6. settlement of shallow footings
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6. Settlement of Shallow Footings. CIV4249: Foundation Engineering Monash University. (change of) Height Applied Load . Void Ratio Applied Stress. Oedometer Test. Particular Sample Measurements:. General Derived Relationship:. h. height vs time plots. height. h o. - PowerPoint PPT PresentationTRANSCRIPT
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6. Settlementof Shallow Footings
CIV4249: Foundation Engineering
Monash University
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Oedometer Test
• (change of) Height• Applied Load
• Void Ratio• Applied Stress
Particular Sample Measurements:
General Derived Relationship:
h
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height vs time plots
ho
heig
ht
log time
typically take measurements at 15s, 30s,1m, 2m, 3m, 5m, 10m, 15m, 30m, 1h, 2h,3h, 6h, 12h, 24h, 36h, 48h, 60h ….etc.
elastic primaryconsolidation secondary
compression
typically repeat for 12.5, 25, 50, 100, 200, 400, 800 and 1600 KPa
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Void ratio = f(h)
RelativeVolume
SpecificGravity
1
e 1.00
2.65
1 + e 1.917
e = 0.8
h = 1.9 cmdia = 6.0 cmW = 103.0 g
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Elastic Settlement
• Instantaneous component
• Occurs prior to expulsion of water
• Undrained parameters
• Instantaneous component
• Expulsion of water cannot be separated
• Drained parameters• Not truly elastic
Clay Sand
By definition - fully reversible, no energy loss, instantaneousWater flow is not fully reversible, results in energy loss, and time depends on permeability
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Elastic parameters - clay
Eu
• Soft clay• Firm clay• Stiff Clay• V stiff / hard clay
Eu/cu
• most claysnu
• All clays
• 2000 - 5000 kPa• 5000 - 10000 kPa• 10000 - 25000 kPa• 25000 - 60000 kPa
• 200 - 300
• 0.5 (no vol. change)
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Elastic parameters - sand
Ed
• Loose sand• Medium sand• Dense sand• V dense sand
nd
• Loose sand• Dense sand
• 10000 - 17000 kPa• 17500 - 25000 kPa• 25000 - 50000 kPa• 50000 - 85000 kPa
• 0.1 to 0.3• 0.3 to 0.4note volume change!
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Elastic Settlement
r = H s/E = H.ez
E
s
Hez
Q
Generalized stressand strain field
E
r = ez .dz0
¥
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Distribution of Stress
r
R z
Q
sz
sq
sr
• Boussinesq solution
e.g. sz = Q Is z2
Is = 3 1 2p [1+(r/z)2]5/2
Is is stress influence factor
y
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Uniformly loaded circular area
dq
dr
r
z
load, q
sz
aBy integration of Boussinesqsolution over complete area:
sz = q [1- 1 ] = q.Is [1+(a/z)2]3/2
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Stresses under rectangular area
• Solution after Newmark for stresses under the corner of a uniformly loaded flexible rectangular area:
• Define m = B/z and n = L/z• Solution by charts or
numerically• sz = q.Is
Is = 1 2mn(m2+n2+1)1/2 . m2+n2+2 m2+n2-m2n2+1 4p m2+n2+1
+ tan-12mn(m2+n2+1)1/2
m2+n2-m2n2+1
z
sz
BL
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0
1
2
3
4
5
6
7
8
0 0.05 0.1 0.15 0.2 0.25
L/B = 1L/B = 2L/B = 10
Total stress changeIs
z/B
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Computation of settlement
1. Determine vertical strains:
r
R z
Q
sz
sq
sr
y
2. Integrate strains:ez = 1 [sz - n ( sr + sq )] Eez = Q .(1+n).cos3y.(3cos2y-2n) 2pz2Er = ez .dz
0
¥
r = Q (1-n2 ) prE
ߥ
â¥
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Settlement of a circular area
dq
dr
r
z
load, q
sz
a
Centre :
Edge :
r = 4q(1-n2).apE
r = 2q(1-n2).aE
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Settlement at the corner of a flexible rectangular area
z
sz
BL
Schleicher’s solution
r = q.B1 - n2
EIr
Ir = m ln + ln 1p
1+ m2 + 1m
m+ m2 + 1
m = L/B
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nz
z
sz= q.I
s
x
Area coveredwith uniformnormal load, q
mzy z
Note: m and n are interchangeable
m = ocm = 3.0m = 2.5
m = 2.0m = 1.8
m = 1.6m = 1.4 m = 1.2
m = 1.0m = 0.9m = 0.8m = 0.7
m = 0.6
m = 0.5
m = 0.4
m = 0.3
m = 0.2
m = 0.1
m = 0.000.01 2 345 0.1 2 43 5 1.0 2 3 45 10
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
Is
VERTICAL STRESS BELOW A CORNEROF A UNIFORMLY LOADED FLEXIBLE
RECTANGULAR AREA.
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Settlement at the centre of a flexible rectangular area
B
L
B/2
L/2
rcentre = 4q.B 2
1 - n2
EIr Superposition for any
other point under the footing
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Settlement under a finite layer - Steinbrenner method
q
H
B
E
“Rigid”
X
Y
rcorner = q.B1 - n2
EIr Ir = F1 + F21-n
1-2n
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
2
4
6
8
10
L/B = 1
L/B = 2
L/B = 5
L/B = 10
L/B = ooL/B = 1
L/B = 2
L/B = 5
L/B = 10L/B = oo
F1
Values of F ( ) and F ( )1 2
Dept
h fa
ctor
d =
H/B
Influence values for settlement beneath the corner of a uniformly loadedrectangle on an elastic layer (Depth D) overlying a rigid base
F2
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Superposition using Steinbrenner method
B
L
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Multi-layer systemsq
H1B E1
“Rigid”
H2E2
r = r(H1,E1) + r(H1+H2,E2) - r(H1,E2)
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• A phenomenon which occurs in both sands and clays
• Can only be isolated as a separate phenomenon in clays
• Expulsion of water from soils accompanied by increase in effective stress and strength
• Amount can be reasonably estimated in lab, but rate is often poorly estimated in lab
• Only partially recoverable
Primary Consolidation
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0
1
2
3
4
5
6
7
8
0 0.05 0.1 0.15 0.2 0.25
L/B = 1L/B = 2L/B = 10
Total stress changeIs
z/B
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Pore pressure and effective stress changes
s¢i
s¢f
Ds = Du + Ds¢
At t = 0 : Ds = DuAt t = ¥ : Ds = Ds¢
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Stress non-linearity
qnet
z
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Soil non-linearity
0.40.50.60.70.80.9
11.11.2
10 100 1000
Clay
Cr
Ccp¢cs¢i s¢f
e
sv
r = S log + log Cr H1+eo
Cc H1+ec
p¢c
s¢i
s¢f p¢c
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Coeff volume compressibility
0.40.50.60.70.80.9
11.11.2
0 200 400 600 800 1000
Clay
(1+eo).mv
e
sv
r = Smv.Ds¢.DH
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Rate of Consolidation
Flowh = H Flowh = H / 2
T = cv ti / H2
U = 90% : T = 0.848
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Coefficient of Consolidation
• Coefficient of consolidation, cv (m2/yr)• Notoriously underestimated from
laboratory tests• Determine time required for (90% of)
primary consolidation• Why?
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Secondary Compression
• Creep phenomenon• No pore pressure change• Commences at completion of primary
consolidation• ca/Cc » 0.05
ca = Delog (t2 / t1)
r = log (t2/t1)caH
(1+ep)
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Flexible vs Rigid
stressstres
sdeflectiondeflection
F F
rcentre 0.8 rcentre RF = 0.8
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Depth Correction
0.5
0.6
0.7
0.8
0.9
1
0 2.5 5 7.5 10z/B
Dep
th F
acto
r Bz
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Total Settlement
rtot = RF x DF ( relas + rpr.con + rsec )
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Field Settlement for Clays(Bjerrum, 1962)
Pore - pressure coefficient
1.2
1.0
0.8
0.6
0.4
0.20 0.2 0.4 0.6 0.8 1.0 1.2
Settl
emen
t co
efficie
nt
Values on curves are DB
0.250.25
4
4
1.0
1.00.5
0.5
Over-consolidated Normallyconsolidated
Verysensitive
clays
CircleStrip
D
B
Clay layer
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Differential Settlements
Guiding values• Isolated foundations on clay < 65 mm• Isolated foundations on sand <40 mm
Structural damage to buildings 1/150(Considerable cracking in brick and panel walls)
For the above max settlement valuesflexible structure <1/300rigid structure <1/500
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Settlement in Sand via CPT Results (Schmertmann, 1970)
yearsin is 1.0
log2.01
5.01
102
01
121
t
tC
C
zEICCnlayer
layer
z
D¢
DD
ss
sr
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