The Geology of the Target Area of Mining - Coursework Example

SUMMARY In this feasibility study gives the design options that can be used to construct decline in the mining of copper and silver in Montana State USA. In this report the geology of the target area of mining has been given where the type of rock formations have been given and also the depth of the mineral deposit. Mechanical excavation and TMBs have been identified as the possible excavation methods. Two methods that can be used in the excavation of the tunnel have been described. The length of tunnel for 150 declination was found to be1931m, 1710m for 170 and 1460 for 200. The cost approximation was found to be $14.62 millions for 200 and 14.345 millions for 150tunnel. In conclusion the report gives the major reasons why TBMs is preferred and the exceptional conditions that may need the use of mechanical excavation. TABLE OF CONTENTS 1.2 Methods of excavation 4 1.3 D & B and other mechanical excavation 4 .4 Calculation of tunnel length 7 1.5 Time duration for D&B 8 1.6 TBMs Method 9 1.7 Challenges with TBMs 11 1.8 Permitting and Environment 12 1.9 Conclusions 14 Recommendations 15 1.0 Introduction In this feasibility study gives the design options that can be used to construct decline in the mining of copper and silver in Montana State USA. In this report the geology of the target area of mining has been given where the type of rock formations have been given and also the depth of the mineral deposit. Two methods that can be used in the excavation of the tunnel have been described. The equipment needed, time estimate and the costs for the two methods have also been given. The reports also gives the length of the tunnel which is associated with different angles of declination. 1.1 Geology and project location The project is located in Montana State in USA. The projects targets silver and copper deposits that is located in extensive series metasedimentary rocks. The rock formations have three distinct subdivisions that depend on the level of siltite, quartzite and silty quartzite. In the lower region where the mineral deposit are found has quartzite as the dominant as the dominant rock type. The copper and silver are found in three mineral: bornite, chalcocite. The targeted mineralized zone depth has an average of 500 with the deepest point being 1300M. The average depth of the mineral deposit is 35 feet. The citizen of Lincoln County are the immediate neighbor of the project and have shown a lot of support to the project as they anticipate it to reduce the rate of unemployment which is has been describe as being above average. 1.2 Methods of excavation There are two major options of decline construction to access the mineral deposit which are use of Tunnel boring machinery (TBM) or use drilling and blasting (D&B) and other mechanical and method. In all the two methods there can be variation in tunnel design in terms of cross section area and the decline angle. In terms on the decline angle the restriction is posed by the type of mechanism to be used in transporting the minerals to ground. In this case the means to be used is a conveyor and therefore the maximum grade will be restricted to between 15-20. The cost of drilling also varies with the decline angle with the highest angle having the highest cost being associated with it. 1.3 D & B and other mechanical excavation Drill and blast and other mechanical method of excavation is one of the method that can be used in the decline construction. For drill blast the required equipment include drilling jumbo, mucking equipment, air and water discharge pumps, electrical generator, a loader, scissor lift, haul dump truck, scalar and backhoe caterpillar. There is need for supplies like drill bits, drill steel and blasting powder and caps. In drill and blast there will be extra cost which is associated with building temporary excavation structures such as access adits, tunnels and shafts. Temporary access structures may not be welcome by local population while in remote areas temporary structures construction may be difficult and also very costly to construct. The use of D&B may be advantageous in the case where rocks in question are hard, presence of rock masses that are of low quality. The use of D&B can also be ideal in a case where there is a cheap labour and a short lead time is required. In the present case labour is available from the Lincoln County where there is high rate of unemployment. This can also be a way of satisfying the local community. A short lead time can also be beneficial in case there is fear of political change or change in policies which may affect the project. The accident occurrence in D&B are approximated to be double those that occur in TBM operations. With drill and blast construction there is high over break which is influenced by lithology, the quality of blasting in practice and intact and rock mass proportions. In drift and blast excavation at least 10% over break is anticipated with a possibility of the over break running up to 25%. The approximate variation of cost of excavation of a 4m x 5m by drill and blast method for a given angle of declination is given in table 1 according to a project done in South Africa. Table 1: Advance rate and cost of excavation for various declination angles DECLINATION ADVANCER/DAY COST/METRE 8 deg. 4.5 $5714 10 deg. 4.0 $6571 15 deg. 2.7 $7429 20 deg 1.3 $10000 Figure 1: Illustration of the tunnel length variation with declination angle In figure 1 point a represent the contact point of the decline with the mineral deposit. From the figure it can be seen that depending on the angle of decline the ground outlet point of the decline can either point C or point B. It can also be observed that BD = CE which is the depth to the mineral deposit from the ground. The difference in the length of line AB and line AC represent the difference in the length of the tunnel for the two angles of declination. 1.4 Calculation of tunnel length Assuming the angle of decline is 150 Taking CE = 500m Triangle ACE being right angled AC = 500 / sin 15 = 1931m For decline angle 200 Taking BD = 500 Triangle ABD being right angled AB = 500/sin 20 = 1461 m For decline angle 170 Length = 500/sin 17 = 1710m The cost of the excavations using the cost of excavation per meter given in Table 1. The cost for 150 declination tunnel 1931x7429 = 14 345 399 The cost for 200 declination tunnel 1461x10000 = 14 610 000 This cost is for mechanical cultivation 1.5 Time duration for D&B Time duration for construction of 150 decline Time = Length of tunnel/Rate of advance From table 1 the rate of advance for 150 is 2.7m/day = 1931/2.7 = 715 days Time duration for constructing of 200 Time = Length of tunnel/Rate of advance = 1462/1.3 = 1124 days From this calculation it can be seen that the cost of 20 declination tunnel is higher even though it is of shorter length. The cost of operations is also expected to be higher for the steeper tunnel. The steeper tunnel can be an alternative where the outlet is restricted due to several reasons including presence of infrastructure like building, the path having unstable materials or extremely hard material that may require extra effort to make way. In the present situation having an inclination angle of 15% does not have any hindrance. 1.6 TBMs Method TBM is another method that can be used in excavation of the tunnel. The labour requirement when using the TBMs include; foreman, operator/mechanic, electrician, 2 labourers for utilities, 2 brakeman and at least 10 labourers. There is less skills requirement and training of the manpower is much simpler as compared to the case of drill and blast. The equipment required is the TBMs itself, trails and associated equipment including TBM cutters. There is high uncertainty in the prediction and use of TBM as compared to the use of drill and blast. This is an indication that in the process of using the TBMs it is possible to meet an anticipated condition which may change the whole situation of operation. There is a lot of saving which can run up to 90-95% for steel rib support when using TBMs. Figure 2: Unshielded gripper TBM schematic drawing (COE, 1997) According to (Tarkoy, 1995) the rate of advancement can be up to six times in TBM method when compared to drill and blast method. The cost of excavation has also been reported to be lower for mechanical boring as compared to drill and blast excavation with cost variation being about 40% in tunnels that are more than 1000m. The actual variation of a specific project will depend on a number of factors including the rock type. The cross section area will also depend on the material to be transported. The tunnel length can is assumed to be the same in both the methods and thus the previous calculations applies. Cost of excavating the 150 can be approximated to 60/100x 14 345 399 = $8 607 239 The cost for constructing the 200 tunnel 60/100x14 610 000 = $8 766 000 Technological advancement has seen great change in TBMs with places where the use of the technology was thought to be impossible now being applicable. The conditions which were previously impossible include granite, poor rock conditions and occurrence of sand mixed with ‘arenisca dura’. In terms of hardness, rock hardness of over 400Mpa has been able to be bored (Brinkgreve, 2001). The factors that has made this possible include increase in diameters of cutters by a factor of 1.5, increase in cutter gauge velocities by a factor of 2 and increase in cutter load capacity by 7-9 factor. There has also been considerable improvement in geometry and in cutter metallurgy. Shafts are known to play a vital role in planning of the mining with location being pre-determined but the actual shaft location is subject to change when there is encounter of adverse geotechnical site conditions (Garshol, 2003) 1.7 Challenges with TBMs Thrust pressure: when using TBMs on a decline the force of gravity acts on the machine and is possible for the thrust pressure to be reduced in the proportion to the angle of incline. In blocky ground there is a chance that the machines dead weight may rest on only a few cutters and this may result in the bearing capacity being exceeded. Slip-back prevention: On a decline the machine may require a mechanism that will prevent occurrence of forward slippage when work is progressing at the heading or when the cutters are being changed. This can be achieved by bracing the machine at the lining. There is also need to minimize the need to access the heading through provision of the cutters that are changeable from the rear. Muck-handling: on the process of excavating, the decline angle can be counteracted by raising the pick-up point and lowering the discharge point. This measure is only adequate up to certain decline angles. As a further measure fluted belts or chain conveyors can come into play; though the working angle will affect the capacity. When choosing the design, water inflows should be considered, as this increases the difficulty of pick –up muck. Fluids: Hydraulic and lubrication fluids may have the tendency to pool in to the region of the system in contrast to the areas of their requirement. Since there is increased potential even flooding, there is need for components to be raised substantially from the invert. 1.8 Permitting and Environment There are several approval stages that the project should undergo. Permits and approvals are required from USFS, the Army Corps of Engineers and the State of Montana. It is the responsibility of USFS and State of Montana to take a joint review of the project and related permits. The USFS is in charge of preparing an EIS, soliciting and considering public comments, is also in charge of undertaking a biological assessment and consults with the U.S. Fish and Wildlife Service in order to be in a position to issue a biological opinion that will address the impact of the project on the endangered species like grizzly bear, bull trout and lynx. It is expected that after the issuance of a final EIS there will be issuance of a record of decision by USFS which will set the decision on the proposed plan of operation. After the issue of the record of decisions including the decision on appeals and legal challenges on the same, it is expected that the required permits will be received from the relevant agencies and there will be finalizing of the project based on the results that was completed by agency review. The process will involve USFS and DEQ issuing a draft EIS relating to the project. A meeting for public comments on the EIS of about two weeks will be held and this will include a site meeting for USFS, EPA and the Army Corps of Engineers whose function will be to review the project. USFS and DEQ will compile a list of public comments and issues which will be utilized in coming up with the final EIS. Effort will be directed towards reviewing technical details regarding the project and the comments made by the public. Part of what will be expected to be reviewed is a hydrological model and they will work on modalities on how its implementation can work in the final EIS. The model will address issues like water quality, water quantity among other public and agency comments in the project. Geochemical analysis will be contacted which will address water quality and waste rock quality. Plan detailing waste tonnages and disposal areas will also be produced. Evaluation involving an analysis of potential project discharges of dredged or/and fill material in the country’s waters. Such discharge regulation is under Section 404 of Clean Water Act that directs that a permit be obtained before dredged or filled materials are discharged. In the process of selecting an alternative that has the least damage to the environment, the relevant government agencies will be charged with the responsibility of evaluating the locations that have been proposed for the construction of tailings impoundments basing on the location that has the best ability to protect the aquatic ecosystem at the same time the project purpose being met. USFS will be expected to initiate a biological assessment and there will be need to collect baseline data with regards to fisheries, the quality of water, grizzly bear and other environment components. As part of the biological assessment the selection of the electricity transmission lines alignment will be expected to be approved by the State of Montana. In this connection there will be need to have a consultation between USFS and U.S. Fish and Wildlife Service with a record of decision approval being issued if issues are resolved satisfactorily. 1.9 Conclusions The objectives of this study were to investigate the methods which can be used in the excavation of a decline in mining of copper and silver in the study area. The various issues that surround the two methods that can be used in mining have been addressed. From the study the following conclusions can be made. TBM will result in cost savings as there is chance of elimination of construction of temporary structures due to the fact that the method results in high excavation rates. Although having a smaller decline angle increases the length of decline to be excavated it is still economical to excavate a longer decline which is less steeper as the cost associated with construction of a steeper decline per unit length overrides the saving made from shortness of the decline TBM excavation is advantageous when progress rate is high even when the cost associated with temporary structures is eliminated. This is a result of high requirements of support structures and lining required in mechanical excavations. The advantage with mechanical excavation is that it can be used where the rock is very hard and the fact that the technique used in different situations in the process of excavation can easily be changed to meet the requirement at a particular point. Recommendations The following are the recommendations Further geological studies be conducted to establish the underground water hydrology and the quality of water that is likely to be discharged to the water ground surface. TBM method should be taken as the preferred method but if further investigation existence of very hard rock mechanical excavation should be used as the second alternative. References Tarkoy J.P. (1995). Comparing TBMs with drill+blast excavation. COE (U.S. Army Corps of Engineers) 1997. Tunnels and shafts in rock. Brinkgreve R.B.J., Vermeer P.A. (2001). Plaxis 3D Tunnel, Version 1, Complete set of manuals, Finite element code for soil and rock analyses. PLAXIS B.V. A.A. Balkema Publishers. Crova R. 2006. Example of Turin Metro Line 1. Gruppo Torinese Trasporti (GTT), Lectures at the Master Course in Tunnelling and TBMs, Edition V, Politecnico di Torino, Italia. Garshol K.F. 2003. Pre-Excavation Grouting in Rock Tunneling. 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cess structures may not be welcome by local population while in remote areas temporary structures construction may be difficult and also very costly to construct. The use of D&B may be advantageous in the case where rocks in question are hard, presence of rock masses that are of low quality. The use of D&B can also be ideal in a case where there is a cheap labour and a short lead time is required. In the present case labour is available from the Lincoln County where there is high rate of unemployment.

This can also be a way of satisfying the local community. A short lead time can also be beneficial in case there is fear of political change or change in policies which may affect the project. The accident occurrence in D&B are approximated to be double those that occur in TBM operations. With drill and blast construction there is high over break which is influenced by lithology, the quality of blasting in practice and intact and rock mass proportions. In drift and blast excavation at least 10% over break is anticipated with a possibility of the over break running up to 25%.

The approximate variation of cost of excavation of a 4m x 5m by drill and blast method for a given angle of declination is given in table 1 according to a project done in South Africa. Table 1: Advance rate and cost of excavation for various declination angles DECLINATION ADVANCER/DAY COST/METRE 8 deg. 4.5 $5714 10 deg. 4.0 $6571 15 deg. 2.7 $7429 20 deg 1.3 $10000 Figure 1: Illustration of the tunnel length variation with declination angle In figure 1 point a represent the contact point of the decline with the mineral deposit.

From the figure it can be seen that depending on the angle of decline the ground outlet point of the decline can either point C or point B. It can also be observed that BD = CE which is the depth to the mineral deposit from the ground. The difference in the length of line AB and line AC represent the difference in the length of the tunnel for the two angles of declination. 1.4 Calculation of tunnel length Assuming the angle of decline is 150 Taking CE = 500m Triangle ACE being right angled AC = 500 / sin 15 = 1931m For decline angle 200 Taking BD = 500 Triangle ABD being right angled AB = 500/sin 20 = 1461 m For decline angle 170 Length = 500/sin 17 = 1710m The cost of the excavations using the cost of excavation per meter given in Table 1.

The cost for 150 declination tunnel 1931x7429 = 14 345 399 The cost for 200 declination tunnel 1461x10000 = 14 610 000 This cost is for mechanical cultivation 1.5 Time duration for D&B Time duration for construction of 150 decline Time = Length of tunnel/Rate of advance From table 1 the rate of advance for 150 is 2.7m/day = 1931/2.7 = 715 days Time duration for constructing of 200 Time = Length of tunnel/Rate of advance = 1462/1.3 = 1124 days From this calculation it can be seen that the cost of 20 declination tunnel is higher even though it is of shorter length.

The cost of operations is also expected to be higher for the steeper tunnel. The steeper tunnel can be an alternative where the outlet is restricted due to several reasons including presence of infrastructure like building, the path having unstable materials or extremely hard material that may require extra effort to make way. In the present situation having an inclination angle of 15% does not have any hindrance. 1.6 TBMs Method TBM is another method that can be used in excavation of the tunnel.

The labour requirement when using the TBMs include; foreman, operator/mechanic, electrician, 2 labourers for utilities, 2 brakeman and at least 10 labourers. There is less skills requirement and training of the manpower is much simpler as compared to the case of drill and blast. The equipment required is the TBMs itself, trails and associated equipment including TBM cutters. There is high uncertainty in the prediction and use of TBM as compared to the use of drill and blast. This is an indication that in the process of using the TBMs it is possible to meet an anticipated condition which may change the whole situation of operation.

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