Soil mechanics lecture 11 slope stability and theory

Soil Mechanics
CE-3323
Lecture 10 (b): Soil Permeability
Engr. Shahrukh Abbas
M.Sc. Geotechnical Engineering
Associate Lecturer
Department of Civil Engineering
Capital University of Science and Technology, Islamabad
shahrukh.abbas@cust.edu.pk
1

2
Water Flow Through Soils
• Determination of ‘k’
1 Laboratory Testing  [constant head test & falling head test]
2 Field Testing  [pumping from wells]
3 Empirical Equations
Determination of ‘i’
1- from the head loss and geometry
To determine the quantity of flow, two parameters are needed
* k = hydraulic conductivity (how permeable is the soil medium)
* i = hydraulic gradient (how large is the driving head)
2- flow nets
q  k  i  A  k 
h
 A
L
Today’s
discussion

Constant Head Permeability Test
• Primarily used for coarse-grained soils.
• A constant head of water is applied to each end of soil in a
“permeameter”.
• After a constant flow rate is established, water is collected in a
graduated flask for a known duration.
3
(ASTM D2434)

(ASTM D2434)
•The total volume of collected water may be
expressed as;
Q  Avt  A kit
k 
QL
Aht
Q = volume of water collected
A = x-sec area of soil specimen
t = duration of water collection
h
L
i  t
4
L 
h 


Q  Ak

k 
QL
Aht
5

Falling/Variable Head Permeability Test
(ASTM D5084)
• Mainly used for fine-grained soils but can
also be used for coarse-grained soils.
• Procedure is same as constant head test
except:
• Record initial head difference, h1 at
t=0.
• Allow water to flow through the soil
specimen.
• Record the final head difference, h2
at t=t2.
• Record the volume of water, Q (in
ml), collected at the outlet during
this time.
6

7
(ASTM D5084)
• Rate of flow of water through the specimen at
any time ‘t’can be given by;
h1 = head at the start of test
h2 = head at the end of test
k  2.303
aL
log
h1
At h2
Falling/Variable Head Permeability Test

Practice Problem #1
8
A constant head permeability test on a medium sand sample
having a x-sectional area of 7585mm2 yielded the following data.
Distance between stand pipes = 100 mm Constant head
difference = 70.4 mm Quantity of water collected = 500 x 103
mm3 Time of collection = 132 sec
Determine the coefficient of permeability of sand specimen.

Practice Problem #2
9
In a falling head permeability test, a soil sample of 7585mm2
cross-section and 210.2mm length was subjected to a flow of
water from a stand-pipe having cross-sectional area of
730mm2. The stand-pipe level changed from 1650mm to
550mm above reservoir datum during a time interval of
182sec. Determine the coefficient of permeability of soil.

Practice Problem #3
10
A constant head permeameter, 85 mm in diameter containing a
fine sand sample 450mm long, allowed water to flow at a rate
of 184ml/min under steady-flow conditions. Given the
difference in head between two points 240mm apart was
375mm, determine the coefficient of permeability in mm/sec.
When the same size sample is tested in a falling head apparatus
using a stand-pipe of 32.5mm diameter. Calculate the time
required for the water in the stand-pipe to drop from 1750mm
to 1000mm above outflow level to the nearest five seconds.

Determination of ‘k’ – Empirical Equations
11
Allen Hazen’s Method
Permeability of filter sands
k = C . (D10)2
k = coefficient of permeability (cm/sec)
C = empirical coefficient varying from 90 to 120; typically assumed as 100
D10 = effective size in cm
Applicability → k > 10-3 cm/sec
D10 ranging from 0.1 mm – 3 mm
Cu < 5
Permeability from Consolidation Test
k = CV . mV . γw
Applicability → Clays with k ≤ 10-7 cm/sec

Dependence of Hydraulic Conductivity (k)
12
1 Effect of Shape and Size of Particles
k = C . (D10)2 Allen Hazen’s equation i.e., coarser the soil, larger
would be permeability
2 Effect of Void Ratio
For sands, the following two equations hold good.
OR
i.e., larger the void ratio, greater would be permeability
2
e 2
k
1
 1
2 2
2 1 e 3
k e 2
k e 3
1 e
k 1 e
1
 1
 2

3 Effect of Stratification
Permeability parallel to the strata > permeability perpendicular to the strata
4 Effect of Degree of Saturation
Sample for permeability test → fully saturated.
Low degree of saturation → low permeability; because entrapped air
blocks water flow
5 Effect of Temperature
Temperature affects viscosity and density of pore fluid Higher the
temperature, higher will be permeability.
Lab tests are standardized at 20°C
13
Dependence of Hydraulic Conductivity (k)

Soil mechanics lecture 11 slope stability and theory

More Related Content

Similar to Soil mechanics lecture 11 slope stability and theory (20)

Recently uploaded (20)

Soil mechanics lecture 11 slope stability and theory