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The relative proportions of solids, water, and air have a strong influence on the engineering behavior of a soil as well as on its resistance to deformation due to loads from structures.
The following basic physical properties are defined and used to study and understand the soil behavior:
1. Physical Properties in Terms of Volume:
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i. Void ratio.
ii. Porosity.
iii. Degree of saturation.
iv. Percentage air voids.
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v. Air content.
2. Physical Properties in Terms of Weight:
i. Water content.
3. Physical Properties in Terms of Density (Weight and Volume):
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i. Bulk density.
ii. Density of soil solids.
iii. Specific gravity of soil solids.
iv. Apparent and Mass specific gravity.
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v. Absolute specific gravity.
vi. Density of water.
4. Physical property in terms of mass
Each of the physical properties listed above are defined and explained below. These physical properties can be visualized and understood easily by referring to the three-phase diagram of soil shown in Fig. 4.2. The units of mass and weight are discussed before defining various physical properties.
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Units of Weight and Mass in MKS and SI Systems:
Weight is the gravitational force acting on a body and has same units as that of force. In MKS system, force is the basic dimension (unit – kgf) and mass has a derived unit (slug). The unit of mass in MKS system is derived from the relation –
W = Mg
or M = W/g = kgf/(m/s2) = kgf.s2/m = slug
In SI system, mass is the basic dimension (unit – kg) and weight has a derived unit (newton). The unit of weight in SI system is derived from the relation –
W = Mg
W = kg.(m/s2) = kg.m/s2 = newton
The units of weight and mass in MKS and SI system are given in Table 4.1
The system of units used in India earlier was the MKS system and all balances are calibrated to give the weight of the object in kgf. As per international standards, the system of units is being converted from MKS to SI system.
If W = weight of an object in kgf as obtained from a balance, then the equivalent weight in SI system will be 9.81 WN. So –
Mass of this object, M = W/g = 9.81W/9.81 = W kg
Thus, the mass of an object in kg (SI system) is numerically equal to the weight of the object in kgf (MKS system) as given by the balance. Since mass of an object is constant at all places and is the basic dimension in SI system, all the density terms are being frequently expressed in terms of mass.
Weight density of water γω = 1000 kgf/m3 = 9810 N/m3 = 9.81 kN/m3 = 1 g (f)/cc = 1 t(f)/m3 Mass density of water rw = 1000 kg/m3 = 1 g/cc
Property # 1. Physical Properties of Soils in Terms of Volume:
The physical properties of soil in terms of volume express volume of voids (void ratio and porosity), volume of water (degree of saturation), or volume of air (percentage of air voids and air content) as a proportion of volume of other components.
Void ratio is defined as the ratio of the volume of voids to the volume of solids. Being a ratio, void ratio has no units. It is represented by the symbol “e.” Thus,
e = Vv/Vs …(4.5)
where Vv is the volume of voids and Vs is the volume of solids. Void ratio is one of the most important properties of soils and has a strong influence on engineering properties such as density, permeability, compressibility, and shear strength. In general, the higher the void ratio, the higher will be the permeability and compression, and the lesser will be the dry density and shear strength of soils, other parameters remaining the same.
Void ratio is a useful parameter for representing volume changes during the consolidation of soils because the volume of solids (denominator in void ratio) remains constant during consolidation. The relative density of cohesionless soils is also expressed in terms of void ratio.
Void ratio of soils is not directly determined but computed using its relationship with density, specific gravity, water content, and degree of saturation.
The following two relationships are commonly used to compute void ratio –
e = (Gγw/γd) – 1 …(4.6)
Gω = Se …(4.7)
with usual notations. Void ratio of soils usually has values greater than 0 and less than 1. However, void ratio of some clayey soils with values greater than 1 is also not uncommon. For any soil, void ratio is always higher than porosity as the volume of solids (denominator in void ratio) is always less than the total volume of soil (denominator in porosity), the numerator being the same.
Porosity is defined as the ratio of the volume of voids to the total volume. It is usually expressed as a percentage and is represented by the symbol “n.” Thus, –
n = Vv/V …(4-8)
where Vv is the volume of voids and V is the total volume of the soil. Porosity of soils is always greater than zero and less than 1. A porosity of 1 is absurd because the volume of voids in soils can never be equal to the total volume.
Both porosity and void ratio represent the proportion of volume of voids in the soil. The total volume of soil (denominator in porosity) changes with-the change in the volume of voids (numerator). For this reason, porosity is less frequently used in geotechnical engineering. Porosity is more commonly used by geologists. It is also used to represent the volume of voids in other building materials such as concrete, brick, and stone, where the total volume remains constant.
Degree of saturation is defined as the ratio of volume of water to the volume of voids. It is usually expressed as a percentage and is represented by the symbol S. Thus, –
S = Vw/Vv × 100 …(4.9)
where Vw is the volume of water and Vv is the volume of voids. In a dry soil, voids have no water, that is, Vw = 0 and Vv = Va, and the degree of saturation is 0%. When the entire void space is completely filled with water, that is, Vv = Vw and Va = 0, the soil is fully saturated and the degree of saturation is 100% or 1. For a partially saturated soil, the degree of saturation is more than 0 and less than 100% or 1.
Thus, the valid range of degree of saturation for soils is 0 > S >100%. Soils below the groundwater table are generally fully saturated. Where capillary action is present, soil mass up to some depth above the groundwater table may also be fully saturated, with negative pore water pressure.
Percentage air voids is defined as the ratio of the volume of air to the total volume of soil. It is usually expressed as a percentage and represented by the symbol na. Thus, –
na = Va/V × 100 …(4.10)
where Va is the volume of air and V is the total volume of soil.
Air content is defined as the ratio of volume of air to the volume of voids. It has no units and is designated by the symbol ac. Thus, –
where Va is the volume of air and Vv is the volume of voids. Since volume of air is equal to zero in fully saturated state and to volume of voids in dry state, therefore the valid range for air content is 0 > ac > 1. The sum of degree of saturation and air content is given by –
S + ac = (Vw/Vv) + (Va/Vv) = [(Vw + Va)/Vv] = (Vv/Vv) = 1
Thus, the sum of degree of saturation and air content is unity –
S + ac = 1 …(4.12)
Property # 2. Physical Properties of Soil in Terms of Weight:
There is only one physical property of soil-water content that is expressed as a ratio of weights.
Water content is defined as the ratio of the weight of water to the weight of dry soil. It is usually expressed as a percentage and designated by the symbol ω. Thus, –
ω = Ww /Wd × 100 …(4.13)
where Ww is the weight of water and Wd is the weight of dry soil. Water content is also known as moisture content. Water content and degree of saturation describe the wetness of the soil. Water content is not a constant for a given soil as water is subject to evaporation. The water content of soil at a site at any given time is called natural water content or in-situ moisture content. In summer, the natural water content of the top soil is low or zero and it increases due to rainfall or groundwater flow.
Thus, for dry soil, the water content is zero and it increases as the degree of saturation of the soil increases from 0% to 100%. Even after the soil becomes fully saturated, the water content does not remain constant. In situations where the void space between the solid particles increases, the water content continues to increase even though the degree of saturation remains constant at 100%.
For a fully saturated soil, the water content may be less than, equal to, or more than 100% depending on the soil type and structure. For coarse-grained soils, such as gravels and sands, the water content may be less than 100%. For fine-grained soils, such as silts and clays, the water content is more than that of coarse-grained soils. For some clays, the water content may be more than 100%.
Bentonite, which is a clay containing montmorillonite and which is used for stabilizing boreholes for construction of piles, can have a water content more than 400%. It indicates that the weight of water, present in a given volume of soil, is four times the weight of solids.
The effect of evaporation is limited only up to a certain depth below the ground surface. In summer, the natural water content is minimum at the ground surface (or ground level, GL) and it increases with the increase in depth below the ground surface. At some depth, the natural water content reaches its maximum value, and thereafter, the natural water content remains constant and is unaffected by seasonal variations.
Water content is one of the most important physical properties of soils. The consistency limits, namely liquid limit, plastic limit, and shrinkage limit, are expressed in terms of water content. Fine-grained soils can be in liquid state, plastic state, or solid state depending on the magnitude of water content relative to the consistency limits.
The magnitude of natural water content relative to the consistency limits greatly influences the strength and deformation characteristics of soil, and also its workability with various construction tools and equipment for excavation, filling, or compaction. The optimum moisture content, which is the right quantity of water for compacting soil for the construction of embankments and earth dams, is expressed in the terms of water content. The dry density for a given soil decreases with the increase in water content. The oven-drying method is the standard laboratory method to determine the water content.
Property # 3. Physical Properties of Soil in Terms of Weight and Volume:
Physical properties, which are expressed in terms of weight and volume, are different types of density and specific gravity of soil.
Bulk density of soil is defined as the ratio of weight of soil to the volume of soil. Bulk density is measured in g/cc and t/m3 in MKS system and kN/m3 in SI system. It is designated by the symbol γ (gamma). Thus, –
γ = W/V …(4.14)
where W is the total weight of soil and V is the total volume of soil.
Bulk density of soil represents:
a. Dry Density – When the soil is in dry state.
b. Saturated Density – When the soil is in fully saturated state.
c. Submerged Density – When the soil is in submerged state.
Each of the above terms are defined below:
a. Dry Density:
Dry density of soil is defined as the ratio of weight of dry soil to the volume of soil. Dry density is measured in g/cc and t/m3 in MKS system and kN/m3 in SI system. It is designated by the symbol γd. Dry density is also called dry unit weight. Thus, –
γd = Wd/V
where Wd is the weight of dry soil and V is the total volume of soil.
b. Saturated Density:
Saturated density of soil is defined as the ratio of weight of saturated soil to the volume of soil. Saturated density is measured in g/cc and t/m3 in MKS system and kN/m3 in SI system. It is designated by the symbol γsat. Saturated density is also called saturated unit weight. Thus, –
γsat = Wsat/V
where Wsat is the weight of saturated soil and V is the total volume of soil
c. Submerged Density:
Submerged density of soil is defined as the ratio of weight of submerged soil to the volume of soil. Submerged density is measured in g/cc and t/m3 in MKS system and kN/m3 in SI system. It is designated by the symbol γsub or γ’. Submerged density is also called submerged unit weight. Thus, –
γ’ = Wsub/V
where Wsub is the weight of submerged soil and V is the total volume of soil. It may be noted that the weight of submerged soil (Wsub) is less than Wsat as the soil is in submerged condition. As per Archimedes principle, when an object is immersed in a liquid, it appears to lose some weight. This loss of weight is equal to the weight of the liquid displaced by the soil. In submerged condition, the soil displaces water equal to total volume of soil. Thus, the weight of submerged soil is –
wsub = wsat – V. γw
Hence, the submerged density is equal to –
Density of soil solids is defined as the ratio of weight of soil solids to the volume of soil solids. Density of soil solids is measured in g/cc and t/m3 in MKS system and kN/m3 in SI system. It is designated by the symbol γs. Thus, –
γs = ws/Vs …(4.15)
where Ws is the weight of soil solids, which is equal to the weight of dry soil (Wd), and Vs is the volume of soil solids. Density of soil solids is used for defining specific gravity of soil solids.
iii. Specific Gravity of Soil Solids:
Specific gravity of soil solids is defined as the ratio of density of soil solids to the density of water. Being a ratio, it has no units and is designated by the symbol G. Thus, –
G = (γS/γw) = [(WS/VS)/γw] = (WS/VSγw) …(14.6)
Specific gravity is also defined as the ratio of weight of a given volume of soil solids to the weight of an equal volume of distilled water. Alternate names for specific gravity of soil solids are specific gravity of solid particles or simply specific gravity. There are different types of specific gravity of soils but when the term specific gravity is used without any attribute, it only refers to specific gravity of soil solids.
Specific gravity of most inorganic silts and clays lies in the range of 2.6-2.9. The presence of organic matter in soils may reduce the specific gravity significantly. Clean quartz and flirt sands have specific gravity values close to 2.65.
Specific gravity of soils is determined in the laboratory by using a pycnometer for coarse-grained soils (sand and gravel) and a density bottle for fine-grained soils (clay and silt). The principle of test in both methods is the same.
iv. Apparent or Mass Specific Gravity:
Apparent or mass specific gravity is defined as the ratio of bulk density of soil to the density of water. It is the ratio of the total weight of a given volume of soil to the weight of an equal volume of distilled water. Being a ratio, mass specific gravity has no units and it is designated by the symbol Gm. Thus, –
Gm = γ/γw = (W/V)/γw = W/Vγw …(4.17)
where γ is the bulk density of soil, γw is the density of water, W is the total weight of soil (Wd + Ww), and V is the total volume of soil. Apparent specific gravity is not widely used except in shrinkage limit calculations. It is also known as bulk specific gravity.
Specific gravity of soil solids calculated by excluding permeable and impermeable voids of soil is known as absolute specific gravity. In practice, it is normally difficult to estimate the impermeable void space and thus absolute specific gravity is not widely used, except in some research applications.
Water is an important constituent of soil and its density is frequently used in calculations involving the measurement of soil density. The density of water is the weight per unit volume of water and is equal to 1000 kgf/m or 1 t/m3, or 1 g (f)/cc in the MKS system and 9.81 kN/m3 or 9810 N/m3 in the SI system. Where the density of soil is expressed as mass per unit volume, the density of water to be used is its mass density. Its value is equal to 1 g/cc or 1000 kg/m3.
Property # 4. Physical Properties in Terms of Mass:
The physical properties containing only the volume terms are the same irrespective of whether mass or weight is used to define density or specific gravity. The physical properties of soil involving weight are expressed below in terms of mass. Figure 4.5 shows the three-phase diagram of a partially saturated soil in terms of mass. Figures 4.6 and 4.7 show the two-phase diagrams of a fully saturated soil and dry soil, respectively, in terms of mass.
Bulk (mass) density is –
r = M/V = (Md +Mw)/V …(4.51)
where M is the total mass of soil, Md is the mass of dry soil, and Mw is the mass of water. So the water content will be –
ω = (Mw/Md) ×100 …(4.52)
where Mw and Md are as defined above. Dry (mass) density is –
rd = Md/V …(4.53)
where Md is the mass of dry soil. Saturated (mass) density is –
rd = Msat /V …(4.54)
where Msat is the mass of saturated soil. Submerged (mass) density is –
r = Msub/V …(4.55)
where Msub is the mass of submerged soil.(Mass)density of soil solids is –
rs Md/Vs …(4.56)
where Vs the volume of soil solids.
Preparation of Soil Samples for Determination of Physical Properties:
Physical properties of soil are determined by conducting tests on soil samples in a geotechnical engineering laboratory. The soil samples are collected and transported from the field and stored in the laboratory.
Soil samples received from the field are dried in the air or sun. Where required, oven is also used for drying samples to conduct tests after a short while.
IS: 2720 (Part 2) – 1973 recommends air drying for conducting the following tests:
1. Grain size analysis.
2. Liquid limit.
3. Plastic limit.
4. Shrinkage limit.
5. Compaction.
6. California bearing ratio (CBR).
7. Field moisture equivalent.
8. Centrifuge moisture equivalent.
9. Organic matter.
Oven-dried samples can be used for conducting the following tests:
1. Specific gravity.
2. Unconfined compressive strength test.
3. Triaxial compression test.
4. Direct shear test.
5. Vane shear test.
6. Permeability
7. Total soluble solids.
8. Calcium carbonate.
9. Cation exchange capacity.
10. Silica sesquioxide ratio.
11. PH value.
12. Total soluble sulfates.
13. Free swell index.
14. Swelling pressure.
After the soil sample is dried, it is broken down to individual particles. The big clods in the soil sample, containing groups of particles, are broken down with the help of a wooden mallet. Further pulverization is done, up to the required extent, in a pestle and mortar.
Organic matter present in the soil, such as tree roots and pieces of bark, is separated from the main sample. Similarly, matter other than soil, such as shells, should also be separated from the main soil sample. The weight of any such matter removed from the main sample should be carefully noted, which will be considered in determination of organic matter and lime content, respectively.
The pulverized soil is passed through the specified sieve for the particular test and the soil retained on the sieve is again pulverized for sieving. This procedure is repeated until very little soil passes through the specified sieve after successive pulverizations. Care should be taken not to break the individual soil particles during pulverization.
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