Geoscience Reference
In-Depth Information
0
is the basic friction coefficient between polished surfaces (range
between 27
o
and 33
o
),
R'
is the equivalent roughness related to rock type, shape
and packing (Fig 16.1a),
S'
is the normalised equivalent strength (Fig 16.1b) and
Here,
'
the actual in situ effective stress.
Densification
The natural densification (settlement) of rock fill subjected to gravity, hydraulic
or earthquake loading may be important. Cohesionless materials dumped or
discharged through water will be in a loose state. For earth reservoir dams usually
0.2 to 0.5% densification is assumed together with careful placement during
construction. In marine structures, controlled densification is rarely performed.
Loose dumped gravel and rock fill can be densified by specific methods (falling
weight, shaking plate). The Menard method is popular and densifications to depths
of 10 m are reported. For gravel 10-15% densification can be achieved and for rock
fill 10-60 kg about 15%. For larger stones heavier equipment may cause crushing.
For comparison, densification machines produce accelerations up to 10g,
earthquakes may produce 2g in the entire structure (leading to 2-4% densification).
Large slamming waves cause 1g acceleration, locally under concentrated impact,
which can be observed along coastal defence structures around the storm water line
(see page 284).
Turbulent porous flow
Porous flow in coarse granular media (stones and rock beds) develops under
inertial effects and partial turbulence (Forchheimer's law), in accordance to
u
,i
/
w
=
cq
,t
(
a + b|q|
)
q
i
(16.2)
n
2
)/(
gD
15
2
n
3
)
and
b =
2/(
gn
2
D
15
)
with
a = 160
(
1
the
kinematical viscosity,
n
porosity,
g
gravity acceleration and
D
15
the characteristic
pore size dimension. Here,
u
,i
/
Here,
c
is a mass factor, and
a
and
b
are permeability parameters with
w
represents the pressure head gradient. The
expressions for
a
and
b
are empirical (Hannoura and Barends).
Turbulence effects may be expected to start in granular soils with a
D
50
> 5 mm,
depending on the grain size distribution. Sea gravel often has a flow regime
intermediate between linear and turbulent. For turbulent flow one may adopt an
apparent permeability, applying
k =
(
a
/2 + (
a
2
/4+
b
|
u
,i
|/
w
)
0.5
)
1
. Formula (16.2)
shows that the apparent permeability in the linear regime (
b
= 0) is proportional to
D
2
and in the turbulent regime proportional to
D
0.5
, showing a dependence of four
orders. The flow regime depends on the actual pore pressure gradient, which will
vary in space and time under wave loading conditions. Scaling difficulties seem
unavoidable in physical modelling of the linear-turbulent porous flow field.
Filter rules
In stratified filter layers, local pore fluid velocities may convey fine particles
through the pores. This may lead to internal erosion and can become the onset of
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