Shaft mining or shaft sinking is the action of excavating a mine shaft from the top down, where there is initially no access to the bottom.[1] Shallow shafts, typically sunk for civil engineering projects, differ greatly in execution method from deep shafts, typically sunk for mining projects.
Shaft sinking is one of the most difficult of all mining development methods: restricted space, gravity, groundwater and specialized procedures make the task quite formidable.[2] Shafts may be sunk by conventional drill and blast or mechanised means.
Historically, mine shaft sinking has been among the most dangerous of all the mining occupations and the preserve of mining contractors called sinkers.[3] Today shaft sinking contractors are concentrated in Canada, Germany, China and South Africa.
The modern shaft sinking industry is gradually shifting further towards greater mechanisation. Recent innovations in the form of full-face shaft boring[4] (akin to a vertical tunnel boring machine) have shown promise but the use of this method is, as of 2019, not widespread.[5]
Mine shafts are vertical or near-vertical tunnels, which are "sunk" as a means of accessing an underground ore body, during the development of an underground mine.
The shape (in plan view), dimensions and depth of mine shafts vary greatly in response to the specific needs of the mine they are part of and the geology they are sunk through. For example, in North and South America, smaller shafts are designed to be rectangular in plan view with timber supports. Larger shafts are round in plan and are concrete lined.[6]
Mine shafts may be used for a variety of purposes, including as a means of escape in the event of an emergency underground and allowing for the movement of:
When the top of the excavation is the ground surface, it is referred to as a shaft; when the top of the excavation is underground, it is called a winze or a sub-shaft. Small shafts may be excavated upwards from within an existing mine as long as there is access at the bottom, in which case they are called raises.
A shaft may be either vertical or inclined (between 80 and 90 degrees to the horizontal), although most modern mine shafts are vertical. If access exists at the bottom of the proposed shaft, and ground conditions allow, then raise boring may be used to excavate the shaft from the bottom up; such shafts are called borehole shafts.
Following the Hartley Colliery disaster where the single shaft at the mine became blocked,[7] the United Kingdom made single shaft mines illegal in 1862, establishing the practice that all underground mines must have a "a second means of egress". Many other global mining jurisdictions have adopted this rule and shafts are therefore often found in pairs (although there are multiple alternative methods of providing a second means of egress).
Currently, the deepest continuous single-lift mine shaft in the world is the main shaft at South Deep Mine in South Africa, owned by Gold Fields Limited, which has a depth of 2991 meters.[8] Along with its twin ventilation shafts, it took ten years to sink and equip.
The most visible feature of a traditionally-built mine shaft is the headframe (or winding tower, poppet head or pit head) which stands above the shaft. Depending on the type of hoist (or winder) used, the top of the headframe will either house a hoist motor or a sheave wheel (with the hoist motor mounted on the ground). The headframe will also typically contain bins for storing ore being transferred to the processing facility.
At ground level beneath and around the headframe is the Shaft Collar (also called the Bank or Deck), which provides the foundation necessary to support the weight of the headframe and provides a means for workers, materials and services to enter and exit the shaft. Collars are usually massive reinforced concrete structures with more than one level. If the shaft is used for mine ventilation, a plenum space or casing is incorporated into the collar to ensure the proper flow of air into and out of the mine.
Beneath the collar the part of the shaft which continues into the ground is called the shaft barrel.[citation needed]
At locations where the shaft barrel meets horizontal workings there is a shaft station (or inset) which allows men, materials and services to enter and exit the shaft. From the station tunnels (drifts, galleries or levels) extend towards the ore body, sometimes for many kilometers. The lowest shaft station is most often the point where rock leaves the mine levels and is transferred to the shaft, if so a loading pocket is excavated on one side of the shaft at this location to allow transfer facilities to be built.
Beneath the lowest shaft station the shaft continues on for some distance, this area is referred to as the shaft bottom. A tunnel called a ramp typically connects the bottom of the shaft with the rest of the mine, this ramp often contains the mine's water handling facility, called the sump, as water will naturally flow to the lowest point in the mine.
Many (although not all) shafts are lined following excavation and the installation of temporary ground support. The shaft lining performs several functions; it is first and foremost a safety feature preventing loose or unstable rock from falling into the shaft, then a place for shaft sets to bolt into, and lastly a smooth surface to minimise resistance to airflow for ventilation.
Final choice of shaft lining is dependent on the geology of the rock which the shaft passes through, some shafts have several liners sections as required[9] Where shafts are sunk in very competent rock there may be no requirement for lining at all, or just the installation of welded mesh and rock bolts. The material of choice for shaft lining is mass concrete which is poured behind shaft forms in lifts of 6 m as the shaft advances (gets deeper).
Shotcrete, fibrecrete, brick, cast iron tubing, and precast concrete segments have all been used at one time or another. Additionally, the use of materials like bitumen and even squash balls have been required by specific circumstances. In extreme cases, particularly when sinking through halite, composite liners consisting of two or more materials may be required.[10]
El revestimiento del pozo no llega hasta el fondo del pozo durante la inmersión, sino que queda retrasado a una distancia fija. Esta distancia está determinada por la metodología de excavación y el espesor de diseño del revestimiento permanente. Para garantizar la seguridad de las personas que trabajan en el fondo del pozo, se instala un soporte temporal de tierra, que generalmente consta de malla electrosoldada y pernos de roca . La instalación del soporte temporal del suelo (llamado empernado ) es una de las partes más desafiantes físicamente del ciclo de hundimiento del eje, ya que los pernos deben instalarse utilizando perforadoras de roca neumáticas .
Por esta razón, y para minimizar el número de personas en el fondo del pozo, varios proyectos han cambiado con éxito al hormigón proyectado para este revestimiento temporal. La investigación y el desarrollo en esta área se centran en la aplicación robótica de hormigón proyectado y la comercialización de revestimientos finos de polímeros proyectados .
Cuando el eje se va a utilizar para izar, frecuentemente se divide en múltiples compartimentos mediante conjuntos de ejes , que pueden estar hechos de madera o acero . Los miembros verticales en un conjunto de ejes se llaman guías , los miembros horizontales se llaman buntons . Para guías de eje de acero, las dos opciones principales son secciones estructurales huecas y secciones de sombrero de copa. Las secciones de sombrero de copa ofrecen una serie de ventajas sobre las secciones estructurales huecas, incluida una instalación más sencilla, una mejor resistencia a la corrosión y una mayor rigidez. Los medios de transporte mineros se desplazan sobre las guías de forma similar a como lo hace una montaña rusa de acero sobre sus rieles, ambos con ruedas que los mantienen seguros en su lugar.
Algunos ejes no utilizan vigas guía, sino que utilizan cables de acero (llamados cables guía ) mantenidos en tensión mediante pesos masivos en la parte inferior del eje llamados pesos de queso (debido a su parecido con un carro o rueda de queso), ya que son más fáciles de mantener y reemplazar.
El compartimento más grande se utiliza normalmente para la jaula de la mina , un medio de transporte utilizado para mover trabajadores y suministros debajo de la superficie, que está suspendido del polipasto mediante un cable de acero. Funciona de manera similar a un ascensor . Las jaulas pueden ser de uno, dos o, rara vez, tres pisos y siempre tienen múltiples sistemas de seguridad redundantes en caso de falla inesperada.
The second compartment is used for one or more skips, used to hoist ore to the surface. Smaller mining operations use a skip mounted underneath the cage, rather than a separate device, while some large mines have separate shafts for the cage and skips. The third compartment is used for an emergency exit; it may house an auxiliary cage or a system of ladders. An additional compartment houses mine services such as high voltage cables and pipes for transfer of water, compressed air or diesel fuel.
A second reason to divide the shaft is for ventilation. One or more of the compartments discussed above may be used for air intake, while others may be used for exhaust. Where this is the case a steel or concrete wall called a brattice is installed between the two compartments to separate the air flow. At many mines there are one or more complete additional separate auxiliary shafts with separate head gear and cages.
The lowest point in a sinking shaft is known as the "shaft bottom". Shaft projects differ from some other forms of mine development in that all activities that take place on the shaft bottom become part of the critical path for the project schedule. The infrastructure required to sink a shaft is referred to as "the sinking set-up".
It is typical for progress (the "sinking rate") in the sinking phase (that is excavation, ground support and lining) of a shaft project to follow a learning curve as the project team repeats the same series of activities over and over in what is called "the sinking cycle", eventually approaching the theoretical maximum rate for that sinking set up over time. The use of experienced shaft sinkers is necessary to reduce the length of this learning curve and thus the duration of the project as much as possible.
Key to a successful shaft sinking project are:
Although significant emphasis is placed on the rate of progress of a project sinking cycle by shaft sinkers, sinking is only one of a number of phases in the conventional construction of a new shaft, as follows;
As with the depth and design of shafts, significant variations may exist in this sequence depending on local conditions. For example, shafts in the Canadian Shield generally do not need a deep and complex shaft collar since the bedrock is both strong and close to the surface. This reduces the amount of time required to establish the shaft collar.
Traditionally, sinking contractors would build a temporary headframe for the sinking set-up, which would then be dismantled to make way for a permanent headframe. With the growth in complexity and duration of shaft sinking projects over time it has become more common to incorporate more of the permanent shaft set-up into the sinking phase. This results in a reduced overall project duration, as for example, if the service piping used to sink the shaft does not need to be stripped out to make way for permanent piping.
With the advancements made in raise boring technology, raise borers have been used to create a pilot hole for shaft sinking, where access exists at the bottom of the new shaft, in this case the sinking phase is dedicated to enlarging this pilot hole to full diameter (a process usually called "slashing"). This methodology can be considerably faster than full face sinking as muck (waste rock) from sinking falls down the pilot hole and is handled using existing mine infrastructure off critical path.