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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
215
Effective Stress Concept
Topics Effective Stress Concept Effective Stress in Saturated
Soil with
no Seepage Effective Stress in Saturated Soil with
Seepage Seepage Force Filter Requirements and Selection of
Filter Material Capillary Rise in Soil Effective Stress in
Capillary Zone
Water
Water Table (W.T.)
Ground Surface (G.S.) Air
Voids
Solid
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
216
Effective Stress Concept Soil is a multi phase system To perform
any kind of analysis - we must understand stress distribution The
concept of effective stress:
The soil is loaded (footing for example) The resulting stress is
transmitted to the soil mass The soil mass supports those stresses
at the point to point contacts of the individual soil grains
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
217
The total stress at A is calculated from: The weight of the soil
above A The weight of the water above A = Hw + (HA - H) sat = Total
Stress at A w = Unit Weight of Water sat = Saturated Unit Weight HA
= Height of A to Top of water H = Height of water
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
218
is the stress applied to the soil by its own weight As you go
deeper in the soil mass, the stress increases The soil carries the
stress in 2 ways:
A portion is carried by the water (acts equally in all
directions)
A portion is carried by the soil solids at their point of
contact.
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
219
/= (P1v+P2v+P3v .....+Pnv) / A If as = a1 + a2 + a3 +...an Then
the space occupied by water = A - as Assume u = HAw HA = Height of
water = / + u(A - as) / A
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
220
Since as is very small, assume = 0 = / + u
Recall the following equation: = Hw + (HA - H) sat Now, / = - u
Substituting: / = [Hw + (HA - H) sat] - HAw Rearranging: / = (HA -
H)(sat - w) = Hsoil/ Effective Stress is independent of height of
water In the equation: = / + u / is the soil skeleton stress u is
the stress in the water, or pore water pressure
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
221
Effective Stress in Saturated Soil with no Seepage
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
222
Effective Stress in Saturated Soil with Seepage Upward flow
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
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223
Note that the h/H2 is the hydraulic gradient that caused flow
therefore,
wc izz JJV c c And limiting conditions may occur when 0 c c wc
izz JJV which lead to icr = critical hydraulic gradient
wcri J
J c
for most soils 0.9-1.1 ith average value of 1
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
224
Downward flow
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
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225
Seepage Force
Azp J c c1
AizforceseepageforceseepagepAizzp
w
w
JJJ
c c c 12 )(
AizforceseepageforceseepagepAizzp
w
w
JJJ
c c c 33 )(
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
226
The volume of the soil contributing to the effective stress
force equals zA, so the seepage force per unit volume of the soil
is
ww i
zAAiz JJ
in the direction of seepage (see the fig.) Therefore, in
isotropic soil and in any direction, the force acts in the same
direction as the direction of flow. Thus, the flow nets can be used
to find the hydraulic gradient at any point to find seepage force
at that point. This concept is useful to estimate F.S against
heave
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
227
Factor of Safety against heave at the downstream of hydraulic
structures Terzaghi (1922)
H1 H2
Sheet pile
Impermeable layer
Permeable layer
D
D/2
D
D/2
W c
U
Heave zone
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
228
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
229
Estimation of iav
a b c
e d
nd = 10
Dri
ving
hea
d
a b c
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
230
point driving head
a H104
b H
107.6
c H
105.2
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
231
Filter Requirements and Selection of Filter Material In
practice, for the safe of the hydraulic structure, a minimum value
of 4 to 5 for F.S against heaving is used, because of the
uncertainty in the analysis. One way to increase the F.S is using
filter. Filter:- is a granular material with opening small enough
to
prevent the movement of the soil particles upon which is placed
and, at the same time, is previous enough to offer little
resistance to seepage through it.
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
232
a b c
e d
nd = 10
D
D/2
W cW
U
D1
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
233
wav
F
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21
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21
.
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
234
Selection of Filter Material Capillary R
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
235
Capillarity rise in Soil
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
236
For pure water and clean glass = 0
wc d
Th J4 ?
For water T = 72 m.N/m
dhc
1v
the smaller the capillarity tube, the larger capillary rise
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
237
For soils, the capillary tubes formed because of the continuity
of voids have variable cross sections. The results of the
nonuniformity on capillary can be demonstrated as shown in the
fig.
Variation of S in the soil
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
238
Hazen (1930) give a formula to estimate the height of
capillary
10
)(eD
Cmmh c
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
239
Effective Stress in Capillary Zone The general relationship of
effective stress is u c VV For soil fully saturated by capillary
wchu J
For soil partially saturated by capillary wchSu J
100
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
240
Examples EXAMPLE1. Plot the variation of total and effective
vertical stresses, and pore water pressure with depth for the soil
profile shown below in Fig.
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
241
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
242
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
243
The values of v, u and /v computed above are summarized in Table
1.
Table 6.1 Values of v, u and /v in Ex. 1
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
244
EXAMPLE2. Plot the variation of total and effective vertical
stresses, and pore water pressure with depth for the soil profile
shown below in Fig.
Dry Sand
Gs = 2.66
Moist Sand Zone of capillary rise
Gs = 2.66
Saturated Clay
= 42%
A
H1 = 2 m
B
H2 = 1.8 m
C
H3 = 3.2 m
G.W.
Rock
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
245
Dry sand 84.1681.955.0166.2
e1G
ws
d J J kN/m3
Moist sand 58.1881.955.0155.0*5.066.2
e1SeG
ws
t J
J kN/m3
Saturated Clay 138.1
142.0*71.2
SGe s Z
66.1781.9138.11
138.166.2e1eG
ws
sat J
J kN/m3
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
246
Point v kN/m2 u kN/m2 /v kN/m2
A 0 0 0
0 33.68 B 2*16.84=33.68 - S w H2 = - 0.5*9.81*1.8 =
- 8.83 33.68-(-8.83) =
42.51
C 2*16.84+1.8*18.58 = 67.117 0 67.117
D 2*16.84+1.8*18.58+3.2*17.66 =123.68 3.2*9.81=31.39
123.68-31.39 = 92.24
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University of Anbar College of Engineering Civil Engineering
Department Iraq-Ramadi
Asst. Prof. Khalid R. Mahmood (PhD.)
247
The plot is shown below in Fig.
Variation of v, u and /v with depth
0
1
2
3
4
5
6
7
8
-20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Stress, kN/m2
dept
h, (m
)
Total stress
Pore water pressure
Effective stress