Dr. Ravi Kant mittal Assistant Professor, BITS Pilani E- Mail: ravi.mittal@rediffmai.com ravimittal@bits-pilani.ac.in Mobile: 9887692025 CE C361 Soil Mechanics and Foundation Engg. 1
Contents Soil Formation Clay Minerals Field Identification of Soils SOIL CLASSIFICATION 2
Definition Soil may be defined as an assemblage of discrete solid particles of organic or inorganic composition with air and/or water occupying the void space amongst the particles. Soil can thus have all three phases present in it - solid, water and air. If there is no AIR present SATURATED SOIL If there is no WATER present DRY SOIL 3
Rock Cycle Soil Formation Soils The final products due to weathering are soils
Soil Formation Soil is formed by the process of weathering of rocks, i.e., disintegration and decomposition of rocks and minerals at or near the earth s surface through the actions of natural or mechanical and chemical agents into smaller and smaller grains. The factor of weathering such as changes in temperature and pressure; erosion and transportation by wind, water and glaciers; chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water.
Soil Formation Soils formed by mechanical weathering bear a similarity in certain properties to the minerals in the parent rock. Chemical changes destroys the identity. In chemical weathering some minerals disappear partially or fully, and new compounds are formed. The intensity of chemical weathering depends on the presence of water and temperature and the dissolved materials in the water. Carbonic acid and oxygen are the most effective dissolved materials found in water which cause the weathering of rocks. 95% of the earth crust consists of Igneous rock and remaining 5% consists of sedimentary and metamorphic rocks. However, sedimentary rocks are present on 80% of the earth s surface area.
Residual and Transported soils Soils which are formed by weathering of rocks may remain in position at the place of region are called as RESIDUAL SOILS. Soils may get transported from place of origin by various agencies such as wind, water, ice, gravity etc. these soils are called as TRANSPORTED SOILS. Residual soils differ very much from transported soils in their characteristics and engineering behavior. Transported soils may also be referred to as SEDIMENTARY soils since the sediments formed by weathering of rocks will be transported by agencies such as wind and water. A high degree of alteration of particle shape, size and texture occurs during transportation and deposition.
Transported soils Transported soils can be classified as below based on the transporting agency and the place of deposition. ALLUVIAL SOILS: Soils transported by rivers and streams: sedimentary clays AEOLINE SOILS: soils transported by wind: Loess GLACIAL SOILS: soils transported by glaciers: Glacial till. LACUSTRINE SOILS: soils deposited in lake beds: Lacustrine silts and lacustrine clays. MARINE SOILS: soils deposited in sea beds: Marine silts and marine clays.
Major Soil deposits in India 10
Major Soil deposits in India: (i)marine deposits: SOIL FORMATION & COMPOSITION Very soft to soft clay (thickness from 5 to 20 meters). Medium sensitive & inorganic. Need pretreatment before load application. Controlled loading to prevent failure. 11
SOIL FORMATION & COMPOSITION Major Soil deposits in India: ( (ii)black cotton Soils: Expansive due to presence of Illite & Montmorillonite clays. Thickness upto max. 20m. Crack depth & pattern varies. Surface is hard in summer & slushy in rainy season. Seasonal w/c change causes volume change upto max. 1.5m depth. Due to swelling & shrinkage characteristics, soil should be pretreated. 12
1. SOIL FORMATION & COMPOSITION (iii)laterites & lateritic soils: Thickness more than 30m. Laterisation is the process of rock removal, silica removal, base removal, aluminum & iron accumulation at the top of soil profile. If approximately 90% coarse grained: laterite. Mostly fine grained: lateritic. Has high strength when cut & dried in heat (due to iron oxide dehydration & halloysite presence). Strength of hardened soil not affected due to water presence. ( 13
1. SOIL FORMATION & COMPOSITION ( (iv)alluvial Soils: Exhibits alternate layers of sand, silt & clay. In some locations organic layers are also found. Depth upto 100m. Alluvial sand: Used as fine aggregate. Alluvial clay: For brick manufacturing. 14
1. SOIL FORMATION & COMPOSITION (VI)Boulder Deposits: Boulder deposits due to rivers flowing in hilly terrains. Their properties depend on relative proportion of boulder and soil matrix. Boulder-to-boulder contact results in large friction and angle of shearing resistance. Due to large size, laboratory sample is not representative of natural deposit, hence field investigations are carried out to find properties needed in design. 15
1. SOIL FORMATION & COMPOSITION (V)Desert Soils: Wind blown deposits in the form of sand dunes. Formed under arid conditions. Mostly fine or silty sand. Water scarcity is a serious problem. ( 16
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SAND: Sand particles are made up of rock minerals. They have the same composition as that of big boulder from a rock mass. They are only smaller in size. The processes of weathering reduces boulders to cobbles, cobbles to gravel, gravel to sand, sand to silt and even silt to rock dust which have particles of clay size. Particles of rock minerals are ELECTRICALLY NEUTRAL. They are acted upon only by the gravitational force. CLAYS: Clay particles have a net electrical charge on them. Usually a negative charge on their faces and a positive charge on their ends. They are made up of clay minerals. Three important clay minerals are Kaolinite, Illite, Montmorillonite Clay mineral particles have a net electrical charge on them on account of a phenomenon that occurs during their formation.
Clay Formation Clay particles < 2 μm Compared to Sands and Silts, clay size particles have undergone a lot more chemical weathering! 19
Clay vs. Sand/Silt Clay particles are generally more platy in shape (sand more equi-dimensional) Clay particles carry surface charge Amount of surface charge depends on type of clay minerals Surface charges that exist on clay particles have major influence on their behavior (for e.g. plasticity) 20
Clay Minerals Kaolinite family Kaolinite (ceramic industry, paper, paint, pharmaceutical) Smectite family Montmorillonite (weathered volcanic ash, Wyoming Bentonite, highly expansive, used in drilling mud) Illite family 21
Clay Morphology Scanning Electron Microscope (SEM) Allows us to study morphology of clay minerals Used in mineral identification 22
Origin of Clay Minerals The contact of rocks and water produces clays, either at or near the surface of the earth Rock +Water Clay For example, The CO 2 gas can dissolve in water and form carbonic acid, which will become hydrogen ions H + and bicarbonate ions, and make water slightly acidic. CO 2 +H 2 O H 2 CO 3 H + +HCO 3 - The acidic water will react with the rock surfaces and tend to dissolve the K ion and silica from the feldspar. Finally, the feldspar is transformed into kaolinite. Feldspar + hydrogen ions+water clay (kaolinite) + cations, dissolved silica 2KAlSi 3 O 8 +2H + +H 2 O Al 2 Si 2 O 5 (OH) 4 + 2K + +4SiO 2 Note that the hydrogen ion displaces the cations.
1.2 Characteristics (Holtz and Kovacs, 1981)
Soil Classification Object: To keep various types of soils into groups according to their properties Soil consisting of similar characteristics Can be placed in the SAME Group Need: To find the suitability of the soil for construction of dams, highways and foundations
Field Identification of Soils Distinguish Gravel from Sand Sand from Silt Grain Size Dispersion Test Silt from Clay Dispersion Test Shaking (Dilatancy) Test Strength Test Rolling (Toughness) Test Organic Content and Color Organic soils usually have a distinctive odour of decomposed organic matter, which can be detected by heating. Acid Test use dilute HCL to check the presence of Calcium Carbonate Shine Test Highly Plastic soil is more Shine than Low Plastic soil
Types of Soil Classification 1. Particle Size Classification 2. Unified Soil Classification System (USCS) 3. Textural Classification 4. Public Roads Administration Classification (AASHTO, 1978)
Particle Size Classification Soils are according to Grain Size Various grain size classifications are in use Grain size distribution of soil is required Percentage of soil in each size group is determined Example: Soil 10% Gravel + 52% Sand + 38% Silt & Clay
Unified Soil Classification System (USCS) Origin of USCS: This system was first developed by Professor Casagrande (1948) for the purpose of airfield construction during World War II. Afterwards, it was modified by Professor Casagrande to enable the system to be applicable to dams, foundations, and other construction Four major divisions: (1) Coarse-grained (2) Fine-grained (3)Organic soils (4)Peat IS 1478 is adopted USCS after rounded up
Coarse Grained Soils Fine Grained Soils Boulders Cobbles Gravel Sand Silt Clay Coarse Fine Coarse Medium Fine 300 mm 80 mm 4.75 mm 0.075 mm 0.002 mm 20 mm 2.0 mm 0.425 mm IS 1478 Soil Classification System
IS 1478 Soil Classification System 50 % Coarse-grained soils: Gravel Sand 4.75 mm 0.075 mm Grain size distribution C u Fine-grained soils: Silt Clay PL, LL Plasticity chart C c Sieve analysis Atterberg limits
Particle size IS 1478 Question For the purpose of engineering descriptions, soils are divided into classes of similar grain size. The NOUNS used to describe a size class refer to a specific range of sizes. What is the range of sizes of SAND? 0.075 mm to 0.425 mm 0.425 mm to 4.75 mm 0.075 mm to 4.75 mm 2.0 mm to 4.75 mm 0.075 4.75 mm What is the range of sizes of FINE SAND particles? 0.075 mm to 0.425 mm 0.425 mm to 4.75 mm 0.075 mm to 4.75 mm 2.0 mm to 4.75 mm 0.075 0.425 mm
Particle size IS 1478 Question For the purpose of engineering descriptions, soils are divided into classes of similar grain size. The NOUNS used to describe a size class refer to a specific range of sizes. What is the range of sizes of MEDIUM SAND? 0.075 mm to 0.425 mm 0.425 mm to 2.0 mm 0.075 mm to 4.75 mm 2.0 mm to 4.75 mm 0.425 2.0 mm What is the range of sizes of SILT particles? 0.002 mm to 0.425 mm 0.002 mm to 4.75 mm 0.002 mm to 0.075 mm <0.002 mm 0.002 0.075 mm
Particle size IS 1478 Question For the purpose of engineering descriptions, soils are divided into classes of similar grain size. The NOUNS used to describe a size class refer to a specific range of sizes. What is the range of sizes of Gravel? 2 mm to 4.75 mm 4.75 mm to 20 mm 4.75 mm to 80 mm 20 mm to 80 mm 4.75 80 mm What is the range of sizes of CLAY particles? 0.002 mm to 0.425 mm 0.002 mm to 4.75 mm 0.002 mm to 0.075 mm <0.002 mm < 0.002 mm
Classification of Soils Experiment Coarse-grained soils: Grain Size Distribution Fine-grained soils: Gravel Sand Silt Clay 0.075 mm (USCS) Sieve analysis Hydrometer analysis
Procedure for grain size determination Sieving - used for particles > 75 μm Hydrometer test - used for smaller particles (< 75 μm) Analysis based on Stoke s Law, velocity proportional to diameter At the beginning Towards the end of test Schematic diagram of hydrometer test
Hydrometer Analysis Stoke s Law Assumption Reality v = ( γ s γ w )D 18η 2 Sphere particle Single particle (No interference between particles) Platy particle (clay particle) as D 0.005mm Many particles in the suspension Known specific gravity of particles Average results of all the minerals in the particles, including the adsorbed water films. Note: the adsorbed water films also can increase the resistance during particle settling. Terminal velocity Brownian motion as D 0.0002 mm
Grain Size Distribution Curves 100 80 % Finer 60 40 20 0 0.0001 0.001 0.01 0.1 1 10 100 Particle size (mm) D 60 C C D D u = 60 c = D 10 2 30 ( D D ) 60 10 x% of the soil has particles smaller than D x where C U is Coefficient of Uniformity and C c is Coefficient of Curvature
Grain Size Distribution Curves 100 80 E B A % Finer 60 40 D C 20 0 0.0001 0.001 0.01 0.1 1 10 100 Particle size (mm) A B C D E Well graded Soil Uniform Soil (or Poorly Graded Soil) Gap Graded Soil (or Poorly graded soil) Well graded with some fines Well graded with an excess of fines
IS 1478 Soil Classification System To determine Well Graded (W) or Poorly Graded (P), calculate C u and C c. C D D u = 60 10 C c = D 2 30 ( D D ) 60 10 where C U is Coefficient of Uniformity and C c is Coefficient of Curvature If prefix is G (Gravel) then suffix is W if C u > 4 and C c is between 1 and 3 otherwise use P If prefix is S (Sand) then suffix is W if C u > 6 and C c is between 1 and 3 otherwise use P
Describe the following Soil IS 1478 D D D 10 30 60 = 0.02 mm(effective = 0.6 mm = 9 mm size) Criteria Coefficient C Coefficient C u c = = D D 60 10 of of (D30) (D )(D 10 = 9 0.02 2 60 uniformity curvature ) = = 450 2 (0.6) (0.02)(9) = 2 Well graded soil 1< Cc 3 and C ( for gravels) 1 C 3 and C c ( for sands) u u > > 4 6 Well Graded Soil
Question What is the C u for a soil with only one grain size? Finer Coefficient of C D D 60 u = = 10 1 uniformity D Grain size distribution
Question The grading curve for a soil gives the size characteristics: d 10 = 0.16 mm and d 60 = 0.47 mm What is the Uniformity coefficient (C u ) and gradation of the soil? 0.34, well graded 2.94, well graded 2.94, uniformly graded 0.34, uniformly graded 2.94, Uniformly (or Poorly) Graded Sand
Classification of Fine Grained Soils IS 1478 -- Atterberg Limits (or Consistency Limits) Moisture content = massof water massof solids Soil-water mixture Increasing water content Dry Soil Liquid State Plastic State Semisolid State Solid State Liquid Limit, LL Plastic Limit, PL Shrinkage Limit, SL
Atterberg Limits (or Consistency Limits) - Cont. Volume Change with water content
Classification of Fine Grained Soils The classification system uses the term fines to describe everything that passes through a # 200 sieve (<0.075mm) No attempt to distinguish between silts and clays in terms of particles sizes since the biggest difference between silt and clay is not their particle sizes, but their physical and chemical structures The soil consistency is used as a practical and an inexpensive way to distinguish between silts and clays Plasticity property is important because it describes the response of a soil to change in moisture content
Why Plasticity? Water Content Significantly affects properties of Silty and Clayey soils (unlike sand and gravel) Strength decreases as water content increases Soils swell-up when water content increases Fine-grained soils at very high water content possess properties similar to liquids As the water content is reduced, the volume of the soil decreases and the soils become plastic If the water content is further reduced, the soil becomes semi-solid when the volume does not change
Liquid and plastic limits The lower and upper limits of the PLASTIC range are used to classify the fine soils. Plastic Limit, Liquid Limit
Plasticity Index The difference between the liquid limit (w L ) and plastic limit (w P ) is called as PLASTICITY INDEX (P.I.) Plasticity Index = Liquid Limit Plastic Limit
Atterberg Limits Atterberg limits are important to describe the consistency of fine-grained soils The knowledge of the soil consistency is important in defining or classifying a soil type or predicting soil performance when used a construction material A fine-grained soil usually exists with its particles surrounded by water. The amount of water in the soil determines its state or consistency Four states are used to describe the soil consistency; solid, semi-solid, plastic and liquid 50
Atterberg Limits (cont.) Wetting Solid Solid State Semi Solid Plastic Liquid Volume, v or e v i v f S = 100 % SL PL LL PI Drying w % 51
Atterberg Limits Liquid Limit (LL) is defined as the moisture content at which soil begins to behave as a liquid material and begins to flow (Liquid limit of a fine-grained soil gives the moisture content at which the shear strength of the soil is approximately 1.7 to 2kN/m 2 = 17-20 gm/cm 2 ) Plastic Limit (PL) is defined as the moisture content at which soil begins to behave as a plastic material Shrinkage Limit (SL) is defined as the moisture content at which no further volume change occurs with further reduction in moisture content. (SL represents the amount of water required to fully saturate the soil (100% saturation)) 52
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Liquid Limit (LL) In the lab, the LL is defined as the moisture content (%) required to close a 2- mm wide groove in a soil pat a distance of 0.5 inch along the bottom of the groove after 25 blows. ASTM D 4318 (IS2720) Soil sample size 150g passing 425 micron Equipment: Casagrande liquid limit device 54
Liquid Limit - LL (or w L ) Casagrande Method Professor Casagrande standardized the test and developed the liquid limit device. Cone Penetrometer Method This method is developed by the Transport and Road Research Laboratory, UK.
LL - Casagrande Method Device N=25 blows Closing distance = 12 mm The water content, in percentage, required to close a distance of 12 mm along the bottom of the groove after 25 blows is defined as the liquid limit
Source: http://www.wku.edu/~matthew.dettman/matt/prof/ce410/ll.htm
Liquid Limit (Procedure) 150g air dry soil passing 425 micron (# 40 sieve) Add 20% of water - mix thoroughly Place a small sample of soil in LL device (deepest part about 8-10mm) Cut a groove (2mm at the base) Run the device, count the number of blows, N Stop when the groove in the soil close through a distance of 0.5in Take a sample and find the moisture content Run the test three times [N~(10-20), N~(20-30) and N~(35-45)] and Plot number of blows vs moisture content and determine the liquid limit (LL) (moisture content at 25 blows)
Determining LL Log Scale
LL - Casagrande Method (Cont.) w N Flow index, I F = w log 1 w ( N / N ) 2 2 1
Determining LL by single test
Where x is the depth of penetration of cone in mm w x is water content corresponding to penetration D, here it is based on 2 LL - Cone Penetrometer Method 30 o Cone of Stainless steel Total sliding weight of 148 g Cylindrical mould of 5 cm diameter and 5 cm height. Penetration of cone (mm) 20 mm LL Water content w% For D = 14 to 28mm
LL - Cone Penetrometer Method 31 o Cone of Stainless steel Total sliding weight of 148 g Cylindrical mould of 5 cm diameter and 5 cm height. Penetration of cone (mm) 20 mm LL Water content w% w L = wx x + 0.01(25 x)( w + 15) For x = 14 to 28mm Where x is the depth of penetration of cone in mm w x is water content corresponding to penetration x, here it is based on 2
4.2.3 Comparison A good correlation between the two methods can be observed upto LL is less than 100. Littleton and Farmilo, 1977 (from Head, 1992)
Plastic Limit (PL) The moisture content (%) at which the soil when rolled into threads of 3.2mm (1/8 in) in diameter, will crumble. Plastic limit is the lower limit of the plastic stage of soil Plasticity Index (PI) is the difference between the liquid limit and plastic limit of a soil
Plastic Limit PL (or w P ) The plastic limit is defined as the water content at which a soil thread with 3 mm diameter just crumbles.
Plastic Limit (cont.)
Plastic Limit (Procedure) Take 20g of soil passing #40 sieve into a dish Add water and mix thoroughly Prepare several ellipsoidal-shaped soil masses by quizzing the soil with your hand Put the soil in rolling device, and roll the soil until the thread reaches 1/8 in Continue rolling until the thread crumbles into several pieces Determine the moisture content of about 6g of the crumbled soil.
Plasticity Index, PI Plasticity Index is the difference between the liquid limit and plastic limit of a soil PI = LL PL After finding LL and PI use plasticity chart to classify the soil
Soil symbols: G: Gravel S: Sand M: Silt C: Clay O: Organic Pt: Peat Ex: SYMBOLS Used for USCS Soil Classification SW: Well-graded Sand SC: Clayey Sand SM: Silty Sand Liquid limit symbols: H: High LL (LL > 50) I: Intermediate (35<LL<50) L: Low LL (LL < 35) Gradation symbols: W: Well-graded P: Poorly-graded Well graded soil 1 p Cc 3 and Cu > 4 ( for gravels) 1 Cc 3 and Cu > 6 ( for sands)
Classification of Fine Grained Soils Fine grained soils Silt (M) & Clay (C) To determine M or C use plasticity chart IS1478 Plasticity Chart A Line PI = 0.73(w L 20) Liquid limit Above A-line use suffix C Clay Below A-line use suffix M Silt LL < 35% use Prefix L 35% < LL < 50% use Prefix I LL > 50% use Prefix H Use O below A Line, if Soil is Organic
Classification of Fine Grained Soils - Cont. Borderline Cases (Dual Symbols) USCS Fine-grained soils with limits within the shaded zone (i.e., PI between 4 and 7 and LL between 12 and 25). It is hard to distinguish between the silt and claylike materials. Use: CL-ML: Silty clay & SC-SM: Silty, clayed sand. Coarse-grained soils with 5% - 12% fines. The first symbol indicates whether the coarse fraction is well or poorly graded. The second symbol describe the contained fines (M/C). Example: SP-SM: Poorly graded Sand with Silt.
Classification of Coarse Grained Soils
Classification of Fine Grained Soils
-- occur in fine grained soils (in particular silts and rock flour). Due to smaller size of grains, besides gravitational forces, inter-particle surface forces also play a major role. Structure of Soils The structure of soil is defined as the MANNER OF ARRANGEMENT and state of aggregation of soil grains. Single grained structure: -- is a characteristic of coarse grained soils. Gravitational forces predominate the surface forces and hence grain to grain contact results. Honey Comb Structure
Flocculent Structure: Structure of Soils -- is characteristic of fine grained soils such as clays. Interparticle forces play a predominant role in the deposition. -- very fine particles of colloidal size (<0.001 mm) may be in a flocculated or dispersed state. The flaky particles are oriented edge - to - edge or edge to face or face to face. Flaky particles of clay minerals tend to form card house /dispersed structure. Flocculated structure Card-house structure Dispersed structure
Textural Classification 1. IS 1498 1970 System IS 1498 1970 System Grain size distribution of soil is required Percentage of soil in each size group is determined Ex:- Soil 10% Gravel + 52% Sand + 38% Silt & Clay
Plasticity Characteristics Plasticity Index Plasticity 0 1 5 5 10 10 20 20 40 > 40 Non-Plastic Slight Low Medium High Very High
Range of Plasticity Index
Shrinkage Index = Indices Plastic Limit Shrinkage Limit Flow Index = (w 2 -w 1 )/(log 10 (N 1 /N 2 )) (from Liquid limit test) Toughness Index = Plasticity Index/Flow Index
Indices Liquidity Index/ Consistency Index Liquidity index LI For scaling the natural water content of a soil sample to the Limits. LI = w w PI w w w w w is the water content p = l p p Consistency index CI For scaling the natural water content of a soil sample to the Limits. CI = wl w PI = wl w w w w is the water content l p LI > 1 soil is in the Liquid state LI = 1 soil is at the liquid limit LI = 0 soil is at the Plastic limit CI < 1 soil is in the Plastic state CI = 1 soil is at the Plastic limit CI = 0 soil is at the Liquid limit LI + CI = 1
4.1 Atterberg Limits The presence of water in fine-grained soils can significantly affect associated engineering behavior, so we need a reference index to clarify the effects. (The reason will be discussed later in the topic of clay minerals) In percentage (Holtz and Kovacs, 1981)
Consistency Classification Consistency Index Liquidity Index 1.00 0.75 0.00 0.25 Stiff Consistency 0.75 0.50 0.25 0.50 Medium soft 0.50 0.25 0.50 0.75 Soft 0.25 0.00 0.75 1.00 Very soft
Indices- ACTIVITY Activity, A A = clay % Purpose PI clay size material fraction : < 0.002mm present - Indicates of the type of clay present in soil Normal clays: 0.75<A<1.25 Inactive clays: A<0.75 Active clays: A> 1.25 High activity: large volume change when wetted Large shrinkage when dried Very reactive (chemically) Examples: Kaolinite Inactive Illite Normal Montmorillonite Active
Indices - Sensitivity Sensitivity S t (for clays) Strength( undisturbed) S t = Strength( disturbed) Clay particle Water w > LL Unconfined compressive strength