Presentation on theme: "Shaft sinking is a very important part of mine development as it gives access to the resource that is to be exploited. A proper feasibility must be carried."— Presentation transcript:
Shaft sinking is a very important part of mine development as it gives access to the resource that is to be exploited. A proper feasibility must be carried out and various considerations have to be made as to: The application/purpose of the shaft, Its dimensions Proximity to the ore body Inclination Method of shaft sinking. Investment requirements The mine surveyor is very instrumental in: Locating of the shaft, Shaft plumbing General construction and the installation of infrastructure. Often a dedicated Project Surveyor is assigned to the shaft specifically for survey services as and when required because: The whole procedure is meticulous and fairly tedious.
Fig 1.Kibble being lowered
A shaft is an excavation from the surface into the earth. It can either be vertical or inclined, rectangular or circular. Often there is initially no access to the bottom of the shaft. Shaft sinking is the general activity associated with the task of excavating a shaft. Initial considerations include: Analysis of local geology Topography Possible depth Ore body proximity Project and operational risks Operational costs System maintenance. General uses of shafts are: Transportation of men and materials Ventilation Utility conduits Dewatering systems Emergency escape route. Shaft sinking is one of the most notorious undertakings in development due to: Working in restricted space Interaction with high volumes of ground water Difficult geological conditions which often worsen with depth. Given the risks involved in the task, the business is often left in the competent hands of shaft sinking contractors who employ specialized methods and equipment.
There are three shaft sinking methods discussed in this presentation: Raise bore Horidiam Blind Sinking
The raise bore method employs a raise borer to create a shaft and is a very capital intensive piece of equipment and requires competent workmen to operate. The method is applicable in areas where there is access at the target point of the shaft that will allow access for the reamer to be attached to it once the directional hole has been drilled. In the case of very deep shafts a mid access point may be required to facilitate the aforesaid procedure. Location of shaft is done through survey work. Utilities are brought to the site (Power and water that meets the requirements of the size of the equipment. ) A foundation is dug for a concrete pad which is constructed over the area where the machine will be mounted. Raise borer is mounted in the desired location and an accurate pilot hole is drilled using directional drilling techniques. This hole has to be large enough to accommodate the drill string. For deep shafts this is done in two phases. The depth of the pilot hole is determined and limited by the capacity of the equipment to lift the drill rods. Should a high level of accuracy be required the cost of drilling also shoots up. When the pilot hole has reached its target point a shaft collar is constructed to sustain loads that will be generated by the raise borer once reaming commences. The reamer head is then attached to the drill string to increase the diameter of the pilot hole from about 400mm to anything up to 2.7 m depending on the size of shaft required and machinery used. Muck generated is collected at the bottom of the shaft by either air loader or LHD to a waste disposal point.
Raise bore machine and Control Panel
This method is initially similar to that of the raise borer but only up to the end of the reaming phase of the job. It is mainly applicable to very competent ground and is used in shafts in excess of 1000m and those of large diameters, in particular, ventilation shafts. The reamed out hole is then used to drill and charge holes from the Horidiam stage that is lowered to the bottom of the shaft. Holes are drilled into the shaft barrel at 1.2m intervals from the bottom – up the length of the shaft. A man cage is used as a conveyance to and through the Horidiam stage. After drilling is completed the holes are charged from bottom going up and blasts are carried out for two rounds at a time. Blast rock is then collected at the bottom of the shaft in the same way as that for the raise bore method. This method results in a rough wall finish thus making equipping difficult.
Blind sinking refers to the fact that there is no access to the bottom of the shaft by some other means. The technique is often employed using conventional drilling and also by raise bore reverse circulation technology. In the former case it is necessary that a shaft collar be established and the following shaft sinking infrastructure be in place. This includes the headgear, stage winders, kibble, compressed air, mucking and drilling equipment. A concrete batching plant may be used for the liner. A presink is often required so as to get clear of the weathered ground up to about 50m below surface. A cycle is practiced for the establishment of safety standards as well as efficiency. The Galloway stage is lowered into the shaft at this point. Once the presink is complete sinking equipment is then commissioned. The size of the equipment differs with the size of the shaft being sunk. For large shafts the stage may be too high to be installed once the headgear is installed in place. The kibble and stage winders are commissioned with the load testing and mucking system. Generally the shaft sinking cycle is: Drill charge-holes Charge the holes Move the stage up Fire the round Activate force ventilation and allow for re-entry period, make safe and clear the round. Depending on competency, the shaft barrel is lined for every 6.0m of progress so as to provide protection and support to the country rock. Ground conditions dictate the type of support necessary in the shaft, it can be in various forms e.g. bolting and meshing.
Shaft Steel Reinforcement
Cactus grab lashing
Finally the shaft sinking comes to an end once the desired bottom is reached. Permanent structures are installed as sinking equipment is removed. Commissioning is carried out thereafter.
New Headgear Installation
The precise location of a shaft wholly lies in the hands of a competent surveyor. It is often necessary to have gyro bases installed close to the desired location of the shaft so that errors from previous surveys are minimized and not transferred down to the new levels. This process requires the use of a gyro theodolite which may be hired for the task or otherwise a competent team be contracted for the purpose. Shaft plumbing involves the transfer of survey from one level to another. At set up point pegs are established preferably from the gyro bases afore mentioned at a location that will not be affected by shaft sinking despite their proximity. The centre of the shaft is then located and the corners marked off. Preferably pegs can be installed that allow the centre to be located by means of strings crossing over it. Mining can then commence to about 15-20m when it’s safe to establish the collar set. The shaft collar is then constructed and the collar set is set in the concrete therein. For the purpose of control of the shaft barrel during sinking, plumb bobs are used to mark out the shaft outline. These are hung at pre- prescribed points along the perimeter of the shaft. Shaft brackets are welded on the collar set frame and very small holes drilled to allow the plumbing wires to pass through. The wire used for plumbing is known as piano wire. The holes on the brackets are often about 200mm from the side wall. Wires used for plumbing are lowered by means of winches secured close to the shaft. During sinking, the exercise has to be planned very carefully so as to avoid unnecessary delays and errors. This requires communication with mining and those that are involved in the shaft construction that shaft sinking activities will be suspended until the exercise is complete.
It is important to have: Equipment for two survey crews, one at the top level and the other at the bottom level. Necessary attachments Plumb bob winches Piano wire to reach the desired level from the top, Half drums Used oil in sufficient quantity to immerse plumb bobs and the bobs themselves. At best this should be checked off before taking off to underground.
The following steps are then taken once in position, I. Two or more wire are secured at the top level and suspended in the shaft from top to bottom at least a meter apart. ii. Heavy bobs are then attached to the wires Iii.Bobs are then immersed in drums of oil to minimize wire oscillations as much as possible Iv. Care should be taken that the wires do not foul the sidewall. v. One crew is stationed at the top level and the other at the bottom level. The instrument at the bottom level should be stationed at the new peg. At best 2 to 3 pegs should be located at the new station. Vi. Once the wires are steady observations are then made to both wires for angles and distances. Offsets are also taken at this point vii. As a check it is important that the distance between the wires is measured at the top and at the bottom. viii. When all observations have been completed it is important that both teams communicate and agree to remove their equipment from the shaft. Calculations are best carried out at the survey surveyors use the Weisbach triangle to calculate the new set of co- ordinates. Surveys on the shaft should be fairly regular so as to control the barrel profile and minimize over and under sly ping of the walls. This also assists in the design of shaft steel and other related furniture. Of late 3d technology is being applied to shaft surveying. This often involves the scanning of the walls of the shaft after mucking has been completed by means of a scanner. Scanners define the shaft profile very rapidly and accurately thus allowing for timely adjustments and quick decision making.
The procedure of shaft sinking require a lot of forethought and preliminary analysis. Given the three methods discussed above it is the up to management to take up the decision on the best route to follow for the task at hand. Where possible it is best to have the process highly mechanized. Mechanization generally offers much more rapid rates and generous savings on time and resources. The advent of technology also allows all shaft work to be monitored and managed by means of shaft dedicated software like Sight Power Mine Shaft Construction software. This application also reduces costs by avoiding unnecessary delays in information relay to the stakeholders in the project.