Properties of Soils - 1 | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Introduction

Soil properties encompass a wide array of characteristics crucial for agriculture, civil engineering, and environmental sciences. These properties, including texture, structure, density, moisture content, permeability, and nutrient composition, influence soil fertility, stability, and erosion potential. Understanding these traits is essential for effective land management, construction, and environmental conservation efforts. 

Formation of soils:

 In the Earth's surface, rocks extend up to as much as 20 km depth. The major rock types are categorized as: 

• Igneous rocks: formed from crystalline bodies of cooled magma.
• Sedimentary rocks: formed from layers of cemented sediments.
• Metamorphic rocks: formed by the alteration of existing rocks due to heat from igneous intrusions or pressure due to crustal movement.

 Soils are formed from materials that have resulted from the disintegration of rocks by various processes of physical and chemical weathering. The nature and structure of a given soil depends on the processes and conditions that formed it: 

• Breakdown of parent rock: weathering, decomposition, erosion.
• Transportation to site of final deposition: gravity, flowing water, ice, wind.
• Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
• Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.

 All soils originate, directly or indirectly, from different rock types. 

Physical Weathering

• Physical weathering reduces the size of the parent rock material, without any change in the original composition of the parent rock.
• Physical or mechanical processes taking place on the earth's surface include the actions of water, frost, temperature changes, wind and ice.
• They cause disintegration and the products are mainly coarse soils.

Chemical Weathering

• Chemical weathering not only breaks up the material into smaller particles but alters the nature of the original parent rock itself.
• The main processes responsible are hydration, oxidation, and carbonation.
• New compounds are formed due to the chemical alterations.

Effects of Weathering and Transportation

• The effects of weathering and transportation mainly determine the basic nature of the soil (size, shape, composition and distribution of the particles).
• The environment into which deposition takes place, and the subsequent geological events that take place there, determine the state of the soil (density, moisture content) and the structure or fabric of the soil.

Transportation / Deposition

• Soil is classified into two types:
  • Residual soil:
    • Residual soils are found at the same location where they have been formed. Generally, the depth of residual soils varies from 5 to 20 m.
    • Accumulation of residual soils takes place as the rate of rock decomposition exceeds the rate of erosion or transportation of the weathered material.
    • Residual soils comprise of a wide range of particle sizes, shapes and composition.
  • Transported soil:
    • Weathered rock materials can be moved from their original site to new locations by one or more of the transportation agencies to form transported soils.
    • Transported soils are classified based on the mode of transportation and the final deposition environment as follows:
      • Soils that are carried and deposited by rivers are called alluvial deposits.
      • Soils that are deposited by flowing water or surface runoff while entering a lake are called lacustrine deposits. Alternate layers are formed in different seasons depending on flow rate.
      • If the deposits are made by rivers in sea water, they are called marine deposits.
      • Melting of a glacier causes the deposition of all the materials scoured by it leading to formation of glacial deposits.

• Black cotton soil
  • A large part of central India and some portion of South India is covered with Black cotton soil.
  • It has clay mineral montmorillonite; due to this, it has high swelling, shrinkage, and plasticity properties.
  • Shearing strength of soil is very low.
• Standard Sand of India: 
  • Ennore Sand (Madras)
  • 1. LOAM SOIL - It is the mixture of sand, silt, and clay.
  • 2. BENTONITE SOIL - It is chemically weathered volcanic ash that is generally used as lubricant in drilling. It is a clay with montmorillonite. Application found in pile foundation.
  • 3. Lithification: The process by which unconsolidated material is converted into a soil rock by compaction or cementation.
  • 4. Indurated clay: Hardening of clay due to heat and pressure.

Phase Diagram

• Soil mass is in general a three-phase system composed of solid, liquid, and gaseous matter.
• The diagrammatic representation of the different phases in a soil mass is called the phase diagram.
• A 3-phase system is applicable for partially saturated soil, whereas a 2-phase system is for saturated and dry states of soil.

•  Total volume, V = Vs + Vv
•  In engineering, expressing weight-volume relations of phases is essential for understanding, predicting, and optimizing systems. 
• The various relations can be grouped into: 
  • Volume relations
  • Weight relations
  • Inter-relations
• VOLUME RELATIONS
• Water Content: 

•  Where, WW = Weight of water, WS = weight of solids. 
•  Its value is 0% for dry soil and its magnitude can exceed 100%. 
•  Void Ratio: 

 
where, V_V = Volume of Voids Vs= Volume of solids.
Void ratio of fine grained soils are generally higher than those of coarse grained soils.
In general e > 0 i.e., no upper limit for void volume.

• Porosity:

where, V_V = Volume of Voids V= Volume of soil.
Porosity cannot exceed 100% i.e., 0 < n < 100
Note:-In comparison to porosity, void ratio is more frequently used because volume of solids remains same, whereas total volume changes.

• Percentage Air Voids ( n_a )

The ratio of the volume of air to the total volume.

• Degree of Saturation
  
  where,V_w = Volume of water V_v= Volume of voids
  For perfectly dry soil : S = O for fully saturated soil : S = 100%
• Air Content(a_c):
  It is the ratio of the volume of air (V_a) to the volume of voids.

WEIGHT RELATIONS

• Bulk unit weight

• Dry Unit Weight is the weight of soil solids per unit volume.
  
  -Dry unit weight is used as a measure of denseness of soil. More dry unit weight means more compacted soil.
•  Saturated unit weight(when soil is completely saturated, S = 100%,V_a=0) : It is the ratio of total weight of fully saturated soil sample to its total volume.
  
• Submerged unit weight or Buoyant unit weight(when soil is below ground water table, S = 100%): It is the submerged weight of soil solids per unit volume.

Quantifying a soil's condition upon arrival in the lab is crucial before conducting further tests, especially regarding water content and unit weight. These properties might alter during transportation and storage. Certain physical properties are derived from others; for instance, dry unit weight can be calculated using bulk unit weight and water content. Here are some interconnections:

• Water content and unit weight are essential for determining the soil's condition.
• Dry unit weight is derived from bulk unit weight and water content.
• Changes in these properties can occur during transportation and storage.

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Determination of Index Properties

Index properties of soil refer to the characteristics used for identifying and classifying soil, aiding in its categorization and analysis.

• Water content
• Specific gravity
• Particle distribution
• Consistency limits
• In-situ density
• Density index

Specific Gravity:

Specific gravity of soil solids (Gs) is the ratio of the weight of a given volume of solids to the weight of an equivalent volume of water at 4ºC.

• Unit weight of water = 9.8 kN/m³
• Average value of Gs for granular soils is 2.65, while the average value of Gs for cohesive soils is 2.80.

Particle Distribution:

Index properties of soil particle distribution are attributes used to characterize the size distribution of soil particles within a given sample. These properties include parameters such as the grain size distribution curve, grain size range, and the percentages of various particle sizes (e.g., gravel, sand, silt, clay) present in the soil. These index properties help classify soils into different categories based on their particle size distribution, which is essential for various engineering and geo technical applications.

Consistency Limits:

Index properties of soil consistency limits include the liquid limit, plastic limit, and shrinkage limit. These parameters determine soil behavior and plasticity under varying moisture conditions, guiding soil classification and engineering practices.

Consistency Limits in Soil

(i) Oven Drying Method

• Simplest and most accurate method.
• Soil sample is dried in a controlled temperature.
• For organic soils, temperature is about 60ºC.
• Sample is dried for 24 hrs.
• For sandy soils, complete drying can be achieved in 4 to 6 hrs.
• Water content is calculated as:
• w = [W2-W3] / [W3 -W1]*100%
• where, W1 = Weight of container, W2 = weight of container moist sample, W3 = Weight of container dried sample, Weight of water = W2 – W3, Weight of solids = W3 – W1

(ii) Pycnometer method

• Quick method
• Capacity of pycnometer = 900 ml.
• This method is more suitable for cohesionless soils.
• Used when specific gravity (G) of soil solids is known.
• Let W1 = Wt. of empty dried pycnometer bottle
• W2 = wt. of pycnometer Soil
• W3 = Wt. of pycnometer Soil Water
• W4 = Wt. of pycnometer Water.

• The water content of the soil is computed using the following equation:
• w = [((W2 - W1) / (W3 - W4)) * ((G - 1) / G)] - 1
• (iii) Calcium Carbide method/Rapid Moisture Meter method
• Quick method (requires 5 to 7 minutes); but may not give accurate results.
• The reaction involved is CaC2 + 2H2O → C2 H2 + Ca(OH)2
• Soil sample weights 4-6 gms.
• The gauge reads water content with respect to wet soil, i.e.,

•  
  Actual water content

• Sand Bath Method
  • Quick, field method
  • Used when electric oven is not available.
  • Soil sample is put in a container & dried by placing it in a sand bath, which is heated on kerosene stove.
  • Water content is determined by using the same formula as in oven drying method.
• Torsion Balance Moisture meter Method
  • Quick method for use in laboratory.
  • Infrared radiations are used for drying sample.
  • Principle: The torsion wire is prestressed accurately to an extent equal to 100% of the scale reading. Then the sample is evenly distributed on the balance pan to counteract the prestressed torsion and the scale is brought back to zero. As the sample dries, the loss in weight is continuously balanced by the rotation of a drum calibrated directly to read moisture% on wet basis.
• DETERMINATION OF SPECIFIC GRAVITY OF SOIL SOLIDS:
  • Pycnometer method is used.
  • Instead of pycnometer, Density bottle (50 ml) OR Flask (500 ml) can also be used.

• Let, W1 = Weight of empty pycnometer,
• W2 = Weight of pycnometer + Soil sample (oven dried),
• W3 = Weight of pycnometer + Soil solids + water,
• W4 = Weight of pycnometer + water.

NOTE:
1.Specific gravity values are generally reported at 27ºC (in India).
2.If TºC is the test temperature then Sp. Gr. at 27ºC is given by,

3.If kerosene (better wetting agent) is used instead of water then,

 k [K Sp.gr. of Kerosene]