GEOTECHNICAL PROPERTIES (CE1203)

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GEOTECHNICAL PROPERTIES(CE1203)
PERMEABILITY
Ms Ikmalzatul
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Introduction
The permeability of a soil has a considerable
effect on the cost and difficulty of many Civil
Engineering construction operations e.g. the
excavation of soil below the water table, or the
rate at which a soft clay stratum consolidates
under the influence of the mass of a superimposed
load.
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Definition
Permeability is the passage of water (or oil or
gas) through a soil. As soil consists of discrete
particles with interconnected pore spaces water
can flow within the soil. Such water will flow
from areas of high pore pressure to areas of low
pore pressure.
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Hydraulic head across a soil
When considering water flow pressure is usually
expressed as a head measured in metres of water.
There are, according to Bernoullis equation,
three components to the head - elevation head
(z), pressure head due to pore water pressures (
u/?w ) and velocity head ( v2/2g ). Velocity head
is usually ignored in groundwater flow problems
as the term in v is quite small. The total head
causing flow through the soil mass is therefore
the sum of the elevation head and the pressure
head.
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Hydraulic gradient ( i )
The hydraulic gradient ( i ) is defined as the
hydraulic head ( ?H ) across the soil divided by
the length of flow path through the soil ( ?L ).
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Critical hydraulic gradient ( ic )
This is the hydraulic gradient at which the soil
becomes unstable - the effective stress becomes
zero. Consider a soil in which the flow of water
is upward, this will create an upward seepage
pressure. If the upward flow of water is large
enough the seepage pressure will negate the
effective stress and the soil will become
unstable. In this situation the soil is said to
be in a quick condition
or
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Critical hydraulic gradient ( ic )
Cohesionless soils, in particular fine to medium
sands, typically exhibit the quick condition at
hydraulic gradients of around 1.0. Coarse sands
and gravels (soils of high permeability) require
large flow rates to achieve this quick
condition and these are seldom found in practice.
Cohesive soils do not exhibit quick conditions
as even at zero normal stress they posses some
shear strength.
Example- A soil has a porosity of 0.4 and
saturated unit weight of 19.7 kNm-3. Calculate
its critical hydraulic gradient. Gs 2.7.
Unit weight density x gravity
or
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Flow of water
The flow of water in a soil is governed by
Darcys Law, which states that under saturated
conditions flow velocity is proportional to the
hydraulic gradient. v ? i or v k
i where v velocity of flow i hydraulic
gradient k coefficient of permeability ?
the quantity of water flowing ( Q ) is given by
where Q quantity of water flowing in
time t t time A area through which flow
is taking place or working in unit time, q k A
i
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Coefficient of permeability

• This is defined as the flow velocity produced by
  a hydraulic gradient of unity. From the flow
  equations above
• and is expressed in ms-1
• The value of k ranges from almost zero in the
  case of clay (impermeable) upto 10 ms-1 for very
  coarse gravels.
• The actual k value for a soil is dependent on a
  number of factors including the
• porosity of the soil,
• particle size distribution,
• shape of the particles,
• degree of saturation and
• temperature/viscosity of the water.

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Typical values of k
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Laboratory determination of k

• The two main laboratory tests used in the
  determination of k are -
• the constant head permeametre -used for gravels
  and sands with k values gt 10-5 ms-1
• the falling head permeametre - used for fine
  sands, silts and clays with k values between 10-4
  to 10-7 ms-1.

A third laboratory test the Hydraulic Cell test,
as developed by Rowe and Barden, can be used for
soils of very low permeability.
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Constant Head Test

• Apply a vacuum to the sample by opening valve C
  with valves A and B closed.
• Close valve C and open valves A and B and allow
  water to flow through the sample from the
  reservoir until steady state flow is achieved
  (the levels in the two manometers remain
  constant).
• Flow of water through the sample is controlled by
  adjusting valve A. Once the steady state flow has
  been achieved the quantity of water flowing ( Q )
  in a given time ( t ) is recorded together with
  the readings on the two manometers.
• The difference in the two manometer readings
  giving the head difference ( H ) over the sample
  length ( L).

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Constant Head Test
Now but ? Having found a value
for k the test is repeated several times at
different flow rates/heads and the average value
for k calculated.
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Example 1
A constant head permeameter test has been run on
a sand sample 250 mm in length and 2000 mm2 in
area. If the head loss was 500mm and the
discharge 260 ml in 130 secs determine the
coefficient of permeability and comment on the
drainage characteristics.
0.5 x 10-3 ms-1
Drainage Characteristics Good drainage
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Example 2
During a constant head permeameter test a flow of
173 ml was measured in 5 minutes. The sample was
0.1 m in diameter and the head difference of
0.061 m was measured between tapping points 0.2 m
apart. Determine the coefficient of permeability
and comment on the drainage characteristics of
the soil.
Answer 0.24 x 10-3 ms-1
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Falling Head Test

• This test is used with fine grained soils were
  the rate of flow of water is too small to be
  accurately measured using the constant head
  apparatus/test.
• The test is normally carried out on a 100mm dia.
  undisturbed sample. With the top and bottom
  filters in place the sample is stood in the water
  reservoir.
• The top of the sample/filter is connected to a
  glass standpipe of known diameter and the
  de-aired water contained in the standpipe allowed
  to seep through the sample. The height of the
  water (h1 , h2 , etc.) is recorded at several
  time intervals (t1 , t2 , etc.) during the test.

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Falling Head Test

• The procedure is then repeated using standpipes
  of different diameters and the average value of k
  computed.

or
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Example 1
In a falling head permeater test, the water level
in the standpipe was originally 1.584m above the
overflow, and dropped 1.0m in 15.2 minutes. The
sample was 0.1m long and 0.1m in dia, and the
area of the standpipe was 67 mm2. Calculate the
coefficient of permeability and comment on the
drainage characteristics of the soil.
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Example 2 (from Whitlow)
During a test using a falling head permeameter
the following data was recorded. Determine the
average value of k. Diameter of sample
100mm Length of sample 150mm Recorded data
Standpipe diameter d (mm) Level in Standpipe Level in Standpipe Time interval (t2 t1) (s)
Standpipe diameter d (mm) Initial h1 (mm) Final h2 (mm) Time interval (t2 t1) (s)
5.00 1200 800 82
5.00 800 400 149

9.00 1200 900 177
9.00 900 700 169
9.00 700 400 368

12.50 1200 800 485
12.50 800 400 908
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The End