When the grains are larger than about 0.1–0.5 mm,
the settling velocity increases, and turbulence
develops around the grains. The frictional resistance
therefore increases, and in the case of larger grains
(>1 mm) the settling velocity increases approximately
in proportion to the square root of the radius. It is not
practicable to measure each grain, but the settling
velocity can be measured indirectly. A hydrometer
floating in a suspension of sediments and water
measures the density of the suspension through time.
The rate of density reduction in the suspension is a
function of the grains’ size. By taking successive
readings of the density we may plot a curve which
expresses density reduction as a function of time.
Since density reduction is a function of settling velocity, this curve can be calibrated to give a grain-size
distribution curve.
When we analyse fine-grained sediments with a
large clay fraction, or separate out clay fractions, it
may be useful to use a centrifuge. We then increase the
a
b
Sediment sample
X 1
X 2
X 3
X 4
X 5
X 6
Shaker
Amount of sediment on
each sieve X 1 − X 6
Sedimentation balance
Weight
Time
Sediments
depositing
on the balance
Cumulative
grain-size
distribution
curve
Sieves with
different
mesh sizes
Wentworth Scale
Boulder
mm
j = – log 2 d
–8
64
256
–6
4
–2
–1
2
+1
+4
+8
1
0.5
0.25
0.125
0.0625
0.004 ( 256
1
Cobbles
Pebbles
V. Coarse sand
Coarse sand
Medium sand
Fine sand
V. Fine sand
Granules
Sand
Silt
Clay
)
Sieve analysis
Fig. 2.1 (a) Sketch showing the principles involved in sieve
analysis and use of a sedimentation balance. Sieve analysis is
usually used for grain sizes down to 0.03–0.02 mm, but with
wet-sieving even finer sediment grains can be sieved. The sedimentation balance gives us a direct expression of settling
velocity, i.e. weight increase as a function of time. This is
therefore a cumulative grain-size distribution. (b) Grain-size
classification of clastic sediments. The grain size (d) is often
described in terms of φ values (φ ¼ –log 2 d)
2 Introduction to Sedimentology
35
the settling velocity increases, and turbulence
develops around the grains. The frictional resistance
therefore increases, and in the case of larger grains
(>1 mm) the settling velocity increases approximately
in proportion to the square root of the radius. It is not
practicable to measure each grain, but the settling
velocity can be measured indirectly. A hydrometer
floating in a suspension of sediments and water
measures the density of the suspension through time.
The rate of density reduction in the suspension is a
function of the grains’ size. By taking successive
readings of the density we may plot a curve which
expresses density reduction as a function of time.
Since density reduction is a function of settling velocity, this curve can be calibrated to give a grain-size
distribution curve.
When we analyse fine-grained sediments with a
large clay fraction, or separate out clay fractions, it
may be useful to use a centrifuge. We then increase the
a
b
Sediment sample
X 1
X 2
X 3
X 4
X 5
X 6
Shaker
Amount of sediment on
each sieve X 1 − X 6
Sedimentation balance
Weight
Time
Sediments
depositing
on the balance
Cumulative
grain-size
distribution
curve
Sieves with
different
mesh sizes
Wentworth Scale
Boulder
mm
j = – log 2 d
–8
64
256
–6
4
–2
–1
2
+1
+4
+8
1
0.5
0.25
0.125
0.0625
0.004 ( 256
1
Cobbles
Pebbles
V. Coarse sand
Coarse sand
Medium sand
Fine sand
V. Fine sand
Granules
Sand
Silt
Clay
)
Sieve analysis
Fig. 2.1 (a) Sketch showing the principles involved in sieve
analysis and use of a sedimentation balance. Sieve analysis is
usually used for grain sizes down to 0.03–0.02 mm, but with
wet-sieving even finer sediment grains can be sieved. The sedimentation balance gives us a direct expression of settling
velocity, i.e. weight increase as a function of time. This is
therefore a cumulative grain-size distribution. (b) Grain-size
classification of clastic sediments. The grain size (d) is often
described in terms of φ values (φ ¼ –log 2 d)
2 Introduction to Sedimentology
35