7 Longwall Sublevel Caving Block Caving

Topic 7: Underground Mining Methods 

Longwall  Sublevel Caving  Block Caving Hassan Z. Harraz [email protected] 2014- 2015

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Outline of Topic 7: 

Longwall 

Longwall in coal Longwall in Hard Rock  Sublevel Caving  Characteristics of the ore body and mining method  Development  Production  Equipments Used  Block Caving  Introduction  Historical evolution of the method  Condition deposit  Principles of the method  Methodology of block caving  Basic issues of geomechanical to the black caving method: 

1) 2) 3) 4) 

Caveability Mine des ig n Fragmentation and extraction control S ubs idence as s ociated

Advantages and Disadvantages of Block Caving

We will explore all of the above in Topic 7. Prof. Dr. H.Z. Harraz Presentation Caving methods

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Longwall (LW)

Prof. Dr. H.Z. Harraz Presentation Caving methods

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Longwall (LW) 

The Longwall is a very old method, srcinated in coal mines in Europe in the 7th century.  The most important application of Longwall relates to coal mining.  Much of the production of coal from countries like USA, Australia and China are obtained by Longwall. Conditions of applicability of the method: 

Stratiform tabular bodies, little thick, horizontal (tilt up to 20°); Uniform distribution of thicknesses / levels;  High degree of continuity of the ore body;  Geological discontinuities (e.g., faults) are highly detrimental to the method;  Applicable in hard rock (metalliferous mines) and fragile (coal). 


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Longwall Stoping 

Longwall stoping is applicable to bedded deposits of uniform shape, limited thickness and large horizontal e.g.,extension a coal seam, ( a potash layer or the reef, the bed of quartz pebbles exploited by gold mines in South Africa).

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It is a caving method particularly well adapted to relatively flat-lying, thin, planar deposits or horizontal seams, coal,usually at some depth.

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It is suitablefor tabular orebodies, with moderate dip(e.g., coal and stratiform hard-rock ores like diamond deposits).

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It is one of the main methods for mining coal. It recovers the mineral in slices along a straight line that are repeated to recover materials over a larger area.

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Need to divide orebody toface " " or the "working face “.

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The collection ofcuts, cross-cuts, and pillars all together make up apanel " " and all the equipment that goes together to operate in that panel is a "unit or Longwall units".

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In this method, a face of considerable length (a long face or wall ) is maintained, and as the mining progresses, the overlying strata are caved, thus promoting the breakage of the coal itself.

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Applied to longer (~100 m) and longer diameter blastholesthus (i.e., requiring less drilling than sublevel stoping).

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Greater drilling accuracy is required.

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Need to alongwall machine (It's designed to let the roof fall behind it, and mines out big rooms in which the roof almost immediately collapses, leaving only a small entryway and the metal barrier that protects the longwall unit). The space closest to the face in kept open while the hanging wall is allowed to collapse at a safe distance behind the miners and their equipment.

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Preparation for longwall mining involves the network of drifts required for access to the mining area and transport of the mined product to the shaft. Since the mineralization is in the form of a sheet that extends over a wide area, the drifts can usually be arranged in a schematic network pattern. The haulage drifts are prepared in the seam itself. The distance between two adjacent haulage drifts determines the length of the longwall face.

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Continuous miner operations, and longwall units.

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Traditionally high production rates.

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Large openings with long open times.

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High ground support cost .

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Bottom up mining method.

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Non-selective mining.

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Not stress friendly.

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Many equipment types.

Longwall General Layout

Top Gate

Longwall mining method includes drivage of two long roadways in coal and joining them at the end by a perpendicular drivage forming a face.

Face

Bottom Gate

Example: Longwall Mining of Coal  Longwall mining is a highly mechanized underground mining system for mining coal.  It set of longwall mining equipment consists of a coal shearer mounted on conveyor operating

underneath a series of self-advancing hydraulic roof supports.  Almost the entire process can be automated.  Longwall mining machines are typically 150-250 meters in width and 1.5 to 3 meters high .  Longwall miners extract "panels" - rectangular blocks of coal as wide as the face the

equipment is installed in, and as long as several kilometers.  A layer of coal is selected and blocked out into an area known as a panel (A typical panel

might be 3000 m long X 250 m wide).  Passageways would be excavated along the l ength of the panel to provide access and to place

a conveying system to transport material out of the mine.

http://en.wikipedia.org/wiki/File:SL500_01.jpg

 Entry tunnels would be constructed from the passageways along the width of the panel.  Extraction is an almost continuous operation involving the use of:self-advancing hydraulic

   

roof supports sometimes called shields, a shearing machine, and a conveyor which runs parallel to the face being mined. Powerful mechanical coal cutters (Shearers) cut coal from the face, which falls onto an armoured face conveyor for removal. The longwall system would mine between entry tunnels. Longwalls can advance into an area of coal, or more commonly, retreat back between development tunnels (called "Gate roads") As a longwall miner retreats back along a panel, the roof behind the supports is allowed to collapse in a planned and controlled manner.

Longwall (LW) in Coal "As applied to longwall coal mining, is not maintained the integrity of the immediate roof above the newly mined coal. This ceiling should desplacar the main ceiling, separating into blocks and falling into the void left behind the line automarchantes brackets. The process of peeling is accompanied by swelling (about 50%). Ceiling and immediately occupies the void left by coal mined, acting as a natural bed against which converges the main ceiling. The greater role of the immediate roof is desplacar and blistering, filling the void mined and retaining the convergence of the main roof, maintaining its integrity. " R ef.: B rady & B rown, 1993, R ock me chanics for un derg round mining , cap.12.4.6.


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LW in Coal carvão


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Longwall in Coal


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Longwall Mining

Longwall (LW) in Coal Preferred conditions (beyond those already mentioned):   

immediate roof of coal consists of shales, siltstones or other brittle rocks, enough to produce peeling fracturing; competent main roof, which can deform without breaking on the immediate roof has collapsed. competent flooring to withstand the stress produced by the monkeys;

Situations in which there is an advantage in applying LW in relation to the R & P:   

bad roof (fragile), preventing bolting ceiling; great depths (e.g., beyond 500m), causing much loss of coal pillars; reduced thickness of coal seams.


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Anatomy of a Coal Mine

Pennsylvania Department of Environmental Protection Bureau of Deep Mine Safety

Types of Longwall  Longwall advancing  Longwall retreating

Longwall advancing


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Longwall retreating


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Longwall in Coal

Setup room: where the face of longwall begins operation; Recovery room: where the longwall finishes and equipment are removed from the panel;

Barrier pillar: pillars to protect main and bleeder galleries axis.


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Advantages of LW in coal  Greater than the recovery room and pillar panel;  High rate of production and productivity - over 100 ton / man / shift "face productivity" - the highest of underground methods;  The lower production costs in underground mines (next to the block caving);  Ease of hand-to-work training.  Adequate to poor roof;  Coal generally produces better quality (lower dilution);  Better able to control venting and elimination of gases and dust;  Good control of subsidence.  It is safer - the workers are all the time under the roof bracing.


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Disadvantagesof LW in coal            

Does not work well in layers of irregular thickness; Stops result in a large variation in production (high production / low availability); Geological discontinuities (faulting or problems with the ceiling) can cause long downtime; Dust control often difficult; Problems of methane under high production; Variability and intermittency in production between simultaneous fronts cause overload in the discharge of mine system; Impact on the construction of the surface (subsidence); High initial investment in equipment; Significant development in the preparation of mining panels; Need for immediate ceiling collapse after the withdrawal of support from apes; Long delay to exchange panel; Rock bursts: e.g. big problem in depth beyond 750m.


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Basic equipments Longwall (coal)

AFC = Armored Face Conveyor Prof. Dr. H.Z. Harraz Presentation Caving methods

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Equipment for Longwall method in coal


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Coal Mine Then

And Now

http://en.wikipedia.org/wiki/File:SL500_01.jpg

Pennsylvania Department of Environmental Protection Bureau of Deep Mine Safety

Mechanized cutting machine on a longwall coal-mining face: Shearer Working at Longwall Face .

Equipment for longwall method in coal (shearer)


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Longwall in Coal: operation The extraction is done with the aid of rotary cutters will fragmenting the carbon layer. The coal falls on a channel of transmission and is transferred to a continuous transport.

Longwall Mining Machine

At the coalface....!!! A virtual reality 3D creation of a working shaver on a longwall.


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A very attractive feature of this method is the protection system roof that provides complete safety to operators.

•

The hydraulic cylinders move as the carbon layer is drawn, creating an area without support on the back of falling relieving stress on the system. Prof. Dr. H.Z. Harraz Presentation Caving methods

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After completion of mining in the panel, it is necessary to change the equipment. This change takes 10 to 30 days to be performed and is performed, on average, 1 to 3 times a year.


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Equipment for Longwall method in coal (plough)


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Longwall Mining Machine http://upload.wikimedia.org/wikipedia/commons/thumb/ 5/5d/Schildausbau.jpg/220px-Schildausbau.jpg

It's designed to let the roof fall behind it, and mines out big rooms leaving in whichonly thearoof almost immediately collapses, small entryway and the metal barrier that protects the longwall unit.

Figure shows Hydraulic chocks

Figure shows Hydraulic chocks, conveyor and shearer http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Longwall_wit h_hydraulic_chocks%2C_conveyor_and_shearer.jpg/220pxLongwall_with_hydraulic_chocks%2C_conveyor_and_shearer.jpg

http://wikimedia.org/wikipedia/common s/thumb/1/19/SL500_01.jpg/

Fig 12a: Passageway

Figure 12c: Longwall system in place. Figure 12b: A typical panel is 3000 m long by 250 m wide

Mechanized cutting machineon a longwall coal-mining face.

Figure shows another continuous miner in an underground coal mine.

Typical dimensions of a panel longwall : Extension panel: 900 - 5300m;

Width. the gal. Face: 2.4 - 3.6m; Length of f ace: 200 - 360m; Height: 0.9 - 4.5mm; Cutting Thickness: 80 - 800mm; Depth: 60 - 800m.


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Capital price/longwall:    

30 million $ for a face equipment; Need for large reserves … minimum of 50 million tons; Producing a front … 2-6 million tons / year; Employment of a shearer …. 200-500 minutes / day.

Compared with a front operating with continuous miner ...    

Capital of 3-5 million $; Production 0300000-0800000 t/year; 3 Continuous miners are needed in developing a front LW; Continuous miner is flexible and can be easily availed in other reserves. Prof. Dr. H.Z. Harraz Presentation 30 Caving methods

Longwall: examples The mine Kuhntown (Pensilvânia) achieves a production of up 46.000 t/d of iron ore by plowing a layer of 900mm coal cutting at a rate of 2.700 t/h.


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Longwall: examples Mine in Colorado operates production until 4.500t / h, reaching 22.700t / day in a coal seam thickness of 1.07m. The power cutter has cutting 1.100kW, moving from 8 to 12 m / min along the face.


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Longwall: exemplos Experimental longwall at Mine Leão I - Rio Grande do Sul, 80s.        

Extension panel = 800m Face-width = 70m Height layer = 2m 1 double drum cutter (300 hp), diameter 1,09m; Hydraulic cylinders 54 automarchantes type "chock" (6 legs capacity of 240t) 1 panzer front 65hp, with 64m long and capac. 600t / h; Side galleries of the panel developed by Roadheader; Daily production of around 800t.


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Longwall: Leão I


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Longwall in Hard Rock 

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In this case, the method must maintain the integrity of the floor and ceiling; cover and lapa must be composed of hard, competent rock. Temporary support (near side) and permanent (eg uprights of wood and / or concrete columns) are used to prevent discontinuities in the stope. Used in metalliferous deposits; differs greatly for the Longwall coal. During the work of the scraper, the roof is anchored with temporary supports that are later replaced by permanent concrete supports. Additional information about the LW method fo r coal on the Internet ... Wollongong University-Austrália www.ouw.edu.au/eng/current/longwall

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Longwall in Hard Rock Schematic of Longwall Panel (Hangingwall Stripped A way For Illustrative Purposes)

Protective Screen

“Permanent” support, often timber packs, will remain in place after mining. With time, these become deformed or completely crushed –as part of the “controlled” closure of the panel.

The extraction proceeds during the strike, with the dismantling of the face done with the aid of explosives. The ore is disassembled collected with a scraper and taken to a orepass.

In hard-rock minerals mining a “s cr aper” is pulled down the length of the stope face after drilling and blasting, to collect the fragmented ore rock.

Temporary support near the working face: often hydraulic props.

In coal mining, a mechanized cutting device is run along the length of the coal face. Figure from Hartman and Mutmansky, 2002.

Work Face at South African Gold Mine 37

Deep level gold mining, South Africa

Stope face with temporary support Stopes (yellow): on-reef excavations where the reef (orebody) is mined. http://www.bullion.org.za/MiningEducation/Images/images/ CrossSectMine.jpg

Sublevel Caving Topics 

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Characteristics of the ore body and mining method Development Production Equipments Used


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Sublevel Stoping

Figure 7: Sublevel Caving

Sublevel Caving            

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It is used to mine la rge orebodies with steep dip tabular or massive deposit and continuation at depth (Fig.17). The ore is extracted via sublevels which are developed in the orebody at regular vertical spacing. Each sublevel has a systematic layout of parallel drifts, along or across the orebody. Sublevel stoping recovers the ore from open stopes separated by access drifts each connected to a ramp. The orebody is divided into sections about 100 m high and further divided laterally into alternating stopes and pillars. A main haulage drive is created in the footwall at the bottom, with cut-outs for draw-points connected to the stopes above. The bottom is V-shaped to funnel the blasted material into the draw-points. Short blastholes are drilled from the access drifts in a ring configuration. The ore in the s tope is blast ed, collected in the draw-points, and hauled away. Blasting on each sublevel starts at the hangingwall and mining then proceeds toward the footwall. Blasting removes support for the hangingwall, which collapses into the drift. As mining progresses downward, each new level is caved into the mine openings, with the ore materials being recovered while the rock remains behind. Loading continues until it is decided that waste dilution is too high Work then begins on a nearby drift heading with a fresh cave. As mining removes rock without backfilling, the hangingwall keeps caving into the void. Continued mining results in subsidence of the surface, causing sink holes to appear. Ultimately, the ground surface on top of the orebody subsides (Fig.18). However, the stopes arenormally backfilled with consolidated mill tailings after being mined out (This allows for recovery of the pillars of unmined ore between the stopes, enabling a very high recovery of the orebody).

C haracteri s tics of the method ... Sublevel Caving in the process of fragmentation of the ore is done by explosives (induced caving) and the ore is detonated with drilling in ascending fans. The sterile overlying should crumble as the ore is removed. Characteristics of the orebody and enclosing     

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massive and/or tabular (varying inclinations are allowed); Diving > 50o case is thin; competent body with mineral rock wall rock (cover) fractured; stable development of the footwall to access; the method requires minimal stability to the ore body, because the galleries sublevel should be self-supporting piece and can receive routine bolting; significant dilution, very little sensitive to fragmentation; likely surface subsidence;  rock cover must accompany the ore in a continuous felling, producing subsidence at the surface. The ideal condition is that the enclosing fragment into larger blocks which ore disassembled to facilitate flow separation at the extraction drift. Prof. Dr. H.Z. Harraz Presentation Caving methods

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Design alternatives for dips and varying thickness of the ore body Tabular ore body and thick makes all production galleries are always in the ore, avoiding fans incomplete perforation (loss of ore), open galleries in sterile (the roof support problems and expenses), losses along the footwall


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Design alternatives for dips and varying thickness of the ore body

Losses ore


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Design: transverse sublevel caving – for thick bodies


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Figure shows sublevel caving is used to mine large orebodies with steep dip and continuation at depth.

Figure shows Continued mining results in subsidence of the surface, causing sink holes to a ppear. Ultimately, the ground surface on top of the orebody subsides.

Design: transverse sublevel caving - this case, the galleries of production (drifts) are perpendicular to the strike of the ore; - Mining recoveries are greater than the longitudinal layout.


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Design: longitudinal sublevel caving – for narrow bodies and sharp dip


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Development in Sublevel Caving      

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The method requires significant development, being part of ore (in high producing mines, 6% of the total ore mined comes from developing). The cost / ton of ore in the development is several times higher than in production. Should maximize production and minimize development. The ore body is divided into panels whose height varies from 50 to 250 m in height, depending on the scale of production and reserves per vertical meter. Each panel is divided into sublevels spaced 20-30 m (increasing the spacing between sublevels minimizes the development) which will be issued successively downward. The lower level of the panel is characterized by a main gallery of transportation that serves all "orepasses", connecting the premises of the extraction well. Access to sublevels is accomplished by a ramp situated between the ends of the ore body. The ramp is linked to gallery transporting each sublevel. This gallery of transporting each sublevel must accompany the footwall contact at a distance 15-20 m. In transverse sublevel, crosscuts traverse the deposit, going to the hanging wall; development is in the footwall. Starting in the transport gallery, galleries are open from distant production center-tocenter 15-25 m, parallel to each other, extending to the contact with the footwall. Have dimensions (width x height) 5x4 m; 6x5 m; 7x5 m. Sublevel of the galleries, just above the ore is drilled and drilling with longholes in range.


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Production in Sublevel Caving  

 

The production begins when the galleries of a sublevel reach the subsequent contact. These galleries do not come into general production while, but should not retreat in a un-organized way. In some mines, adopts the recoil so that its edges are coarsely contained in a plane. In other cases, the recoil is done so that the galleries production of more distant "orepasses" are the first to reach the main gallery. Equipment used for production drilling (ascending): carts with two spears, with sectioned stems and crowns of up to 115 mm. Currently, the drills used are electrohydraulic and DTH's.

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

Drilling targeting the production is made in the form of irradiated fans of the galleries of the sublevel. The holes made are long (up to 50 m long) and is used in this drilling

process "longholes". Charging is done by pneumatic devices. ANFO explosive is the most common. In the case of explosive in cartridge, it uses a similar device, equipped with blades to break through the cartridge.


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One way to start is dismantling create a free face (slot relief) with a hole pattern up to an inclination of approx. 80 to 90. Another way is to open the slot from a raise.


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Production        

LHD's perform the loading, transport and unloading of ore in the end "orepasses" being sublevels designed for better efficiency of loaders. Drilling operations and loading are performed independently and at different levels. Due to the large number of galleries sublevel there are many fronts production The explosive consumption is high due to the fact the dismantling be carried out against the mass of fragmented rock. Must be careful in removing the ore from drawpoints (you need to control levels at the point of load): removal of the material causes little lower recovery; removal of too much material causes excessive dilution. There is a cut-off grade given below which do not remove more ore in crosscut and should detonate new range.


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Equipment for development and production


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In the design of production drifts, the Sublevel Caving uses the principles of "gravitational flow" ore dismantled, with subsequent collapse of the host rocks. Approaches used to issue a gravitational flow:

A ) The phys ical-s cale models B ) fi eld experi ments in r eal s cale C ) models mathematical / numeri cal. The solutions found so far are not fully satisfactory from the standpoint of optimization of the gravitational flow of the ore.


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A ) E xample test with phys ical s cale models:

These models are the oldest, made with particles ( e.g., sand), leaking containers of small size.


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Two major factors related dilution and ore recovery (*)  Width (c) extraction of the gallery; 

Clearance (V) between ranges of production

(*) View A rtic le : Theory and pratic e of very-larg e-s cale S ublevel Caving . Und erg round Min. Met hods : E ng . Fundamentals and International Cas e S tudies , 2001, W.A .Hus trulid & R .B ullock . C hapter 46, p.381-384. Prof. Dr. H.Z. Harraz Presentation Caving methods

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Width (c):  A gallery of production should be as wide as possible; 



When the ceiling of the gallery is concave, the flow of ore is very centralized and inefficient sides, requiring closer galleries. The amount of ore away from LHD's increases with the increase ofheight the height of the gallery production. Therefore, the should be as low as possible.


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Examples of ancient settings production galleries...


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B ) R eal S cale E xperi ments : Are directly executed in underground mines, where they put up markers (markers) numbered, to be retrieved and counted after detonation. A recent experiment shows that the flow of ore is mainly formedbut by he theis not material above drawPoint, very predictable. (Quinteiro, C R, Larsson, L and Hustrulid, W A., 2001. Theory and practice of very large scale Sublevel caving. Underground Min. Methods- Eng. fundam. and Intern. Case Studies SME)


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Experiments on real scale:   

Recently, Sublevel Caving mines have increased lateral spacing interval drifts and sublevels. This has increased production, but also increased dilution. There are still several outstanding questions about the best production design in this method.


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C ) Mathematical / numeri cal models : There are several numerical models being investigated to explain the influence of the main variables of the method. Among the principal's PFC - numerical code developed by Itasca group since the 1990s PFC = Particle Flow Code.


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Principles of choice of the configuration of production galleries:    

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

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the idea of minimization of production costs leads to employ the maximum vertical spacing between sublevels (currently around 30m); essential for maximum spacing between sublevels ... ability to drill and carry long, straight holes and large diameter factor; the largest possible diameter hole, which allows drilling and loading, is ideal (the maximum is now 115mm); large galleries (7x5m, for example) allow the use of drill pipe and longer rectilinear holes (few rods provide increased rigidity to the drillstring). There are projects of fans with holes up to 50m in length; distance between planes of fans (B):  depends on the hole diameter (D) and the type of explosive.  Initial estimate for ANFO ... B = 20 D.  For higher energy explosives ... B = 25 D.  D = 115mm and assuming emulsion as an explosive has been B ≈ 3m . Number of holes in the array:  should follow the S / B ratio ≈ 1.3; where S is the distance between the ends of neighboring holes in the same range .  In this case (B = 3m), S is 4m. Interval between sublevels:  is chosen based on the maximum drilling capacity and the ability to maintain satisfactory alignment of the holes.  As an example, assume 25m. Lateral spacing between production galleries:  makes an angle of 70 o between the upper gallery of the reference and the midpoint of the gallery just above the sublevel (this is the angle of minimum theoretical expected to drive the ore detonated). The center-to-center spacing resulting from side galleries is approx. 22m (see Figure A1 in the next slide).  This initial configuration (Figure A1)Harraz was adapted in the 1990s to become more practical Prof. Dr. H.Z. Presentation 65 Caving operational point of view (Figure A2).methods

Figura A1

Figura A2


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Principles of choice of the configuration of production galleries: 

For adaptation:  effecting up to 55 lateral holes of inclination. Function of these holes ... ore fragment lying in the slope of approximately 70 and reduce the length of (longer) central holes of the fan.  Holes smaller than 55o inclination are difficult to load with explosives, due to the angle of repose of the ore in drawPoint.  The fans may be vertical or inclined ( generally uses up 70 to 80o to the horizontal). The steepening improves the stability of the roof drawPoint and easy access for loading the holes w ith explosive.


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advantages High production rate;  Many And efficient fronts of simultaneous mining;  Possibility of high degree of mechanization;  Method Safe for operators. 

disadvantages 

Dilution may be high (15-30%) and moderate recoveries (75-85%);

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subsidence on the surface;

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high consumption of explosives;

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high cost of development;

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intensive drilling and disassemble to generate a suitable granular product to flow ore;

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controlling the cut-off level can result in low recovery of ore.


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Block Caving


Block Caving 69

Introduction: Block Caving* is method in which volumes of

rock are left without support and suffer rebate under its own weight; the overlying rock fragments-along with the ore. The fracturing and the disposal of ore are obtained by the action of gravity and efforts resulting from tectonic and lithostatic stress. * Translation: Allowance for blocks.


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Block Caving  Block-caving method is employed generally for steeply dipping ores, and thick sub-

horizontal seams of ore.The method has application, for example in sulfide deposits and underground kimberlite (diamond) mining.  It is most applicable to :A large-scale or bulk mining method that is highly productive, low in cost, o and used primarily on massive steeply dipping orebodies that must be mined underground. Weak or moderately strong orebodies that readily break up when caved. o Large, deep (>2 km deep), low-grade deposits with high friability (Fig.19). o  It is often done to continue miningafter open pit mining becomes uneconomic or

impossible. However, some mines start as block cave operations (e.g.,There are several of these in Chile. Rio Tinto is considering a deep at the Resolution deposit to the east of Phoenix).  A grid of tunnels is driven under the orebody The rock mass is then undercut by blasting.  Ideally the rock will break under its own weight Broken ore is then taken from draw points.  There may be hundreds of draw points in a large block cave operation (Fig.20).

 An undercut with haulage access is driven under the orebody , with "drawbells"

excavated between the top of the haulage level and the bottom of the undercut. The drawbells serve as a place for caving rock to fall into.  The orebody is drilled and blasted above the undercut, and the ore is removed via the

haulage access.  Due to the friability of the orebody the ore above the first blast caves and falls into the

drawbells. As ore is removed from the drawbells the orebody caves in providing a steady stream of ore[3].  If caving stops and removal of ore from the drawbells continues, a large void may form,

resulting in the potential for a sudden and massive collapse and potentially catastrophic [4]

windblast throughout the mine.  Where caving does continue, the ground surface may collapse into a surface depression (such as those at the Climax and Henderson molybdenum mines in Colorado. Such a configuration is one of several to which miners apply the term " glory hole“).  Orebodies that do not cave readily are sometimes preconditioned by hydraulic

fracturing, blasting, or by a combination of both. Hydraulic fracturing has been applied to preconditioning strong roof rock over coal longwall panels, and to inducing caving in both coal and hard rock mines.  Essentially block caving creates an underground 'inverted open pit'. Surface subsidence

can be a problem….???.

Figure shows application of the Block caving to large, deep, lowgrade deposits

Figure shows hundreds of draw points to take broken ore in a large block cave operation Figure: Models of block caving (Brown, 2003). www.ivanhoe-mines.com/s/Mongolia_ImageGallery

Surface TOP OF OREBODY OREBODY

Figure from Hartman and Mutmansky, 2002.

An undercut tunnel is driven under the orebody, with "drawbells“ excavated above. Caving rock falls into the drawbells. The orebody is drilled and blasted above the undercut to initiate the “caving” process. As ore is continuously removed from the drawbells, the orebody continues to cave spontaneously, providing a steady stream of ore. If spontaneous caving stops, and removal of ore from the drawbells continues, a large void may form, resulting in the potential for a sudden and massive collapse and a potentially catastrophic windblast throughout the mine (e.g., the Northparks Mine disaster, Australia).

Prof. Dr. H.Z. Harraz Presentation Mining Geology, Mining Methods

74

Block-cave mining: Mud-rushes –an under-reported hazard Mud-rushes are sudden inflows of mud from ore drawpoints (or other underground openings), in block-cave mines that are open to the surface. Considerable violence, in the form of an airblast, is often associated with mud-rushes. Mud-rushes are (under-reported) hazardous occurrences that have occurred frequently in mines in South Africa, as well as in Chile and Western Australia, and have caused fatalities (Butcher et al., 2005).

Mud is produced by the breakdown of rock in the n ear-surface muckpile in the presence o f rainwater. Kimberlite rock on diamond mines is particularly susceptible to weathering by rainwater.

SCHEMATIC CUT-AWAY VIEW OF SUB-LEVEL BLOCK-CAVE MINE Figure from Hartman and Mutmansky, 2002.

Effect of Mineral extraction upon displacement of country rock and surface Workings and voids formed after extraction of mineral gets filled with time by the caving rock so that the rock over the deposit may deformed and subside. This phenomenon is called "Displacement of rock". Displacement causes smooth subsidence of the earth's surface without ruptures, or abrupt subsidence with considerable movements, caving and collapses.

The rock displacement zone include:- "a caving zone" within which the displacement is accompanied by the fault and destruction of layers and the separation of lumps and blocks from the solid; "a cracking zone" which is an area of rock discontinuity and cracks; "a smooth-displacement zone" wherein rock features plastics deformation without discontinuities. The earth's surface area whi ch experienced displacement is called a "trough". Figure shows Effect of Mineral extraction upon displacement of country rock and surface as well as rock displacement in mining.

Block caving


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Historical evolution of the method ...        

End of the century. xix: block caving applied to iron ore mines in Michigan, USA; Beginning of the century. xx: block caving applied in the USA for iron ore and copper states in the west side; 20s: block caving starts in Canada and Chile; 50s: block caving starts in South Africa, diamond mines and asbestos; Beginning of the 60s: LHD vehicles developed for underground mining; 1970: LHD's used with block caving mine in El Salvador, Chile; 1981: panel mechanized caving introduced in the primary ore of El Teniente, Chile; 90: Planning new generation of mines with greater height block and ore bodies more resistant (eg Northparkes, Palabora).


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Operating mines closed and planned using the method …


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Some data using block caving mines...


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Condition deposit      

Resistance ore: weak to moderate, preferably soft or friable ore with intense fracturing; Resistance the wall rock: similar to the ore, distinct interface between / barren ore; Diving: vertical is better, but can be flat if the deposit is thick; shape: large areal extent and thick (> 30 meters); uniform and homogeneous distribution of levels (suitable method at low levels); Depth: moderate (> 500m and <1200m).

Principles of the method ... 



In block caving method, the ore is moved by subsidence (caving) to a cavity formed almost always without the use of drilling and blasting. Drilling and disassemble are used in establishing the initial "enhancement". The base of the ore is excavated by removing their support, this results in fracturing of the ore which migrates to the enhanced vacuum and which is then removed.


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Methodology of block caving:        

develop below the panel to be mined, a layout extraction of ore; above the level of extraction, a horizon of "undercut" (highlight) will provide free face below the ore body, causing the collapse; temporary pillars in the "undercut" horizon are removed and the collapse of the ore starts; ore haggard blister and fills the void of the "undercut"; remove fragmented material in the extraction horizon, inducing flow of ore and ore loss of support has not beaten down that is also subject to collapse; vertical progress of "caving" is related to the extraction of fragmented ore and its blistering. During the flow of the fragmented ore is reduced the size of the blocks. Primary fragmentation is done by natural mechanical process, advantageously in terms of cost. Sometimes explosive is used in production, making long and spaced holes to induce fracturing.


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Basic issues of geomechanical to the black caving method:

1) 2) 3) 4)

C aveability Mine desig n Frag mentation and extraction control S ubs idence as s ociated


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1) C aveability:   

The process is still not well understood, but it is known that the main factors involved. The caving is principally defined by the rock quality (RMR, Q, etc.) and its hydraulic radius - RH. The basic requirement for the method to work is that the rebate (caving) occurs. The slump of the ore is the result of the action of gravity, being influenced by:  pattern of fracturing the middle ... for good fracturing at least two families cross sub-vertical joints between themselves and one horizontal family ;



stress distribution in the area to be mined.

It is not easy to predict whether the resulting fragmentation which occurs rebate or. 

A rule of thumb: For an ore body be subject to abatement, approx. 50% of the fragments should have a maximum size of 1.5m.

There are several geomechanical classifications to forecast caving and fragmentation. The most commonly used: RMR, Q system, classification of Laubscher (1981) system.


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Geomechanical parameters observed in some mines: 

Palabora Mine (copper) in South Africa ... MRMR between 57-70, which is among the highest values for block caving (this method is not advised when

  

MRMR >50). Henderson Mine (molybdenum) in Denver- USA ... with RMR 27-60. Northparkes (copper-gold) in Australia ... features RMR between 33-54, for Lift 1 (upper part of the ore body). El Teniente, Chile ... MRMR between 55-74, for various lithologies of the mining area (andesites, diorites, breccias).


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Note .:

RMR classification  Developed by Bieniawski  Ranges 0-100  Main parameters:  RQD (Rock Quality Designation)  Spacing between discontinuities  Uniaxial compressive strength of rock  Quality of discontinuities 

 

Presence in the rockrelative mass orientation of the Orientationofofwater discontinuities excavation

Rating MRMR (dev by Laubscher,. Page 413 SME book.) Similar to RMR, but includes stress induced by mining and blasting in the calculation of MRMR parameter.


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2) Mine Des ig n : Key elements in establishing the layout of mine ...

2.1) i s pos s ible divis ion of the area to be mined :  division separated by pillars of security blocks sequentially to be mined;  division into blocks without pillars, with continuous mining.


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2.2) S election of extracting ore s ys tem : 

The extraction system is complex, time consuming and costly preparation. Usually requires several years of work to be put into production.

a) traditional by g ravity s ys tem; b) by s lus her s ys tem; c) by LHD's s ys tem.

a) Traditional g ravity s ys tem - ideal for very frag mented ores :    

loading and transportation system developed under each block orepasses are open and finger raises with grids the level of fragmentation is controlled louvers Finally enhancement is done in the block which begins the fragmentation and migration of the ore and racks through the undercut up to the level of transport.

b) B y s lus her s ys tem- for medium or little frag mented ores:  

development is simplified by omitting grids level. The cones migration ore bind directly to points of discharge. the high wear on the pillars discharge requires very resistant concrete coating.

c) B y L HD 's s ys tem - more modern s ys tem, to little frag mented ore: 

Provides greater productivity drawPoint simpler design eliminates the need for a orepass for each drawPoint but need area larger tube (strut costs!) Due to the size of the equipment. Prof. Dr. H.Z. Harraz Presentation Caving methods

89

Systems ore extraction by gravity


90

Systems ore extraction by gravity


91

Systems ore extraction


92

Ore extraction system for LHD's


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Isometric view of a system of extraction of ore per LHD`s :


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Plan view of the level of extraction of ore to a system with LHD`s:

Drawpoints


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2.3) Location of permanent facilities The block caving is generally used in low resistance of rocks, but the developments and openings production (draw points, etc.) must be kept in places where the rock has better quality.

2.4) Other important as pects ... drawPoint size, spacing (s mall frag ments i mplies clos er drawpoints ), geometry of the pillars, sequence and direction of mining.


96

Typical design parameters …


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Typical design parameters …


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3) Frag mentation and extraction control: 

Fragmentation is difficult to predict and influence the choice of ore extraction system and spacing drawpoints.  The rate of ore extraction affects fragmentation:  very rapid extraction : very

rapid

extraction

can

create

voids

near

the

surface

enhancement; of ore larger because they suffer fewer burdens fragments on the mass of become collapsed overlying materials; for better fragmentation stack height of caved ore must be maximized;

 very

slow extraction :

very slow extraction can cause compaction of the ore and restore locally stable structures. 

Modes of observation of progress caving: see SME Min. Eng. Handbook, 1992 pg.1820.



Accident Northparkes (1999) by sudden collapse of the ore in the abatement process


. 99

Examples of fragmentation obtained in some mines… Mina

Grace Urad Clímax El Salvador Bell Mine Com rastelamento Com Trav Carregamento

Tamanho do fragmento médio D50mm 1.500 700 350 Minério fino


Espaçamento entre os pontos de drenagem (m) 6,0 x 9,0 9,0 x 9,0 10 x 10

7,6 x 7,6 12,2 x 12,2

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Examples of fragmentation inferred from borehole...


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Types of fragmentation : •

•

•

In situ ... represented by the blocks that are naturally present in the rock before mining activities; Primary ... represented by blocks in the vicinity of the cavity abatement and separate themselves from the massive intact when the rebate is started; Secondary ... occurs when blocks of primary fragmentation move for drawpoints. Prof. Dr. H.Z. Harraz Presentation Caving methods

102

Examples of fragmentation observed in drawpoints :

Northparkes E26, Austrália

Esmeralda Sector, El Teniente Mine, Chile


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Cumulative distribution of fragmentation observed in Premier Mine, South Africa


104

4) S ubs idence as s ociated : 

subsidence is discontinuous and affects large areas of the surface;  the final geometry of the subsidence area is quite varied, depending on ...     

resistance ore; resistance overburden; presence of significant structural features (e.g., dykes, faults); depth of mining; natural slope of the surface.


105

Examples of sub sidence - kimberlites in West Africa


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Examples of subsidence - Mine El Salvador, Chile


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Examples of subsidence - San Manuel Mine, USA


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A dvantag es of B lock C aving       

higher production rate than any other method in underground-coal High productivity lower production cost of underground methods, side of the Longwall (e.g., production costs of $ 6.0 / t in the Premier Mine-South Africa) High recovery (90% or more), but with significant dilution production (not development) runs for abatement; i.e. there is no need for drilling and blasting can be highly mechanized good ventilation and security for workers

Di s advantag es of B lock C aving  subsidence and collapse in large scale  high dilution  control resumption is critical to the success of the method  development is slow  costly operations of support  reduction and fragmentation difficult to predict and control  method with little flexibility and no selective  possibility of oxidation of the ore due to the long time of percolation water


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Important R eferences 

     

…

the International Caving Study (ICS) Stages I and II (ICS II sponsors: Codelco Chile, De Beers Consolidated Mines, LKAB, Newcrest Mining Limited, Northparkes Mines, Rio Tinto Technical Services, Sandvik Tamrock, WMC Resources Limited); the ICS I monograph, Block Caving Geomechanics, published by the JKMRC, 2003; Proceedings, MassMin 2000, Brisbane; Proceedings, MassMin 2004, Santiago, and PowerPoint presentations made to that conference; Rock Mechanics for Underground Mining, 3rd edition, by B H G Brady and E T Brown, 2004; Block Caving Geomechanics by E T Brown; Individual works of GP Chitombo, BA Eadie, GE Flores, NJ Harries, E Hoek, The Karzulovic, DH Laubscher, The Logan, The Moss, IA and I Oñederra Ross.

Prof. Dr. H.Z. Harraz Presentation The author has requested enhancement of the downloaded file. All in-text referencesunderlined in blue are linked to publications on ResearchGate.

Caving methods

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7 Longwall Sublevel Caving Block Caving

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