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  1. Rock support for tunnels and underground cavern design is a demanding and very complex task. In principle, the problem can be approached from two directions: The first way is to define the relationship between geo-mechanical properties of the rock mass and the support methods used. This is mostly based on the utilization of statistical and empirical data gathered in similar conditions. The second way is to estimate the deformation characteristics of the rock structure, and then the related effect on supporting structures. This method typically requires very good rock property and rock mass property data. The most important factors affecting rock reinforcement method and design are: Geological factors, such as rock mass structure. Dimensions and geometry of excavated space. Location and direction of caverns in the rock mass. Excavation method. Use and expected lifetime of space. Common support methods in underground construction work are: Bolting. Sprayed Concrete. Steel Arches. Concrete Lining. Grouting. 1. BOLTING Rock bolting is one of the most common methods of rock reinforcement. The main principle of bolting is to reinforce loose rock or fractured in-situ rock to precent caving or spalling, and to assist the rock mass to form its own self-supporting structure. Bolts can be divided into three categories according to the way they behave in the rock, for example, grouted bolts, mechanically anchored bolts and friction bolts. 1.1 Cement-Grouted Bolts Cement-grouted rebar is still the most inexpensive and widely used rock bolt, because it is simple and quick to install and can be used with or without mechanized equipment. Correctly installed, a cement-grouted bolt gives rock support for years. The grout cement provides protection from corrosion. Special galvanized and/or epoxy coated bolts can be used in extremely severe conditions. The major disadvantages of the cement-grouted bolt is its relatively long hardening period. The grout takes between 15-25 hours to harden, therefore it does not provide immediate support. When immediate support and/or pre-tensioning is needed, a grouted wedge-type or expansion-shell bolt can be used. Mixing additives in the grout can reduce the hardening time, but it also increases bolting cost. The water/cement ratio considerably affects the quality of installed bolts. The best water/cement ratio is 0.3 (w/c). This grout density can be easily used and maintained when using mechanized bolting equipment. 1.2. Resin-Grouted Bolts Resin-grouted bolts give the required support relatively quick due to a short hardening time. When correctly installed with full-length grouting, the resin-grouted bolt is considered to give permanent support with a life span of 20 to 30 years. By using resins with two different hardening times, with faster one at the bottom of the hole and another that is slower at the stem, the bolts can be pre-tensioned. The same can be done for short-time support by only bottom-grouting the bolt. 1.3. Cable Bolts Cable or steel strained bolts are used to bind and secure large volumes of rock around large caverns. Cable bolts can be used both before and after excavation, and also used for preventing rock slides in mountain slopes and quarries. The anchor itself is a steel strand, typically two strands of 15.2 mm in diameter, with typical bolt length being between 10-25 meters. Today, with mechanizes equipment, the installation and grouting of cable bolts of any length is fast and efficient, and the cable bolt's bearing capacity clearly exceeds capacity of rebar steel bolts. Its lack of efficient protection against corrosion limits its extensive use in permanent rock support. 1.4. Mechanically-Anchored Bolts Mechanically anchored bolts are usually wedge or expansion-shell bolts that are point-anchored at the bottom of the hole. The bolt has an expanding anchor at its end. After insertion, the bolt is either rotated or pressed/hammered against the bottom of the hole. This expands the wedged end and anchors the bolt firmly to the end sides of the hole. To install anchored bolts successfully, the hole size must be accurate and the rock must be relatively solid. Wedge or expansion-shell bolts are typically meant for temporary rock support. Together with cement grouting, it provides both immediate and long-term support. 1.5. Friction-Type Bolts Typical examples of friction-type bolts are the split-set and Swellex bolts. Both are quick and easy to install and give instantaneous support. They can not, however, be used for long-term reinforcement. The split-set bolts is hammered into the hole, which has a slightly smaller diameter than bolt. Using the correct hole size for a specific bolt diameter is essential for successful installation. Split set bolts are very suitable for layered formations. The Split-set bolts provide immediate support but only for fairly short period of time. A disadvantage is that the split-set cannot be effectively protected against corrosion. The life span can somewhat be extended by using cement grouting. The Swellex bolt has a longer life span than the Split-set. It is installed by applying high-pressure water to bolt after inserting it to the hole. The high pressure expands the bolt to its final dimensions in the hole, therefore enabling it to utilize the roughness and fractures in the bolt hole surface. As with the Split-set bolt, poor corrosion protection limits this bolt. 1.6. Equipment for Bolt Installation Development of mechanized equipment began as early as the 1970s. Today there is a wide selection of fully mechanized equipment, and a wide variety of different methods for bolt installation. The main factors affecting the choice of method are usually tunnel size, amount of bolts to be installed and work cycle arrangement at this site. Manual operation, the hand-held drilling and installation of bolts, is typically used in small drifts and tunnels where drilling is also performed by hand-held equipment, and there is a limited amount of bolting work. Semi-mechanized installation is still typical at tunneling work sites. The drilling jumbo is used for drilling bolt holes, and bolt installation is performed from the jumbo's basket boom or from a separate utility carrier or truck. With today's full mechanized equipment, one operator can handle the entire bolting process from drilling to grouting and bolt installation. The operator is positioned away from the unbolted area under a safety canopy that protects him from failing rock. Although safety is a major reason for the development of mechanized bolting equipment, the superior installation technique of mechanized bolting rigs also produces consistently higher bolting quality. Thanks to powerful cement mixers, pumps and effective grouting methods, the bolts are securely fixed and grouted to their full length, providing a sound reinforcement structure, even with long bolts. 2. SCREENING Screening, which is the installation of wire mesh, is most typically used in underground mining, but also construction sites together with bolting and/or sprayed concrete. Screening is primarily performed manually by applying the wire mesh together with bolting of the tunnel. It can also be done by mechanized equipment, such as by having a screen manipulator on the bolting or shotcreting unit, or on a dedicated screening machine. 3. SPRAYED CONCRETE Sprayed concrete, otherwise called shotcreting, is widely used support method in construction. It is used for temporary or long-term support, lining and backfilling. Usually shotcrete is used together with bolting to obtain the best support or reinforcement. Shotcrete can be reinforced by adding steel fiber to the concrete. The most common forms of shotcreting are dry-mix and wet-mix methods. In the dry-mix method the aggregate, cement and accelerators are mixed together and propelled by compressed air. Water is added last through a control valve on spray nozzle. The dry method is suitable for manual shotcreting because the required equipment is usually inexpensive and small. On the other hand, dry method can pose a health hazards as it creates considerably more dust and rebound than the wet method. The quality also depends heavily on the shotcreting crew, and may vary widely. In the wet mix method, aggregate, cement, additives and water are measured and mixed before transport. Today, wet mix is more widely used because it is easy to mechanize and the capacity can easily out-do the dry method. Rebound rate is low and the quality procedure is even. Critical factors in shotcreting are: Water/cement ratio. Grain size distribution of aggregate. Rebound ratio. Grain size distribution. Mix design. Nozzle design. Nozzle distance and angle. Layer thickness. Manual shotcreting has been largely replaced by mechanized shotcreting machines. With mechanized equipment, multiple capacities per hour can be reached, together with consistent and even quality of the concrete layer. Safety, ergonomic and environmental conditions are other important aspects of shotcreting. These factor are efficiently improved with mechanized shotcreting units. 4. STEEL ARCHES Steel arches are common permanent support method for weak rock formations. These are usually installed in the tunnel immediately after each round, at the same time as rock bolting. Steel arches are also commonly installed during shotcreting to give temporary support before final concrete lining of e.g. traffic tunnels. 5. GROUTING Grouting is the method in which a solidifying liquid is pressure-injected into the rock mass. The main purpose of grouting is to prevent ground water leakage into the tunnel, and to increase overall rock mass strength. In grouting, a chemical agent or cement mass is pressure-pumped into the drill-hole to penetrate fractured and fill cavities. In drill and blast tunneling, grouting is typically performed before (pre-grouting) or after (post-grouting) excavation. 5.1 Pre-Grouting Pre-grouting means that rock mass is grouted before excavation begins. Usually, pre-grouting is done from the tunnel, but in situations with low overburden it is also possible to do it from the surface. Probe holes are drilled to map possible fractures and register water flow. This helps to analyze the need for grouting. Later, grout holes are drilled in conical-fan shape in front of the tunnel face. Typical grouting fan length is 15-25 meters. After drilling, the grouting agent is pumped into the hole until leakage has reached an acceptable level. Tunnel excavation can begin once the grouting mass has settled. Grouting fans overlap each other so that in 15-meter long grout holes, grouting is performed every second or every third round depending on the round length. 5.2 Grouting after Excavation (Post-Grouting) When grouting is done after excavation, grouting holes are drilled from the tunnel in a radial form. In good rock conditions with small water leakage, post-grouting is often adequate. Post-grouting enables better rock mass structure evaluation. On the other hand, water leakage blockage is more difficult because the water flow tends to flush away the grouting agent before it hardens. 5.3. Grouting Agents The grouting agents can be divided into two categories: Suspension and Chemical. Cement water or bentonite water suspension is the most typical in rock grouting because both are cost-effective and environmentally safe. The drawback is, however, a relatively large maximum grain size, which leads to poor penetration in small cracks. Penetration characteristics can, however, be improved by adding additives. Silicate-based chemicals are also used to speed up the hardening time. Chemical agents are silicate-based, resin polymers, polyurethane-based or lignin-based chemicals that typically penetrate very s cracks and have adjustable hardening times.
  2. Tunnel Sections Tunnels range in dimensions of cross-sections from those of small galleries driven by miners working with hand tools, to tunnels large enough to accommodate rail road trains, double lane of highway traffic, or to transport very large volume of water as in diversion structures in dams. A minimum size of tunnel is 9 ft high and 4 ft wide at the working face. Designed shapes or sizes of tunnels in x-section conform to a planned uses to tunnel and the some extent to the nature of the material that is anticipated will be encountered during excavation, x-sectional shapes vary from square or rectangular as for example in mining operations in strongly bedded sediment rocks, to circular. A common type of x-section is horse shoe shaped to provide maximum stability in the roof portion of the tunnel. Geological Exploration The geological conditions that are likely to met in any given work of construction must be predicted. The line of the tunnel and the neighboring ground is geologically surveyed and sub-surface data obtained by exploratory boring. Careful control of such trial boring operations is necessary in order to extract the maximum amount of information from the ground. The cost of tunneling in general is least where construction is carried out in sound rock, and in one kind of rock throughout. Straight forward geological conditions such as simply dipping strata allow cost to be estimate easily; more uncertainties arise in connection with folded and faulted beds. Geological structures such as faults and joints should be mapped along the line of a tunnel. Strongly developed joints systems are potential channels for underground circulation and should be recorded. Badly fractured ground is to be avoided if possible. If unavoidable it may require special timbering or other treatment, and a prediction of where faults are likely to met underground is therefore of greater importance. Hard rocks where excavated may stand with little support (some tunnels are unlined throughout) because they are strong enough to withstand the lateral pressure exerted by surrounding rocks but if soft bands are present there may be a tendency to slipping on these weaker layers and suitable support for the walls of the excavation will be necessary. Inter- bedded hard and soft rocks, such as sand-stones and shales may give rise to many difficulties. Ground-water percolating through the sand-stones soaks into the shales and softens them and hence the slipping is promoted.
  3. A tunnel is an elongated, narrow essentially linear underground opening with a length greatly exceeding its width or height. Most tunnels are nearly or exactly horizontal but for special purposes, tunnels may be driven at angles up to 30 degree from the earth's surface. The one which is greater than 30 degree from horizontal are designed as shafts. When rocks in tunnels are highly in-competent, especially when underground water is present, tunneling becomes a very costly and hazardous operation, and excavation and containment of such rocks present a challenge that requires maximum use of highly technical skills and ingenuity. History of Tunneling There is abundant archaeological evidence that in Europe stone age man sank shafts and drove tunnels to recover flint for the fabrication of sharp-edged implements such as knives, axes etc. later as an elementary knowledge of metallurgy was acquired by premature people, possibly for the first time central Asia, underground excavation became necessary to supply the increasing demands for metals and alloys. Very early underground excavations for metal-bearing have been identified in Caucasia, between the Black and Caspian Sea, and date back to approximately 3500 BC. Many tunnels were built in ancient times by the Babylonians, Indians, Persia and Egypt in search for precious metals. Stone-age man used very primitive tools in underground excavation. Particularly useful to him were picks made of deer antlers, flint axes and hammers and wedges made of bone and wood. The production of metals and alloys provided materials for increasingly efficient rock excavation. Later on explosives were used in the seventeenth century. For hundreds and perhaps thousands of years underground working in hard rocks, especially those containing few fractures and fissures, were advanced by building fires against rock faces to cause expansion and spalling. In some operations spalling of the heated rock was accelerated by dowsing it with water. The fractured rock was than separated from the working face with picks, gads and wedges. With the increasing use of explosives first the black powder later nitroglycerin, steel temping techniques were perfected and permitted efficient and economical hand drilling of holes for explosives. Tunneling machines have been used to excavated tunneling with diameters of about 6 ft to more than 36 ft. Rate of excavation of over 400 ft per day have been recorded in soft ground. In hard rocks it can be less as 100 ft per day. It includes a rotating cutter head and provision for controlling forward thrust and alignment. In the hardest of rocks, near the middle of the nineteenth century steam powered piston drills and later percussion drills, powered by compressed air, made their appearance and at the same time several tunneling machines such as moles were invented. Tunnels have been driven in a variety of natural materials ranging from unconsolidated water-soaked clay, sand and gravel to dry very hard un-fractured rocks. It is one of the most costly and at the same time one of the most hazardous of all engineering under-takings. In case of long tunnels in area of geological complexity, all types of uncertainties arise, including design and construction techniques and including estimate of cost. The location of a tunnel like the site of bridge often does not allow much freedom of choice. It becomes necessary at given place to maintain an alignment. Before designing and planning a tunnel the undesirable underground conditions must be anticipated. Tunnels through massive un-fractured granite or through horizontally layered sandstones that are well cemented and un-joint present no special problems in design and preparation of cost estimation; whereas in geological complex areas it is an art and intelligent guess work. Purpose of Tunneling Tunnels have been constructed for great variety of purpose, and they are classified as follows: Tunnels driven to gain access to economic mineral deposits and to provide haul-ways for extracted minerals. In some mining operations tunnels are driven to provide adequate circulation of air in underground workings. Transportation Tunnels, including pedestrian highness navigational and rail road tunnels. These are among the largest and at time the most difficult of all tunnels to excavate. Water or Sewage Tunnels: These tunnels may or may not be constructed so as to transport liquid under pressure, and a distinction is made between gravity flow tunnels and pressure tunnels. The latter are designed to contain without leakage water under hydrostatic pressure or force-pressure head. Military Tunnels: These tunnels are driven in connection with underground military operations. Tunnels to provide protection from atomic explosion. Utility Tunnels. Built to contain power and communication transmission line, gas line, etc.
  4. Chemical weathering, or chemical decomposition, involves several important reactions between the elements in the atmosphere and those in the mineral of the earth's crust. In these processes, the internal structures of the original minerals are destroyed, and new minerals are formed, with new crystal structures that are stable under conditions at the earth's surface. Water is of prime importance in chemical weathering. It takes part directly in the chemical reaction. It acts as a medium to transport elements of the atmosphere to the minerals of the rocks where reactions can occur, and it removes the products of weathering to exposed fresh rock. The chemical reactions involved in the decomposition consist of the following three main groups i.e. Hydrolysis, Dissolution, and Oxidation. 1. Hydrolysis The chemical union of water and a mineral is known as hydrolysis. The process involves not merely absorption of water, as in sponge, but a specific chemical change in which a new mineral is produced. In hydrolysis, ions derived from one mineral reach with the H+ or OH- ions to produce a new mineral. A good example of hydrolysis is the chemical weathering of k-feldspar. Two substances are essential: carbon dioxide and water. The atmosphere and the soil contain carbon dioxide, which unites with rain water to form carbonic acid. If k-feldspar comes in contact with carbonic acid, the following chemical reaction occurs: 2KAlSi3O8+ H2CO3 + H2O → K2CO3 + Al2Si2O5(OH)4 + 4SiO2 Hydrolysis is an extremely important weathering process because it acts on the feldspars and ferromagnesium minerals, the dominant minerals in most rocks. 2. Dissolution Water is one of the most effective and universal solvent known. Because of the polarity of the water molecule, practically all minerals are soluble in water to some extent. The water molecule is polar and behaves like a tiny magnet. It acts to loosen the bonds of ions at the surface of minerals with which it comes in contract. Some rock types can be completely dissolved and leached away by water. Rock-salt is perhaps the best known example. It is extremely soluble, surviving at the earth's surface only in the most arid regions. Gypsum is less soluble than rock salt, but is also easily dissolved by surface water. Few out crops of these rocks occur in humid regions. Limestone is also soluble in water, especially if the water contains carbon dioxide. The chemical analysis of river water illustrates the effectiveness of dissolution in the weathering of rocks. Fresh rain water contain relatively little dissolved mineral matter, but running water soon dissolves the more soluble minerals in the rock and transport them in solution. Each year the rivers of the world carry about 3.9 million metric tons of dissolved minerals to the oceans, so it is not surprising that sea water contains 3.5% (by weight) dissolved salts, all of which were dissolved from the continents by pure rain water. 3. Oxidation Oxidation is the combination of atmospheric oxygen with a mineral to produce an oxide. The process is especially important in the weathering of minerals that have high iron content, such as olivine, pyroxene, and amphibole. The iron in silicate minerals unites with oxygen to form hematite or limonite. Hematite is deep red, and if it is dispersed in sandstone or shale, it imparts a red color to entire rock.
  5. All rocks, when exposed for sufficient length of time to the atmosphere, undergo decay from disintegration and decomposition, together referred to as weathering. Disintegration is the break down into small particles by the action of mechanical agents of weathering such as rain, frost etc, decomposition is the breakdown of mineral particles into new compounds by the action of chemical agents such as acid in air and in rain and river water. Denudation is the general term used for the wearing down of land areas by the processes originating and acting at the earth’s surface. It includes both weathering and erosion. In addition to the atmospheric processes, agents of erosion (rivers, moving ice, water waves) contribute to the deduction of the land in their particular spheres of action, they also transport weathered and eroded material away from areas where it is derived, to from deposits of sediments elsewhere. Agents of Weathering 1. Rain The mechanical action of rain consist mainly in the washing of loose particles of soil and rock to lower levels. This phenomenon is known as rain-wash. It is the means by which rivers receive much of the sediments they carry in suspension. The chemical weathering effects of the rain are seen its solvent action on some rocks notably limestones. The process depends on the pressure of feeble acids, derived from gases such CO2 and SO2 which are present in air in small quantities and which enter into solution in rain water. The denuding effects of heavy showers and rain-storms may be very sever, especially in regions where a covering of vegetation is lacking. It cuts gullies in the surface of the ground, some of considerable size and may cause great damage by the destruction of roads and livestock. Heavy rains also promote land slides. Vegetation protects the ground from the immediate disintegrating effects of rainfall. 2. Frost In cold climates the action of the frost is to break off angular fragment from exposed rock surface, a process sometimes referred to as ice-wedge. Water enters rock along pores, cracks and fissures. On freezing it expands and occupies about 10% greater volume exerting a pressure of about 2000 lbs per square inch. This is therefore like a miniature blasting and brings about the disintegration of the rock. The loosened particles fall from the mass and accumulate as heaps of talus at lower levels and this material may later be consolidate into a deposit known as breccia. 3. Wind It is one of the two natural agents which transport rock material against gravity. Its effect is three-fold. First it removes loose particles of rock decay as it blows over a surface, then charged with these grains the wind act as an abrading sand-blast driving the grains against rock surfaces which becomes worn and polished in course of time. Thirdly the blown grains are accumulated to from sand-dunes. Lines of communication may be seriously affected by wind-blown sand in arid countries. It is on record that the telegraphic wire on the trans-Caspain railway was worm down to half of its diameter in eleven years, and renewal was then made. To avoid accumulation of sand alongside railway embankments in Sudan, culverts have been made to allow for easy passage of the wind and its load sediments. 4. Insolation When a rock surface is exposed to a considerable daily range of temperature, as in arid and semi-arid regions, the expansion which occurs during the day and contraction at night, constantly repeated have a weakening effect on the texture of the rock over a period of time . The outer heated layers tend to pull away from the cooler rock underneath a process known as exfoliation. By the unequal expansion and contraction of its mineral constituents the strain is set up in a rock and its texture is loosened. This kind of weathering is prevalent in climates where high day and low night temperatures are prevalent. Weathering by Organic Agents Plants retain moisture and any rock surface on which they grow is kept damp, thus aiding the solvent action of the water. The chemical decay of the rock is also promoted by the formation of vegetable humus organic product of the decay of plants. The mechanical break up of rocks is helped by the roots of plants which penetrates into cracks and crevices and tend to wedge apart the rock.
  6. The geological work performed by rivers may be placed under three heads: Erosion, Transport and Deposition. Rivers are active agents of erosion, especially in times of floods. They carry away much material and redeposit some part of it farther downstream, the rest being transported to the sea. Some matter goes into solution in the river water. The finer particles remain in suspension and the coarse roll along the bed during floods and hence perform the work by abrading the channel of river. The work of erosion, performed by rivers, results in the widening and deepening of their valleys during their course of development. Stages of youth, maturity and old age may be distinguished in the history of river. There is a steep sided valley of a youthful stream the broader valley and more deeply dissected landscape of mature river system and the flat meandering course of a river in the stage. Meanders When a river has cut down nearly to its base level, it flows more slowly with a reduced gradient and begins to swing side by side of its valley. The energy imparted to the load of sediments which it carries is consumed in the widening of the valley by lateral erosion, and the course of the rivers develops a series of big looped curves called meanders. The length of the loop when fully formed is about sixteen times the width of the stream. On the concave side of a meander the bank is undercut and eroded while deposition is on the convex side. River Deposits The general term given to deposit laid down by rivers is alluvium, though this is often restricted to the finer material such as silt and mud as distinct from gravels and larger fragments. After heavy rains, the velocity of river may be trebled; larger boulders which would not be moved under normal conditions of flow are shifted with an intermittent motion by stream is spate. Transported sediments are dropped by a stream whenever its velocity is checked. A river emerging from a mountain on to a flatter ground, such as the edge of a plain, builds up a heap of detritus known as alluvial cone. The material deposited is mainly sand and gravel, partly rounded during transport. The finer particles being carried on down to the sea. In the lower courses of a mature river the finer alluvium is spread out to form an alluvial flat. This is subject to periodic flooding and a fresh layer of alluvium is deposited at each flood. Water Falls These are performed where a stream in a youthful stage flows over rocks of different hardness. A hard rock is worn away less rapidly than a soft, with a result that a river’s gradient is increased where it crosses a ledge of hard rock, softer material below the resistant layer is undercut by the water, leaving an overhanging ledge over which the stream falls. Deltas A river entering a body of still water such as a lake or sea drops much of its load of sediments as its velocity is reduced and forms a delta which is gradually built forward into the still water. The building of a delta proceeds as sediments is deposited at a river’s mouth. The stream than subdivides and flows through the water on either side of the obstacle which it has made. Further deposition takes place along these distributions and after further barriers have been made, the streams branch again. In this way by repeated bifurcation and sedimentation, the deltaic deposits cover a large area which has a roughly triangular like the Greek latter ∆ as is the case of Nile delta, Ganges and Mississippi.
  7. Introduction Accurate and complete subsurface information is necessary for all types of civil engineering projects, for without this information it is not possible to arrive at a rational design for structure and proper construction procedures. Structures have failed because of inadequate or misleading subsurface data, and many so called successful structures could probably have been completed at much less cost because proper consideration have been given to obtaining more complete subsurface information. Boring and Sampling Methods Auguring is a simple method of putting down holes a few inches in diameter to depths up to 20ft in soft sediments. Trial pits and trenches in addition to providing samples of the deposits excavated, allow inspection of the rocks in the walls and floor over an area. Boreholes allow much greater depths to be penetrated. Boring methods are broadly of two kinds: 1. Percussion Drilling In percussion drilling the bit is suspended from the rods (or a cable) and is jumped up and down so that the rock is broken up by repeated rows. Water is added to the hole to keep the bit cool and make slurry and debris removed by means of a bailer. The pounded rock mixed with water from slurry, from which chips may be recovered for identification. The rate of progress of drilling, and the cost varies according to the hardness encountered. 2. Rotary Drilling Rotary drilling methods include mud rotary drilling and core drilling. In the former the bit is rotated, and is attached to hollow drilling rods through which a fluid mud is pumped continuously. The mud is returned to the surface through the annular space between the rods and the hole, bringing with it small fragments of rock which can be screened out and examined. The rods are usually in 20 ft lengths, and successively added to the assembly as the hole is lowered. 3. Core Drilling Core drilling uses a tabular bit with a lower cutting edge, which is rotated in the hole; many forms of bits are available, some containing diamonds and other hard abrasives for penetrating hard rocks. Diamond drills with a diameter of 2 to 4 inches are commonly used, for sampling, or in exploratory bore from underground workings and holes can be drilled at any angle. As the bit is rotated, a core is cut out and enters a barrel mounted above the bit. The length of the barrel controls the length of the core which can be obtained at any time. The core is recovered by drawing the drilling rod and barrel. The cost of core drilling is greater than for any other method and increase greatly with increasing depth of the hole; but the recovery of the core is a great advantage, as it yields information from great depth and is a sample of rocks drilled through. 4. Core Barrels The aim of structural drilling is to recover undisturbed core upon which measurements of structural features can be made. This can be achieved either by the multiple-tube core barrels or by the use of large diameter barrels. In the multiple-tube core barrel, the inner tube or tubes are mounted on a bearing so that they remain stationary, while the outer barrel, which carries the diamond bit, rotates. The core, cut out by bit, is transferred into the non-rotating inner barrel where it remains undisturbed until the barrel is removed from the hole. Removing the core from the barrel is the most critical part of the operation. The most satisfactory system is to use a split inner barrel which is removed from the assembly with the core inside it and then split to reveal undisturbed core. 5. Geophysical Methods Geophysical methods allow subsurface features to be located, mapped, and characterized by making measurements at the surface that respond to a physical, electrical or chemical property. These non-invasive measurements can be effectively used to provide reconnaissance to detailed geological information guide, subsurface sampling and excavation, and provide continuous monitoring. If all sites were simple (horizontally stratified geology with uniform properties), site characterization would be easy. Data from just one boring would be sufficient to characterize the site. However in most situations, this will not be the case. Even at sites where geology appears to be uniform, one must be alert to often-subtle variations that can cause significant changes in structural or hydrological properties. Traditional approaches to subsurface field investigations commonly rely only on the use of direct sampling methods such as boring of rock and soil samples, monitoring wells for gathering hydro geologic data points. Soil and rock sample programs and the placement of boring and wells are done mainly by educated guess work. Numerous pitfalls are associated with this approach that can result in an incomplete or even erroneous understanding of site conditions. These oversights are the cause of many structural and environmental failures. In many cases direct sampling alone is not sufficient to accurately characterize site conditions. This is the primary reason for the application of surface geophysical methods. Surface geophysical measurements can be made relatively quickly; they provide means to significantly increase data density. Because of the greater sample density, anomalous conditions are more likely to be detected, resulting in an accurate characterization of subsurface conditions. 5.1. Application There are four major areas where surface geophysical methods may be applied to environmental and engineering problems: 1) Assessment of natural geologic and hydro geologic conditions. 2) Detection and mapping of contaminant plumes, spills and leaks. 3) Detection and mapping of landfills, trenches or other underground structures and utilities. 4) Evaluation of soil and rock properties and man-made structures. The following is a summary of various geophysical methods: 6. Seismic Methods Seismic measurements involve the measurement of seismic waves travelling through the subsurface. Stratigraphy, structure, and material properties can be assessed with seismic methods. All applications of seismic methods are based on the fact that the elastic properties of soils and rocks determine the velocities of wave propagation through them. The higher the elastic modulus, for example, higher the velocity is. In rocks the dominant factors that influence the velocity are the crystallinity and porosity. Rocks having crystalline textures and low porosity have higher shock wave velocities just as they have high elastic moduli and compressive strengths. Shock waves in earth materials follow multiple paths from source to receiver. In the near surface, waves take a direct path from source to receiver and the measurements of elapsed travel time for measured distance results in the wave velocity through that material. Waves moving downward into the earth may be refracted and reflected at velocity interfaces. The ray-path diagram is a convenient method of showing wave propagation by direct, refracted and reflected paths. Unless otherwise noted, all velocities refer to compressional waves. Seismic data are obtained by recording shock-wave travel time between a source and a receiver, or geophone, for various chosen distances. Two general types of seismographs are used in engineering applications. One type permits the simultaneous, multichannel recording of shock-wave arrivals at a number of geophone locations from a single energy source such as a buried explosive charge or a weight-dropping system. The output from the geophones may be recorded in analog and digital forms in a variety of ways, all having a time base to permit extraction of elapsed travel times to each geophone location. Two advantages of multi-channel recording are; (1) single energy source for all geophone channels, and (2) more sophisticated filtering, recording, data processing, and printout capabilities than simpler single-channel seismographs. Disadvantages include higher firs cost, greater operating cost, large size, and often the need for greater peripheral support such as computer hardware and software. The second type of seismographs is a single-channel instrument that records shock-wave travel time from a source to a single geophone location. As a result, the operation must be repeated for different geophone distances until a suitable number of travel times have been obtained. Single-channel seismographs may record geophone output in several ways such as on an oscilloscope; on a chart, as in multichannel unit; or only as a first-arrival pulse time digitally (or by some other means). Timing systems are inherently a part of each system. Single e-channel seismographs are low initial cost, small size and provision for tailoring of geophone recording distances to the given site as work progresses. Disadvantages include time require for obtaining data, lack of single energy source for the entire geophone spread, and usually a restriction of use to refraction seismic surveys. 6.1. Seismic Refraction It is a method used to determine the P-wave velocity structure of the subsurface. Seismic P-waves are generated on the surface, propagate through the soil and rock, and are recorded by geo-phones at known distances from the source. When seismic waves encounter interfaces separating materials of different seismic velocities, the waves are refracted according to Snell's Law. At the critical angle for each interface (energy refracted 90 degrees) the seismic wave will travel along the interface with velocity of underlaying layer. A seismograph is used to record the travel times of these first arrivals, after which seismic velocities can be derived. Depths to the refracting layers can also be determined. Primary applications include determination of depth to bed rock and thickness of geologic strata, rippability and dredge-ability, and in-situ elastic modulus of soil and rock. It can provide data to depth of 100 ft or more and resolves up to 2 to 3 layers. It also provides a 2D cross-section of P-wave velocity. The source of seismic energy can be as simple as 8-pound sledge hammer. Deep measurements may require explosive as an energy source.If a velocity interface is not parallel with the surface, the velocities recorded at the surface are apparent rather than true velocities. The velocities recorded or plotted will be less than or greater than true velocity when energy is travelling down-dip or up-dip, respectively. The shallower down-dip end will exhibit a lower intercept time and critical distance compared to the deeper or up-dip end. 6.2. Seismic Reflection Seismic reflection measures the travel time of seismic waves from the surface downwards to geologic contacts where part of the seismic energy is reflected back to hydrophones at the surface. A reflection will occur from geologic strata when the reflection coefficient (derived from density and seismic velocity contrasts) between strata is sufficient. After raw data are processed, a cross-sectional picture of subsurface strata and anomalous conditions can be developed. Primary application is for determination of depth and thickness of geologic strata, structural and anomalous condition. The depth ranges from as shallow as 30 ft to greater than 100 ft. 6.3. Down-hole seismic surveys These are the simplest and cheapest method as they required only a single borehole. Seismic energy is generated on surface at a fixed distance from the top of the borehole. The travel times of the first-arrival seismic waves measured at regular intervals down the hole using string of hydrophones or in case of S-wave surveys, a single clamped tri-axial geophone that is gradually moved down the hole. P and S wave arrival time for each receiver location are combined to produce travel time versus depth curves for the complete hole. These are then used to produce total velocity profiles from which interval velocities and various elastic moduli can be calculated (in conjunction with density data from geophysical logging of borehole). 6.4. Cross-hole Seismic Surveys This involves measurement of the travel time of seismic energy transmitted between two or more boreholes in order to derive information on the elastic properties of intervening materials. One hole is used to deploy the source while the other hole(s) are used to detect the arrival of the seismic energy. The travel time of the seismic waves are derived from the first arrivals identified on the seismic trace for each short-receiver position and are used with the known distances between the short/receiver boreholes to calculate the apparent velocities (P & S) for each depth interval. This data is then used to derive a vertical profile of the various elastic moduli. The relationship between the velocity of seismic waves and the density and elastic properties of the materials through which they are travelling means that seismic techniques can be utilized to provide information on various geotechnical properties of the subsurface, such as Poisson's ration and shear modulus. The most common method of measuring these properties in engineering studies through the use of cross-hole seismic surveys. 6.5. Cross-hole Seismic Tomography Borehole seismic tomography involves the measurement of the travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocity in the intervening ground. Data is collected using one hole for the seismic source (normally a speaker) and measuring one arrival time using strings of hydrophones in the other. Travel times are collected at regular intervals (usually 0.5 m to 2 m) all the way down the hole(s) for each short position. This results in a network of overlapping ray paths that can be used to model the velocity profile. The resulting velocity image is termed a tomogram and enables identification of anomalous velocity zones laying between the boreholes as well as imaging individual velocity layers. The primary applications of borehole seismic tomography is in engineering studies for the identification of features such as fault zones and voids when combined with S-wave survey, the data can additionally be used to provide information in material stiffness properties. 7. Electrical Resistivity Electrical resistivity measurements are made by placing four electrodes in contact with soil or rock. A current is caused to flow in the earth between on pair of electrodes while the voltage across the other pair of electrodes is measured. The depth measurement is related to the electrode spacing. The resistivity measurement represents the apparent resistivity averaged over a volume of the earth determined by the soil, rock, and pore fluid resistivity, along with the electrode geometry and spacing. In contrast to wave velocities in refraction seismology, resistivity values are not representative of specific physical properties of earth materials no do they remain constant over time. The flow of current through soil and rock is by ion conduction, which is dependent on a combination of the conductivity of the fluid present, porosity, and percentage of saturation. Dissolved salts in water provide for ion conductance of electrical current. The conductance that is the reciprocal of resistance is directly proportional to amount of dissolved salt in water, or salinity. The amount of fluid regardless of its salinity that can be present is controlled by the porosity of material. The more interconnected the pore spaces, the greater the ease of ion migration through the material. In addition, the degree of saturation that varies with season, in turn affects conductance (or in the context of this discussion resistivity). Seasonal fluctuations in resistivity of as much as 200% have been reported. The rock forming minerals normally are highly resistive to current flow. An exception, which complicates resistivity work, is the presence of clay minerals. The exchangeable ions in the clays may separate from the lattice and make the pore water conductive even though the formation water may not be saline. As a result, clays have low resistivity – whether occurring as clay-rich soils or as shales. If we are certain that the groundwater in an area is fresh, low resistivity is representative of clay. Conversely, freshwater (a poor conductor) will cause high resistivity when present in the poor spaces of a clean or clay-free soil or in the pores or joints of a porous or dense, relatively clay-free rock. Note, however, that there is nothing distinctive about the kind of material that has high or low resistivity values, as is the case with seismic velocities. For instance, it would be possible on the basis of high resistivity to drill expecting to encounter porous, freshwater-bearing sand and instead encounter tight sandstone. Also saline pore water in sand or porous or highly fractured rock gives low resistivity values that are also indicative of clay or shale. Because of these perplexing problems, there is the need for subsurface control of materials and thickness from either exposures or boreholes. 8. Magnetic There are two primary applications for magnetic measurements; 1) locating and mapping buried ferrous metals, and 2) mapping geologic structures. The presence of buried ferrous metals creates a local variation in the strength of the earth's magnetic field permitting the detection and mapping of buried ferrous metal. Total field measurements made with one magnetometer, and gradient measurements made with two magnetometers are commonly used. Magnetic gradient measurements are made by a gradiometer, which is simple two magnetic sensors separated by a constant offset. Magnetic measurements can be used for geologic mapping by responding to the magnetic susceptibility of soil and rock. Generally total field measurements are used for geologic mapping. The primary application is for mineral exploration and for characterizing geologic structures such as faults. 9. Microgravity A microgravity survey provides a measure of change in subsurface density. Natural variations in subsurface density include lateral changes in soil or rock density, buried channels, large fractures, faults, dissolution-enlarged joints and cavities. A microgravity survey consists of making sensitive gravity measurements at the micro Gal (µGal) level (1/1000 of a milliGal or 10E-9 of the earth's gravitational field) with a gravimeter. Gravity measurements are acquired at discrete points along a profile line or within a grid, and are corrected for instrument drift, tidal effects, elevation changes, and latitude. Gravity anomalies are directly related to lateral variations in subsurface density. This method is used to identify caves, voids, skin-holes and weathered zones. It is also used to map top of rock. It can also be used to identify man-made structures such as tunnels and mines. 10. Ground Penetrating Radar Ground penetrating radar (GPR) uses high frequency electro-magnetic waves to acquire subsurface information. Energy is radiated downward into the ground from a transmitter and is reflected back to receiving antenna. The reflected signals are recorded and produce a continuous cross-sectional profile of shallow subsurface conditions. Reflections of the radar wave occur where there is a change in the di-electric constant or electric conductivity between two materials. These changes are associated with natural hydro-geologic conditions such as bedding, cementation, moisture, clay content, voids and fractures. Large changes in dielectric properties often exist between geologic materials and man-made structures such as buried utilities or tanks. This technique is also used for the location of rebar in concrete nondestructive testing of man-made structures. Ground penetrating radar is also used to map void space behind concrete tunnel lining.
  8. 1. Introduction Subsidence is displacement of ground surface vertically over a broad region or at localized areas. It may be either a gradual lowering or a collapse. This can have costly effect on facilities and structures over a subsiding area. Subsidence results from a number of different mechanisms. It can occur as a consequence of natural processes. The dissolving of limestone, salt, or other soluble materials creates underground openings that may collapse. Collapse may also occur in the roofs of lava tubes in areas underlain by volcanic rock. Withdrawal of fluids from subsurface reservoirs can create human-induced subsidence. This type if subsidence has resulted from extracting oil, gas and ground-water. Underground mining is another mechanism for creating subsidence by creating subsurface openings. Natural solution of rock leading to collapse of the overlaying surface can be a rather spectacular form of subsidence. A single sinkhole 324-ft wide and 100-ft deep was formed by collapse on May 8-9, 1981, in Winter Park, Florida. It destroyed a house, several cars, streets, parts of neighboring buildings, and the city swimming pool, causing losses estimated to exceed $2 million. Groundwater withdrawal for surface uses or dewatering of quarries and mines causes a general lowering of the water table. Construction activities are a less-common cause of collapse. Subsidence because of construction can result from loading the ground surface over a cavity or from the diversion of surface water, thus changing the groundwater system and increasing sinkhole development. Subsidence caused by underground mining results in severe economic losses in some areas. It is estimated that damage amounting to $30 million annually results from subsidence over abandoned coal mines. Underground mining notably coal mining, creates subsurface openings. Rock layers bridging these voids may fracture and collapse into the opening, with resultant lowering of the ground surface. Subsidence causes differential settlement, with the greatest amount near the center of the opening. Associated with this differential settlement are ground cracks. The extent and size of cracks will change until subsidence is complete in an area. Withdrawal of oil, gas and water has produced subsidence that has resulted in extensive losses in Arizona, California, and along the gulf coast of Texas. Subsidence causes damage in several ways. The most obvious causes are in tilting, cracking, and shearing of structures where subsidence produces differential settlement. Large-scale collapse can completely destroyed some structures. Destruction results when water-containment structures such as reservoirs and canals are breached. Subsidence causes damage impairing the function of some surface facilities. It can create low points in pipelines and alter the alignment of microwave transmission stations. A more-subtle consequence of subsidence is ground lowering that makes more land subject to flooding. 2. Evaluating Subsidence Processes It should be clear from our discussion that evaluation of subsidence processes depends on a variety of methods. Evaluating possible underground openings will require a very different approach than estimating subsidence from fluid withdrawal. The potential of solution-caused subsidence depends on the presence of limestone or other soluble rock type. Examining the natural subsidence occurring where these rocks are present serves as an initial indicator. For example, a sinkhole-density map compiled by means of aerial photography is useful measure of relative collapse potential. Similarly, the potential for subsidence from underground mining exists only where mining is active or was conducted in the past. Historical records or the details surface surveys normally conducted by many present day mines provide a basis for evaluating subsidence potential. The extent of subsidence depends on factors such as: 1) thickness of mined coal, 2) mine geometry and mining methods and 3) thickness, lithology, structure, and hydrology of bedrock and surficial material in the mining area. Detecting solution cavities or abandoned mine openings is mostly reliably done with the drilling. However, expense limits such drilling over large areas. Earth-resistivity surveys can detect openings at depths up to 25m. Gravity surveys are marginally successful; only large openings near the surface are detectable. Subsurface radar techniques proved too unreliable for practical use. There are two field-based techniques for subsidence prediction where fluid is being withdrawn. One is the depth porosity method, and other is aquitard-drainage method. Estimating subsidence in an area where fluid withdrawal is being initiated is best done with the depth-porosity method. For the more complicated situation in which subsidence is already active, the aquitard-drainage method is recommended. Ground failures in areas subsiding owing to fluid withdrawal can be predicted. These ground failure range from tension cracks to surface faults. Prediction of ground failure in areas where deformation is underway requires monitoring of surface conditions for sign of failure. In areas not yet subject to subsidence, prediction requires determining the particular surface conditions conducive to failure. 3. Mitigating the Effects of Subsidence Processes Controlling land use to avoid large-scale changes in the regional water table is one way to avoid subsidence in areas underlain by soluble rock. Avoiding withdrawal overdraft from compressible groundwater aquifers is equally effective in avoiding subsidence. Reservoirs from which oil, gas, or geothermal fluids is being withdrawn can be re-injected with water to compensate for the lost fluids. In some instances, subsidence over mines need not result in structural damage. This requires knowing the specific factors that influence subsidence at that locality and conducting mining in a manner that permits a general lowering of the entire area in which a structure is situated. This minimizes the differential settlement responsible for most of the distress to structures. Structures damaged or impaired by subsidence can be restored in many cases, this involves sealing the cleaned-out sinkhole, restoring the ground surface, ensuring that surface water to site is minimized, and promoting groundwater flow down gradient from the repair location. Some problems with solution-related subsidence are human related. This is especially true where natural sinkhole is used to drainage. Diverted water can increase the groundwater gradient in areas, leading to greater subsidence. Subsidence controls over mined areas generally takes the form of either providing selective support for the structure or filling the underground space to halt further subsidence. In an effort to control subsidence affecting an electric substation in Pennsylvania, both approaches to dealing with subsidence were used. Selective support involved placing drilled piers and piling seated into rock below the base of the mined coal seam. Fly ash was injected through drill-holes to fill some underground openings. In 15 selected locations, grout columns were constructed.
  9. 1. Introduction Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limits. If a strained rock breaks, it then snaps into a new position and, in the process of rebounding, generates vibrations called seismic waves. Land waves produced by an earthquake have been reported with heights of more than 0.5 m and wave lengths of 8 m. The period between the passages of wave crests can be as much as 10 seconds. The vibrations can continue for as long as an hour before the wave dies out. 2. Focus and Epicenter Earthquake waves are propagated in all directions from the center of origin or focus of a shock. The average depth of focus for normal earthquakes is about 30 km, though many have a deeper or shallow origin. Shallow-focus earthquakes occur from the surface to a depth of 70 km. They occur in all seismic belts and produce the largest percentage of earthquakes. Intermediate-focus earthquakes occur between 70 and 300 km. Both intermediate-focus and deep-focus earthquakes are limited in number and distribution. The center of origin of an earthquake is called focus and the point on the earth's surface immediately above the focus is called epicenter. 3. Types of Seismic Waves Three types of waves are generated by an earthquake shock: 3.1. Primary Waves (P-Waves) These are a kind of longitudinal waves, identical in character to sound waves passing through liquid or gas. The particles involved in these waves move forward and backward in the direction of wave travel, causing relativity small displacements. These are the first waves to arrive at seismic station. 3.2. Secondary Waves (S-Waves) These are a kind of transverse waves that are a little slower than the primary waves. The particles are displaces at right angles to the direction of wave propagation. These waves cause strong movements to be recorded on a seismograph. 3.3. Surface Waves (Rayleigh Waves) The last waves to arrive are the surface waves and travel only in the outer layer of earth and are similar to waves in water. Particles involved in surface waves more in an orbit similar to that of particles in water waves i.e. in a circular path at the surface. The motion diminished with depth. Surface waves caused the most damage during an earthquake. Together with secondary effects from associated landslides, tsunamis, and fires, the results in approximately 10,000 lives and $100 million each year. 4. Elastic Rebound Theory The origin of an earthquake can be illustrated by a simple experiment. Bend a stick until it snaps. Energy is stored in the elastic bending and is released if rupture occurs, causing the fractured end to vibrate and send out sound waves. Detailed studies of active faults show that this model, known as elastic-rebound theory applied to all major earthquakes. The elastic-rebound theory explains earthquakes as a result of either rupture or sudden movement along existing fractures. 5. Intensity The intensity or destructive power of an earthquake is an evolution of the severity of ground motion at a given location. It is measured in relation to the effects of the earthquake on human life. Generally, destruction is described in terms of the damage caused to buildings, dams, bridges, and other structures. 6. Magnitude The magnitude of an earthquake is the measure of the amount of energy released. It is much more precise measure than intensity. The total energy released by an earthquake can be calculated from the amplitude of the waves and the distance from the epicenter seismologists express magnitudes of earthquakes using the Richter scale, which arbitrarily assigns 0 to the lower limits of detection. Each step on the scale represents an increase in amplitude by a factor of 10. The vibrations of an earthquake with a magnitude of 2 are therefore 10 times greater than those of an earthquake with a magnitude of 1, and the vibrations of an earthquake with a magnitude of 8 are 1 million times greater in amplitude than those of earthquake with a magnitude of 2.
  10. There are generally four types of earthquake induced processes: 1) Surface rupture, 2) Ground shaking, 3) Ground failure, and 4) Tsunami and Seiches occurrence. Each type has significance to a safe design of structures and is a concern for engineering geologist defining hazard levels. 1. Surface Rupture It is the actual displacement and cracking of the ground surface along a fault trace. Surface rupture is confined to a narrow zone along an active fault. Rupture may happen rapidly during an earthquake or it may not occur at all. Displacement beneath a building that exceeds 1 or 2 inches can have a catastrophic effect. Some active faults undergo imperceptibly slow movement, termed fault creep. Although a serious problem for structure, the cumulative effect of this long-term displacement, unlike rupture during an earthquake, is not catastrophic. 2. Ground Shaking It is the actual trembling or jerking motion produced by an earthquake. It causes widespread damage is one of the most difficult seismic effects to quantity and predict. The damage varies with wave length, duration of shaking, nature of underlying materials, and character of structures. For the same earthquake, the effect of ground shaking can be several times greater at sites with thick, water saturated soil than at those on competent bedrock. Buildings have a fundamental period that is roughly equivalent to the number of their stories. A resonance or amplification of ground shaking develops where the building and underlying soil have a similar fundamental period and causes more-extensive damage. Building with differing fundamental periods will sway differently. 3. Ground Failure This is because of ground acceleration from an earthquake produces landslides, ground cracking, subsidence and differential settlement. Hilly slopes can fail under a dynamic load, the stress from earthquake motion. Liquefaction is another mechanism of ground failure during an earthquake. Ground motion transforms loose water-saturated granular material to a liquid state. This occurs because the cyclic stress of succeeding waves of ground motion causes pore-water pressure to build up. This rapid increase in pore-water pressure nearly reduces the effective stress of soil mass to zero, a state where it has a least resistance to applied stress. Otherwise solid level ground becomes unable to bear the weight of overlaying structures. Building and structural foundations sink into the liquefied materials, causing tilting and related damage. Differential settlement can cause damage to buildings as some of the soil settles more than other parts underlying the building foundation. Liquefied soil may flow on very low slopes. This special type of landslide is called lateral spread failure. 4. Tsunamis and Seiches These are similar effects that occur in bodies of water. Tsunami is a Japanese term for large ocean waves generated by submarine earthquakes. Rapid displacement of an undersea fault can cause a wave travelling thousands of miles from the epicenter. On the deep ocean, the tsunami height typically is 1 ft. The wave height increases as it reaches shallower water near shore lines. Seiches or earthquake generated standing waves are not quite as destructive as tsunamis. They occur in enclosed or restricted bodies of water and consist of a standing wave that oscillates across the surface. Seiches generally are of low amplitudes (less than 1 ft.)
  11. In countries where earthquakes are of frequent occurrence, important buildings are now designed to withstand all but the most severe shocks. Structures founded on hard rocks are generally less damaged than those on soft grounds and it is found that properly designed steel-framed or ferro-concrete structures posses the highest degree of immunity from damage. Thus a rigid frame carried on a strong foundation would undergo as a whole the same movement as the ground. In soft ground a concrete raft foundation should be used. Bridges present special problems; after the Bihar earthquake of 1934, it was found that bridges with screw pile foundations had stood up the best to the shock. Brick arches were easily broken; and the girder bridges supported on stone work piers also failed. The heights of large buildings should not exceed 100 ft and heavy loads near the top should be avoided. The acceleration of the ground when the shock begins is an important factor which should be taken into consideration while designing a frame structure rigid enough to withstand the force. In big earthquakes the acceleration may amount to nearly half that of gravity but it is generally not greater than 0.2 g. The following are the different techniques used in designing earthquake resisting structures: 1. Cross Bracing If two diagonals are used in the form of cross-bracing they only need to resist the tension. This is because one brace is in tension for sideways force in one direction on the frame while the other brace is in tension when the force is reversed. Steel cables can be used for cross-bracing as they can be stretched but not squashed. 2. Rubber Bearings Rubber bearings are made from layers of rubber with thin plates between them, and a thick steel plate on the top and bottom. The bearings are placed between the bottom of a building and its foundation. The bearings are designed to be very stiff and strong for vertical load, so that they can carry the weight of the building. However they are designed to be much weaker for horizontal loads, so that they can move sideways during an earthquake. Rubber bearings have been used to protect the Museum of New Zealand from large earthquakes. 3. Friction Pendulum Bearings Friction pendulum bearings are made from two horizontal steel plates that can slide over each other because of their shape and an additional articulated slider. The bearings are placed between the bottom of a building and its foundation. They are designed to be very stiff and strong for vertical load, so that they can carry the weight of the building. However the fact that they slide means that earthquake movements will occur mainly in the bearings. Friction pendulum bearings have been used in the San Francisco Airport International Terminal. The building has been designed to resist a magnitude 8 earthquake occurring on the San Andreas fault. 4. Viscous Fluid Dampers Viscous fluid dampers are similar to shock absorbers in a car. They consist of a closed cylinder containing a viscous fluid like oil. A piston rod is connected to a piston head with small holes in it. The piston can move in and out of the cylinder. As it does this, the oil is forced to flow through holes in the piston head causing friction. When the damper is installed in a building, the friction converts some of the earthquake energy going into the moving building into heat energy. The damper is usually installed as part of a building's bracing system using single diagonals. As the building sways to and fro, the piston is forced in and out of the cylinder.
  12. Sedimentary strata may lie horizontal as originally deposited or they may be bent into more or less regular folds, they may be displaced along fractures, or they may be traversed by planes or parting known as joints. Geologic structures influence engineering projects in many ways. Folds and faults obviously have much to do with the selection of dam sites and even such seemingly unimportant matters as the spacing of joints may have vital bearing on uplifting pressure and safety of dams. Gushed and chemically altered rocks contiguous to originating along faults may damage or destroy engineering structures. The design of deep cuts in rocks is greatly influenced of geologic structures on circulation of the ground water. Some of the most common terms that are involved in the study of geologic structures are as follows; Bedding Planes: The planes or surfaces which divide on bed from the other are called bedding Plane. Dip: The dip of a bed in the angle between the bedding and the horizontal plane. Strike: It may be defined as the direction of line formed by the intersection of bedding and horizontal plane. Outcrop: The area of exposure of bed on the earth’s surface is called outcrop. 1. FOLDS Perhaps the most common type of deformation is folding. As the name implies, folds are undulation, flexures waves which resembles to ocean waves. They are best displayed in stratified formation i.e. sedimentary rocks. But any layered or foliated rock such as banded gabbros or granite gnessis may display folds. Some folds are few miles away, the width of others to be measured in feet or inches or even in fraction of an inch. 1.1. Parts of Folds The axial plane or axial surface of a fold is the plane or surface that divides the folds as symmetrical as possible. In some folds the axial plane is vertical in other it is inclined and in still other it is horizontal. The sides of a folder are called the limbs or flanks. A limb extends from the axial plane in one fold of the axial plane in the next. The highest point of a fold is called the crest and the lower portion is called trough. 1.2. Nomenclature of Folds On the basis of dip relationships two major types of folds can be distinguished; Anticline: In which the strata on opposite flanks dip towards the axis in other words the folds that concave upwards. Monoclines: Folds in which horizontal or gently dipping beds are modified by simple step like bends. Over turned Fold: The axial plane is inclined and both limbs dip in the same direction but usually at different angles. 2. FAULTS Faults are fractures in the earth’s crust along which slippage or displacement has occurred. As a result, formerly continuous beds have been dislocated in a direction parallel to fault’s surface. The displacement may vary from a few inches or less, to many miles. When subjected to great pressure, the earth’s crust may have to withstand shear force in addition to direct compression. If the shear forces so induced become excessive, failure will result, movement will take place along the plane of failure until the unbalanced forces are equalized and a fault will be the result. 2.1. Terminology The vertical component of the displacement between two originally adjacent points is called Throw of the faults. The block above the fault is called Hanging Wall and the underlying block is termed as Foot Wall. The horizontal component of displacement is called the Heave and the angle of inclination to the vertical is called the Hade of the fault. 2.2. Types of Faults The two common types of faults are normal faults and reversed fault. In a normal fault the hanging wall is displaced downward relative to the footwall. In the reversed faults the hanging wall is displaced upwards relative to footwall. If the faults dip at angles less than 45 degree the term high thrust fault is applied. Strike slip faults are the high angle fractures in which displacement is horizontal, parallel to the strike of the fault plane. There is little or no vertical movement. Normal faults rarely are isolated fractures. Typically, a group of parallel normal faults develops a step like arrangement, or a series of fault blocks. A narrow block dropped down between two normal faults is called graben, and an upraised block is called a horst. 2.3. Active and Inactive Faults Fractures that are known to have experienced dislocation within historic time are known as active faults. As they present hazard to construction, the differentiation of active and inactive faults is matter of considerable engineering importance, and it is quite unfortunate that frequently no very reliable decision can be made. The most direct and best evidence of activity is that furnished by seismographs and benchmarks. If the seismograph records show that earthquakes ocean along a fault it should, of course be regarded as active. Similarly accurately located bench mark exhibit horizontal in vertical displacement, and fault known to exist in the area should be regarded as active. If a fault is known to be overlain by younger strata that are not displaced, it is permissible to regard it as inactive. 3. JOINTS Joints are planes or surface which intersect rocks, but along which there has been no appreciable displacement parallel to the joint surface. When displacement parallel to the fracture is measureable, the fracture is known as a fault. Joints result either from tension or shear stress acting on rock mass. The cause of stresses may be due to contraction, compression, unequal lift, subsidence, earthquake or other earth phenomena. Tension joints arise, for instance by drying and resultant shrinkage of sedimentary deposits, or igneous rocks by contraction and cooling. Shear joints may arise from compression of sedimentary or igneous rocks. 3.1. Engineering Significance Because of their almost universal presence, joints are of engineering importance, especially in excavation operations. It is desire able for joints to be spaced closely enough to reduce secondary plugging and blasting requirement to a minimum, but not so closely spaced as to impair stability of excavation slopes or increase breakage in tunnels. Needless to say, the ideal conditions are seldom encountered. Joints oriented approximately at right angles to the working face present the most unfavorable conditions, whereas joints oriented approximately parallel to the working face greatly facilitate blasting operations and ensure a fairly even and smooth break parallel to the face. Joint offer channels for underground water circulation and in working below the ground water table may greatly increase water problems. They also may exert an important influence on weathering.
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