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vertically partly in the soil and a certain amount of water is poured in the pipe. The
initial free water head
h
0
in the pipe and the time
t
required for the penetration of
all the water provides an approximate value for the local permeability
k
0.7
h
0
/t
.
Note, that this value includes impurities. The pipe toe should not reach the
groundwater or a relatively impervious layer, i.e. the outflow at the pile toe into the
ground must not be blocked.
pump
Buffer
Buffer
Parallel Plates
top view
phreatic surface
side view
H
1
Q
s
eepage face
H
2
L
Figure 4.4 The Hele-Shaw model, applied for free-surface groundwater flow
Two dimensional flow in a homogeneous porous medium can be simulated by
flow between two flat parallel plates, separated by a small distance
d
, (see Fig 4.4).
The experiment was originally carried out by Hele-Shaw in 1898. For flow of a
viscous fluid (no turbulence: Hagen-Poiseuille flow), the corresponding intrinsic
permeability is
k = gd
2
/12
the kinematic viscosity (for water of 25
o
,
=
10
-6
m
2
/s). The analogy between two dimensional groundwater flow and the flow
between parallel plates enables the study of two-dimensional groundwater flow
problems, since gravity influences the flow in the same way as in permeable soils.
Hence, with the Hele-Shaw model the flow of moving surfaces is possible, either
with
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