Chapter -4 GRAIN SIZE PROPERTIES V_V

Transcription

1 Chapter -4 GRAIN SIZE PROPERTIES Q V_V

2 Chapter - 4 GRAIN SIZE PROPERTIES 4.1 Introduction The size of soil materials in a soil mass may range from the finest (colloidal size) to the coarsest (boulders). The grain size of the soil forms one of the major factor which affects the behaviour of soil under stress; apart from other related factors such as gradation of grains, mineralogical composition of the particles, arrangement of grains in relation to each other etc.. The characteristics of the particles vary with their sizes. The grain size of soil particles and their aggregate structures affect the ability of a soil to transfer the load, transport and retain water, compact to denser state of packing etc., which in turn depict the engineering behaviour of the soil. Generally, more finer grained the soils are, greater is the swell-shrink potential that they are susceptible to, though the swell-shrink potential is affected by mineralogy of the soil and other factors as well. From the point of view of bearing capacity of soil; which is also one of prime concern for the civil engineers, the grain size distribution of a soil has great effect on the strength of the soil. The soil grains may be in loose packing condition or in a dense packing condition. If an external load is applied on to the loose packing, normal and shear stresses develop in the contact points between the grains. When the shear stresses become greater than those that can be taken through friction between the grains, shear failure occurs and the grain skeleton re-orientates. In that case, the grains fall in to a denser packing condition, in which more friction can be mobilized because of the greater number of contact points. The shear resistance between the grains is further increased if smaller grains are present in the pores between the coarser grains as the number of contact points further increases. This indicates the fact that, a soil containing grains with different sizes, enabling a dense grain packing, has a greater load bearing capacity than a soil that contains grains of same size. It is thus important to study and analyse the grain size distribution (grading) of any soil to predict its response under loading conditions. The soil particles with the size higher than mm (75 micron) constitute coarse fraction (viz. gravel, sand) and those with size finer than mm constitute 54

3 the finer fraction of soils (viz. silt and clay). The Indian standard nomenclature of grain size is considered as given in the Table-4.1. Soil type Grain size (nun) Gravel } COARSE Sand FRACTION } Silt Clay } FINE FRACTION Less than } Table- 4.1 Indian standard nomenclature of soil grain size The coarse fraction of a soil is generally cohesion less and the finer fraction cohesive, and both the fraction exhibit distinct characteristics in their behaviour from engineering point of view. The black cotton soils are classified as fine grained soils as almost all of the soil particles constitute finer fraction, with negligible or nil fraction of gravel and sand (Singh, 1981). The soil particles finer than mm are further distinguished as given in the Table- 4.2 below: Grain size range (mm) Distinction Silt Clay < Colloids Table- 4.2 Composition of fine fraction of black cotton soils The particle size distribution of a soil mass can be found in two stages : (i) Sieve analysis - for coarse fraction of the soil (ii) Sedimentation or wet analysis - fine fraction of the soil. 4.2 Significance of clay fraction and colloidal content in soil The results of grain size analysis facilitates the classification and identification of the soils. The sedimentation or the wet analysis provides the segregation of the clay fraction and colloidal fraction of a soil mass, which have considerable bearing on the swelling and strength behaviour of that soil (Bishop and Little, 1967).The grain size composition of the whole soil and the amount of clay fraction (i.e. percentage of particle size range of mm mm) and the colloidal content (i.e. percentage of particles of size < mm) have significant influence on the swelling 55

4 and shrinkage characteristics of fine grained soils particularly the black cotton soils (Winterkom and Fang, 1986) Clay fraction in black cotton soils and its influence on engineering properties and swell potential Soils those exhibit high volume changes during transformation from dry to wet states usually possess a considerable percentage of clay fraction in them. The swell potential and swell pressures are reported to increase with the clay content of the soils (Seed et.al., 1962; Mitchel,1976 and Chen,1988). The swelling soils such as black cotton soils do not have continuous granular skeleton with sufficient interstitial porosity to accommodate the volume changes of clay fractions due to increase (or decrease) in moisture content. The magnitude of swelling and shrinkage of such soils thus varies directly with the percentage of clay fraction constituting the soil. The actual contribution of clay fraction to the engineering behaviour of the whole soils thus depends mainly upon the quantity of clay fraction apart from other factors such as physico-chemical properties of micelles of clay fraction, relative amount and characteristics of the other soil constituents with which the clay particles interact. However, the decisive influence of the clay fraction is often exerted by the following factors: i) State of aggregation of the clay particles as to whether they are dispersed and independently acting or dispersed but aggregated to dense secondary structures or whether they form loosely aggregated flocculent structures. ii) Location of the clay particles as to whether they form films around the coarser constituents, which while interfering with their packing, endow the system with cohesive properties or whether they act as fillers in the pore space of a granular skeleton formed by the coarse constituent. iii) Degree and strength of physico-chemical interaction by the clay particles with the coarser sized constituents of the soil. The clay fraction also has effect on the amount of specific surface area of the soil mass; which is one of the major contributing factor influencing the engineering behaviour of fine grained soils. As such the swelling potential of clays is also related to the specific surface area; which in fact is controlled by the percentage of clay fraction in the soil (Low,1980; Ross,1978 and Dasog et.al.,1988). 56

5 The clay content affects the shrinkage limit of the soil as well. Sridharan and Prakash (1998) stated that there is an optimum clay content that results in an optimal dense packing which yields minimum shrinkage limit for that soil. They showed that, if the clay content is less than the optimal level, the shrinkage limit will be higher and however if the clay content is higher, it would result in poorly proportioned grain size distribution of soil and hence optimal packing may not be possible and it may result in an increase in the shrinkage limit. The clay fraction has strong correlation with the other constituent limits also. Davidson et.al.( 1952) and Smith et.al.( 1985) showed the relations between clay fraction and Atterberg limits. Van der Merwe (1964) evolved empirical relations between the degree of expansion, plasticity index, percentage of clay fraction and classified the expansivity of the soils based on their clay fraction and the plasticity index as indicated below at Table Clay fraction (% between 2micron -1 micron) Plasticity Index (%) Potential expansiveness Low Medium High >28 >34 Very high Table- 4.3 Relation between clay fraction, plasticity index and potential expansiveness (V an der Merwe, 1964) Several other models have been developed to evaluate the swell potential based on the clay fraction; plasticity index, liquid limit which lead to the conclusion of the fact that the clay fraction do have a strong correlation with the swelling properties of expansive soils which in turn affects their engineering behaviour Measure of swell potential by the coefficient of linear extensibility (COLE) using the clay fraction of the soil Thomas et.al.(2000) made use of the clay content percentage of the soil for estimating the coefficient of linear extensibility (COLE); which is one of the important engineering parameter of a soil, and related it with the consistency of the soil to characterize the shrink-swell behaviour of the soil. They presented a range of soil swell-shrink potential based on the values of COLE as indicated at Table

6 COLE Soil swell-shrink potential <0.03 Low Moderate High >0.09 Very high Table- 4.4 Coefficient of linear extensibility and soil swell-shrink potential (Thomas et.al.,2000) Hardcastle (2003) presented the expansion potential of the soil as a function of COLE and the percentage of clay fraction as shown in the Fig. 4.1 Fig. 4.1 Expansion potential as a function of clay percentage and COLE (Hardcastle, 2003) 4,2.3 Colloidal content in black cotton soils and its influence on engineering properties and swell potential The clay fraction of a soil contains particles less than mm in size. The soil particles smaller than mm possess the colloidal properties and are known as colloids. The colloidal content constitutes a major component of clay fraction of a soil mass. Colloids are thus the smallest of particles composing a fine grained soil. They are not just extra small fragments of rock and organic matter of soil but are highly reactive materials with electrically charged surfaces. Because of their micro size and shape, they give the soil an enormous amount of reactive surface area. Each tiny colloid particle carries a swarm of positively and negatively charged ions viz. cations and anions respectively; which is attracted to electrostatic charges in its surface. 58

7 Soil colloids greatly impact nearly all ecosystem functions. Different soils are endowed with different types of colloids and elicit very different types of physical and chemical behaviour and hence engineering behaviour. The soil colloids differ widely in their physical properties including plasticity, cohesion, swelling, shrinkage, dispersion and flocculation; which in fact are the properties that greatly influence the usefulness of soil for engineering purposes. The tendency of certain clays and the black cotton soils to swell in volume when wetted has been a major concern while constructing roads and foundations in such soils. The example that may be considered in this regard is that of smectite clays (which possess a higher clay and colloid content); which form wavy stacks or microscopic clay domains containing extremely small ultra micro pores. These ultra micro pores attract and hold large quantities of water; accounting for much of the swelling and plasticity of these clays. Thomas et.al.(2000) and Mc.Nabb(1979) showed that the coefficient of linear extensibility (COLE) and the plasticity index (Ip); the two of the measures of soil expansiveness are much higher for smectic clays as compared to other clayey soils such as kaolinitic clays, black cotton soils etc. which have relatively less clay fraction and less colloidal contents. Their studies have shown that, the cost of constructing buildings on such smectic soils may be twice than that of construction on soils dominated by the clays having lower values of COLE and Ip, wherein the conventional foundation designs can be safely used. The study also revealed that the same swelling properties that make the smectic soils so problematic for construction activities make them much useful and much suited for certain geo-environmental applications for clay lining of ponds, lagoons, wet lands etc. The colloidal attraction also enables soils to act as effective filters, sinks, exchangers for protecting the ground water and fertile/clean natural soil from the excessive exposure to a number of pollutants. The critical role and importance, the soil colloids play in determining the usefulness of a particular soil for various engineering purposes can thus be well realized General properties of soil colloids influencing the soil behaviour Gill (1957) and Evangelou et.al.( 2005) have presented the salient properties of the soil colloids of which the following are relevant with respect to the behaviour of the soil. 59

8 i) Size : The colloids are characterized by their extremely small size and are visible only with an electron microscope. ii) Surface area : Smaller the size of particles in a given soil mass, greater is the surface area exposed for adsorption, catalysis and other surface phenomena. Because of their small size, the soil colloids expose a large external surface area per unit mass. The external surface area of 1 gm of colloidal clay is at least 1000 times that of 1 gm of a coarse sand. iii) Surface charges : Soil colloidal surfaces, both external and internal, characteristically carry negative and/or positive charges. For most soil colloids, electro negative charges predominate. The charges on the colloidal surface attract or repulse substances in the soil solution as well as neighboring colloid particles. These reactions, in turn, greatly influence the swelling behaviour of the soil. iv) Adsorption of cations and anions : As the soil colloids predominantly possess negative charged ions (anions), they attract the ions of opposite charged ions (positively charged ions i.e. cations) to the colloidal surfaces. Hundreds of cations such as H+, Al3+, Ca2+ and MgC>2+ etc. are attracted, thereby giving rise to an ionic double layer; which plays a vital role in the volume changes of the soil. A colloidal particle is thus accompanied by a swarm of cations that are adsorbed or loosely held on the particle surfaces. Anions such as Cl'may also be attracted to certain soil colloids that have positive charges on their surfaces. v) Adsorption of water : In addition to adsorbing cations and anions, soil colloids attract and hold a large number of water molecules. Greater the external surface of soil colloids, greater is the amount of water held by the soil. Water adsorbed between the clay layers causes the layers to move apart, making the clay more plastic and swelling its volume. The colloids that adsorb a large amount of water, make the soil highly expansive; rendering it unsuitable for civil constructions. vi) Cohesion : Cohesion is one of the important property contributing to the strength of the soil. It is derived from the phenomenon of sticking together of colloidal particles that are of similar nature. The cohesion thus indicates the tendency of clay particles to stick together. When the colloidal substances are 60

9 wetted, water first adheres to the particles and then brings about cohesion between two or more adjacent colloidal particles. Cohesion in soils is thus due to the colloidal content present in them. vii) Adhesion : It refers to the phenomenon of colloidal particles sticking to other substances. It is due to the sticking of colloidal material, that the clayey or black cotton soil adheres to the surface of any other body or substance with which it comes in contact. viii) Swelling and shrinkage : The soil colloids swell when wet and shrink when dry. After a prolonged dry spell, the soils high in colloidal content often are characterized by wide, deep cracks. These cracks at first allow rain to penetrate rapidly. Later, because of swelling, such soil is likely to close up and become impervious. The swelling and shrinkage of the soils is thus due to the presence of colloidal content in considerable percentage in them. ix) Flocculation and dispersion : The flocculation of black cotton soils and other clayey soils upon the treatment with certain chemicals or flocculation agents for their stabilisation, is the result of activity of colloidal content present in them. As long as the colloidal particles remain charged, they repel each other and suspension remains stable. When the magnitude of the charge is reduced; eg. due to the chemical/ lime treatment or due to any other such account; the colloidal particles coalesce, form flocculates or loose aggregates and settle out. This phenomenon of flocculation due to coalescence and formation of floes has thus a bearing on the colloidal content of the soils and plays a vital role in the stabilisation of black cotton and other such expansive soils. The reverse process of breaking up of floes into individual colloidal particles is the de-flocculation or dispersion Types of soil colloids Soils contain numerous types of colloids, each having its particular composition, structure and properties. The important colloids which affect the soil from engineering point of view can be grouped in four major types as follows (Horace et,al., 1932) : 61

10 i) Crystalline silicate colloids : These colloids are the dominant type in most soils. They are predominantly negatively charged and differ widely with regard to their particle shape, intensity of charge, stickiness, plasticity and swelling behaviour. ii) iii) iv) Non-crystalline silicate colloids: This type of colloids have high amounts of both positive and negative charge and high water holding capacities. They exhibit a very low degree of stickiness. Iron and aluminium oxide colloids. These are found in many soils and are relatively low in plasticity and stickiness. Organic (humus) colloids. Organic colloids are important in nearly all soils, especially in the upper parts of the soil profiles. These are neither minerals nor crystallines, but they consist of chains and rings of carbon atoms bonded to hydrogen, oxygen and nitrogen. This type of colloids are often among the smallest of soil colloids and exhibit very high capacity to adsorb water but almost no plasticity or stickiness. Since the organic colloids are thus noncohesive, the soils composed mainly of humus colloids have very little shear strength and bearing capacity. The presence of large percentage of organic colloids in soils, therefore, render them unsuitable for the construction of buildings, roads and other such structures on them Colloidal content and the swell potential / expansiveness of black cotton soils Extensive study made by Holtz and Gibbs (1956) regarding the swelling problems encountered with potential expansive soils such as the black cotton soils, proposes a criteria for determining the qualitative expansivity of clays and black cotton soils based on the percentage of colloidal content present in these soils. The United States Bureau of Reclamation (USBR) used the colloidal content as criteria for the identification and characterisation of expansive soils; based on the revealations of work of Holtz and Gibbs(1956). Bishop and Little (1967), Chen(1988) stated that the colloidal fraction of a soil mass have considerable bearing on swelling of that soil and the following criteria was proposed for knowing their percentage probable expansion and for identifying the degree of expansion of the expansive soils based on their colloidal content as shown in Table

11 Colloidal content (%) Probable expansion (%) Degree of expansion ( % of total volume change) >28 >30 Very high High Medium < 15 <20 Low *(% change in the thickness of the sample from oedometer swell under a surcharge of 6.9 kpa or 1 psi from air dry condition to saturation) Table- 4.5 Degree of expansion based on colloidal content The colloidal content of a soil mass can thus be utilized for the evaluation of swelling potential of expansive soils, without resorting the direct measurement of swelling pressure Chen, (1988). 4,3 Activity in black cotton soils and its significance Activity is another significant parameter of expansive soils, derived from the plasticity index and the clay fraction of the soil, with which the potential expansiveness of black cotton soils and other swelling soils has been predicted by the researchers. Soil activity is a tenn generally applied on the ability of a soil to take in and dispose water under changing moisture conditions. It is mainly due to the soil possessing excess negative changes on its clay mineral surfaces; resulting in ability to attract water molecules and exchangeable cations in the clay structure, where with increased water, both the exchangeable cations and the clay surfaces are hydrated. The properties of a clay are determined fundamentally by the physicochemical characteristics of the various constituent minerals and by the relative proportions in which the minerals are present. The determination of these characteristics is a lengthy and difficult process requiring the use of an X-ray spectrometer, thennal analysis etc. and such techniques can not be normally practiced as a routine laboratory procedure due to their complexity. A quantitative measure of the composite effects of the basic properties of clay and valuable additional information can be obtained by simple index property tests; combining the Atterberg limits and the grain size distribution of a clay. Higher the plasticity index, more pronounced are the colloidal properties of a clay and the colloidal properties are contributed largely by the finest particles i.e. by the clay fraction. Skempton (1953) carried out the studies on the relation between the plasticity index and clay fraction and presented a parameter called activity for the soils. From the activity, prediction of dominant clay type present in the soil sample can be made. Generally, high activity 63

12 signifies large volume change when wetted and large shrinkage when dried. Soils with high activity are very reactive chemically. Black cotton soils with high swell potential can be identified by their high activity and/or high liquid limits and/or low shrinkage limit (Seed et.al., 1962; Holtz and Gibbs, 1956; van der Merwe, 1964; Navdocks,1962) Importance of activity in the study of black cotton soils, its correlation with the degree of expansiveness and Van der Merwe s activity chart As the soil particle size decreases, the surface area of the particles and the amount of water attracted to the soil surface increases. Thus the amount of water attracted depends considerably on the number of clay size particles present in the soil. Fine grained soils predominantly contain clays and silts and the clays are plastic while the silts are non-plastic. The plasticity index of a fine grained soil such as black cotton soil depends on the relative proportions of clays and silts within the soil. On the basis of these factors, Skempton (1953) proposed a relationship between the plasticity index and the percentage of particles of size finer than 2 micron (clay) and coined the word activity to separate and signify the plasticity of the clay fraction of the soil. He defined the activity expressed as activity number (Ac) as below: A _ Plasticity index (lp) A.c ~ Percentage by weight of particles finer than 2 micron (i.e. % of clay fraction) The activity of clayey soils gives a qualitative measure of the behaviour of soil as active, normal or inactive. Based on the definition, Skempton (1953) and Seed et.al.(1962) gave a relative activity classification and the degree of expansiveness of the soil based on the activity number as given in Table- 4.6 Activity (Ac ) < >1.40 Activity classification Inactive clay Normal clay Active clay Table- 4.6 Activity classification based on activity number (After Skempton (1953) and Seed et.al.(1922) 64

13 Skempton and Northey (1952) showed that mineral assemblage of illite, kaolinite, montmorillonite, calcite, quartz, mica etc. in different order of predominance give different value of clay fraction, activity and undrained shear strength. Skempton (1953) further deduced that, there is considerable degree of correlation between the activity and the mineralogy of clays and gave typical values as given in the following Table Mineral Activity (Ac) Muscovite 0.25 Kaolinite 0.40 Illite 0.90 Montmorillonite > 1.25 Smectite Table-4.7 Typical values of activity number for clay minerals Skempton (1953) further showed that, if the plasticity index is plotted against the clay fraction content for a particular clayey soil, then the points lie about a straight line which extrapolates back to the origin. Seed et.al.(1962) suggested the following formula for the activity. _ Plasticity index (I p) ( % of clay fraction -10) With a reference to Casagrande (1958), they further added that the above modification may be attributed mainly due to (i) the difference in the characteristics of liquid limit devices used in U.K. (by Skempton, 1956) and those in U.S.A. (by Seed et.al., 1962) and (ii) the relatively poorer grading of artificially prepared clayey soils (used by Seed et.al.) as compared with natural clayey soils (used by Skempton). Van der Merwe (1964) evolved empirical relations between the degree of expansion, plasticity index and percentage of clay fraction. He proposed the classification of potential expansiveness of black cotton soils based on these parameters as indicated at Table His method was especially useful in predicting the movements (settlements) in the buildings constructed on regional black clays of South Africa. 65

14 Clay fraction (%) Plasticity Index Potential Expansiveness Low Medium High >28 >34 Very high Table- 4.8 Potential expansiveness of black cotton soils based on clay fraction and plasticity index (After Van der Merwe, 1964) Van der Merwe (1964) also developed a chart as shown in Fig. 4.2 based on the plasticity index (Ip) and the percentage of clay fraction. H 70 PI of whole sample w SJ U i O', o <5 o 5 o o <50 70 Clay fraction of whole sample (% < 2 micron) Fig. 4.2 Activity and swelling potential chart (Van der Merwe, 1964) The chart comprises of four zones of classification of soils as follows; based on the value of activity (expressed as activity number Ac= Ip / Clay %) : i) Very high swelling potential (for soils having Ac between ) ii) High swelling potential (for soils having Ac between ) iii) Medium swelling potential (for soils having At between ) iv) Low swelling potential (for soils having Ac < 0.5) The chart developed by Van der Merwe is known as activity-swell potential chart and is commonly used for assessing the expansiveness of black cotton and other clayey soils. 66

15 4.4 Grain size analysis data and its use in triangular variation diagrams The data viz. percentages of sand, silt and clay fractions; obtained in the grain size analysis has been used for the classification of a fine grained soil based on these fractions constituting that soil. Such classification which is purely based on the grain size of the soil is also called textural classification and it is depicted by triangular diagram or ternary diagram. Any soil with the three constituents viz. sand, silt and clay can be represented by one point on the triangular chart. It is a particular type of graph, which consists of an equilateral triangle in which a given plotted point represents the relative proportions of sand, silt and clay. The percentages of sand, silt and clay are represented on each side of the triangle. The characteristic point of a soil is the intersection of 3 lines drawn parallel to the sides which are obtained by plotting on each side, the values in percentage of sand, silt and clay. Each soil thus plots as a point within or along the sides of the diagram, depending on its specific grain size composition. Shepard (1954) attempted to standardize the nomenclature of soil types (particularly of sediments) with respect to the sand, silt and clay content and proposed a triangular variation diagram containing 10 classes as shown in Fig. 4.3 (a) and (b). The triangular diagram proposed by USBS and PRA (United States Bureau of Soils and Public Roads Administration) system is more commonly used for the textural classification of the soils. It is composed of 12 classes as shown in Fig Ci-AY SAND 50% SILT Fig. 4.3 (a) Shepard s triangular diagram ( Shepard, 1954) 67

16 clay 60% silt 10% sand 30% = sandy clay 100% *->7- CLAY 33% clay 33% silt 33% sand = sand/silt/clay clay 20% silt 20% sand 60% = clayey sand or silty sand = sandy silt 0% clay 70% silt 30% sand Fig 4.3 (b) Illustration of the diagram ( Shepard, 1954) 100 Fig. 4.4 USBS and PRA triangular diagram 68

17 Robinson (1948), Trefethen (1950) and Folk et.al. (1970) also proposed different triangular diagrams based on the sand, silt and clay percentages. The study soil samples are plotted on these triangular diagrams proposed by Shepard, Robinson, Trefethen and Folk et.al. and the same are presented in the subsequent chapters. The triangular variation diagrams, designed based on the sand-silt-clay ratios are thus quite useful in standardizing the nomenclature of fine grained soils and sediments with relative to their sand, silt and clay content only. This system uses well established names for the soil fractions and it has simplicity and symmetry which make it easily remembered. The triangular charts are therefore commonly and widely used for the classification and broad identification of fine grained soils such as black cotton soils. 4.5 Grain size properties of the black cotton soils under study The range of variation in the values of the grain size property parameters of the soil samples under study are given at Table Grain size property parameter Range of variation Unit Gravel fraction % Sand fraction % Silt fraction % Clay fraction % Colloidal fraction % Activity number Table- 4.9 Range of variation of grain size property parameters of the black cotton soil samples under study An observation into the range of variation of the grain size property parameters lead to a quick assessment of these soils with respect to their classification as fine grained soils. The range of variation also indicates that these soils predominantly have high clay content with due representation of colloids. It therefore points out that these soils have a very high potential for expansiveness. The variation of the grain size property parameters are studied in detail and analysed for their behaviour in the subsequent chapters. * * * * * 69