Tunneling and Underground Excavations - Research Paper Example

TABLE OF CONTENTS CONTENTS PAGE Introduction 2. Basic Rock Mechanics 2 Engineering Uses of Rocks 2 2.2 Measurement of Rocks 2 3. Shape, Size, and Orientation of an Opening 2 4. Elastic Deformation and the Kirsch Solution 3 5. Rock Mass Classification 4 6. Support Design and Ground Reaction Curve 4 7. Tunnel Construction Methods 5 7.1 Drill and Blast method 5 7.2 NATM Tunneling Method 5 8. Tunneling in Soft Ground 6 8.1 Problems with Urban Tunneling 6 8.2 Deformation and Surface Settlement 6 8.3 Structural Design of Load Lining 7 8.4 Working Face and Methods of Soft Ground Tunneling 7 8.4.1 EPB Method 7 8.4.2 Slurry Shield method 9 9. Conclusion 9 References i Appendix A (A Railway Tunnel) ii Appendix B (How Tunnels Work) iii Appendix C (Ground Reaction and Support Reaction) iv Tunneling and Underground Excavations 1. Introduction For many centuries, miners have been continuously excavating below the surface of the ground in order to explore minerals. Brown and Hoek (2005, p.7) assert, “Originally, these underground operations were simply a downward extension of the small excavations created to exploit the surface outcrops”. However, when the minerals search methods became complicated, some underground mining methods were developed by the civil engineering experts in order to make use of the discovered deposits. Some of the major categories of underground excavations include temporary mine openings, permanent mine openings, storage rooms, water tunnels for hydroelectric projects, and major road and railway tunnels. 2. Basic Rock Mechanics Rock mechanics is a discipline of applied science. It is recognized as one of the most logical and rational engineering disciplines. It is the knowledge and understanding of the mechanical properties of rock, different methods for the study of rock-stress under some specific circumstances, various principles that express the rock mass response to load, and some coherent schemes for the application of rock-stress analysis methods to actual physical problems. Several factors have significantly contributed to the recognition of rock mechanics as a science of mining. If we talk about major causes for its recognition as a mining science, we can say that it is due to the increased development and advancement in underground mining activities. Krishnan, et al., (2000) found that underground mining operations act as one of the greatest motivating sources for the advancement in the field of rock mechanics. 2.1 Engineering Uses of Rocks Rocks are used for two basic engineering purposes that include building construction and preparation of foundations. Understanding of the basic properties of rocks is very important for the professionals of the field of civil engineering because it allows the structures to be founded in an appropriate manner. 2.2 Measurement of Rocks In order to derive the analytical data on the physical characteristics of rock mass, the engineers use two basic measures. These measures include field-scale measures and laboratory measures. Field-scale measures are also known as rock mass properties and they are descriptions of the rock mass’s bulk strength properties. Whereas laboratory measures of rocks are analyzed in laboratory settings and are known as rock properties. They are obtained using field site’s samples. 3. Shape, Size, and Orientation of an Opening There are two types of mining openings, which are service openings and production openings. Brady and Brown (1992, p.197) state that service openings are characterized by the duty life approaching the mining life of the ore body. The engineers design these openings in such a way that the engineers not only assure the operational functions of the openings and but also maintain them at low cost over a long operational life. However, production openings have to do a temporary function in the mining operation. Hartman and Mutmansky (2002, p.275) found that three common types of openings include shafts, declines, and drifts. Size of an excavation opening is also very important because the stresses that are induced in the rock are always independent of the excavation’s size. Not only stresses, but also the excavation’s stability depends on the size of the excavation. Finding the optimal shape of an excavation’s opening is also important because if we find the optimal shape for an excavation based on stress distribution, it will be useful in increasing the stability and decreasing the support costs. In a hydrostatic stress filed, the optimal shape for an opening is a circle because the lowest stresses found on the boundary of an opening take place for the largest radius of the boundary curve. However, in case of non-hydrostatic stress field, the lowest boundary stresses are associated with an oval shaped opening. 4. Elastic Deformation and the Kirsch Solution Elasticity is the property of rocks that describes the rock material’s ability to come back to its original shape after removal of the stress applied to the rock. If the load is excessively heavier, it is not always possible for the rock material to get back to its normal form. Elastic deformation is one of the two concepts related to the rock material’s deformation. It occurs when the rock material completely restores to its original shape after the stress is removed. Civil engineering professionals make use of Kirsch solution in order to calculate the deformation of rock material. In the roof or floor stresses, radial and shear stresses are found to be zero. The stresses are tensional when value of k is zero. However, when the stresses are zero, k is 0.33 and it is greater than 0.33 when the stresses are compressed around the entire boundary. For sidewall stresses, stresses are 3X applied vertical stress when the value of k is zero and they are 2X applied vertical stress when the value of k is one. 5. Rock Mass Classification In order to get detailed information on the rock mass and its hydrologic characteristics, civil engineering professionals make use of rock mass classification scheme during the feasibility and earliest design stages of a project. The engineers make a limited use of rock mass classification and do not eliminate the use of other design procedures. They only extract the desired information from the classification in order to use it in other design procedures. The engineers use different rock mass classification systems during the early stages of a project. Some of them include Terzaghis rock mass classification, Classifications involving stand-up time, Rock quality designation index, and Rock structure rating. In order to classify a rock mass using rock mass rating system, the factors that are considered by the engineers include spacing of discontinuities, orientation of discontinuities, condition of discontinuities, condition of groundwater, rock quality designation, and rock material’s compressive strength. 6. Support Design and Ground Reaction Curve Support designs are of extreme importance in the field of civil engineering. Support designs not only hold a structure properly but also they can be utilized to improve the performance of the structures. If we talk about ground reaction curve, we can say that the ground reaction curve is the relationship between the deformation of the walls and the internal pressure on the unsupported tunnel’s walls. Civil engineering professionals always display the ground reaction curve in the ground reaction view. However, in case of addition of a support, the support reaction curve is also displayed in the ground reaction view. 7. Tunnel Construction Methods Wood (2000) found that some of the most famous tunneling methods include drill and blast method and NATM tunneling method. Let us describe both of them in some detail: 7.1 Drill and Blast method Drill and blast method is one of the most practiced tunneling methods. It is used in such places where the use of tunnel boring machines is quite expensive. For example, if we want to construct a tunnel through hard rock, drill and blast method would be a better option instead of using boring machines. The civil engineering professionals make use of a drilling machine in order to drill a pattern of holes of a specific depth. These holes are dilled on the rock face of the selected tunnel path. After the holes are created, dynamite is used to fill the holes. Explosions are made through the dynamite in order to crack the rock. The process goes on until the tunnel is constructed. 7.2 NATM Tunneling Method NATM (New Austrian Tunneling Method) is also a famous tunneling method. This method is mainly used in complex geological conditions where the conditions change very rapidly resulting in difficult forecasting of rock mass. The primary principles on which MATM is based include short Crete protection, mobilization of strength of the rock mass, measurements of the deformation of the excavations, primary lining of the tunnel by rock bolts and wire mesh, rock mass classification, closing of the inverts, and dynamic designing of the tunnel. 8. Tunneling in Soft Ground In soft ground tunneling, “workers dig soft-ground tunnels through clay, silt, sand, gravel or mud” (Harris n.d.). Stand-up time is of extreme importance in the construction of tunnels in the soft ground. Stand-up time is the ability of the ground to stand up safely by itself at the excavation point. An important point to be considered by the civil engineers is that stand up time of the soft ground is very short and it creates the threat of cave-ins. For eliminating the cave-in threat, engineers make use of a special material known as shield. “A shield is an iron or steel cylinder literally pushed into the soft soil” (Harris n.d.). Shield supports the surroundings of the soft ground while engineering professionals remove debris and install a permanent lining made of cast iron or concrete. Tunneling in soft ground is a difficult process and engineers have to work very carefully in order to construct a tunnel. 8.1 Problems with Urban Tunneling There are some big problems associated with urban tunneling. Urban tunneling disturbs the soils and rock masses, soil loses its original balance condition, and ground settlement affects the safety of other buildings present around the tunnel’s ground surface. Keeping the ground surface balanced and undisturbed is one of the biggest challenges regarding urban tunneling. 8.2 Deformation and Surface Settlement The determination of surface settlements is also a very important task for the engineering professionals. In geotechnical engineering, forecasting of deformations is not very accurate. The reason is that the ground has a non-linear relationship between stress and the strain. That is why the distribution of the stiffness cannot be predicted easily. Engineering professionals are always highly concerned about the surface settlement issue because if the ground surface is not settled properly, it causes a risk for the buildings constructed on that specific ground surface. 8.3 Structural Design of Load Lining Engineers have to consider some general issues while designing the structural design of the final lining for tunnels. These issues include short-term and long-term load conditions, liner support with structure-ground interaction, design of connection with buildings and connecting tunnels, and seismic design (Ozdemir 2006, p.354). The main load components include hydrostatic load, final liner’s dead load, and load from the interior structures. 8.4 Working Face and Methods of Soft Ground Tunneling Two of the widely used soft ground tunneling methods includes EPB and slurry Shield methods. Let us describe both of them in some detail: 8.4.1 EPB Method EPB (Earth Pressure Balance machine) is a famous metro tunneling method. Some of the advantages of using EPB method include low cost, simple technology, and small construction yard. Huang, et al., (2009, p.343) assert, “EPB machine applies pressure to the working face by the mucks, which are cut from the face sometimes with injection of soil conditioning additives”. The working face maintains stability due to the pressure applied on it and it is one of the key features of EPB tunneling. 8.4.2 Slurry Shield method Slurry shield tunneling method makes use of a rotating shield that cuts the soil. It is a mechanized method of tunnel construction. The role of slurry is that it stabilizes the front of the working face during the process of excavation. The major functions of slurry shield method include transportation of the excavated soil and working face stability. 9. Conclusion Summing it up, we can say that construction of tunnels, temporary mine openings, and permanent mine openings are some of the major categories of underground excavations. In the process of underground excavations, shape, size, and orientation of openings are of extreme importance. Tunnels are created through excavations and engineers make use of most appropriate methods for the construction of tunnels on hard and soft grounds. Drill and blast method and NATM tunneling method are widely used for hard rock tunneling purposes whereas slurry shield and EPB methods are two of the most famous methods for soft ground tunneling. Structural design of load lining and surface settlement issue are given extreme importance by the engineers because improper load calculations and surface unsettlement create risks for the buildings constructed on those ground surfaces where the tunnels are to be created. References Brady, B. & Brown, E., 1992. Rock Mechanics: For Underground Mining. 2nd ed. Dordrecht: Kluwer Academic Publishers. Brown, E. & Hoek, E., 2005. Underground Excavations in Rock. Oxon: Taylor & Francis. Harris, W n.d., How Tunnels Work, viewed 15 July 2010, . Hartman, H. & Mutmansky, J., 2002. Introductory Mining Engineering. New Jersey: John Wiley and Sons, Inc. Huang, H. Liu, G. & Ng, C., 2009. Geotechnical Aspects of Underground Construction in Soft Ground. London: Taylor & Francis. Krishnan, R. Shirlaw, J. & Zhao, J., 2000. Tunnels and Underground Structures. Rotterdam: Taylor & Francis. Ozdemir, L., 2006. North American Tunneling 2006: Proceedings of the North American Tunneling. London: Taylor & Francis. Wood, A., 2000. Tunneling: Management by Design. London: Taylor & Francis. APPENDIX A (A Railway Tunnel) APPENDIX B (How Tunnels Work) APPENDIX C (Ground Reaction and Support Reaction) Read More