jointing within the footwall. The presence of well defined steeply/gently dipping joint set approaching perpendicular orientation with relation to the fault will increase the kinematic potential for failure of major near surface footwall segments. In such circumstances a model combining the fault/jointing system is extremely important.

Fault influences on caving

Steeply dipping faults, daylighting into the cave and located within an area of imminent caving are likely to be caved and therefore are unlikely to play any major role in the resultant subsidence. Faults partially intersecting the caving area may create unfavourable conditions with potential for failure of the entire hanging wall. Depending on rock mass fabric, faults located in the vicinity of the caving zone may have a minimal influence or decrease the extent of the area of subsidence deformation. The former behaviour was observed in models with horizontal/vertical joint sets and the latter for orthogonal steeply/gently dipping joints. A topographical step in the surface profile is formed where the fault daylights at the surface. Significant movements should be anticipated if the fault daylights into the cave.

Planning

Ground Control and Rock Mechanic Consideration in Stope Planning

Subsidence

Subsidence occurs when an underground excavation caves and the movement of material connects all the way to surface, where an opening or deformation in the land is formed. In block caving this is an important consideration because the method is based on caving the orebody. As the ore is pulled into the drawpoints, the waste material caves and swells, filling the space where the ore was previously. The dip of the cave angle determines how and from where the caved material fills into the cave. A narrower cave angle means the waste material tends to cave from a vertical direction, increasing the chance of the caving reaching surface, where it would become subsidence. Laubscher’s chart is an empirical method of determining the subsidence parameters for block caving mining. It relates the cave angle to the mine geometry, height of caving and density of rock. It can be used to assess the potential and severity of subsidence. There are also analytical methods as well as numerical modelling that are also used to predict the behaviour of caving. One flaw with Laubscher’s method is the value of density that attempts to represent the entire cave. This assumption creates inherent uncertainty in the calculations.

Fragmentation

Primary fragmentation occurs when the back of the cave fails into the draw column. It is determined by the various characteristics of the rock mass such as intact rock strength, number of joint sets and orientations, induced and in-situ stresses in the rock mass and the orientation of the cave front. At this stage particle sizes are quite large.

Secondary fragmentation occurs as the ore descends through the draw column and collides with other particles. As it drags down it slowly deteriorates itself into a much smaller particle size. Secondary fragmentation determines the size of material that comes into the drawpoint. The effectiveness of secondary fragmentation is also determined by the range of particle sizes created during primary fragmentation. If the ranges are large, the finer particles will tend to cushion the larger particles creating a large range at the drawpoint, which is not desirable. Ideally a well jointed rock mass with high rock strength will a narrow range of particle sizes which will fragment better during secondary fragmentation.

The draw rate is also important as it will determine if the rock is given the proper time to fail, as well as the propagation of the draw front which will affect how adjacent caves behave. Mines have to be careful with drawing at a high rate from well fragmented caves because it could be at the expense of other coarser caves.

Equipment

The main production tasks for cave mines are accomplished by loaders taking ore from the drawpoints and transporting it to another location. Other production tasks may involve secondary breaking, crushing, conveying and shaft haulage. A train network is common for large block caving operations as seen for example in El Teniente Mine in Chile. This technology often provides better efficiencies in equipment utilization, safety and energy consumption.

Drill and Blast

Three main factors must be taken into consideration when drilling for both undercut and drawbell development: rock properties, drilling accuracy, and the capabilities of the drilling equipment. First the rock properties, these will influence the powder factor required to fracture the rock mass when blasting. As drawbells need to remain intact for the entire mine life in most cases it is important to practise proper blasting techniques. A smooth wall blasting technique using lower power explosives couples with close spaced holes along the desired final boundary in order to create a pre-split. The resulting rock boundary is more likely to be stable and less fractured.

It is very important to select the drilling machine that maximizes drilling accuracy, regardless of the drilling method. Drilling accuracy is most important for upholes as these will affect the undercut and extraction level stability. This ties into the capabilities of the drill, where factors such as the combination of drilling angles, hole diameter range, stability of the drilling platform, length of hole and automation capabilities.

Load and Haul

The layout of a block cave mine lends itself well to partial or full automation. One of the systems that has proved most effective for automation is LHD. Operations that use this system have seen significantly higher productivities at a lower unit operation cost due to lower manning costs, increased tramming speeds and minimal damage. Another benefit is a higher effective utilization due to the elimination of operator constraints. Although, automation does have its setbacks with extra precautions needed for safety often completely removing people from the working area. Also, roadways must be well maintained in order to gain the full benefits of high tramming speeds. People will always be required during an operation to perform inspections and maintenance but automation can reduce the exposure time of workers in the production environment.

Infrastructure Requirements

Ventilation

Drawpoint blasting may be required in cave mining, so ventilation systems must be designed so that mine personnel and mobile equipment are not exposed to return air.

Ore Handling

Grizzly systems are used for ore that breaks into finer particles. Grizzly levels are connected to undercuts in caving operations by finger raises. These raises are connected at the top to create pillars that initiate the caving after blasts. When material reaches the grizzly rails, finer pieces fall through while oversized rock is held back and broken down further (e.g. using remote controlled rock breakers). Transfer raises then connect grizzly levels to haulage levels. At the haulage level, loading chutes direct the finer rock pieces to rail cars. Grizzly systems are labour-intensive and require significantly ore development than other systems.

Slusher systems are mostly applied to ore that breaks into medium-coarse pieces, but can also handle finer particles like grizzly systems. Slusher drifts are driven perpendicular to haulage drifts and alternate 180o from the previous drift. Finger raises are driven from these drifts to the undercut levels. Mechanized systems can be used when orebodies are less fractured and cave in large chunks. These results in high production rates and less development required. Drawpoints are driven horizontally from the production drifts and commonly at angles of 45o to allow for LHD access. The undercut drift is located approximately 15 to 20 m directly above the production drifts. Pillars left between these levels protect the production level.

Advantages and Disadvantages

Advantages and disadvantages of block cave mining

Case Studies

Block caving mines world wide

A table of current block caving mines can be seen below.

El Teniente Case Study

Information on the El Teniente mine.

Palabora Mine Case Study

Information on the Palabora mine.