GeoTech Midterm - Introduction to Soil

# Introduction to Soil ## Geotechnical Engineering - A branch of Civil Engineering concerned with the engineering behavior of earth materials - It uses the principle from soil mechanics and rock mechanics in the solution of engineering problems and in the design and construction of buildings and o...

# Introduction to Soil ## Geotechnical Engineering - A branch of Civil Engineering concerned with the engineering behavior of earth materials - It uses the principle from soil mechanics and rock mechanics in the solution of engineering problems and in the design and construction of buildings and other structures. ## Soil Mechanics - A scientific field of the civil engineering discipline that studies the physical properties and utilization of soil. - It is critical in civil engineering as it describes the principles that govern the way civil infrastructure projects such as buildings, bridges, tanks, embankments, dams and tunnels, are supported by the soil. ## History of Soil Mechanics ### Soil Engineering Prior to 18th Century - The record of the first use of soil as a construction material by mankind is lost in antiquity. - In true engineering sense, there is no ‘Soil Engineering’ prior to the 18th century. - For years, the art of soil engineering and geotechnical engineering was based only on past experiences through a succession of experimentation without any real scientific character. - Based on those experimentation, many structures were built some of which have crumbled, while others are still standing. - The hanging gardens of Babylon were supported by huge retaining walls, the construction of which should have required some knowledge, though empirical, of earth pressures. - The large public buildings, harbors, aqueducts, bridges, roads, and sanitary works of Romans certainly indicate some knowledge of the engineering behavior of soil. - One of the most famous examples of problems related to soil bearing capacity and foundations in the construction of structures prior to 18th century is the Leaning Tower of Pisa in Italy. The construction of the tower began in 1173 and last over 200 years. This is due to lack of sufficient knowledge of the behavior of compressible soil in those days. ### Pre-Classical Soil Mechanics - **Henri Gautier (1660 - 1737)** - A French Royal Engineer, investigated in 1717, the physical slopes of the soil just before overturning of a pile. - As a result, he formulated the design procedure of a retaining wall. - The term angle of repose is today related with the natural slopes. - **Bernard Forest de Belidor (1671 - 1761)** - A French Engineer who extended the theory which was presented in 1717 by Gautier for the lateral earth pressure appearing on retaining walls. - He also published in France a textbook for civil engineers and military. - **Francois Gadroy (1705 – 1759)** - French engineer who observed the existence of slip planes in the soil at failure. - In 1746, he also reported the 1st laboratory model test results on a retaining wall built with sand backfill. - **Jean Rodolphe Perronet (1671 - 1761)** - A French Engineer who investigated the stability of slopes, offered in this period an additional significant contribution around 1769, distinguishing it between intact ground and fills. ### Classical Soil Mechanics - **Charles Coulomb (1736 – 1806)** - A French physicist and engineer who undertook the first scientific study on soil mechanics and published a theory of earth pressure in 1773. - He first studied the problem of lateral earth pressures on retaining structures. - He used limit equilibrium theory, which considers the failing of soil block as a free body in order to determine the limiting horizontal pressure. - **Wiliam Rankine (1820 – 1872)** - A Scottish mechanical engineer who contributed to civil engineering, physics and mathematics. - He published Coulomb's work and theory of earth masses in 1857. - In the field of civil engineering, he is famous on his lateral earth pressure theory and the stabilization of retaining walls. - **Alexander Collin (1808 – 1890)** - An engineer who introduced in 1846, the details for deep slips in slopes of clay, cuttings, and embankments. - He considered that in each case the failure appears when the mobilized cohesion of the soils is greater than the existing one. - He determined profiles of slip surfaces in clay slopes and approximated the actual failure surfaces by arcs of cycloids. - **Otto Mohr (1820 – 1872)** - A German scientist who improved the Coulomb's theory of impulses and developed the necessary techniques for depicting graphically a two-dimensional stress-strain situation through the famous Mohr's circle. - **Jean Victor Poncelet (1788 – 1867)** - An engineer and professor who determined graphically the magnitude of lateral earth pressure and provided the theory of the first ultimate bearing capacity of shallow foundations. ### Modern Soil Mechanics - **Karl Terzhagi (1883 – 1963)** - An Australian mechanical engineer, geotechnical engineer, and geologist known as the father of soil mechanics and geotechnical engineering. - He is famous on his formulation of effective stress principle and its influence on settlement analysis, strength, and permeability of the soil. - **Albert Mauritz Atterberg (1846 - 1916)** - A Swedish soil scientist and chemist. - He understood the significant role of clay particles in the soil. - He justified the cohesion of cohesive soils, depending on the water level of clay and defined the limits of shrinkage, plastic and liquid. - He further defined the index of plasticity as the difference between limits of plastic and liquid. - **Arthur Casagrande (1902 – 1981)** - An American civil engineer who was the inventor of an ingenious worldwide soil testing apparatus, along with the fundamental research on seepage and liquefaction. ## Importance of Soil Mechanics ### Soil - Defined as the uncemented aggregate of mineral grains and decayed organic matter (solid particles) with liquid and gas in the empty spaces between the solid particles. - Soil is used as a construction material in various civil engineering projects and it supports structural foundation. ### Application of Soil Mechanics - **Foundation** - All foundations for any structure that a civil engineer constructs are bound to rest on the soil. The bigger the building or structure, the bigger the foundation and consequently the more important it is for a civil engineer to take into consideration the soil mechanics of the site. - **Earthen Dams** - Dams are among the largest and consequently, some of the most expensive civil engineering projects in the modern world. Soil to be used for constructing these earthen dams must be suitable enough to use in its construction. Their construction requires that one comes up with a proper design to ensure that they can withstand the pressure from water and other elements to serve their purpose. - **Embankments** - Embankments are usually constructed to raise the level of a road, railway or land above ground level to prevent chances of flooding as well as it is required to keep the foundation of the pavement above the water table. There is a need to design an economical embankment which is possible by studying various soil properties. - **Canals or Other Retaining and Underground Structures** - Factors such as shear strength of the soil should be taken into consideration to ensure that the canal that is put up can withstand the force of water that flows through and minimize seepage as much as possible. The retaining walls, whether made of compacted soil or concrete, should also be designed accordingly having taken into consideration the soil mechanics that will be at play depending on the given surroundings soil type. ## Engineering Properties of Soils - **Cohesion** - Cohesion of soil is the tendency for particles of soil to stick together. It is used in describing the shear strength of soil. Cohesiveness is one measurement to consider when evaluating the type of soil and quality of the soil. Ex. Clay Soil; Sand Soil - **Compressibility** - Compressibility in soil is the decrease in volume of soil when subjected to loadings. - **Elasticity** - A soil is said to be elastic when it suffers a reduction in volume while the load is applied, but recovers its initial volume immediately after the load is removed. - **Permeability** - The property of soil which permits the water or any liquid to flow through its voids is called permeability. - **Capillarity** - The process by which soil moisture may move in any direction through fine pores of the soil, under the influence of surface tension forces between the water and the soil particles. ## Basic Types of Rocks - **Igneous Rocks** - These types of rocks are formed by solidification of molten Magma ejected from deep within earth's mantle. Magma has originated well below the surface and ascended towards the surface, and has crystalized as solid rock either on the surface or deep within the earth's crust as temperature fell. Ex. Obsidian; Granite; Basalt - **Sedimentary Rocks** - These types of rocks are formed at or near the Earth's surface by the accumulation or deposition of mineral or organic particles or the lithification of sediment. Ex. Shale; Sandstone; Limestone - **Metamorphic Rocks** - These type of rocks starts as one type of rock and when subjected to increase in temperature, pressure, and time, it gradually changes into a new type of rock. Ex. Marble; Slate, Quartzite ## The Rock Cycle ### How Soils Formed - Soil is the thin layer of material covering of earth's surface and is formed through the process of rock weathering and natural erosion. It is made up mainly of mineral particles, organic materials, air, and water. - Weathering is the breakdown of rocks at the Earth's surface through physical, chemical, and biological processes induced or modified by wind, water, and climate. There are two important weathering processes classifications. - **Physical Weathering** - The physical disintegration of the original rock mass into smaller particles by direct contact with the atmospheric and physical conditions without any change in the chemical composition. Ex. Root Expansion; Frost Action; Thermal Expansion; Exfoliation - **Chemical Weathering** - It involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils, and minerals - **Carbonation** - mixing of water with carbon dioxide to make a carbonic acid which reacts with minerals in rocks. - **Oxidation** - chemical reaction with oxygen that causes the rocks to rust. - **Hydrolysis** - chemical breakdown of rocks when combined with water. ### Transportation of Weathering Products - **Residual Soils** - material resulting from the in-situ weathering of the parent rock. These soils formed from the weathering of rocks practically remain at the location of the origin with little or no movement of individual soil particles. - **Transported Soils** - weathered soil deposits formed at one location but are transported and deposited at another location by natural agents like wind, water, glaciers, and gravity. - **Alluvial Soils** - soils transported by running water and deposited along streams of water. - **Marine Soils** - soils transported by ocean or seawater. - **Lacustrine Soils -** soils deposited in lakes. - **Aeolian Soils** - soil deposits carried by the wind. - **Glacial Soils** - soils formed by transportation and deposition of glaciers. - **Colluvial Soils** - soils that are transported to short distance under the action of gravity. # Soil Classification and Identification ## Soil Classification - Soil classification is the systematic organization of soils based on distinguishing characteristics and criteria. - Soils can be classified according to its, - **Physical Properties** - **Chemical Properties** - **Engineering Properties** - **Basic Soil Classification (Clay; Loam; Sand; Silt)** - **Sandy Soil** - Sandy soil is made up of very tiny pieces of weathered rocks and hard minerals. Its inability to hold water and lack of nutrients make it a poor choice for growing most types of plants. However, it is most extensively used construction material. The particle size of course sand ranges from 2 – 4.75mm; Medium sand ranges from 0.425 - 2mm, and; fine sand ranges from 0.075 - 0.425mm. - **Silt Soil** - Silt soil is known to have much smaller particles compared to sandy soil and is made up of rock and other mineral particles, which are smaller than sand and larger than clay. This silt soil is more fertile compared to the other three types of soil. Therefore it is also used in agricultural practices to improve soil fertility. - **Clay Soil** - Clay is the smallest particle among other types of soil. The particles in this spoil are tightly packed together with each other with very little or no airspace. This soil has very good water storage qualities and makes it hard for moisture and air to penetrate into it. Clay is the densest and heaviest type of soil. - **Loam Soil** - Loam is the fourth type of soil. It is a combination of sand, silt and clay such that the beneficial properties of each are included. This soil is also referred to as agricultural soil as it includes an equilibrium of all three types of soil materials, being sandy, clay, and silt, and it also happens to have humus. Apart from these, it also has higher calcium and pH levels because of its inorganic origins. ## Systems Used in Soil Classification - **Unified Soil Classification System (USCS)** - **United State Department of Agriculture (USDA)** - **American Association of State Highways and Transportation Officials (AASHTO)** ## Particle Size Classification ## Occupational Safety and Health Association Soil Classification (OSHA) ## Importance of Soil Classification # Formation of Soil and Soil Deposits # Site Investigation and subsurface Exploration ## Site Investigation - Site investigation can be defined as the process of investigating a proposed construction site for the purpose of collecting, assessing and reporting information and data regarding the site. ## Purpose of Investigation 1. To know about the order of occurrence of soil and rock strata. 2. To know about the location of the groundwater table level and its variations. 3. To determine engineering properties of soil. 4. To select a suitable type of foundation. 5. To estimate the probable and maximum differential settlements. 6. To find the bearing capacity of the soil. 7. To predict the lateral earth pressure against retaining walls and abutments. 8. To select suitable soil improvement techniques. 9. To select suitable construction equipment. 10. To forecast problems occurring in foundations and their solutions. ## Stages of Site Investigation ### 1. Reconnaissance - Site reconnaissance is the first stage of site investigation. In this stage, visual inspection of the site is done and information about topographical and geological features of the site are collected. The general observations made in site reconnaissance are as follows: - Location of groundwater table by observing well in that site - High flood level marks on the bridges, high-rise buildings, etc. are observed - Presence of vegetation and nature of the soil. - Past records of landslides, floods, shrinkage cracks, etc. of that region. - Study of aerial photographs of the site, blueprints of present buildings, geological maps, etc. - Observation of deep cuts to know about the stratification of soils. - Observation of Settlement cracks of existing structures. ### 2. Data and Map Study - Details obtained from reconnaissance now need to be thoroughly understood in order to determine the necessary tests and samples to be collected for the next stages of site investigation. The study includes: - Presence of licensed water abstraction and discharge consents, landfills, and waste disposal sites - History of hazardous incidents - Quality of surface water and groundwater vulnerability - Past local borehole records - Presence of coal and other mines - Historical mapping - Buried and overhead supply and utility lines ### 3. In-Depth Investigation - At this phase, samples will need to be taken from the intended site. A critical technique used in site investigation for construction involves creating test borings to sample disturbed and undisturbed soil. Samples may need to be collected from various depths for testing and observation. Depending on the project, one rule of thumb to start is to drill one exploration in each corner of the intended structure and in the center. Depending on the uniformity of the results, more borings may be added and further tests conducted. The borings should be at a minimum, deep enough to acquire samples past the unsuitable topsoil into firmer layers, extending to depths that accommodate the structure and design intent, including anticipated building loads. ### 4. Laboratory Testing - The soil recovered during the in-depth soil investigation is tested in the lab at this stage. The material obtained is classified and characterized, and based on the project, geotechnical parameters are provided for the design phase. The basic tests are: - **CLASSIFICATION TEST** - For cohesive soils, the tests are done for moisture content, plasticity index, particle size distribution and bulk density. - For granular soils, tests are done for particle size distribution and bulk density - **SHEAR STRENGTH TEST** - For cohesive soils, tests are done for short-term stability, long-term stability and residual shear strength properties using the shear box test. For granular soils, the shear box test is done for both short-term and long-term stability analysis. ### 5. Subsurface Exploration - Subsurface exploration is the process of identifying the soil deposits that underlie the proposed structure and their physical characteristics. - A subsurface exploration program is conducted to make engineers aware of the site characteristics and properties needed for design and construction. #### Purpose of Subsurface Exploration - Selecting the type and depth of foundation for a given structure - Evaluation of foundation bearing capacity - Estimating the potential origin of the structure - Determine potential foundation issues (for example, elaborate Soils, Collapsible Soils, Sanitary Landslides, etc.) - Determine the location of the groundwater floor - Determination of Lateral Earth Pressure to Maintain Determination Walls, Foil-like Structures - Pile bulkheads and braced cuts - Installation of construction methods to change ground conditions #### Planning of Subsurface Exploration - Exploration program involves the areal extent of the investigation, methods of exploration to be used, test boring locations, spacings and depths and types of soil sample. - The program should be planned so that the maximum amount of information on the subsurface conditions can be obtained at minimum cost and time. #### Depth of Exploration - Exploration, in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundation. - This depth is known as depth of exploration, which depends upon the following factors. - Type of structure - Load on structure - Size and shape of the foundation - Position of loaded areas - Soil profile and its properties #### Thumb Rule for Deciding Depth of Exploration | Sl. No. | Type Of Foundation | Depth Of Exploration | |---|---|---| | 1 | Isolated spread footing or Raft | 1.5 B | | 2 | Adjacent footings with clear spacing less than 2B | 1.5 L | | 3 | Pile foundation | 10 to 30m OR 1.5B | | 4 | Base of retaining wall | 1.5 B(Base width) | | 5 | Floating basement | 1.5 H (Exposed height of wall face) [whichever is Greater] | | 6 | Dams | Depth of construction 1.5 times of bottom width of earth dams 2 times of height from stream bed to crest for concrete dams, for dams less than 30 m high Upto bed rock, in all soft, unstable and permeable strata. | | 7 | Roads Cuts | 1. 1.0 m where little cut or fill is required 2. In cut sections, 1.0 m below formation level 3. In deep cuts, equal to the bottom width or depth of the cut | | 8 | Road Fill | 2.0 m below ground level or equal to the height of the fill whichever is greater | Note: B = width of the foundation L = length of the foundation ## Methods of Exploration ### Test Pits/Trial Pits - Trial pits are a simple and economical method of soil exploration to shallow depths. - In this method of exploration, a square pit is excavated and soil samples are collected at required depths. - Trial pits are applicable to all types of soils, which provides visual inspection of soil in their natural condition in either disturbed or undisturbed state. - Test pits have an average depth of 3 to 4 meters. #### Advantages of test pits/Trial Pits - The method is simple and fast. - Most economical means of soil exploration to shallow depths. - The near-surface stratigraphy is exposed, facilitating sample collection and recovery and logging of soil, water level, and bedrock surface. - Information is provided on the lateral and vertical extent of subsurface features. - Test pits are usually cost-effective over this depth, and equipment is readily available. #### Disadvantages Of Test Pits/Trial Pits - Excessive ground disturbance - Limited exploration depth (depending upon equipment size and excavation footprint) - Impractical in unstable soil conditions and hard rocks - Impossible to excavate below the water table. - Health and safety risks associated with pit face instability, gas and toxic chemicals #### Methodology - **Excavation** - Pits can be excavated manually or by the use of small tractor-mounted mechanical excavators, available, which makes the exploration faster and economical. #### Size Of Pits - The size of the excavation depends primarily on the space required for efficient excavation and sample collection and on economic limits. - Test pits normally are square or circular in plan, of size 1.2-3 m. #### Stabilizing Side Walls - Excavations to depths of approximately 1.5 m often do not require lateral support to side walls. - Shallow test pits can be stabilized more economically by sloping the side walls. - Deeper excavations are generally more economical if sheeted. #### Collection Of Soil Samples - After excavating the test pit up to the required depth, the soil sample can be collected from the bottom of the test pit. - Necessary soil samples may be obtained by sampling techniques and used for finding strength and other engineering properties by appropriate laboratory tests. #### Backfilling - A test pit should be backfilled immediately and compacted after its completion. - No test pits should be left open overnights when the site is unattended. ### Boring Methods - Boring methods are widely used for subsurface investigations to collect samples, in almost all types of soil, for visual inspection or laboratory testing. - These methods are usually utilized for exploration at higher depths. - Exploratory bore holes are excavated in relatively soft soil close to the ground. - The location, spacing, and depth depends on the type, size and weight of the structure. #### Boreholes Location - Bore holes are generally located at. - The building corners - The center of the site - The place at which heavily loaded columns are proposed - At least one boring should be taken to a deeper stratum - When the depth of excavation is large, vertical boring methods are adopted. - Samples are extracted from bore holes and tested in laboratory. - Groundwater table is located and in-situ tests are carried using bore holes. #### Different Boring Methods 1. **Auger Boring** - The auger boring method for soil exploration is a technique to drill a hole into the ground and get information about the sub-soil strata. This method is usually used for cohesive soils and other soft soils above the water table. - **Auger boring is suitable for small depth exploration works like highways, railways and airfields subsurface exploration.** ##### Advantages of Auger Boring - The investigations through auger boring becomes quite rapid and economical. - It is suitable for shallow foundations. - It is used for soft cohesive soils. - Easy collection of the soil sample by simply placing the sampler into the hole. ##### Disadvantages of Auger Boring - The auger boring method is not suitable for cobbles, boulders, or other obstructions that prevent the drilling of the borehole. - The main disadvantage of auger boring is that the soil samples are highly disturbed. - In the auger boring, it is difficult to locate the exact changes in the soil strata. - The auger boring becomes an expensive and slow process for soil that cannot stand unsupported, especially for sandy soils below the water table. #### Types of Auger Boring - **Manual Auger Boring** - The manual auger boring is operated by hand and is suitable for soft soils. With the help of manual auger boring, the hole is created of 15 to 20 cm in diameter up to a depth of 3 to 6 meters. The process of auger boring to creating a hole with the manual auger boring is done by turning the cross-arm manually and at the same time applying the thrust in the downward direction. When the auger is filled with the soil, then the auger is taken out. The soil sample is collected by using Post-hole Auger by simply putting it into the hole. - **Mechanical Auger Boring **- The mechanical auger boring is used in the case of hard layers of soils up to a depth of 12 meters. The mechanical auger boring is operated with the help of power. In the mechanical auger boring, a continuous flight auger is used for making the hole. In the continuous flight auger, the cuttings rise to the surface through the spiral and we can collect the soil sample by simply inserting the sampler. ##### The post hole auger is used for taking the soil sample by inserting it into the hole. 2. **Wash Boring** - Wash boring is a method for advancing boreholes for site exploration and geotechnical investigation in most types of soil except gravels and boulders. The method is fast and simple to carry out and utilizes inexpensive, easily portable handling and drilling equipment. The method consists of first driving a casing through which a hollow-drilled rod with a sharp chisel. Water is forced under pressure through the drill rod which is alternatively raised and dropped and also rotated ##### Wash Boring Process - In the wash boring method of soil exploration, a wash borehole is drilled inside the soil then a casing is pushed inside the borehole of the size of 2 to 3 meters long. Then, a hollow drill rod is inserted into the bore hole with a chisel-shaped chopping bit at its lower end. - Water is pumped down into the hollow drill and due to a small hole into the chopping bit, the water becomes a strong jet. Due to the action of water jet and chopping, the soil is starting to disintegrate in the form of soil water slurry and the hole starts to increase. The soil water slurry goes up to the surface through the space between the casing and hollow drill rod. - The soil water slurry collected in the tub through the T-shaped pipe. (the sample of soil water slurry collected in the tub does not represent soil's true condition) ##### Advantages of Wash Boring - Wash boring is a fast and simple method - Can be used in all types of soils except boulders and rock - Soil samples can be collected with the help of a sampler after the hole has been drilled. - Equipment used in wash boring method is very light and economical. - The change in soil stratification can be determined from the rate of progress and the color of wash water ##### Disadvantages of Wash Boring - Cannot be used for hard types of soil, rocks, and soils containing boulders. - Slow in stiff soils and coarse-grained soils. - This method is not good for taking good quality undisturbed samples above the ground table. - In the wash boring method, the particles are completely broken down. 3. **Percussion Drilling** - Percussion drilling is a manual drilling technique in which a heavy cutting or hammering bit attached to a rope or cable is lowered in the open hole or inside a temporary casing. This uses a drilling method which involves lifting and dropping heavy tools to break rock, and uses steel casing tubes to stop the borehole from collapsing. This method is suitable for hard soil and soft rock where auger boring and wash boring can not be employed. ##### Percussion Drilling Process - Percussion drilling is a manual drilling technique in which a heavy cutting or hammering bit attached to a rope or cable is lowered in the open hole or inside a temporary casing. Usually, a tripod is used to support the tools. Then, a drill bit attached to rope or cable is repeatedly raised and lowered, impacting soil and rock, and loosening the soil and consolidated rock in the borehole making a hole deeper which is then extracted later by using a bailer. A temporary casing of steel or plastic may be used to prevent the hole from collapsing. When the permanent well screen and casing are installed, this temporary casing has to be removed. ##### Advantages and Disadvantages of Percussion Drilling - **Advantages** - Unlike any other drilling method, percussion drilling can remove boulders and break harder formations, effectively and quickly through most types of earth. Can drill further into the water even drilling past one water table to reach another. - **Disadvantages** - The equipment can be very heavy and relatively expensive. Method is slow, especially in harder rock. When temporary casing has to be used, the time taken driving and removing it can significantly increase drilling time. Equipment costs are high and the method is slow (resulting in high cost / drilled meter) 4. **Rotary Drilling** - Rotary drilling is used to form a deep observation borehole for obtaining representative samples of rock. The drilling method involves a powered rotary cutting head on the end of a shaft, driven into the ground as it rotates. This method is useful in case of highly resistant strata. It is related to finding out the rock strata and also to access the quality of rocks from cracks, fissures and joints. It can conveniently be used in sands and silt also. ##### Rotary Drilling Process - The principle of rotary drilling is based upon a rotating drill stem made of lengths of drill pipe. A bit is attached to a heavy stabilizer or drill collar at the end of the column of drill pipe. The extra weight and larger outside diameter of the stabilizer just above the bit helps to maintain a straight drill hole. - The drill stem is hollow and has a drilling fluid of either mud or air circulating down the drill stem out through the nozzles in the bit and up along the outside of the drill system. - The rotating action of the bit breaks up the material and the drilling fluid carries the cuttings to the surface where they settle out in a mud tank or container. ##### Advantages and Disadvantages of Rotary Drilling - **Advantages** - Quick and efficient method. Rotary drilling method can effectively advance through both hard and soft lithology. Capable of larger diameter boreholes, rotary drilling can also achieve deeper depths compared to direct and auger drilling. - **Disadvantages** - Costly in terms of operation and initial cost. High fuel and maintenance cost. High water requirement ## Geophysical Methods - Geophysical methods are indirect approach to the investigation of subsurface conditions through measuring, analyzing and interpreting physical fields at the surface. - These methods are used when the depth of exploration is very large and also when the speed of investigation is of primary importance. ### Seismic Refraction Method - The seismic refraction method is based on the principle that elastic shock waves travel at different velocities in different materials. Shock waves are generated at a point on the ground surface, using a sledge hammer. - These waves travel deep into the ground and get refracted at the interface of two different materials and to the ground surface. - The time of arrival of these waves at different locations on the ground surface are recorded by geophones, which pick up the refracted waves. - The geophones convert the ground vibrations into electrical impulses and transmit them to a recording apparatus. ### Electrical Resistivity Technique - The electrical resistivity method consists of measuring the resistivity of the soil strata and correlating the resistivity to the properties of the soil. - It employs an artificial source that is introduced into the ground through a pair of electrodes, usually in the form of metal spikes, driven into the ground at the same spacing. - The two outer electrodes are known as current electrodes, and the two inner electrodes are known as potential electrodes. - The procedure involves the application of a direct current between the outer electrodes and the voltage drop or the potential difference between the inner electrodes is measured using a potentiometer. - Apparent resistivity is calculated by using the potential difference for the interpretation. ### Magnetic Survey - The magnetic geophysical method measures distortions in the Earth's magnetic field caused by ferrous materials, such as steel, iron, or iron rich rocks and soil. - In order to increase resolution and detection ability, magnetic data is often collected using two sensors, with one located about a meter above the other. - The values measured by the top sensor are subtracted from the values measured by the bottom sensor which generates an approximation of the vertical magnetic gradient. - The magnetic method can be used for large scale geology mapping (often from the air using airplane or helicopter mounted sensors) or for detecting buried metallic objects such as underground storage tanks, pipes, utilities, or landfills. - Magnetic methods can also detect some geologic features, such as lava tubes ### Remote Sensing - Remote sensing includes methods that utilize images obtained in the ultra-violet, visible, and near infrared bands of the electromagnetic spectrum. - Remote sensing data are treated in digital image format so that they can be processed conveniently. - Remote sensing can also be used in geoenvironmental studies to map surface alteration and to identify anomalous vegetation patterns in areas related to abnormal metal content in soil. - With the rise in UAV (drone) use, remote sensing on a high-resolution regional or project specific scale has now become more accessible and affordable than ever before. # Diagrams ## Percussion Drilling - ! ## Wash Boring Technique - ! ## Electrical Resistivity - ! ## Rotary Drilling - ! ## Manual Drilling - !