Abstract
This dissertation deals with the philosophy behind conventional vertical shaft sinking practices. While most of the topics covered within the dissertation may be "old hat" to experienced engineers, the younger engineer or manager who finds themselves accountable for a shaft sinking project will see the contents of this dissertation as helpful and contribute to the body of knowledge around conventional shaft sinking practices.
The primary objective of this dissertation is to provide engineers with a comprehensive understanding of the intricacies of conventional shaft sinking as it is practiced globally – a guide memoire. Through the in-depth examination of the subject matter, the study delves into the technical aspects of shaft sinking, including excavation methods, ground support systems, dewatering techniques, cover drilling, equipping, surveying, stage design, shaft communication and safety considerations. The dissertation covers theoretical and practical examples with sketches, photos and tables presenting a holistic overview of shaft sinking practices.
The lack of comprehensive resources concerning conventional shaft sinking covering all aspects of the process motivated the compilation of shaft sinking techniques. The goal was to provide a document to fill this gap and provide a valuable resource under one cover for professionals, students, and researchers in the field.
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Underground orebodies are accessed through underground mining techniques that involve the construction of shafts to reach the orebodies. The mining industry invariably has deeper shafts than those used in the civil and public sectors. Shafts are either circular, oval, or rectangular. The choice of skips, cages, auxiliary equipment, and ventilation governs their dimensions. The speed at which the shaft is required to be sunk and the sinking technology used to sink the shaft are also critical factors when selecting a shaft size. Circular shafts are the most common as they have proven more efficient in sink speed and are better suited to mechanised equipment and mining.
The evolution of shaft sinking techniques and technologies provides valuable insight into the industry's progress. Tracing the historical development of shaft sinking and discussing the advancements in equipment, methods, and materials provides a broader perspective and appreciation for conventional shaft sinking. The historical development of shaft sinking offers a solid foundation and context for understanding the subject matter, demonstrating how vertical shafts have evolved, the challenges that early engineers faced, and the solutions they devised. The historical perspective highlights the advancements, innovations, progression, collective knowledge, and experience gained over time, how current shaft sinking techniques have developed to technological, engineering, and societal changes, and an appreciation of the complexities involved in a shaft sinking project.
The decision to sink a mine shaft is an iterative process requiring several variables and options to consider before arriving at a techno-economic decision. To start,
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the business and design objective of the shaft must be established. Knowledge of the orebody, mine design criteria, and mining methods follows. Comparing net present values (NPV), internal rate of return (IRR) and payback periods from different options significantly impacts the decision-making process. The design and shaft execution also go through several steps and processes that substantially affect the schedule, cost, and makeup. Front-end engineering is critical to the project's success, from concept to feasibility, construction, and performance.
Mine owners must involve specialist shaft sinking companies early in their decision-making. The design of shafts includes multiple disciplines that need to fit like a cog in a gearbox to function correctly. Several conventional methods can be applied to shaft sinking, and selecting the best alternative must consider all possible options; sufficient design and costing must be undertaken to evaluate the relative options. The cost of sinking shafts will vary in size, depth, nature of the rock, support requirements, the amount of water handling, the required sinking rate, labour requirements, work organisation, and other considerations. The cost consequence of the above points is self-evident. In addition, calculating the duration of the sink and equipping phase cycles as accurately as possible is essential and involves considering project requirements, equipment capabilities, geological conditions, and the health and safety of the workers. A typical shaft sink cycle comprises drilling, blasting, muck removal, lateral support, concrete lining, installation of services, and face preparation, all of which are in-line activities
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affecting the critical path. A delay in any activity or moving from one cycle activity to another will affect the shaft duration.
Today, independent mining contractors sink most shafts. Two principal methods of sinking a shaft are blind shaft drilling or Raise bore and slipe if bottom access is available. The sinking process invariably comprises a sequence of events that establish the surface infrastructure, excavate the collar excavation, and conduct the pre-sink before conventional shaft sinking operations can commence. Site Establishment includes all the services and support infrastructure for the operations on the surface and underground. After the site’s establishment, work begins on the collar excavation, followed by a pre-sink phase. Various plant items and equipment required for the main sink operation are prepared during the pre-sinking phase of the project. Collar excavation precedes the pre-sink phase. Geological information is crucial in the methodology chosen to excavate a collar. The pre-sink steps that follow collar excavation form the transition phase between collar excavation and sinking operations. The pre-sink is carried out to a depth between 60 and 110m below the collar excavation. On completion of the pre-sink, the stage, headgear, roping up, and commissioning of the winding system will be conducted.
The sinking cycles play a crucial role in the design and operation of a sinking shaft. Besides determining the duration of the sink and equipping phase cycles, cycles are essential for maintaining safety, efficiency, ground control, monitoring, and
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Quality control. A typical sinking cycle comprises drilling, blasting, mucking, installing support, aligning the curb ring, installing services and face preparation.
In the main sink phase, some stages have an underbelly mechanical system that facilitates drilling. In other situations, hydraulic drill jumbos are slung down from the surface to the shaft bottom. Drilling is a crucial function in the sinking cycle. Reducing even a few minutes of drilling time can result in substantial savings. The current shaft sinking blasting practice comprises bulk emulsion explosives with electric or non-electric initiation.
Mucking or loading broken rock from the shaft bottom is critical in the drill-blast-load cycle of a shaft sinking operation and a function that occupies a substantial portion of the time required to complete one cycle. The effectiveness of the cleaning cycle is dependent on the design options around the kibble and hoisting capacities. Various mucking units are available in the market with their limitations. Pneumatic Eimco 630 loaders, due to operator safety, are no longer favoured in the industry. The cactus grab became famous for shaft cleaning due to its high loading rate, but with today's drive for zero harm and no shaft accidents, it has lost favour as it's not the safest mucking system. Clamshell-type muckers - Riddell and Cryderman muckers became widespread in North American mines. However, small hydraulic excavators have become the shaft bottom cleaning method of choice.
Shaft sinking practices typically require cementation work to control water-bearing formations encountered in shaft sinking operations. The main water feeders in a sinking project are faults, fractured rock strata, dykes, and sills. In
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some formations, such as dolomite, water movement along such open cracks can create large openings or cavities by dissolving the rock. Ring cover and injection grouting are critical steps in the sinking process to safeguard the shaft against flooding. Cover drilling and grouting of shafts have been well established over the past 50 years, incorporating various technologies. The mining industry commonly uses cement or chemically applied compounds for grouting or cementation. The process fills all cavities and fissures in the rock strata, strengthening the rock strata and rendering the rock structure impermeable to the passage of water.
The stage/galloway is a series of platforms/decks suspended in the shaft barrel by ropes attached to the stage winder. All barrel activities occur from the stage, which is the in-barrel heart of the sinking operation. The stage's principal function is concreting the shaft sinking operations, extending services, and equipping the shaft once the sinking is complete. The design of a stage is critical, and attention to detail is necessary to ensure a functional platform from which much of the shaft sink operation takes place.
Kibbles and crossheads are crucial in efficiently transporting materials and personnel between the surface and underground excavations in a shaft-sinking operation. Kibbles are large metal buckets that transport rock, soil, and other excavated debris from the bottom of the excavation to the surface. These buckets are attached to a hoist system that lifts the shaft and empties the contents at the surface.
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Ropes, kibbles, and attachments are critical components of a shaft sink project. The document offers engineers' theories behind rope construction, choices, and methodologies for front-end, back-end, and rope-up talks.
Shaft equipping is a specialised task in shaft sinking that requires a team of skilful personnel, mostly artisans with experience in shaft work installations. The permanent equipping of a shaft can be conducted concurrent with the sink, albeit it will slow the sinking phase from the bottom up or from top to bottom. The top-to-bottom method has been the preferred equipping method for South African sinking companies.
Numerous engineered safety systems are incorporated in the engineering of a shaft sink project; encoders have become the new digital replacement instruments for reading the rotational travel and speed of the winder drum shaft and have, in conjunction with the winder PLC, now replaced the old Lilly and Ardic mechanical units almost entirely in modern sinking systems. Communication between the shaft bottom stage, bank and winder house is crucial to the safe operation of a shaft sink project, meriting some coverage of the latest communication trends in the industry.