2. Origin and Properties of Sediments
Introduction
• Sediment is fragmented material, primarily
formed by physical, chemical and organic
weathering of rocks from the earth’s crust
• Size - Large boulders to colloidal particles
• Shape - Rounded to angular
• Sp. gr. and mineral composition vary
• Quartz mineral (predominant)
• Clay mineral (kaolinite, illite, montmorillonite
and chlorite) flocculate under the influence
of electrostatic (cohesive) forces in a saline
environment
Sediment Classification - Genetic Origin
• Lithogeneous sediments (disintegration of preexisting rocks)
• Biogeneous sediments (remains of organisms –
carbonate, opal, calcium phosphate, etc.)
• Hydrogeneous sediments (precipitates from sea
water or interstitial water)
Sediment Classification - Descriptive
• Related to the characteristics
Color, texture, grain size, organic content, etc.
Example: Clayey-sand (contains 25 to 50% sand)
Classification of Sands - Based on Mineral &
Chemical composition
• Silicate sands - consist of quartz and feldspar
minerals; insoluble
• Carbonate sands - consist of calcite and
aragonite which are two crystalline forms of
CaCO3; soluble in fresh water; weakly-cemented
or well-cemented
• Gypsum sands - crystal forms of gypsum
(CaSO4.2H2O); moderately soluble in water; can
survive in arid regions
Properties of Sediment:
Size


Shape


Density

Sp. gravity
 Depend on parent rock
Mineral composition 

Texture


Etc.

Density and Porosity
▪ Dry density - dry sediment mass per unit volume
 = (1− p ) 
dry
s
in which:

dry

P
s
= dry sediment density (kg/m3)
= Msolids/VTotal
= sediment density (kg/m3)
= MTotal/VTotal
= porosity factor
= VVoids/VTotal
▪ Wet density – weight of water and sediment per unit
volume (assuming total saturation)
 s − w 
  dry
 wet = p  w + (1− p )  s
or
 wet =  w + 


 s 
Sand Type
Density, ρ
3
s
(Kg/m )
Quartz mineral
Carbonate mineral
Sand Type
2650
2500 to 2650
Porosity, p
(%)
Coarse sand
40
Fine sand
45
Mud deposits Up to 80
Remark
Poorly sorted
Well sorted
(Clay, silt, sand & organic
materials mixture)
Lane and Koelzer (1953): Based on the analysis of samples from
the top layer (recent deposits) of reservoirs at initial period of 0 to
1 year,

in which:
p
sand
dry
=
817 (100 p sand + 2)
0.13
= fraction of sand particles (d > 50 m)
Lara & Pemberton (1963):
For reservoir samples always
submerged (A) and submerged 50% of the time (B) at initial period
of 0 to 1 year,

A:
dry

B:
dry
=
1550 p sand + 1120 p silt + 420 pclay
=
1550 p sand + 1135 p silt + 560 pclay
in which:
p
sand
p
silt
= fraction of sand particles (d  62.5 m)
= fraction of silt particles (4 m < d < 62.5 m)
Murthy & Banerjee (1976): Using top layer
samples (sampling depth = 0.40m) from Indian
reservoirs which were about 50% of the time
empty at initial period of 0 to 1 year,

p
sand
dry
=
1506 p sand + 866 p silt + 561 pclay
= fraction of sand particles (20 m  d 
200 m)
p
silt
p
= fraction of silt particles (2 m < d < 20 m)
clay
= fraction of clay particles (d  2 m)
Allersam (1988): Based on analysis of a large
amount of soil samples,
.8
480 + (1300 − 280 ) p0sand
in which  = consolidation coefficient ( = 0 to
2.4).

dry
=
see Fig. 2.1
Fig.2.1: Sediment densities according to Allersam (1988)
•
F
Other soil parameters
• Sediment concentration (c or ρdry) = ratio of dry
solid mass (Ms) and total volume (Vt) = Ms/Vt
• Solids content (Sc) = ratio of the mass of dry
solids (MS) and the total mass = Ms/Mt
• Water content (wc) = ratio of the water mass
(Mw) and the solid mass (Ms) = Mw/Ms
• Void ratio (e) = ratio of the void volume (Vv) and
the solid volume = Vv/Vs
See Fig. 2.2
Fig. 2.2: Conversion of values between dry density, wet bulk density, moisture content
and void ratio (ρs =2650 kg/m3)
• Saturation degree (sd) = ratio of water
volume (Vw) and void volume (Vv) = Vw/Vv
 −
p=

s
dry
s
w=
c
e sd  w
s
 s  
   dry
w

sc =
  s  
 s +    −1  dry
w


p
e=
1− p
Shape
• Most of the sand particles are more or less
rounded
• Shape of particles generally is represented by
c
the Corey shape factor, SF.
SF =
c
a
ab
in which
b
a = length along longest axis perpendicular to other
two axes
b = length along intermediate axis perpendicular to
other two axes
c = length along shortest axis perpendicular to
other two axes
• SF is a factor of flatness and does not take into
account the distribution of surface area and
the volume of the particle. For sand SF  0.7.
• Another shape factor SF* overcomes the
above limitation
SF
*
= SF
d
d
s
v
in which
ds = diameter of a sphere having the same
surface area as that of the particle
dn = diameter of a sphere having the same
volume as that of the particle
Size
• The following generic categories refer to sediment
particle size
– Gravel
– Sand
– Silt
– Clay
• Table 2.1 – Grain size scale of the American Geophysical
Union (AGU)
• Methods to determine particle size
o Ruler - For cobbles
o Wet/dry sieving - For gravel, sand and silt
o Settling/electronic techniques - For Clay
Table 2.1: Grain size scale American Geophysical Union (AGU)
• Typical sediment particle diameter
o Sieve diameter – diameter of a sphere equal to
the length of the side of a square sieve opening
through which a given particle will just pass
o Nominal diameter – the diameter of a sphere
which has the same volume as the particle
o Standard fall diameter – the diameter of a sphere
that has a specific gravity of 2.65 and the same
velocity as the particle velocity in still distilled
water of 24C
• Natural Sample of Sediment
o Contains particles of range of sizes
o Size distribution presented as cumulative
frequency distribution by percentage of weight
o Characteristics of the frequency distribution
▪ Median particle size (d50)
▪ Mean particle size, dm = ∑(pidi)/100 with pi =
percentage by weight of each grain size fraction di
▪ Standard deviation, σs2 = ∑ pi(di-dm)2/100 or 0.5(d50/d16
+ d84/d50) which is a measure based on graphic values
Particle Size Distribution
Normal size-frequency
distribution curve
Cumulative frequency
distribution, i.e. percent
finer-than curve reading
o Phi-scale: Used for size distribution representation
with particle diameter d in mm. It allows
description and plotting of grain size distribution
within a narrow range and simplifies the statistical
analysis of size distribution.
ln( d )
 = − log2 (d ) = −
ln( 2)
▪ Characteristic values are
➢
➢
➢
➢
➢
Mean value, φm
Median value, φ50
Standard deviation, σφ = 0.5(φ84 – φ16)
Skewness (asymmetry), φ = (φm – φ50)/ σφ
Kurtosis (peakedness), βφ ={0.5 (φ95 – φ5) – σφ}/ σφ
For a sample which has normal (Gausian) distribution:
d
d
d
d
= s
−1.3
10
d
= d
= d
= d
50
−1
16
84
s
s
50
50
1.3
90
with
s
(
d 50
=
0
.
5
s
50
d 16
+ d 84
d 50
)
Remark:
• Grain size decreases with distance from the source
(Fining Down the Transport Path)
o Near source: Wide size range (well sorted)
o Far from source: Narrow size range (poorly sorted)
• Coarser particles are transported as bed load and finer
particles as suspended load
• Relationship between grain size, sorting and hydraulic
conditions – used to distinguish between different
sedimentary environs ( wave-dominated, current
dominated, eroding or depositing) and paths between
them
Particle Fall Velocity
– Sphere Falling in a Still Fluid
The terminal fall velocity (ωs) of a sphere of
diameter d is the fall velocity when the fluid drag
force on the particle is in equilibrium with the
gravity force.
1
2
(c
d
giving
s =
  s2
) ( d ) =
1

2
4
1
3
(
)

−

g

d
s
6

4 (s −1) g d 0.5
3 CD
in which s = specific gravity
(= 2.65)
In the Stokes region (Re < 1),
Cd =
24
in which
Re
and
Re =
s d
s =

( s −1) g d 2
18
 Re 105 ,
s  d
For 10
3
–
s =
Non-spherical Particles
( s −1) g d 2
18
( )
 s = 10d 1 + 0.01 s −21 g d  − 1

 

 s = (1.1(s −1) g d ) 0.5
3
for
1  d  100 m
for
1 00  d  1000 m
for
d  1000 m
0.5
Effect of Sediment Concentration (C):
A small cloud of particles in a clear fluid have a larger fall velocity
ωs,m than that of a single particle.
According to Richardson and Zaki:
In which
ϒ = 4.6 to 2.3
ϒ =4
 s ,m = (1 − c )   s
for Re = 10-1 to 103
for d = 50 to 500 m
For full range of concentrations Oliver gives:
 s ,m = (1 − 2.15c ) (1 − 0.75c
0.35
s )
Influence of Oscillatory Flow
• ωs decreases in oscillatory flows (turbulence,
eddies)
•
ωs may slightly increase near channel bed due to an
asymmetry fluid motion (high downward velocities)
Angle of Repose φn
• Equilibrium slope (behavioral property of sand
particles)
φn = 30o to 40o
for d = 0.001 to 0.010m
Angle of internal friction φ
• Characteristic angle related to the particle stability
on horizontal or sloping bed
• Determined from initiation of motion experiments
using
sin( −  )
 b ,cr =  b ,cr , o
sin 
in which
 b ,cr = crititcal bed shear stress on a sloping bed
 b ,cr , o = crititcal bed shear stress on a horizontal bed
β = angle of longitudinal (bed) slope