Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

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2 Index properties

3 Review Clay particle-water interaction Identification of clay minerals Sedimentation analysis

4 Hydrometer analysis Hydrometer is a device which is used to measure the specific gravity of liquids φ φ (All dimensions are in mm) 60 50

5 Hydrometer Analysis -For a soil suspension, the particles start settling down right from the start, and hence the unit weight of soil suspension varies from top to bottom. Measurement of specific gravity of a soil suspension (Hydrometer) at a known depth at a particular time provides a point on the GSD.

6 Process of Sedimentation of Dispersed Specimen 1 V W W w V S W S V S = W s /(G s γ w ) V w = [1 -W s /(G s γ w )] Initial unit weight of a γ i = [W s +γ w V w ]/1 unit volume of suspension γ i = [γ w + W s (G s -1)/(G s )]

Process of Sedimentation of Dispersed Specimen z Note: X dz X Size d of the particles which have settled from the surface through depth z in time t d (From Stroke s Law): d =

7 Process of Sedimentation of Dispersed Specimen z Note: X dz X Size d of the particles which have settled from the surface through depth z in time t d (From Stroke s Law): d = 18µ ( G 1) γ Above the level X X, no particle of size > d will be present. In elemental depth dz, suspension may be uniform and particles of the size smaller than d exist. s w z t d

8 Process of Sedimentation of Dispersed Specimen If the percentage of weight of particles finer than d (already sedimented) to the original weight of soil solids in the suspension is N Then: Weight of solids per unit volume of suspension at depth z = (N )(W/V) (i.e. W s = W/V) Unit Weight of suspension after elapsing time t d at depth z is γ z = [γ w + N (W/V)(G s -1)/(G s )] N = [G S /(G S -1)[γ z - γ w ](V/W) N in %

9 Process of Sedimentation of Dispersed Specimen But γ z = G SS γ w = (1 + R h /1000) γ w Where G SS = Sp. Gravity of Soil Suspension (Graduated on hydrometer from ) R h is the reading on Hydrometer N = [G S /(G S -1)](R h /1000) (V/W) = (G S /(G S -1) (R h /W) For V = 1000 c.c.

10 Calibration or Immersion correction for Hydrometer h e x y x V h /(2A J ) Before the immersion of hydrometer y V h /(A J ) x x y y After the immersion of hydrometer H h/2 h = height of the bulb H = Height of any reading R h A J = Area of C/S of Jar V h = Vol. of hydrometer h e = [H+h/2+V h /(2A J )-V h /A J ) = (H+h/2) - V h /(2A J )

11 Conversion of R h into H e R h = 0; G ss = 1.00 R h = (G SS -1)10 3 Plot of R h with H e Valid for a particular hydrometer H e1 H e2 h e R h = 30; G ss = h e = H e1 -[(H e1 -H e2 )/30]R h up to 4 min. h e = H e1 -[(H e1 -H e2 )/30]R h V h /(2A J ) after 4 min.

12 Hydrometer corrections N = (G S /(G S -1) R/W R = R h + C m ± C t - C d N combined = N [W 75 /W T ] Where, W 75 = Wt. of soil passing 75µ W T R = Corrected observed reading = Total wt. of Soil taken for combined Sieve and Hydrometer Analysis C m = Meniscus correction (Always + ) Because density readings increase downwards C t = + for T > 27 C (R h will be less than what it should be) = - for T < 27 C (R h will be more than what it should be) C d = Always Negative (Dispersion agent concentration!!)

13 µ Example on Hydrometer analysis (kaolin) Given Data: Volume of suspension = 1000 ml Volume of hydrometer, V h = 90 cc Weight of dry soil, M s = 50 g Specific gravity of soil, G = 2.62 Cross- sectional area of jar, A j = cm 2 Room temperature, T = 27º C Dispersing agent correction, C m = Meniscus correction, C d = Temperature correction, C t = Viscosity of water, = x 10-7 kn-sec/m 2

14 Example on Hydrometer analysis (kaolin) H e1 = Maximum depth to centre of bulb from Rh = = 21 cm H e2 = Maximum depth to centre of bulb from Rh = = 9 cm At t = 2 min, R h = Since H e varies linearly with reading Rh

15 Example on Hydrometer analysis (kaolin)

16 Example on Hydrometer analysis (kaolin)

17 Example on Hydrometer analysis Percent finer (%) Particle size (mm)

18 Limitations of Stroke s law -Soil particles are not truly spherical and sedimentation is done in a jar (For d > 0.2 mm causes turbulence in water and d < mm Brownian movement occurs (too small velocities of settlement) --- Can be eliminated with less concentrations. -Floc formation due to inadequate dispersion -Unequal Sp.Gr of all particles (insignificant for soil particles with fine fraction)

19 Measures of Gradation D 60 = dia. of soil particles for which 60 % of the particles are finer. (i.e. 60 % of the particles are finer and 40 % coarser than D 60 ) D 10 : Effective Particle Size (10 % Finer and 90 % coarser than D 10 size) D 50 : Average Particle Size

20 Measures of Gradation -Engineers frequently like to use a variety of coefficients to describe the uniformity versus the well-graded nature of soils. D 30 = 0.3 mm

21 Measures of Gradation Some commonly used measures are: The uniformity coefficient C u = D 60 /D 10 Soils with Cu < 4 are considered to be poorly graded or uniform. C u > 4 6 Well Graded Soil Coefficient of Gradation or Curvature C c = (D 302 )/(D 60 *D 10 ) Cc = 1-3 Soil is well-graded. Higher the value of C u the larger the range of particle sizes in the soil

22 Typical characteristics of GSD curves -Steep Curves Low C u values Poorly graded soil (Uniformly graded). -Flat Curves (C u < 5 for uniform soils) High C u values Well graded soil. -Most gap graded soils have a C c outside the range. (an absence of intermediate particle sizes)

23 Typical GSDs for Residual soils Young residual Intermediate maturing Fully maturing A residual deposit has its particle sizes constantly changing with time as the particles continue to break down GSD can provide an indication of soil s history

24 Typical GSDs for Transported soils Glacial Glacial-Alluvial River deposits may be well-graded, uniform or gap-graded, depending up on the water velocity, the volume of suspended solids, and the river area where deposition occurred.

25 Grain Size Curves for different soils

26 Particle size distribution of Bentonite, Illite, and Kaolinite clay After Koch (2002)

27 Gradation % Gravel = 0 % Sand = (100 60) = 40 % Silt = (60 12) = 48 % Clay = 12 %

28 Example problem Determine the percentage of gravel (G), Sand (S), Silt (M), and Clay (C) of soils A,B and C Soil C: 0%G; 31%S; 57%M; 12%C (Well graded sandy silt) Soil B: 0%G; 61%S; 31%M; 7%C (Well graded silty sand) Soil A: 2%G; 98%S; 0%M; 0%C (Poorly-graded sand)

29 Some applications of GSA in Geotechnology and construction -Selection of fill material -Road Sub-Base Material -Drainage Filters -Ground Water Drainage -Grouting and Chemical Injection -Concreting Materials -Dynamic Compaction Embankment Earth Dams

30 Practical Significance of GSD -GSD of soils smaller than mm (#200) is of little importance in the solution of engineering problems. GSDs larger than mm have several important uses. 1) GSD affects the void ratio of soils and provides useful information for use in cement and asphalt concretes. (Well graded aggregates require less cement per unit of volume of concrete to produce denser concrete, less permeable and more resistant to weathering)

31 Practical Significance of GSD 2) A knowledge of the amount of percentage fines and the gradation of coarse particles is useful in making a choice of material for base courses under highways, runways, rail tracks etc., 3) To determine the activity of clay based on percentage clay fraction (<2µ) 4) To design filters (Filters are used to control seepage) and pores must be small enough to prevent particles from being carried from the adjacent soil.

32 Different physical states of fine-grained soil

33 Consistency of Fine-Grained Soils Consistency is the property of a material which is manifested by its resistance to flow. -It represents the relative ease with which the soil may be deformed. -Degree of firmness of a soil and is often directly related to strength. -It is conveniently described as soft, medium stiff (medium firm), stiff (or firm), very stiff. Note: These terms unfortunately are relative and have different meaning to different observers.

34 Consistency of Fine-Grained Soils In Soil Mechanics, it is required to determine the range of potential behaviour of a given soil type based only a few simple tests. Typical concerns are the following: i) Soils might shrink or expand excessively in an uncontrolled manner after they have been placed in geotechnical structures (roadway subgrades, dams, levees, foundation materials, etc.) ii) Soils might loose their strength and ability to carry loads safely.

35 Consistency of Fine-Grained Soils Tests used to detect potential problems for coarsegrained soils (gravels and sands) are different than those used to detect potential problems for finegrained soils (silts and clays). Coarse-Grained soils: - Water content is generally not a major factor - Major factor leading to shrinkage is the structure of the soil skeleton. Fine-Grained soils: Water content is a major factor Soils shrink Gain strength Water Content Soils expand Loose strength

36 Different physical states of fine-grained soil If the water content of a clay slurry is gradually reduced by slow desiccation, the clay passes from a liquid state through a plastic state and finally into a solid state. The water contents at which different clays passes from one of these stats into another are very different. Water contents at these transitions can be used for Identification and Comparison of different clays. Atterberg limits are water contents where the soil behaviour changes

37 Soil-Moisture scale LL PL SL Air dry Physical State Consistency S r Oven dry Liquid Plastic Semi-Solid Solid Very Soft Soft Stiff Hard Hygroscopic Moisture Very Stiff Extremely Stiff 100 % Natural Soil Deposits 100 % 100 % Soil is no longer fully saturated

38 Consistency of Fine-Grained Soils It was discussed that fine-grained soils have high SSAs and electrical charges on their particles. Because of this, fine-grained soils, and clays in particular can change their consistency quite dramatically with changes in water content. Each soil type will generally have different water contents at which it behaves like a solid, semi-solid, plastic, and liquid. For a given soil, the water contents that mark the boundaries between the soil consistencies are so called Atterberg Limits. [After Swedish Soil Scientist A. Atterberg (1902)]

39 Consistency of Fine-Grained Soils Atterberg Limits Atterberg limits are water contents where the soil behaviour changes.

40 V w V O Vol. of Sample SOLI D STATE SEMI-SOLID STATE Transition Zone PLASTIC STATE C B LIQUID STATE A Transition Stages from Liquid to Solid state G F E D Vol. Change of soil = Vol. of moisture lost V d V a V s w s w p w l w o Water Content

41 Atterberg Limits Liquid Limit (LL) is the water content at which a soil is practically in a liquid state, but has infinitesimal resistance against flow which can be measured (2.7 kn/m 2 ) Plastic Limit (PL) is the water content at which a soil would just begin to crumble when rolled into thread of approximately 3 mm diameter. Shrinkage Limit (SL) is the water content at which a decrease in water content does not cause any decrease in the volume of the soil mass. (at SL S r =1)

42 Shrinkage Phenomena R 1, R 2, R 3, R 4, R 5 : Radii of menisci (R 1 >R 2 >R 3 >R 4 >R 5 ) Water Surface Idealized section through soil Imagine a compressible soil consisting of tiny grains with capillary pore space between the grains.

43 Shrinkage Phenomena a) When the pore spaces are completely filled with water and there is free water on the surface of the soil, the meniscus is plane surface (1) and tension in the water is zero. b) As the evaporation removes water from the surface, a meniscus begins to form in each of the pores at the surface with a resulting tension in water. c) At some time after evaporation has started the menisci would have reduced to some position (say 2).. At this stage, tension in the water is 2T s /R 2. Soil is compressed by stress equivalent to 2T s /R 2

44 Shrinkage Phenomena Tension in water T W can be estimated, by equating Tensile force in water to the vertical component of surface tension force, as T w = (2T s /R 2 ) T s σ T s R 2 σ T s T s d) As the further evaporation occurs, the fully developed meniscus in the largest pore recedes to a small diameter!! Produces increased σ and caused further shrinkage

45 Shrinkage Phenomena e) As the evaporation continues the menisci continue to recede and the tension in the water continue to increase and the compression between the soil grains and the resultant shrinkage continue to increase. f) Eventually, the meniscus will reach the smallest radius (R 5 ) By the time, meniscus reduces to least possible radius of meniscus the pores in the soil will not be there to compress Hence, Shrinkage!!!

46 Atterberg Limits The Atterberg limits provide a good deal of information on the range of potential behaviour a given soil might show in the field with variations of water content.

48 Plasticity Index or PI It is the range of moisture content over which soil exhibits plasticity. Plasticity is defined as that property of a material which allows it be deformed rapidly, without rupture. I P = w L w P (Greater the difference between w L and w P, greater is the plasticity of the soil).

Plasticity Index or PI Plasticity Index = LL PL This measures the range of water contents over which a given soil can pull water into its macrostructure, assimilate it, and still act like

49 Plasticity Index or PI Plasticity Index = LL PL This measures the range of water contents over which a given soil can pull water into its macrostructure, assimilate it, and still act like a solid. Clay soils with high SSA s and charged particles will be able to hold a large amount of water between platelets due to their charge field and the polar nature of water molecules.

50 Plasticity Index or PI Clay soils with high SSA s and charged surfaces are able to bind/assimilate water molecules and the overall soil will still behave as a plastic solid. Such soils will have high PIs. Soils with comparatively lower SSA s will not be able to bind/assimilate water molecules and thus will have much smaller PI values.

51 Classification of soil based on PI PI Plasticity 0 Non-Plastic < 7 Low Plastic 7-17 Medium Plastic > 17 Highly Plastic

52 Laboratory determination of Liquid Limit Two Methods: -Casagrandes Method (After Arthur Casagrande) -Cone Penetrometer Method

53 Laboratory determination of Liquid Limit 54 mm Casagrandes Method 10mm 2mm Hard Rubber Base 2 rev/s Soil Passing 200# Sieve

54 Laboratory determination of Liquid Limit -Number of blows required to close the two soil halves over a distance of 13 mm is recorded and the water content of the soil is determined. -The test is repeated several times. Each time change the water content of the sample. A graph of water content vs number of blows is plotted.

55 Flow curve and Flow Index w L Water Content [%] + w 1, N N > N 1 ; w < w 1 Slope of the flow curve = Flow Index I f (indicates rate at at which soil looses shearing resistance with an increase in water content) + w, N + + Flow Curve Equation of Flow curve: w w 1 = -I f [log (N/N 1 )] 25 No. of Blows (Log Scale)

56 Cone Penetrometer Test -The penetration of a standard cone into a saturated soil sample is measured for 30 seconds. - If the penetration is less than 20 mm, the wet soil is taken out and mixed thoroughly with water and the test is repeated till the penetration is between mm. The water content corresponds to 25 mm penetration is taken as Liquid Limit. 148g 50 mm dia. 50 mm ht.

57 Determination of Plastic Limit Water content at which the soil crumbles when rolled into threads of 3 mm diameter.

58 Typical Atterberg Limits for Soils Soil type w l w p I p Sand NP Silt Clay NP = Non-Plastic; -Soils possessing large values of w l and I p are said to be highly plastic or fat clays. -Those with low w l and I p are called lean or slightly plastic.

59 Atterberg limit values of clay minerals with various adsorbed cations Cation Mineral Na + K + Ca ++ Mg ++ W l [%] I p [%] W l [%] I p [%] W l [%] I p [%] W l [%] I p [%] Kaolinite Illite Montmorill onite

60 Liquidity Index and Consistency Index I w L 0 w w = I p Solid p Semi Solid w Plasti c I c Liqui d SL PL 0<LI< LL 1 LI < 0 LI = 0 LI = 1 LI > 1 I c > 1 I c = 1 I c = 0 I c < 0 = l I p w

61 Soil classification based on soil consistency I c I l Consistency >1 <0 Very Stiff Stiff Medium soft Soft Very Soft < 0 > 1.0 Liquid state

62 Indicates the rate of loss of Toughness Index I t shear strength upon increase With the assumption that in w flow % line is straight between wl and wp and Shearing resistance α No. of blows N l = ks l N p = ks p ; w l = -I f log N l +C --- (1) w p = -I f log N p +C --- (2) I I t p = I I = I p f f S log S p S = log S l p l I t <1 Soil is easy to crumble or pulverize. I t = 1 3 for most clay soils

63 Distinction between Silt and Clay

Tikrit University College of Engineering Civil engineering Department

Tikrit University College of Engineering Civil engineering Department Tikrit University SOIL CLASSIFICATION College of Engineering Civil engineering Department Soil Mechanics 3 rd Class Lecture notes Up Copyrights 2016 Classification of soil is the separation of soil into