Flow through Soils ch7

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Flow through Soils(ch7)
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Energies
Kinetic E (velocity) Strain E (fluid
pressure) Potential E (elevation) Head convert
each form of energy into the equivalent
potential energy and express it as the
corresponding height. units of LENGTH
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Heads
hv velocity head (KE) hp pressure head
(SE) he elevation head (PE) h total head
hv hp he (Bernoulli) units of LENGTH
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Heads in a tank of water
A
B
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Head loss
Fluid flows from point of high total head to
point of low total head
head loss Dh h1 h2
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Hydraulic gradient

Rate at which the total head changes along a
length
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Heads in soils
Since velocity is slow through soils, we neglect
the velocity head. Thus,
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Soil piezometer
A
The pore water pressure at A is
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Pore water pressures

uhydrostatic uh due to hydrostatic condition
only
uexcess ue due to additional processes
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Hydrostatic pore water pressure

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One dimensional flow
Flow vectors parallel of equal magnitude

SOIL
Flow is in one direction
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Flowrate through soil
What is the flowrate through a soil?

Concrete dam
Flowrate Q m3/sec
SOIL
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Darcys Law
Assumptions flow is laminar soil
properties do not D with time

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Finding k
Measure Q
Figure 7.11 (text)
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k
Measure of a soil-fluid systems resistance to
flow
depends on
Void size Fabric (structure) Void
continuity Specific surface (drag)
Viscosity Mass density
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k
Units are in cm/sec
but
k velocity
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k
sands
10-3 100
silts
10-8 10-3
clays
10-10 10-6
Probably soils most varying parameter (largest
numerical range)
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Lab testing
k1 10-2 k2 10-6
k precision is on the order of /- 50 or
more! Report values to one decimal place.
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Lab testing (constant head test)
Measure Q
Figure 7.11 (text)
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In-situ testing
Slug test Pumping test
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Hazens Correlation
k a pore size
(pore diameter)2
(pore diameter) D10
For loose clean sands with 0.1mm lt D10 lt 3mm and
Cu lt 5
k cm/sec C Hazens coefficient 0.8 1.2
(typical 1) D10 mm
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Example
Given ksand 4x10-2 cm/sec reservoir length
(into board) 1000 m
Compute seepage loss (Q) through the sand seam
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Solution
Q kiA
k 4x10-2 cm/sec
i Dh/L (167.3m 165m) / 256m 0.009
A (3.2 m) (1000 m) 3200 m2