Unit4
- 1. UNIT - 4: GROUNDWATER Prepared by:K.SAHITYA, Asst prof. Civil engineering dept. BVC college,odalarevu Note: for ground water exploration techniques read 5th unit Hydrogeology deals with occurrence, storage and movement of groundwater in the subsurface. All water below the earth surface is referred to as the groundwater or subsurface water. The surface water percolates or infilters into the ground through the fractures/cracks and its distribution and movement in the subsurface is controlled by the Porosity and Permeability of the geological rock materials such as soils, rocks etc…. Porosity in the rock formations facilitates the storage while permeability contributes to movement of groundwater. Based on the porosity and permeability characteristics all geological formations are named as aquifers, aquiclude, aquitard and aquifuse. Aquifers: A geological formation that yield significant quantities of water has been defined as an aquifer. A few Sedimentary rocks are permeable considered as good aquifers ( eg: porous Sandstone, gravels, cavernous limestones ) while Igneous and Metamorphic rocks are relatively impermeable and hence serve as poor aquifers Aquiclude:A rock formation has porosity but no permeability, then it is called aquiclude. That means it can store water and the flow of water does not take place. Eg: clay. Aquitard:A saturated but poorly permeable strata that impedes groundwater movement and does not yield water freely but may transmit appreciable water to the adjacent aquifers. Eg: Sandy clay with a small quantity of silt. Aquifuse: A rock neither containing water nor transmitting water. Eg: Solid granite AQUIFER PARAMETERS: The quantity of water stored by an aquifer and the quantity of water released by the aquifer depend on the nature and composition of the rocks through certain parameters such as Porosity, Permeability etc.
- 2. Groundwater can be drawn either from consolidated rocks or unconsolidated sediments. The occurrence of groundwater in a geological formation and scope for its exploitation primarily depend on porosity and permeability properties of that formation. Porosity (α ):In simple terms, porosity may be described as the amount of openings ( or ) interstices ( or ) empty spaces present in a rock. However, Porosity may be defined as “the ratio of openings or pores or voids ( Vi ) in the soil/rock to the total volume of the soil / rock ( V ) expressed as percentage”. If α is the porosity, then α = Vi / V where Vi is the volume of interstices and V is the total volume. The average porosity values for some common geological formations are as follows: Rock Porosity % Granite, Quartzite 1.5 Shale, Slate 4 Limestone 5 - 10 Sand with gravel 20 - 30 Only Gravel 25 Only Sand 35 Only Clay 45 Prob: A rock sample has ( dry weight ) of 0.655 kg. After saturation with water its weight is 0.732 kg. It is then immersed in water and found to displace 0.301 kg of water. What is the porosity of the sample? Solution: Sample ( dry ) weight W1 = 0.655 kg Wt of saturated sample with water W2 = 0.732 kg Wt of the water required to saturate the sample ( W2 – W1 ) = 0.732 – 0.655 = 0.077kg Wt of water displaced by the saturated sample (W3) = 0.0301 kg Porosity of the sample: (W2 – W1 ) / W3 = 0.077 / 0.301* 100 = 25.58 % Permeability : The permeability of a rock or soil defines its ability to transmit a fluid or water. Permeability depends on the porosity and interconnected pores character of the rock. Permeability in a rock is measured in darcies ( 1 darcy = 0.987 µ m2 )..square micrometer
- 3. Eg: 1.Shales are porous but less permeable because of fine grained nature which does not allow water to pass through the rock due to less interconnected pores. Eg: 2. Vesicular basalts are highly porous but less permeable because the vesicles in them are not interconnected (i.e., the effective porosity is less). HYDRAULIC CONDUCTIVITY OR COEFFICIENT OF PERMEABILITY ( K):The movement of groundwater depends on the prevailing effects of gravity, velocity and pressure of water. According to DARCY’s law, the flow of water through porous medium is proportional to a factor known as Hydraulic conductivity or coefficient of Permeability (k). It is expressed as Q=KiA where Q= volume of water flowing / unit time through a cross sectional area (A) under a hydraulic gradient ( i ) usually i = 1. Hence K= Q / iA (K will be called as velocity of flow) . For some geological formations, the permeability coefficient values are as follows: Formation Co-efficient values Formation Co-efficient values Granite- 0.04 Quartzite 0.04 Slate 4.0 Shale 4.0 Sandstone 400 Sand 4000 Sand & gravel 40000 Gravel 400000 Clay 0.04 Limestone 4.0 Hydrologic Cycle: Groundwater is one of the components of the hydrologic cycle in nature. Hydrologic cycle enables a clear understanding of the recharge, storage and movement of water in the subsurface. The continuous circulation of water from land, water bodies etc., which joins the atmosphere and finally condenses into the form of precipitation.A part of water is lost by evapo-transpiration and certain portion percolates into the ground to form ground water reservoir and the remaining water flows on the ground as runoff and joins the streams, rivers and finally into sea. This cycle is continuously repeated.
- 4. Uses of water:Water is needed for daily use for organisms, for irrigation, industries, electricity production and domestic use. Hence, water is an important resource in all economic activities ranging from agriculture to industry.About 97% of it is salt water in the seas & oceans, 2.6% is trapped in polar ice caps & glaciers. Only 0.4% is available as fresh water. Fresh water occurs mainly in two forms namely Ground waterand Surface water. The distribution of fresh water is geographically uneven varying greatly from country to country & even one region to another region. 1. DOMESTIC USE: Water used in the houses for the purposes of drinking, bathing, washing, cooking, sanitary & other needs. The recommended value according to Indian Standard specification for domestic use is 135 liters/ day. 2. INDUSTRIAL USE: Water is required for various industries such as cement, mining, textile, leather industries. 3. PUBLIC USE: This includes water used for public utility purpose such as watering parks, flushing streets, jails etc. 4. FIRE USE: Water is used in case of accidents and to prevent the fire issues. 5. IRRIGATION: To grow crops which is the main sources for food. 6. OTHER USES: Hydro electric power generation requires water. Effects of over use of ground water: Over use of groundwater has following ill effects: 1. Lowering of water table: Excessive use of ground water for drinking, irrigation and domestic purposes has resulted in rapid depletion of ground water in various regions leading to lowering of water table & drying of wells.
- 5. 2. Ground subsidence: When ground water withdrawal is greater than its recharge rate, the sediments in the aquifer become compacted. This is called ground subsidence which may cause damage of buildings, destroy water supply systems etc. The reasons for shortage of water are: 1. Increase in population, 2. Increasing demand of water for various purposes. 3. Unequal distribution of fresh water. 4. Increasing pollution of existing water sources cause over exploitation. 5. People depend on ground water as it is considered to be fresh water. How we contaminate groundwater ? Any addition of undesirable substances to groundwater caused by human activities is considered to be contamination. It has often been assumed that contaminants left on or under the ground will stay there. Groundwater often spreads the effects of dumps and spills far beyond the site of the original contamination. Groundwater contamination is extremely difficult, and sometimes impossible, to clean up. Point sources On-site septic systems Leaky tanks or pipelines containing petroleum products Leaks or spills of industrial chemicals at manufacturing facilities industrial waste Municipal landfills Livestock wastes Chemicals used at wood preservation facilities Mill tailings in mining areas Fly ash from coal-fired power plants Sludge disposal areas at petroleum refineries Land spreading of sewage or sewage sludge Graveyards Wells for disposal of liquid wastes Runoff of salt and other chemicals from roads and highways Spills related to highway or railway accidents Coal tar at coal gasification sites Asphalt production and equipment cleaning sites Non-point (distributed) sources Fertilizers on agricultural land Pesticides on agricultural land and forests
- 6. Contaminants in rain, snow Water Table: The land surface is covered by loose soil due to natural weathering phenomenon. Since the effect of weathering decreases gradually with depth, a fractured zone of rocks exists below the soil zone. Further, below this zone, occurs the hard formations (bed rock) which are free from fractures. Groundwater flows slowly through water-bearing formations (aquifers) at different rates. In some places, where groundwater has dissolved limestone to form caverns and large openings, its rate of flow can be relatively fast but this is exceptional. Many terms are used to describe the nature and extent of the groundwater resource. The level below which all the spaces are filled with water is called the water table. Above the water table lies the unsaturated zone. Here the spaces in the rock and soil contain both air and water. Water in this zone is called soil moisture. The entire region below the water table is called the saturated zone, and water in this saturated zone is called groundwater. Hence, when rainfall occurs in any region, the rain water moves downwards through fractures under the influence of gravity until it reaches the bed rock. Then, the percolation/infiltration of water leads to the development of a zone above the bed rock which is called as zone of saturation, in which all openings or pores of the rocks are filled with water. Such water is called as groundwater. The upper surface of the zone of saturation is called WATER TABLE. Above the zone of saturation and below the ground surface is the zone of aeration in which water fills only a portion of the pore space. Surface Soil soil water Perched water Perchedwatertable Vadose water ZONE OF AERATION impermeablestrata capillary water W A T E R T A B L E groundwater ZONE OF SATURATION Groundwater fractures
- 7. BED ROCK ( HARD ROCK ) Types of groundwater occuring in zone of AERATION: 1. Soil Water:Water which occurs in the soil and is available to the roots of plants existing on the surface. 2. Vadose Water: (Gravity water): A fraction of rainfall which percolates downwards under the influence of gravity and slowly reaches the water table. In other words, the groundwater recharged this way. 3. Perched Water: When a groundwater body is separated from the main groundwater by a relatively impermeable strata of small areal extent within the zone of aeration at a particular place is called as Perched water. Wells tapping ground water from these sources yield only a small quantities of water. . 4. Capillary Water: This exists just above and in contact with the water table. 5. Meteoric Water: Due to rainfall, water is being soaked into the underlying rock. Types of groundwater occuring in the zone of SATURATION: 1. Free groundwater (unconfined aquifer): The ground water which lies below the water table under atmospheric pressure more freely upwards or downwards within the aquifer. 2. Confined water: This waters occur below the water table and is confined between aquicludes or aquifuse under hydrostatic pressure. 3. Connate water: Occasionally a sediment or rock may retain some quantity of water from the beginning of their formation. 4. Juvenile water: Water which has not previously been a part of the hydrosphere but which forms / rises from a deep, magmatic source. 5. Magmatic water: It is the water derived from magma. Types of Aquifers:Most aquifers are of large areal extent and may be visualized as underground storage reservoirs. Aquifers may be classified as unconfined or confined depending on the presence or absence of water table. Unconfined Aquifer:An unconfined aquifer is one in which a water table varies depending on recharge and discharge ( pumping form wells ) in a region. Rises and falls in the water table correspond to changes in the volume of water in storage within an aquifer. Wells dug in such an aquifer will have the water level equal to the level of the water table. The water table is under atmospheric pressure only.
- 8. Confined Aquifer: These are also called artesian aquifers. In this case, the groundwater should be under pressure more than atmospheric pressure and sandwiched between two aquicludes or two impermeable formations i.e., clay / shally formations. Unlike the unconfined aquifer, the confined aquifer stores less water. The wells in the confined aquifer are thus artesian wells operating under the piezometric pressure of the aquifer. If the piezometric surface of any well is above the ground surface the water level rises above the ground surface. In such a situation, the well is known as anartesian flowing well. Confined aquifer diagram Piezometric surface: A piezometer is a special tool that is used to take measurements within an aquifer. It is submerged within a well beneath the saturated zone, through less porous rock. Many piezometer wells are drilled within a confined aquifer at certain locations. The piezometric surface of water is the level of water within a piezometric well in a confined aquifer. A hydro-geologist can determine recharge and discharge rates and most importantly groundwater-flow direction and rates.
- 9. GEOLOGICAL CONTROLS ON GROUNDWATER MOVEMENT: Groundwater movement in the zone of aeration takes place under the influence of gravity while in the zone of saturation are of different kinds based on: a) The permeability character of rocks is one of the most influencing factors of groundwater movement. b) Secondary Porosity associated with the rocks viz. well-developed joints, sheet joints, presence of faults etc also influence the groundwater to move along them. c) The groundwater movement in the zone of saturation also depends on attitude of bedding places ( The percolated water moves along the inclined bedding planes, folded beds etc ). Sometimes, tilted beds, if accompanied by faults, joints, intrusives lead to the occurrence of springs and seepages. d) The buried river channels and conformities also influence the groundwater movement as they are porous and permeable. e) Presence of dolerite dykes, quartz veins in the associated country rocks may act as barriers to the natural flow of groundwater and accumulate on one side part of the dyke. f) Another factor which influences groundwater movement is the hydraulic gradient ( ie., slope or difference in the water table ) . Fluctuation in the level of the water table in unconfined aquifer is of two different types ie., SEASONAL and CONE OF DEPRESSION.
- 10. The water table level rises considerably during rainy seasons because of heavy rainfall and high recharge. In summer, the water is pumped out without any recharge. This leads to a significant fall in the level of water table, thus, this type of fluctuation in the water table is seasonal. When water is pumped out in a considerable measure from a dug well, the level of water goes down, and leading to the depression in the water table around the dug well in the form of an inverted cone. This phenomenon is called the Cone of depression (or) the cone of exhaustion. This is a temporary fluctuation in the level of water table because the original position is restored within a short period due to the seepage of ground water from the sides of the dug well The boundary of the cone of depression is known as the GROUNDWATER DIVIDE. The area enclosed by the Groundwater Divide is known as the AREA OF PUMPING DEPRESSION. The distance between the well and the Groundwater Divide is termed as the RADIUS of INFLUENCE. When water is pumped out from an open well, immediately the level of water in it goes down, leading to the hydraulic gradient i.e., the difference in the level of the water table of the aquifer and dug well water level. The difference between the original level of water in the dug well and level after pumping is called DRAWDOWN. If the pumping is continued, the drawdown increases further and the radius of influence of the well also increases. In a good aquifer , the draw down keeps at the same level. It is interesting to know that in all aquifers, the drawdown rate decreases with pumping time. (5’ in 1 hour and further 5’ in 5 hours and still 5’ in another 10 hours). Springs:A spring is a continuous flow of water on the ground surface. Springs are located at the points of interaction of groundwater table with the surface of ground. Groundwater and engineering: Groundwater can also have dramatic implications for engineering and geotechnical studies. The study of groundwater is essential for engineers who construct dams, tunnels, water conveyance channels, mines, and other structures. Groundwater must be considered whenever the stability of slopes is important, whether the slope is natural or constructed. Groundwater must also be taken into account when devising measures to control flooding. In all of these situations, groundwater flow and fluid pressure can create serious geotechnical problems. Groundwater, for example, may create structural weaknesses in dams, or it may flow underground right around the structure. Water flowed so efficiently through the rock formations surrounding the reservoir that the dam would hold no water, even though it was structurally sound. Terminology of an earthquake:
- 11. Focus: Place of origin of the earthquake in the interior of the earth. Epicenter: A point on the ground surface, which is vertically above the focus. Seismic waves: The enormous energy released from the focus at the time of earthquake is transmitted in all directions in the form of waves. Elastic waves: A wave propagated by a medium having inertia and elasticity in which displaced particles transfer momentum to neighbouring particles and are themselves restored to original position. Isoseismal: The imaginary line joining the points of same intensity of the earthquake. Hodograph: time - distance curve Seismograph: an instrument is used to detect/ record the seismic waves Seismogram: Recorded data in seismic methods Of all geophysical methods, seismic prospecting is more complicated and expensive. However, seismic method consists of a number of geophones; an amplifier and a galvanometer are used in addition to explosive material. Seismic methods are effective for depths more than a km but are not suitable for shallow exploration. Seismic methods are important to locate the anticline and synclinal structures in oil exploration, and to identify fault zones. It is also helped a great deal in understanding the internal structure of the earth. Many seismological observations have been established in India. The major ones among them are at New Delhi, Colaba (Mumbai) Alipore (Kolkatta); Shillong etc maintained by the Indian Meteorological Department and at Hyderabad by the National Geophysical Research Institute. CLASSIFICATION AND CAUSES OF EARTHQUAKES: Earthquakes are grouped based on their depth of origin, and described as shallow or intermediate or deeper earthquakes. Earthquakes with a focus depth < 60 km are called shallow earthquakes. If the depth is > 60 kms but < 300 kms, they are called intermediate earthquakes. Other which have a focus depth > 300 kms are called deeper earthquakes. Earthquakes originating at depths > 700 kms are extremely rare. TYPES: Based on the causes responsible for their occurrence, earthquakes are described as TECTONIC EARTHQUAKES AND NON-TECTONIC EARTHQUAKES. Tectonic earthquakes are exclusively due to internal causes ( ie disturbance of geological formations) that takes place in the earth’s interior. Generally, Tectonic earthquakes frequency is less with high intensity and more destructive in nature.
- 12. Non-tectonic earthquakes are generally due to external causes. These are very frequent but minor intensity and not destructive in nature. These earthquakes occur due to variety of reasons as follows: Due to huge water falls. Due to meteorites: Meteorites are bodies of various sizes wandering in space. When they come under the influence of the earth’s gravity field, they suddenly fall on the earth’s surface. Due to landslides Due to volcanic eruptions: Some volcanoes pour lava by throwing out the fire, smoke etc to greater heights. Such violent eruptions sometimes cause earthquakes. eg: Indonesia volcano(1883) due to eruption of volcano. Due to tsunamis: Tsunamis are giant sea waves. They move shore wards and dash against the coastline. Eg: Lisbon earthquake of 1775. Due to man-made explosions: During mining, and quarrying, many explosions are carried out. Due to collapse of caves, tunnels etc.. Due to dams and reservoirs: when the reservoir was filled with water shows the signs of tremors. This is due to lateral thrust of reservoir water contributing stress. SIZE OF EARTHQUAKES:The size of an earthquake is defined by its intensity and magnitude. Intensity is expressed based on the degree of destruction caused and varies from place to place. It is maximum around the epicentral area. Earthquake intensities Intensity Effects I Not serious II Felt by few persons at rest, particularly on upper floors of buildings III Vibrations similar to a moving truck IV Windows and doors rattle; loose objects disturb
- 13. V Breakage of dishes; wall plaster breaks VI Walls crack VII Slight to moderate damage in well-built structures VIII Falling of walls IX Ground cracks; breakage of underground pipes; considerable damage to buildings X Bending of rails; occurrence of land slides XI Buildings destroy XII Total destruction, surface displacements; objects thrown into air The intensity at a place depends on several factors such as distance from the epicenter; depth of focus, geological formations and also on the type of construction of a structure.. Magnitude: Energy released during the time of an earthquake is commonly expressed as Richter’s magnitude. Magnitude of an earthquake does not vary from place to place. Energy released (E) is obtained from the expression: √E = c (a/h)(d2 + h2) Where E = total energy released ( ergs) c = constant ( taken as 0.625 ) a = ground acceleration d = distance (km) of the recording station from the epicenter h = depth of focus (km ) Richter’s scale has magnitude numbers upto 10. But the maximum known magnitude is around 9.6 only. An earthquake magnitude of 6.0 involves energy of around 2.5x1020 ergs (equivalent to that of an atom bomb) while for magnitude of earthquake is 7.0, it is around 80x1020 ergs (equivalent to that of a Hydrogen bomb ). For an earthquake of magnitude is 8.0, then the energy may be around 2500x1020 ergs ( most powerful). The magnitudes of some of the important earthquakes in India are given below: City Date Magnitude of earthquake
- 14. Shillong June12, 1897 8.7 Kangra April 4, 1905 8.0 North Bihar Jan 15, 1934 8.3 Assam Aug 15, 1950 8.6 Koyna (Maharashtra) Dec 11, 1967 6.4 Chamoli (Uttaranchal) Oct 20, 1991 6.5 Sand and mud with water was ejected Killari ( Latur ) Sept30, 1993 6.5 Jabalpur (MP) May 22, 1997 6.0 Bhuj (Gujarat) Jan 26, 2001 7.5 Vertical & partial collapse of a building Andamans Dec 26, 2004 9.0 SEISMIC BELTS & SEISMIC ZONING MAP OF INDIA:On a seismic map, the country has been divided into 7 zones in terms of severity (magnitude). First seismic map (zoning map) was prepared in 1962 on the basis of historical data available regarding the occurrence of earthquakes all over the country. Subsequently the zoning map was revised in 1966. Many of the areas in zone V and VI were merged into one because of their high risk. In the zone map brought out in the year 2000, the earlier zones II and I were merged. So the number of zones got reduced from 7 to 5. Zone – I is least severe and the Zone VI is most severe. Entire NE regions, parts of Uttaranchal, Rann of Kutch ( Gujarat ) & Srinagar are included in zone V where the earthquake severity is high. All regions in Southern India are included in Zone III. Rest of the parts of India are included in Zone I & II. Zone 4 is also treated as severity. Precautionary measures for the construction of buildings, dams/reservoirs etc in seismic areas: To make suitable constructions in seismic areas, IS codes 1893 – 2970 give guidelines.
- 15. For Construction of Buildings:In addition to the safety factors considered there are other precautionary measures which help in increasing the stability of buildings in seismic areas. They are as follows: Buildings should be founded on hard bedrock and never on loose soils or fractured rocks. This is so because loose ground can easily expose to earthquake vibrations. Foundation should be of same depth throughout for continuity. Buildings situated near hill sides, near steep slopes, on undulating ground or on marshy ground always suffer more when earthquake occurs. Therefore these situations may be avoided. Buildings should have light walls. Different parts of abuilding should be well tied together so that the whole structure behaves like a single unit to the vibrations. Proper proportionate of cement and mortar should be used. Doors and windows should be kept to a minimum and they should not be in vertical rows but preferably along the diagonals. The building should have uniform height and additional features such as parapets, cantilevers, domes and arches are undesirable. Buildings should have flat RCC roofs and they should be designed not to yield to lateral stress. Projections above the roofs are undesirable. For Construction of Dams: Dams being very costly projects their consideration in seismic areas needs careful study to ensure their safety precautionary measures which are as follows: Forces in the dam due to reservoir water and due to the dams weight are to counter balanced by introducing additional stress in the design of the dam. Design of the dam is to be made such that during an earthquake they move along with the foundations below. Dams should not ordinarily be built along or across the faults because possible slipping along these planes during earthquakes will introduce additional complications. The resonance factor value (vibrations due to sound) should be given due consideration because a coincidence in the period of vibration of the dam and the earthquake vibrations can produce cumulative effects. LANDSLIDES The term landslide refers to the downward sliding of huge quantities of land masses. Such sliding occurs along steep slopes of hills or mountains.. It may be sudden or slow in its occurrence. Also, in magnitude, it may be major or minor.
- 16. Often, loose and unconsolidated surface material undergoes sliding. But sometimes, huge blocks of consolidated rocks may also be involved. If landslides occur in places of importance such as highways, railway lines, valleys, reservoirs, inhabited areas and agricultural lands leads to blocking of traffic, collapse of buildings, harm to fertile lands and heavy loss to life and property. In India, landslides often occur in Kashmir, Himachal Pradesh and in the mountains of Uttar Pradesh. CLASSIFICATION OF EARTH MOVEMENTS: All movements of land masses are referred to as landslides and grouped them under “earth movements”. The classification of earth movements us as follows: EARTH MOVEMENTS EARTH FLOWS Solifluction Creep Rapid flows LANDSLIDES Debris slides and slump Rock slides Rock falls SUBSIDENCE Compaction Collapse Earth Flows: There are three types of earth flows viz., solifluction; creep and rapid flows. Solifluction refers to the downward movement of wet soil along the slopes under the influence of gravity. Creep refers to the extremely slow downward movement of dry surface material. This is very imp from the civil engg point of view due to slow movement of mass. On careful examination, bending of strata ; dislodgement of fence posts ; telephone poles, curvature of tree trunks; broken retaining walls etc offer clues to recognize creep. Rapid flows are similar to creep but differ with reference to the speed. Rapid flows generally accompany heavy rains. Mud flows are similar to rapid flows.
- 17. Landslides include Debris slides, rock slides and rock falls. Debris slides are common along the steep sides of rivers, lakes. Debris slides of small magnitude are called slumps.. Rock slides are the movements of consolidated material which mainly consists of recently detached bedrocks. For eg: a rock slide that took place at Frank, Alberta in 1903 killing 70 people. Rock falls refer to the blocks of rocks of varying sizes suddenly crashing downwards along steep slopes. These are common in the higher mountain regions during the rainy seasons. Subsidence may take place to the compaction of underlying material or due to collapse. Subsidence due to compaction: Sediments often become compact because of load. Excessive pumping out of water and the withdrawal of oil from the ground also cause subsidence. Subsidence due to collapse: In regions where extensive underground mining has removed a large volume of material, the weight of the overlying rock may cause collapse and subsidence. CAUSES OF LANDSLIDES: Landslides occur due to internal causes (inherent). The internal causes are again of various types such as Effect of slope; Effect of water; Effect of Lithology; Effect of associated structures ; Effect of human factors etc.. 1.Effect of slope: This is a very important factor which provides favourable conditions for landslide occurrence. Steeper slopes are prone to land slips of loose overburdens due to gravity influence. However, it should be remembered that hard consolidated and fresh rocks remain stable even against any slope. 2.Effect of water: The presence of water greatly reduces the intergranular cohesion of the particles of loose ground causing weakness of masses and prone to landslide occurrence.
- 18. Water, being the most powerful solvent, not only causes decomposition of minerals but also leaches out the soluble matter of rocks. This reduces the compaction of rock body and makes it a weak mass. 3.Effect of Lithology; Rocks which are highly fractured, porous and permeable are prone to landslide occurrence because they give scope for the water to play an effective role. In addition, rocks which contain clay minerals, mica calcite, glauconite, gypsum etc are more prone to landslide occurrence because, all these minerals are easily leached out. 4.Effect if associated structures ; The geological structures such as bedding planes, joints, faults or shear zones are planes of weakness and cause landslide occurrence. 5.Effect of human factors Human beings sometimes, interfere with nature by virtue of their activities and cause landslides. For eg: laying roads ; railway tracks etc.. When construction works are carried out on hill tops, the heavy loads on the loose zone of overburden create a sliding of rock masses. Land slides in India:Land slides are reported in the hilly terrains in different regions of India. The most disastrous land slides that have taken place in recent past are in the Himalayan terrain in the North and the Nilgiri hill region in south. In July 1970, heavy debris from Patalganga valley has been transported into Alaknanda in the Garhwal region of Uttaranchal. The flooding in Alaknanda due to these landslides has resulted in a silt and rock fragment accumulation of about 9 M cum. Another disastrous land slide took place on 18th Aug 1998 in Malpa village which is located on the banks of Kali River in Pithorgarh district of Kumaon Himalayas. The piled debris was around 20 m in height. In 1968, numerous landslides occurred during heavy rainfall of about 500 to 1000 mm in the Darjeeling and Sikkim regions where the 60 km highway between Darjeeling (West Bengal) and Gangtok (Sikkim) was disrupted.
- 19. EFFECTS OF LANDSLIDES: From the civil engineering point of view, landslides may cause (1) disruption of transport (2) damaging roads and railways and telegraph poles (3) obstruction to the river flow in valleys (4) damaging sewage and other pipe lines. (5) destruction of buildings and civil structures Recent landslides in the Himalaya terrain are listed below: Himachal Pradesh region: Nathpa (Nov 1989): Road destroyed about a km Kullu (Sep 1995): Road 1km destroyed and 32 persons killed Uttaranchal region: Kalisaur ( July 1968): Road damaged extensively Malpa (Aug 1998): Road to Manasarovar damaged & 205 persons killed Jammu & Kashmir region: Nashri ( Jan 1982) Every year causes damage to the roads Malori ( Jun 1995): National Highway 1-A damaged and 6 persons killed West Bengal Region: Kalimpong, Darjeeling (Aug 1993):40 persons killed with heavy loss of property Arunachal Pradesh Region: Itanagar ( July 1993): 2 km road damaged and 25 persons killed Mizoram region: Aizwal ( May 1995): 25 persons killed and road extensively damaged Nagaland region: Kohima ( Aug 1993): 200 houses and 5 km road damaged. 500 persons killed. Selected landslides in South India are listed below: Major Land slides took place in the Nilgiri Hill region include Runnymede, Glenmore, Coonoor areas. Amboori landslide in Kerala: On Nov 9th, 2001, a disastrous land slide occurred around Amboori (20 km from Thiruvananthapuram ) due to heavy rains and water logging.
- 20. PREVENTION OF LANDSLIDES: 1. Provision of adequate surface and subsurface to enable water to freely drain out . Construction of suitable ditches and waterways along slopes to drain off the water from the loose overburden. 2. Construction of retaining walls against slopes, so that the rock masses which rolls down is not only prevented from further fall but also reduces the slope. 3. Modifying the slopes to stable angles. 4. Growing vegetation to hold the material together. 5. Avoiding heavy traffic and blasting operations near the vulnerable places naturally helps in preventing the occurrence of landslides.