Difference between revisions of "Shaft construction"
Revision as of 21:29, 10 February 2011
A shaft is defined as a vertical or inclined permanent opening that gives access and services various levels of a mine. Shafts are designed according to their required purpose and depending on the demands set out.
Shafts can be either vertical or inclined. Inclined shafts decrease the crosscut lengths from orebody and are typically used in developing countries and where the ore body dips at less than 60 degrees. Vertical shafts have the advantage of experiencing greater hoisting speeds, lower maintenance costs, and for the purposes of pre-production are sunk more quickly in almost any ground types. In the past they have been the most commonly considered and appear in most new mines.
Design begins with a cross sectional view of the compartments to suit the needs of the mine that the shaft must provide. The shaft size is dictated by the minimum size to contain all needed conveyances and cages as well as guides to maintain alignment. Provisions for ventilation, mine services, and sometimes a man way must also be taken into consideration. For deeper shafts the minimum shaft size increases to accommodate larger shaft sinking equipment and allow a greater airflow for ventilation purposes.
The most commonly used shaft design in modern day mining is of circular or elliptical shape. In moderate to high production, larger working areas are necessary to provide adequate support. Circular shaft designs are typically chosen when the shaft diameter exceeds 15 feet (De Souza, 2009). The circular nature of the design provides redistribution around the excavation due to the lateral pressures exhibited by the host rock. Although large horizontal stresses are uncommon and shallower depths, once development deepens, a circular design becomes advantageous. A cross sectional view of a typical circular shaft layout can be seen below in Figure 1. Numerous advantages to a circular design are also seen in the construction of the development opening. Most mechanized equipment used in shaft sinking is more suited to a round shape rather than a rectangular because all points on the periphery are along the same radius for the axis of the shaft due to the symmetrical work area (Berry, 1987). The shape can also be drilled and blasted more accurately due to better blasting practice.
For shafts with short life-span expectancies in competent rock, rectangular shafts using timber supports were still commonly used in the early 1990s (Haycocks & Aelick, 1992). Timber supports, bricks, and concrete blocks are commonly used as shaft liners. However, in rocks exhibiting high lateral pressures, these liners do not withstand forces well due to the bending moments created along the length of the excavation. Due to the geometric nature of the shape, the design allows for greater utilization of the cross-sectional area. A typical horizontal shaft layout can be seen below in Figure 2.
Shafts are generally lined with concrete though wood can be safely used in depths up to 2000 ft. When constructed out of concrete, the concrete is generally poured in place after being designed to the minimum needed thickness. Engineered liners are needed when: the surrounding rock is less stiff than the concrete, the shaft must be watertight, or local seismicity & freezing temperatures occur in the shaft. Where stronger compressive strength is need the addition of steel is inefficient and it is less expensive to employ a higher strength concrete. The substitution of roughly 30% fly ash for Portland cement can cut the permeability of the concrete in half and extend the life of the lining significantly.
Guides can take the form of rope guides suspended in the shaft or rigid guides reinforced to take the lateral and vertical forces created by the conveyances. For conveyance speeds of 10m/s or less, analysis of the required steel design is relatively simple. The forces exerted on the guides vary in direct proportion to the mass of the conveyance, the square of the conveyance speed is inversely proportional to the distance that the guide deflection takes place. Extra steel thickness is added to give a factor of safety and account for potential corrosion. At higher speed a more thorough analysis should be undertaken by a qualified engineering firm. When using fixed guides, the minimum clearance between a conveyance and a compartment wall is 1.5 inches for small compartments or otherwise 2 inches. When using rope guides 12 inches is required and 20 inches is needed between conveyances.
Shaft collars consist of the uppermost portion of the shaft and act as a protective barrier to prevent water and soil from entering the shaft. They typically extend from ground surface down to solid bedrock, which the collar is anchored in to. They also provide a rigid support around the shaft to protect it from external loading conditions caused by both the headframe, which is constructed on top of the shaft, and horizontal stresses resulting from nearby structures such as hoist housing or mills. Collar linings are constructed in a similar manner and with the same materials that are used to construct the rest of the shaft. The thickness of the lining is larger to accommodate for the additional stresses and loading which may be present. The collar is constructed in a 'step down' geometry, meaning it is thickest closest to ground surface and decreases in distinct steps as it nears bedrock. The collars usually contain a maximum of three 'steps'. The first lining step typically ranges between 1 to 1.5 m in width, but can be as high as 2m. It is recommended that the first step occur below the limit of frost penetration. The second step ranges between 0.6 - 1m (or approximately 2 times the thickness of the shaft lining) while the third has a thickness somewhere between the second step and the shaft lining thickness (Unrug, Shaft Collars, 1992). The final step, known as the foot, is usually a double conical shape which acts to transfer the load of the collar to the bedrock and should be placed 3 meters below the overburden to ensure secure anchorage into bedrock. Where weak or badly fractured rock is present, it is recommended the collar extend further into the bedrock. Dimensions of the collar are important to ensure they can handle the loading which they are subject to. Some factors for consideration when determining depth, cross section and thickness include the method used to sink the shaft, overburden soil characteristics, in situ ground stresses, hydrology and additional loading conditions. Calculations of lining thickness are done according to structural needs and stresses which the collar will be subject to. The stresses are calculated and required Factors of Safety are determined. From these values the necessary collar dimensions and material properties are obtained. As a general rule of thumb a collar must have a minimum length of 28 m for a concrete lined shaft and 15 m for a timber lined shaft.
Foldable jumbo drills are used for all of the drilling down at the base of the shaft. The jumbos are lowered down to the working face and then its legs are expanded to allow for vertical drilling. They are removed after drilling is complete to allow for blasting (Souza, 2009). Sinking buckets are used lower personnel to the working face of the shaft as well as remove any muck produced by drilling and blasting. During sinking the bucket size should be at least big enough to fill size loads of each foot to be sunk. For the bucket to be stable during the sinking process the width to height ratio should be 2:1 (Vergne J. d., 2003). It is a good idea to slope the much pile so it sits on an angle from the vertical position of the bucket. The contractor developing the shaft should be able to get 10 buckets of water per shift without compromising advance in a wet shaft.
The excavation of any muck produced at the bottom of the shaft is done using two methods. One method utilizes grabbing hands which are attached to the galloway. Operated from the galloway, the operator will scoop and dump the muck into the sinking buckets to remove the waste. Multiple grabbing arms can be used for quicker excavation rates. Mucking is also done using an EMC0 630 loader (Souza, 2009). This method is used typically at shafts with diameters greater than 18 meters but with innovation can be used at 15 meters (Vergne J. d., 2003). They are lowered to the base of the shaft and removed afterwards for drilling and mucking.
Shaft sinking is generally done by private contractors. This is because private contractors tend to have experience and technical knowledge to complete a shaft quickly and safely. Shafts are more expensive on a per meter basis in comparison to ramps. This is due to the slow rate of advance and the complicated development and technical considerations when developing. Typical costs per meter of development are outlined below.
Shaft sinking equipment is capable of an average advance of 3 meters per day with peak performance of up to 4 meters. (The history of shaft sinking, 2008)
Headframes support the friction hoist or the sheave wheel that the hoist ropes pass over to reach the drum hoist. Their height above the surface allows for material to be dumped on surface.
Headframes can potentially be made from steel, timber or concrete but modern technical requirements have made wood almost obsolete. A comparison of steel and concrete structures can be found below
There are two general design types, one uses backlegs and the other is a tower type headframe. Backleg towers include A-frames, Four Posts or Six Posts and tower type headframes are simply a vertical tower, constructed directly on top of the shaft and collar. Headframes must be able to support numerous loads:
- Dead load is the headframe, sheave wheels, conveyances and their contents.
- Live load occurs when hoisting at maximum capacity
- Braking load which arises as the conveyances stop in the shaft and the forces transfer to the sheave
- Wind load and snow load which depend on the dimensions and location of the structure
- The effects of temperature and seismic stresses in the area
The headframe design should allow for the placement of mine services nearby. For example, a lamp room, waiting area, first aid room, dry, maintenance, and administrative offices. The skip dumping area should be located in close proximity to the waste storage and handling facilities. There should be adequate access for installation maintenance and removal of conveyances and the hoists & sheaves During shaft sinking the headrames are generally made from wood or steel. Prefabricated hollow steel gives strength and allows for easy set up and take down The installation of a permanent headframe tends to be more economical and time saving. The permanent skeleton is designed to support the installation of the sinking sheaves. As the shaft is sunk the headframe and surrounding services can be fully installed Development After the establishment of a headframe shaft sinking can begin