Intro Geotech

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7/21/2019 Intro Geotech http://slidepdf.com/reader/full/intro-geotech 1/12  1 of 12 CINTRODUCTION TO GEOTECHNICAL ENGINEERING Prof. K. Rajagopal Soils are formed from rock by disintegration caused by expansion and contraction due to temperature changes, grinding action from glaciers, wind, water, chemical weathering etc. Types of Soils Based on Formation Residual Soils - those remaining in position, e.g. chemically weathered soils Transported Soils - water, wind are the transportation agents .. these can be further classified according to the mode of transportation and deposition as,  Alluvial Soils : transported by running water e.g. river deposits  Aeolian Soils : transported by wind, e.g. desert sands, loess  Lacustrine Soils : deposited at the bottom of fresh water lakes  Marine Soils : deposited in sea water Glacial Soils: formed due to the movement of glaciers (large block of ice in the ice age)   Deltaic Soils : soils formed when the river merges with the sea .. these are different from alluvial soils and have smaller grain size (e.g. silt in North India) Types of Soils Based on Grain Size Coarse grained: soil particles can be seen with naked eye, ex. boulders, gravels, sands The particle size of sands ranges from 0.075 mm (75) to 4.75 mm .. gravels have particle size greater than 4.75 mm. Fine grained: particles can be seen only under microscope, ex. silts and clays Particle sizes of silts range from 0.002 mm (2) to 75 and clay soils are finer than 2 Clay soils can be moulded to different shapes when mixed with water just as plastics can be  bent to different shapes … hence called plastic soils Volume of clay soils changes with water content while that of sands is not affected very much. This volume change is important while designing foundations COMPOSITION OF SOIL 3-phase medium consisting of gas (air), fluid (water) and soil solids. void ratio (e) = volume of voids/volume of soil = v v /v s  .. this value can be more than 1.0 .. some soils have void ratios as much as 6.  porosity (n) = vol. of voids/total volume = e/1+e .. ranges from 0 to 1.0 solid water air s v v  v a v w weight of solids = w s  weight of water = w w  1 e
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    CINTRODUCTION TO GEOTECHNICAL ENGINEERING Prof. K. Rajagopal

    Soils are formed from rock by disintegration caused by expansion and contraction due to temperature changes, grinding action from glaciers, wind, water, chemical weathering etc. Types of Soils Based on Formation Residual Soils - those remaining in position, e.g. chemically weathered soils Transported Soils - water, wind are the transportation agents .. these can be further classified according to the mode of transportation and deposition as, Alluvial Soils: transported by running water e.g. river deposits Aeolian Soils: transported by wind, e.g. desert sands, loess Lacustrine Soils: deposited at the bottom of fresh water lakes Marine Soils: deposited in sea water Glacial Soils: formed due to the movement of glaciers (large block of ice in the ice age)

    Deltaic Soils: soils formed when the river merges with the sea .. these are different from alluvial soils and have smaller grain size (e.g. silt in North India)

    Types of Soils Based on Grain Size Coarse grained: soil particles can be seen with naked eye, ex. boulders, gravels, sands The particle size of sands ranges from 0.075 mm (75) to 4.75 mm .. gravels have particle size greater than 4.75 mm. Fine grained: particles can be seen only under microscope, ex. silts and clays Particle sizes of silts range from 0.002 mm (2) to 75 and clay soils are finer than 2 Clay soils can be moulded to different shapes when mixed with water just as plastics can be bent to different shapes hence called plastic soils Volume of clay soils changes with water content while that of sands is not affected very much. This volume change is important while designing foundations COMPOSITION OF SOIL 3-phase medium consisting of gas (air), fluid (water) and soil solids. void ratio (e) = volume of voids/volume of soil = vv/vs .. this value can be more than 1.0 .. some soils have void ratios as much as 6. porosity (n) = vol. of voids/total volume = e/1+e .. ranges from 0 to 1.0

    solid

    water

    air

    vs

    vv va

    vw

    weight of solids = ws

    weight of water = ww

    1

    e

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    water content (w%)= wt. of water/wt. of soil solids = ww/ws .. can be more than 100% deg. of saturation (S) = vol. of water/vol. of voids = vw/vv .. varies from 0 to 100% TYPICAL SHAPES OF SOIL PARTICLES Spherical, semi-angular, angular, flaky .. sand particles have spherical, angular and semi-angular shapes whereas the clay soil particles are flaky (leaf like structure) .. silt particles are in between the sand and clay particles. Sand particles have small surface area and relatively large mass .. clay soil particles have very small mass but very large surface area .. some clay soils have as much as 800 m2 specific surface (surface area per unit volume e.g. cube has specific area of 6) per gram mass .. the same for sands may be in order of a few cm2. Sand particles may be in loose condition or in compact state with good interlocking .. interlocking is better when particles of different sizes are present in the soil or when the particles have angular shape. Cross-section of sand soil with spherical particles stacked in different manners is shown below: Loose packing densest packing uniform grain size uniform size grains Notice that sand mass with different shaped/sized particles will have lower void ratio as the smaller particles fit in to the void between two larger particles. Problem: In the loose packing state, if the diameter of each particle is d and there are n

    particles on each side of a cube, the size of cube would be nd. From this data, prove that a sand with uniform sized spherical particles would not have void ratio greater than 0.912.

    CLASSIFICATION OF SOILS Soils are classified using letter symbols. Sands: classified based on the gradation of different particles in the sand mass .. well graded (SW) means soil has a good mix of soil sizes whereas the poorly graded (SP) soils consist of uniform sized soil particles .. first letter S stands for sand and the second letter stands for Well or Poor gradation A Swedish agricultural scientist by name A. Atterberg has developed the following water content limits to describe the clay soil behaviour. LIQUID LIMIT (wl) is the water content at which clay soil will behave like a liquid, PLASTIC LIMIT (wp) is the water content above which clay soil behaves like a plastic material and SHRINKAGE LIMIT (ws) is the water

    sand with spherical particles of different sized particles

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    content below which the volume of clay remains constant. ws

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    down thereby the load is slowly transferred to the spring. The rate of water flow through the tap will be higher for larger sizes of the tap. The size of tap is similar to the permeability of soil. Large tap is similar to soils with high permeability (like sands) and extremely small tap is similar to soils with low permeability (e.g. clays). As the piston is moving, water flows out and the volume in the cylinder goes on decreasing. This is similar to the reduction of void ratio of soil as compression takes place. This process of compression taking place gradually due to the expulsion of pore water is called consolidation. As the consolidation is taking place, effective stress goes on increasing. In other words, we may say that consolidation in soils happens due to the change in effective stress. Because of slow pace of consolidation, the settlement of structures in clay soils take place over a long period of time. General subsidence of the ground surface may take place even with the lowering of ground water table. When ground water level is permanently lowered, the effective stresses in the soil increase that leads to compression of soil skeleton. This problem was particularly acute in the cities of Mexico and Bangkok where the soil strata is soft clay for considerable depth. General subsidence in the city of Mexico was of the order of 5 m. The 1st floor of some old building has become basement! Shear Strain-Volume Strain Relation of Soils Some soils (like dense sands) expand in volume when subjected to shear strain and some others (like loose sands) compress when subjected to shear strain. The analogy for these two cases is illustrated in the following: Imagine two horizontal plates with a number of teeth and assume that the teeth are so strong that they will not break under shearing. In case A (representative of interlocked soils such as dense sands), when the two plates are pressed together normally and moved relative to each other in the horizontal direction (due to shear forces), the distance between the plates has to increase in order for the shear deformation to take place. The increase of the distance between the two plates is similar to the volume expansion. On the other hand, consider the case B in which the plates are stacked one above the other with large initial space between the two plates (representative of loose sands). When shear deformation takes place, the top plate slides into the bottom plate thus decreasing the space between the two plates. The decrease of space is similar to the reduction in volume of soils. When the volume of saturated soils decreases rapidly such as under earthquake shaking, the pore water will not have chance to escape and is consequently subjected to squeezing action leading to an increase in pore water pressure (imagine the air pressure inside a balloon when squeezed). Under such cases, the total stress on the soil remaining the same, the effective stress reduces. In extreme case, when the pore water pressure increases to the level of total stress, the effective stresses may become zero. When the effective stress becomes zero, sands will loose their shear strength (as there is no contact between the particles) and hence behave

    (A (B)

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    like water. This process is called LIQUEFACTION and is common in loose marine deposits subjected to the action of earthquakes. It happened at many places during the recent Bhuj earthquake in Gujarat. ENGINEERING PARAMETERS OF SOILS 1. Shear Strength 2. Compressibility 3. Permeability 4. Proctor density & OMC These properties are functions of the state of soil, void ratio, gradation, water content etc. Shear strength decides the magnitude of permissible loads on soil. Compressibility decides the magnitudes of settlements. Permeability is the capacity of the soil to permit the water to travel through it. Decides the rate of settlement of structures built on it and the loss of water stored in dams due to seepage etc. The Proctor Density is the maximum density that can be achieved in the laboratory by compacting after adding some water to the soil. The water in the soil acts like a lubricant allowing the soil particles to slide over each other freely during compaction. But too much of water will impede the compaction due to development of pore pressures. The Optimum Moisture Content (OMC) is the water content of the soil at which it can be compacted to the highest dry density in the laboratory. SOIL INVESTIGATION Soil at the site is investigated by means of digging Open Pits, Boreholes and Soundings. Different phases of Soil Investigation are as follows: Preliminary Investigations These are performed at a rapid rate to identify a suitable site for the project. Methods of tests adopted during this phase: seismic refraction tests (velocity of sound in soil is measured that gives a rough idea of the type of soil), electrical resistance tests (used to locate ground water as water has lower resistance to electricity), cone penetration tests (standard sized cones are driven into the soil using a standard driving device....the resistance to the cone penetration is measured and correlated to the strength of soil). All these test methods will not be able to collect soil sample from depth. Detailed Investigations These are performed after a site is identified to obtain as much information on the soil as possible .. some field strength tests are performed and soil samples (disturbed and undisturbed) are collected from various depths for laboratory testing. The data obtained in

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    this stage is directly used in the design and making many engineering judgements. The soil at deep depths is investigated by drilling holes (called boreholes) .. the process of drilling boreholes is called BORING. As the borehole is drilled, different tests are conducted at various depths and soil samples are collected for laboratory testing. Different methods of boring are,

    Augering (helical, post hole etc.) used at shallow depths .. a shaft with helices is

    penetrated into the ground by means of rotation .. periodically the shaft is retracted and the soil trapped between the helices is removed to form the borehole.

    Shell boring: a steel shell with a cutting edge is driven into the ground to form the borehole

    Wash boring: water jet is used for drilling boreholes Rotary boring: Rotary drill bit is used to advance the borehole Rock boring: in case of rocks, special drill bits made of diamond are used.

    The soil is prevented from collapsing into the borehole either by lining the hole with a steel casing pipe or by filling the hole with bentonite slurry that has specific gravity slightly higher than that of water. The data collected from these drilling operations is prepared in a standard form called bore charts that describe the type of soil at different depths, its strength, etc. An example of the bore chart is shown in the figure. Different tests performed in the boreholes are,

    Standard Penetration Test (SPT): A split spoon sampler of standard dimensions with sharp bottom edge is driven into the soil from blows from a standard weight dropped from standard height. The number of blows required to drive the sampler by a distance of 300 mm is called SPT and is called the N value. As the sampler is driven into the soil, soil is collected inside the barrel of the sampler. The shear strength properties are very well correlated to the observed N-values. Vane Shear Test: This test is performed in case of soft clays. A standard vane is inserted into the clay soil and is rotated. The maximum torque that can be applied on the vane is correlated to the cohesive strength of the soil.

    The other tests that are commonly performed are, Plate Load Tests: A plate (typically 300 mm diameter) is pushed into the soil by applying pressure. The applied load may be cycled sometimes. The pressure-settlement data is used for designing the foundations. Pile Load Tests: The estimated load capacity of piles (or pile groups) is verified by means of full-scale load tests after the piles are installed at the site. LABORATORY INVESTIGATIONS Tests performed on representative soil samples (packing structure of soil is disturbed but the original composition of the soil is not changed) are,

    Atterberg limit tests (shrinkage, plastic and liquid limits)

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    Grain size distribution: Dry and Wet Sieve analysis for coarse grained soils Hydrometer Analysis for fine grained soils (finer than 75)

    Differential Free Swell Index (qualitative test to identify the swell potential of soil) Tests performed on bulk soil samples (like those collected from open pits):

    Proctor Density Tests to determine the maximum dry density that can be achieved by compaction and the corresponding water content (OMC)

    Tests performed on Undisturbed Soil Samples (both structure and original content of the soil are not disturbed during sampling):

    Shear Strength Tests: Direct shear, Triaxial compression, laboratory Vane shear test etc. to determine the shear strength properties of soil

    Permeability Tests to determine the capacity of soil to allow the water to travel

    through it. Two types: Constant Head used for highly permeable soils like sands and Falling Head used for soils with low permeability such as clays.

    Consolidation Tests: to determine the compression index that allows us to estimate

    the maximum consolidation settlement of foundations and the coefficient of consolidation that allows us to estimate the rate of consolidation settlements.

    Swell Pressure Tests: to determine the likely pressure exerted by the soil when its

    volume expansion upon wetting is prevented. FOUNDATION DESIGN REQUIREMENTS The purpose of the foundations is to spread the load from the super-structure over a wide area of soil such that,

    foundation soil does not fail in shear (bearing capacity failure), and settlements are within tolerable limits.

    Allowable Bearing Pressure (abp) is the pressure that can be applied on the soil that will not lead to failure of foundation soil or lead to excessive settlements of the foundation. TYPES OF FOUNDATIONS Shallow Foundations: loads from structures transferred to soil at shallow depths (Df) near the ground level (Df/B 1).. these can be provided when competent soil occurs at a shallow depth near the ground surface. Deep Foundations: loads from structures transferred to soil at deep depths. Provided when the soil near the ground surface is weak. Shallow Foundations

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    Isolated, combined (rectangular and trapezoidal), strap footings, strip footings under load bearing walls, raft foundations. Provided when competent soil occurs at shallow depths.

    pressure bearing allowableload applied = foundation of areaPlan

    Wall footings (also called strip footings): provided below the load bearing walls and run along the length of the wall. Isolated footings: to support one column (plan shapes are square, circular, hexagonal etc.) Combined footings: supports 2 or more columns all lying on a straight line (plan shapes are rectangular, trapezoidal) These footings are provided such that the center of gravity of loads and the plan area of the footing coincide so that the foundation pressure is uniform over the full plan area of the foundation. The uniform pressure will ensure that there are no differential settlements (leading to tilting, e.g. Pisa tower) in the structure. Rectangular combined footings Trapezoidal Combined Footing Strap footings: supports two far away columns with space restrictions on the footing near the boundary of the property. Raft Footings: extends over the full area of the building and supports all the load bearing walls and columns in the building Compensated Foundations:

    large offset on one side to satisfy the c.g. requirement

    (plan view)

    (sectional view)

    smaller offset than in rectangular footings

    (sectional view)

    (plan view)

    (longer side of trapezium on the side of heavier columns)

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    When the footings are constructed, the excavated soil during the construction is usually filled back in the foundation trench. However, in case of tall buildings with basement floors, the excavated soil is not backfilled in the trench. The result is that the net load transferred to the foundation soil is the difference between the weight of the building and the weight of the soil excavated from the foundation. As the weight of load is partly compensated with the weight of excavated soil, such foundations are called as compensated foundations. In extreme cases, the net load transferred to the soil may become zero. Such foundations are called as floating foundations as they literally float in the soil without exerting any additional pressure on the soil. PILE FOUNDATIONS The structural load from the surface is transferred to deep depths by means of piles. These are similar to the columns in the buildings but extend into the soil to transfer the load at deep depth. The load is transferred to the soil by means of skin friction along the interface between the pile and the soil and end bearing at the end of the pile. This is illustrated in the figure below. The diameter and length of pile and the number of piles in the group is decided by equating the total applied to the resistance developed by skin friction and end bearing. Minimum number of piles in a group is three (3). Materials used for piles are wood, reinforced concrete, and steel ..in India, ancient people have used casurina tree trunks as piles to support structures on soft soils The piles are installed by either driving or boring a hole and filling it with pile material.

    raft foundation

    ground level

    basement floors

    hard strata

    piles

    pile cap

    skin friction

    end bearing

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    Based on the method of installation and formation of pile, the piles are classified as under, Driven: pre-cast (pile is made in a factory and brought to the site) cast in situ (hole is made by driving a steel casing pipe and then the hollow is filled with reinforced concrete) Bored: pre-cast (pile is made in a factory and brought to the site) cast in situ (hole is made by large diameter boring equipment and the hole is

    filled with reinforced concrete) Some times, piles are installed at an angle to the vertical to resist lateral loads as shown under. These piles are called as raker piles. WELL FOUNDATIONS These are similar to piles but of very large plan dimensions of the order of 10 to 20 m such as those used under railway bridges. These are used to support massive loads and especially in rivers etc. where the foundations are to be taken to large depth even below the rock strata. Different types of well foundations are 1. Box caissons 2. Floating caissons 3. Pneumatic caissons MACHINE FOUNDATIONS The foundations used to support machinery that produce vibrations come under the category of machine foundations. The natural frequency of the soil-foundation system should not be near the frequency of the machine to keep the amplitude of motion within tolerable limits. The basic requirements of these foundations are as follows,

    Resonance should not happen

    Amplitude of vibrations should be within tolerable limits

    n

    2 1

    P/K =

    Vibrations should not be annoying to the people or the machinery Types of Machines

    Reciprocating machines: reciprocating engines, compressors .. typical operating

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    speeds about 600 rpm Rotary machines: high speed rotary machines .. turbo-generators, rotary compressors etc. typical speeds 3000-10000 rpm Impact machines: machine producing impact forces..forge hammers etc. typical speeds 60 to 150 blows

    Types of Machine Foundations Block foundations: massive block on a footing .. has a large mass and has smaller frequencies .. suitable for reciprocating machines and impact machines Box or Caisson foundations: similar to block foundations .. lighter with higher natural frequencies. Framed foundations: consists of beams and columns .. suitable for high speed machines GROUND IMPROVEMENT TECHNIQUES Many times, the soil at a site may not be suitable or inadequate for construction because of various reasons. In such cases, we resort to ground improvement techniques. One common method of ground treatment is by roller using heavy rollers before the construction of roads. This method is suitable only to treat the surface soil. Soil at deep depths is treated as shown in the following table. PROBLEMATIC SOILS and their TRETMENT

    SOIL TYPE OF PROBLEMS TREATMENT Loose Sands Soft clays Stiff clays

    Excessive settlements, low bearing capacity, potential for liquefaction Excessive consolidation settlements, low bearing capacity Volume expansion and excessive swell pressures

    Compaction piles Dynamic compaction Vibroflot Blasting Grouting Pre-consolidation Stone columns

    Chemical treatment to neutralise the negative charge on clay soil particles

    Vibration is the best means of compacting sands. The process is similar to the shaking of sugar bottle we all do to put more sugar in the bottle. Dynamic Compaction Heavy mass dropped from pre-determined heights to compact soils. Suitable to treat large plan areas of loose sands of limited depth. Dynamic compaction is suitable for sands, gravels and silty soils. Silt and clay fractions reduce the effectiveness of treatment. Vibroflot

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    A probe with facility to inject water at high velocities and having an eccentric mass that produces vibrations. The vibroflot literally shakes the soil particles and brings them closer together thus creating a dense soil mass. Treatment of Collapsible Soils Loess, dune sands (wind borne soils) have very high void ratio and collapse under even small disturbance. Treated by submersion in water for sufficiently long time to collapse their structure to a compact state. SOME CONSEQUENCES OF SOIL LIQUEFACTION