Class notes of Geotechnical Engineering course I used to teach at UET Lahore. Feel free to download the slide show. Anyone looking to modify these files and use them for their own teaching purposes can contact me directly to get hold of editable version.

1. 1
Geotechnical Engineering–I [CE-221]
BSc Civil Engineering – 4th Semester
by
Dr. Muhammad Irfan
Assistant Professor
Civil Engg. Dept. – UET Lahore
Email: mirfan1@msn.com
Lecture Handouts: https://groups.google.com/d/forum/2016session-geotech-i
Lecture # 23
17-Apr-2018

2. 2
SOIL PERMEABILITY
A measure of soil’s ability to permit water to
flow through its pores/voids.
 Soils → solid particles with interconnected
voids
 water flows from point of high energy to
point of low energy
water
How does water flow through soil?

3. 3
SOIL PERMEABILITY
Different soils have different permeability.
water
Loose Soil Dense Soil
Easy to flow → high permeability Difficult to flow → low permeability
Coarse-grained soils
Fine-grained soils
→ high permeability
→ low permeability

4. 4
PERMEABILITY – Importance
 influences the rate of settlement under applied loads
 design of earth dams
 soil filters
 stability of slopes and retaining structures
 Underground water flow
 Wells/aquifers

5. 5
Why does the water flow?
Because the energy at A is higher than B
water
A
B
 Energy is dissipated in overcoming the soil resistance
 Causing head loss
A
B

6. 6
Bernoulli’s Equation
The total head at any point in water under motion is the summation of
pressure, velocity and elevation heads.
Pressure
Head
Velocity
Head
Elevation
Head
Velocity of flow through soils → extremely small
Velocity head can be ignored
Head = Energy per unit mass

7. 7
Bernoulli’s Equation
Total head at A:
Total head at B:
Head loss between A & B:
Head loss expressed in non-dimensional form:
The total head at any point in water under motion is the summation of
pressure, velocity and elevation heads.
Lih .
ℎ 𝐴 =
𝑢 𝐴
𝛾 𝑤
+ 𝑧 𝐴 =
𝑝 𝐴
𝛾 𝑤
+ 𝑧 𝐴
ℎ 𝐵 =
𝑢 𝐵
𝛾 𝑤
+ 𝑧 𝐵 =
𝑝 𝐵
𝛾 𝑤
+ 𝑧 𝐵

8. 9
HYDRAULIC GRADIENT IN FIELD
In the field, gradient/slope of the head is the head difference
over the distance separating the 2 wells.
X
HH
i


 21
Well-1 Well-2

9. 10
HYDRAULIC GRADIENT IN FIELD
Datum
hA = total head
W.T.
h = hA - hB
W.T.
Impervious Soil
Impervious Soil
hB= total head
A
B
L
h
L
hh
i BA 



)(
L
zA
pA/gw
zB
pB/gw

10. 11
Darcy’s Law
 Henry Darcy (1856)
 The velocity of flow through soil is proportional to
hydraulic gradient
Then the quantity of water flowing through the soil per unit
time is
v = discharge velocity
(i.e., the quantity of water flowing in unit time
through a unit cross-sectional area of soil)
k = hydraulic conductivity/permeability (units in L/T)
i = hydraulic gradient = h/L
Discharge
𝑣 ∝ 𝑖
𝑣 = 𝑘 ⋅ 𝑖
𝑞 = 𝑘 ⋅ 𝑖 ⋅ 𝐴[L3/T]= q = v. A

11. 12
Datum
hA = total head
W.T.
h = hA - hB
W.T.
Impervious Soil
Impervious Soil
hB= total head
A
B
L
zA
pA/gw
zB
pB/gw
A
h
kAikq 


LL
h
L
hh
i BA 



)(
WATER FLOW THROUGH SOILS

12. 13
WATER FLOW THROUGH SOILS
To determine the quantity of flow, two parameters are needed
* k = hydraulic conductivity
* i = hydraulic gradient
Determination of ‘k’
1- Laboratory Testing  [constant head test & falling head test]
2- Field Testing  [constant/falling head tests, pump out tests, etc]
3- Empirical Equations
Determination of ‘i’
1- From the head loss and geometry
2- Flow Nets
(how permeable is the soil medium)
(how large is the driving head)
A
h
kAikq 


L

13. 14
Seepage Velocity (vs) & Superficial Velocity (v)
Darcy’s law
v = k . i
v = velocity of flow
A
Voids
Solids
Av
As
Superficial velocity/Average velocity (v) → Average velocity of flow through soil
Seepage velocity/Actual velocity (vS) → velocity of flow through voids only
vS > v
v = n. vS
How to practically determine ‘v’?

14. 15
Seepage Velocity (vs) & Superficial Velocity (v)
v = Average
velocity/Superficial
velocity
vS = Seepage
velocity/Actual flow
velocity
v = n. vS

15. 16
Soil Classification w.r.t Permeability
Relative Permeability k value
(cm/sec)
Typical Soil
Highly permeable 1 x 10-1 Gravel and sand
Medium permeable 10-1 - 10-3 Sand with fines
Low permeable 10-3 - 10-5 Silts and silty sand
Very low permeable 10-5 - 10-7 Fine silts
Very very low
permeable/Impermeable
< 10-7 Clay

16. 17
CONCLUDED
REFERENCE MATERIAL
Principles of Geotechnical Engineering – (7th Edition)
Braja M. Das
Chapter #7 & 8