Geoscience Reference
In-Depth Information
where
d
15.9
and
d
84.1
are the particle sizes at which 15.9% and 84.1% by weight of
the sample are finer, respectively. For a normal size distribution,
σ
=
d
84.1
/
d
50
=
g
d
50
/
d
15.9
.
Kramer (1935) defined a uniformity parameter as the ratio of the mean sizes of the
two portions in the cumulative size frequency curve separated by
d
50
:
p
k
d
k
100
50
=
M
p
k
d
k
(2.14)
A
k
=
0
A
k
=
50
where
A
k
is the cumulative percentage of sediment finer than size
d
k
.
For uniform sediment,
M
=
1. A smaller value of
M
corresponds to a more non-
uniform sediment mixture.
Porosity and dry density
A sediment deposit is a porous material and has voids among solid particles. Its
porosity,
p
m
, is a measure of the volume of voids per unit volume of the deposit:
V
v
p
m
=
(2.15)
V
v
+
V
s
where
V
v
and
V
s
are the volumes of voids and solids, respectively.
The dry density,
γ
d
, of a sediment deposit are the mass
and weight of the solids per unit total volume. They are related to the porosity by
ρ
d
, and dry specific weight,
p
m
)
p
m
)
ρ
=
ρ
(
1
−
,
γ
=
γ
(
1
−
(2.16)
s
s
d
d
Han
et al
. (1981) proposed the following semi-empirical formula to calculate the
initial porosity of a uniform sediment deposit:
⎨
0.525
d
d
δ
1
3
−
<
1
d
1mm
p
m
=
+
4
(2.17)
⎩
(
)
/
d
0
0.175
e
−
0.095
d
−
d
0
0.3
+
d
≥
1mm
where
d
is the sediment size in mm;
d
0
is a reference size, set to be 1 mm; and
1
is
the thickness of the water film attaching to sediment particles, given a value of about
0.0004 mm.
In a non-uniform sediment deposit, fine particles probably fill the voids among
coarse particles. Han
et al
. (1981) investigated this filling phenomenon and proposed
a method for the overall porosity of the deposit. However, their method is relatively
complicated and inconvenient to use. If a sediment deposit is composed of only fine
particles or if its size range is narrow, the filling phenomenon is negligible and the
δ
