Index Properties:
In order to evaluate the suitability of a soil for use as foundation or construction material, certain physical properties have to be determined. These properties help to classify the soil and assess its engineering behaviour and are called index properties.
There are different soil classification systems based on index properties of the soil. The tests carried out in a laboratory to classify a soil are known as classification tests.
Index properties of a coarse grained soil are:
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i. Grain size distribution and relative density.
ii. Index properties of fine grained soils are consistency.
iii. Atterberg limits and sensitivity.
Particle Size Distribution:
The objective of particle size distribution is to determine the range of size of particles in the soil and the percentage of particles in these ranges.
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This can be done in two stages –
(i) Sieve analysis for the coarse fraction, and
(ii) Sedimentation or wet analysis, for the fine fraction.
The complete procedure for particle size determination can be divided into three stages, viz:
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(a) Sieve analysis of soil fraction retained on 4.75 mm aperture sieve;
(b) Sieve analysis of soil passing through 4.75 mm aperture sieve and retained on 75 micron aperture sieve; and
(c) Sedimentation analysis of soil passing 75 micron aperture sieve.
The procedures are discussed as follows:
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The size used may conform to the Indian standards. An over-dried sample of soil is first separated into two fractions by sieving it through a 4.75 mm IS sieve. The soil which is retained on 4.75 mm sieve will be of gravel size, and that passing through it will be of sand, silt or clay sizes.
The portion retained on this sieve is called gravel fraction, and is analysed for coarse analysis. The portion passing through it is analysed for fine analysis.
A standard set of IS sieve is used. A weighed quantity of air-dried soil is taken and placed on the top most set of sieves, and sieved by through-shaking or by using an automatic sieve-shaker.
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The percentage of the material retained on each sieve, and the cumulative percentage retained is obtained. The percentage passing through each sieve, or the “percent finer” as it is called, is obtained by deducting the percentage of the material retained, from 100.
The resulting data are conventionally presented as a grain size distribution curve, with the sieve size on a horizontal logarithmic scale and the percent finer by weight on a vertical arithmetic scale.
A wide range of particle sizes can be shown in a single plot (including the fine fraction-silt and clay) by using the logarithmic scale. The result of sieve analysis can be tabulated as shown in Table 3.1 (1 µ = 0.001 mm). A typical grain-size distribution curve is shown in Fig. 3.1.
Sedimentation analysis is the most convenient method for determining the grain size distribution of the soil fraction finer than 75 micron in size. The analysis is based on Stokes’ law, according to which the velocities of free fall of spherical, fine particles, through a liquid are different for different sizes.
According to Stokes’ law, the terminal velocity v is given by –
Equation (3.3) is convenient to remember and may be considered a simplified version of Stokes’ Law. Thus, for a particle of diameter 0.06 mm, v = 0.3275 cm/sec and the time required for fall through a height of 10 cm is 30.5 s. The viscosity of water in absolute cgs units (poise) is given in Table 3.2.
Procedure of Sedimentation Analysis:
It consists in preparing a soil specimen in suspension (of fine fraction) in a jar and sampling at different time intervals (by pipette), or determining the specific gravity of the suspension (by hydrometer), at the sampling depth. This would provide the means of determining the content of particles of different sizes.
Since the soil particles are dispersed uniformly throughout the suspension, and according to Stokes’ law, particles of the same size settle at the same rate, particles of a given size, wherever they exist, have the same degree of concentration as at the commencement of the test.
As such, particles smaller than a given size will be present in the same degree of concentration as at the start, and particles larger than this size would already have settled below the sampling depth.
Thus, the percentage of particles finer than a specified size may be got by determining their concentration at that depth at different times either with the aid of a pipette or a hydrometer.
Although there are some limitations in the use of Stokes’ law for sedimentation analysis, satisfactory results are obtained with small but appropriate modifications in the actual procedure of the conduct of the test.
If a pipette is used, it is called ‘pipette analysis’ and if a hydrometer is used, it is called ‘hydrometer analysis’. The pipette analysis is more direct and simple in concept, but needs sensitive weighing apparatus to obtain satisfactory results.
Similarly, if a hydrometer is used, several corrections to the observed hydrometer readings, namely, meniscus correction, temperature correction, and deflocculating agent correction may have to be applied before they could be used for the computation of unit weight of the soil suspension.
Limitations in the Use of Stokes’ Law in Sedimentation Analysis:
1. Stokes’ law is applicable for spherical particle only. Fine clay particles are not spherical in shape. While applying Stokes’ law, the concept of equivalent diameter is used.
2. The soil particles in the soil suspension may have different values of specific gravity. But in the computations, an average value of G is used.
3. The lower limit of particle size for validity of Stokes’ law is 0.0002 mm. However, the upper limit for the same is 0.2 mm. For particles of size less than 0.0002 mm, Brownian movement affects their settlement and in the case of particles larger than 0.2 mm, turbulence affects the settlement.
Use of Grain Size Distribution Curve:
The results of gain-size analysis are usually represented graphically as shown in Fig. 3.1. The aggregate or cumulative weight, as a percentage of the total weight, of all grains smaller than any given diameter is plotted on the ordinate using an arithmetic scale. The size of the soil particle, in mm, is plotted on the abscissa using a logarithmic scale.
The position and the shape and slope of the curve indicate the types and gradation of the soil. A well-graded soil has a good representation of grain sizes over a wide range and its gradation curve is smooth.
On the other hand, a poorly-graded soil has either an excess or a deficiency of certain particle sizes or has most of the particles about the same size. In the latter case, the soil is also called uniformly graded soil. A gap graded soil is the one in which some of the particle sizes are missing.
For the coarse-grained soils, the range of particle diameters present in the sample is of interest. In addition, certain grain diameters (D) which correspond to a certain per cent finer than on the grain-size distribution curve (GSD) are determined.
The diameter D10 corresponds to 10% of the sample finer in weight on the GSD curve. The diameter D10 is called the effective size.
Where D30 = grain diameter (mm) corresponding to 30% finer than. For a soil to be well-graded, Cc must lie between 1 and 3 and in addition to this, Cu must be greater than 4 for gravel and greater than 6 for sands.
Consistency of Soils:
Generally, in fine grained soil, there is more cohesion between the particles of the soil. Therefore, the soil offers resistance against deformation due this property; otherwise the soil may be hard, soft or fine, etc. Consistency is the physical state in which it exists or it denotes the degree of firmness of the soil.
Consistency or degree of firmness of a soil sample can be changed by changing its water content. In 1911, a Swedish engineer, Atterberg, mentioned that a cohesive soil (fine grained soil) can exist in four states, liquid state, plastic state, semi-solid state and solid state.
The water content at which the soil changes from one state to the other is known as Atterberg’s limit or consistency limits. Thus, consistency limits are water content at which the soil mass passes from one state to the next, i.e., liquid to plastic, plastic to semisolid, semisolid to solid state, as shown in Fig. 3.2.
Atterberg’s limits that are most useful for engineering purposes are:
i. Liquid limit
ii. Plastic limit
iii. Shrinkage limit
These limits are expressed as % (percent) water content.
Fine grained soil when mixed with water, this addition and water reduces the cohesion. Further addition of water, again reduces the cohesion until the material no longer retains its shape under its own weight, but flows as a liquid.
Now, if water evaporates from such soil suspension, the soil passes through various states of consistency, either liquid to plastic, plastic to semisolid or semisolid to solid.
Atterberg Limits:
Definitions of Atterberg limits-
1. Liquid Limit (wL) – Liquid limit is the water content corresponding to limit between liquid and plastic state of consistency of soil. It is defined as the minimum water content at which the soil is still in liquid state, but has a small shearing strength against flowing.
2. Plastic Limit (wp) – Plastic limit is the water content corresponding to limit between the plastic and semisolid of consistency of a soil. It is defined as the minimum water content at which the soil begins to crumble when rolled into thread of 3 mm diameter.
Note – Plasticity in soil is due to the presence of clay minerals. If it does not contain clay minerals, it would not become plastic or these cannot be rolled into threads.
3. Shrinkage Limit (ws) – Shrinkage limit is defined as the maximum water content at which reduction in water content will not cause a decrease in the volume of soil mass. It is the lowest water content at which a soil can still be completely saturated.
4. Plasticity – Plasticity is a property of soil which allows it to be deformed rapidly without rupture, without elastic rebound, without volume change. Plasticity is due to the presence of thin scale-like particles (clay particles). These clay particles carry a negative charge on their surface. The water molecules are dipolar and are attracted towards clay surfaces.
This phenomenon is known as adsorption of water. The clay particles are separated by a layer of adsorbed water which allows them to slip over one another. When the soil is subjected to deformation, the particles do not return to their original position, with the result that deformations are plastic.
As the water content of the soil is reduced, the plasticity of the soil is also reduced. So, presence of adsorbed water is necessary to impart plasticity, if the soil sample is mixed with non-polarizing liquid, such as kerosene or paraffin oil. Thus, the clay does not become plastic when mixed with liquid of non-polarizing molecule, i.e., kerosene or paraffin.
Atterberg Indices:
Following are the list of Atterberg indices:
1. Plasticity Index
2. Consistency Index
3. Liquidity Index
4. Flow Index
5. Toughness Index
6. Shrinkage Index
7. Flow Index
1. Plasticity Index (Ip):
The numerical difference between liquid limit and plastic limit is known as Plasticity Index. The range of consistency within which a soil exhibits the plastic property is called Plasticity Index. When water content is reduced below the plastic limit, the soil attains semisolid state. The soil cracks when moulded.
2. Consistency Index (Ic):
It is defined as the ratio of liquid limit minus natural water content to plasticity index of a soil, i.e. –
Note – If consistency index of a soil is equal to unity, it is at plastic limit.
i. If consistency index is equal to zero, it is at liquid limit.
ii. If Ic exceeds unity, the soil is in semisolid state.
iii. If lc is negative (less than unity), it indicates that the soil has natural water content greater than liquid limits.
3. Liquidity Index (IL):
The liquidity index is also known as the water plasticity ratio. It is defined as the ratio of percentage of natural water content minus plastic limit to plasticity index.
Where, W is the natural water content of the soil.
4. Flow Index (IF):
Actually, it is the slope of flow curve obtained between the number of blows and water content in Casagrande’s method of determination of liquid limit.
It can be determined from the relation –
Where, W1 = water content corresponding to blow n1
W2 = water content corresponding to blow n2
n1 and n2 are number of blows.
5. Toughness Index (IT):
It is defined as the ratio of plasticity index to flow index.
IT = Ip/IF
6. Shrinkage Index (Is):
It is the range of water content in which a soil is in the semisolid state of consistency; in other words, it is the difference between the plastic limit and the shrinkage limit.
7. Flow Index (IF):
It is the slope of flow curve obtained by plotting water content as ordinate on natural scale against number of blows as abscissa on logarithmic scale.
Where W1 = water content corresponding number of blows N1
W2 = water content corresponding number of blows N2
Determination of Liquid Limit and Plastic Limit Theory and Application:
Liquid limit is the water content at which the soil passes from zero strength to an infinitesimal strength; hence the true value of liquid limit cannot be determined. For determination purpose, liquid limit is that water content at which a part of soil, cut by a groove of standard dimensions, will flow together for a distance of 1.25 cm under an impact of 25 blows in a standard liquid limit apparatus.
The soil at the water content has some strength which is about 0.17 N/cm2. At this water content soil first passes from the liquid state to the plastic state.
Procedure of Experiment:
The liquid limit is determined in the laboratory by Casagrande’s tool. The device used in Casagrande’s method consists of a brass cup which drops through a height of 1 cm on a hard base when operated by the handle.
The device is operated by turning the crank which raises the cup and lets it drop on the rubber base. The height of drop is adjusted with the help of an adjusting screw.
Take about 100 gm of an air-dried sample passing through 425 µ, IS sieve. The sample is placed in the dish and mixed with distilled water to form a uniform paste. This paste is placed in the cup of the liquid limit device, and the surface is smoothened and levelled with a spatula to a maximum depth of 1 cm. A groove is cut through the sample along the axis of the cup using a standard grooving tool.
Turn the handle at the rate of 2 revolutions per second and count the blows until the two parts of the soil sample come into the contact at the bottom of the groove along a distance of 10 mm.
The groove should close by a flow of the soil, and not by slippage between the soil and the cup. When the groove closes by a flow, it indicates the failure of slopes form on two sides of the groove.
The soil is mixed in the cup again and repeated. Obtain at least 4 sets of readings in the range of 10 to 40 blows transfer about 15 gm of the soil. The forming the edge of the groove together to a water content and determine the water content by oven drying.
The soil in the cup is transferred to the dish containing the soil paste and mixed thoroughly after adding more water. Plot the semi log graph between the number of blows and moisture content as shown in the Fig. 3.4. From Fig. 3.4, corresponding to 25 blows moisture content gives us the liquid limit.
An amount of 30 gm air-dried sample passing through 425 micron IS sieve. Mix thoroughly with distilled water on the glass plate until it is plastic enough to be shaped into a small ball. Take about 10 gm of the plastic soil mass and roll it between the hand and the glass plate to form the soil mass into a thread.
If diameter of the thread becomes less than 3 mm without cracks, it shows that water added in the soil is more than its plastic limit. Hence, the soil is kneaded further and rolled into thread again. Repeat this rolling and re-moulding process until the thread starts just crumbling at a diameter of 3 mm.
The water content at which the soil can be rolled is known as the plastic limit. The test is repeated, taking twice more, with a fresh sample of 10 gm each time. The plastic limit is taken as the average of three values. The plastic limit is reported to the nearest number.
3. Determination of Shrinkage Limit:
The test sample is prepared by filling saturated paste of soil (passing 425 micron) in a shrinkage dish (about 30-40 mm diameter; 15 mm deep). Its wet mass M and wet volume V equal to the capacity of dish are measured. The sample is dried and weighed to get dry mass Ms. The dry volume Vd is measured by displacement of mercury.
On drying a saturated sample, initially at a moisture content greater than shrinkage limit, the volume decreases and it continues to decrease upto the shrinkage limit. The sample still remains saturated.
Further drying reduces water keeping the volume constant (equal to Vd). The moisture content at the stage of the saturation with minimum volume Vd is the shrinkage limit.
Initial mass of water = M – Ms
Loss of water upto shrinkage limit = (V – Vd) ρw,
Mass of water at shrinkage limit = (M – Ms) – (V – Vd) ρw
Where, w is the initial moisture content of the sample.
Shrinkage Ratio (SR):
SR is defined by the expression,
SR = (ΔV/Vd)/Δw
Where ΔV = change in soil volume for change in the moisture content Δw upto shrinkage limit, ΔV = Change in volume of water
Volumetric Shrinkage:
VS is the change in volume, expressed as percentage of dry volume, which occurs when a soil is dried from a given moisture content w1 upto ws.
VS = (ΔV/Vd) x 100
Combining with shrinkage ratio expression,
VS = (Δw) SR = (W1 – Ws) SR
Linear shrinkage:
LS is the decrease in one dimension of the soil sample on dying expressed as the percentage of the initial dimension Li. A soil paste is filled in a shrinkage mould having the shape of a semi-cylindrical trough (140 mm long and 12.5 mm internal dia). The sample is dried and its dry length Ld is measured.
LS is given by –
LS = [(Li – Ld)/Li] x 100
Specific Gravity Determination:
Specific gravity is useful in the determination of void ratio, degree of saturation, unit weight of the soil under different conditions, and several other computations like that of the particle size by wet analysis, and critical hydraulic gradient. Hence, it should be determined accurately.
The values of the specific gravity of soils range from 2.65 for sands, 2.70 for silts, 2.30 to 2.90 for clays on the average. It tends to be lower when organic matter is present. It could be as low as 1.30 for peat, a purely organic soil.
The specific gravity may be determined by using a pycnometer, density bottle or a gas jar.
The test procedure is the same involving the following observations:
i. Mass of empty pycnometer = M1(g)
ii. Mass of pycnometer + dry soil = M2(g)
iii. Mass of pycnometer + soil + water = M3(g)
iv. Mass of pycnometer + water = M4(g)
Therefore
i. Mass of dry soil Ms = M2 – M1
ii. Mass of water in observation (c) = M3 – M2
iii. Mass of water in observation (d) = M4 – M1
iv. Mass of water having the same volume as that of soil solids = (M4 – M1) – (M3 – M2)
Kerosene, being a better wetting agent than water for most soils may be used in place of distilled water. If Gk is the specific gravity of kerosene, the specific gravity (G) of soil (with respect to water) is given by –
Specific gravity is usually reported at the standard 27°C. If T°C is the test temperature,
Moisture Content Determination:
Although moisture (water) content is not a property of soil as such, it has direct bearing on its strength and stability, the moisture content of soil in situ is called the natural moisture content.
Moisture content of a soil may range from a trace to that sufficient to saturate it. If the trace moisture is obtained by absorption from the atmosphere, it is called hygroscopic moisture.
Knowledge of water content is necessary in compaction control, in the determination of consistency limits, and in the analysis of stability of foundations.
The general procedure is to keep the container with the wet soil specimen in an oven for 24 hours, and maintain the temperature between 105°C and 110°C, for the purpose of arriving at the dry weight. Sandy soil may need only four hours of drying, while clays need at least 15 hours.
Once the dry weight and wet weight are obtained, the difference gives the weight of the water driven out.
The water content may now be computed as follows:
Mass of container + wet soil — M2
Mass of container + dry soil — M3
Mass of container — M1
Mass of dry soil — M3 – M1
Mass of water — M2 – M3
Determination of Field Density:
Field density refers to the unit weight of a soil in the undisturbed condition or of a compacted soil in-place. This is determined for borrow-pit soils to estimate the quantity of soil required for placing and compacting a certain embankment, and also for the determination of the in-place density to ensure that the desired degree of compaction is achieved in the field.
The following methods are generally used for the determination of unit weight:
(i) Core Cutter Method
(ii) Sand Replacement
(iii) Field Density of Bouldery Strata
The field methods are discussed below:
(i) The Core-Cutter Method:
This method is very simple in principle and is a more direct approach. The apparatus consists of a mild steel cutting ring with a dolly to fit its top and metal rammer to drive it into the soil at the site.
The principle consists in driving the cylindrical core-cutter into the soil, trimming the end smooth, and determining its weight; the weight of the soil mass in the core cutter is estimated by subtracting from the weight of the core-cutter with soil, the weight of the empty core-cutter. The volume of the soil is taken to be same as the inside volume of the core-cutter, which may be computed from its internal diameter and height.
The calculations are done as follows:
This method is suitable for soft cohesive soils. It cannot be used for soft clays, sandy soil, and soils containing gravel particles, which could damage the cutting edge.
(ii) Sand-Replacement Method:
This method is based on the principle of obtaining the volume of the soil excavated by filling with sand, the hole at the site, the sand having been previously calibrated for its unit weight.
The weight of the sand required to fill the hole is obtained indirectly by the difference of weights of a sand-pouring cylinder with a conical space in the bottom portion. The weight of the excavated soil divided by this volume gives the in situ unit weight.
(iii) Field Density of Bouldery Strata:
This method is based on the principle that a trench is prepared in the ground. The material of the trench is removed and the surface is levelled and a polyester sheet of thick material is laid.
Now measured quantity of water is poured and the trench is filled upto the top. Volume of the trench can be measured from inside the surface. Mass of water divided by volume will give us the density of bouldery strata.
Density Index:
Density index or relative density of a soil, ID, indicates the relative compactness or degree of packing of the soil grains. This concept is relevant specifically to coarse-grained soils or sands.
The void ratio decreases from a large value in a relatively loose condition to a small value in a dense condition. The in-place void ratio eo, may be determined and compacted with the largest value emax, in the loosest state, and the smallest emin, in the densest state.
