origin of soil and grain size
TRANSCRIPT
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CHAPTER 2ORIGIN OF SOIL AND GRAIN SIZE
Associate Professor Mazen AbualtayefCivil and Environmental Engineering Department
Islamic University of Gaza, PalestineThe materials are mostly adapted from the presentation of Ahmed Abu Foul
ECIV 3302
Engineering Geology
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According to Terzaghi (1948):
“Soil Mechanics is the application of laws of
mechanics and hydraulics to engineering problems
dealing with sediments and other unconsolidated
accumulations of solid particles produced by the
mechanical and chemical disintegration of rocks
regardless of whether or not they contain an
admixture of organic constituent.”
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Rock Cycle and the Origin of Soil
The final products
due to weathering
are soils
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Soil Formation
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Parent Rocks
Rocks formation
IgneousFormed by cooling of molten magma (lava) e.g. granite
Sedimentary
Formed by gradual deposition and in layers . e.g. limestone, shale
Metamorphic
Formed by alteration of igneous & sedimentary rocks by pressure / temperature e.g. marble
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.
Specific Gravity (Gs)Definition
Specific gravity is defined as the ratio of the unit
weight of a given material to the unit weight of water.
Weight of an Equal Volume of Water
Weight of a Substance Specific gravity =
Unit weight of Water is:
W = 1 g /cm3 (strictly accurate at 4o C)
W = 62.4 pcf
W = 9.81 kN/m3
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Mechanical Analysis of Soil
Mechanical analysis is the determination of the
size range of particles present in a soil, expressed
as a percentage of the total dry weight.
Two methods that generally utilized to determine
the particle size distribution of soil:
1. Sieve Analysis (for particle sizes > 0.075mm in diameter)
2. Hydrometer Analysis ( “ “ “ < 0.075mm “ “ )
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Particle Size Distributions and Soil
Particle Characteristics
Particle size distribution curve is a representation
in graphical or tabular form of the various
(diameter) grain sizes in a soil, determined through
sieving and sedimentation.
The particle diameters are plotted in log scale, and
the corresponding percent finer in arithmetic scale.
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Particle Size Distribution Curve
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Hydrometer Analysis
Hydrometer analysis is based on the principle of
sedimentation of soil grains in water. When a soil
specimen is dispersed in water, the particles settle at
different velocities, depending on their shape, size and
weight, and the viscosity of the water.
For simplicity, it is assumed that all the soil particles are
spheres and that the velocity of soil particles can be
expressed by Stokes’ law.
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Hydrometer Analysis
u = velocity
gw = density of water
h = viscosity of water
D = diameter of soil particles
Gs = Specific gravity of soil
GL = Specific gravity of liquid
where
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(2.1)
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Some commonly used measures are:
Effective size: (D10) It is the diameter in the particle size
distribution curve corresponding to 10% finer. (maximum
size of the smallest 10% of the soil) The effective size of a
granular soil is a good measure to estimate the hydraulic
conductivity and drainage through soil.
Uniformity Coefficient: Cu =D60/D10
It is the ratio of the maximum diameter of the smallest
60% to the effective size. A well graded soil will have
• Cu > 6 for sands
• Cu > 4 for gravels
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Coefficient of Curvature:
D30 = Diameter corresponding the 30% finer
Clay Fraction: (CF)
It is the percentage by dry mass of particles smaller than
0.002mm (2μm), and is an index property frequently
quoted relation to fine grained soils (soils with 50% or
more finer than 63μm). It has a strong influence on the
engineering properties of fine grained soils.
Some commonly used measures are:
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Sorting Coefficient (S0):
This parameter is another measure of uniformity and is
generally encountered in geologic works and expressed
as:
Some commonly used measures are:
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Definitions
Well – Graded Material
Contains particles of a wide range of sizes. The smaller
particles fill the spaces left between the larger particles;
therefore the soil has greater strength than a poorly
graded soil, and lower permeability.
Poorly – Graded Material
Contains a large portion of uniformly sized particles.
This particular soil has larger voids in its structure and
poor strength along with high permeability.
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Definitions
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The calculations are summarized in the table below
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Particle size distribution curve
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Rounded
Angular
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H.W
All Problems