Geotechnical and Foundation Engineering
SCE5331
Geotechnical and Foundation Engineering
Dr. Hong Chengyu, Joey
Office: 301, Tel: 2176-1545
Email: [email protected]
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TOPICS & SYLLABUS:
Topic 1: Review of Soil Mechanics
Topic 2: Shallow Foundations
Topic 3: Lateral Earth Pressure and Retaining Walls
Topic 4: Pile Foundations
Topic 5: Subsoil Exploration
Topic 6: Slope Stability
Textbook: Braja M. Das. (2007). Principles of Foundation
Engineering, 6th Edition, ISBN 0-495-08246-5.
Reference book:
Foundation Design and Construction (2006), GEO
Publication No. 1/2006, 376 p.
2
2.1 Bearing Capacity of Shallow Foundation
General Concept
Terzaghis Bearing Capacity Theory
Factor of Safety
Modification of Bearing Capacity Equations for Water Table
The General Bearing Capacity Equation
Eccentrically Loaded Foundations
2.2 Settlement of Shallow Foundation
Stresses From Elastic Theory
Types of Foundation Settlement
Elastic Settlement Based on the Theory of Elasticity
Elastic Settlement of Foundations on Saturated Clay
Range of Material Parameters for Computing Elastic Settlement
2.3 Primary Consolidation Settlement and Creep Settlement
Primary Consolidation Settlement Relationships
Three-Dimensional Effect on Primary Consolidation Settlement
Vertical Stress Increase in a Soil Mass Caused by Foundation Load
Allowable Bearing Pressure in Sand Based on Settlement Consideration
Field Load Test
Tolerable Settlement of Buildings
3
Primary Consolidation Settlement Relationships
Three-Dimensional Effect on Primary Consolidation Settlement
Vertical Stress Increase in a Soil Mass Caused by Foundation Load
Allowable Bearing Pressure in Sand Based on Settlement Consideration
Field Load Test
Tolerable Settlement of Buildings
2.3 Primary Consolidation Settlement and Creep Settlement
4
3.14 Primary Consolidation Settlement Relationships (One-Dimensional Straining Vertical Compression Only)
einitialeratiovoide
e
estrainvertical
HdzS
o
o
z
cz
ifH
zpc
zc
;
1
constant
0
)(
conditionoedometerinsettlementSoedc
'''
''''
'''
)(2
1
)4(6
1
),,,,()(
mbt
bmtav
scoczz CCfb
')( zvz ma
5
cavc
czpc
cav
He
CHS
with
iiAreaclayedconsolidatnormallyFor
'
0
''
0
0
)(
'''
0
log1
:
)(
e'''
zoz
'
c'
o'
z
cC
es CorC
'
z)(i
)(ii
cavs
pc
cav
He
CS
with
iAreaclayedconsolidatoverFor
'
0
''
0
0
)(
'''
0
log1
:
)(
stresseffectiveverticialinitial
pressuredationpreconsoli
indexelasticswellingCC
indexncompressioCwhere
He
CH
e
CS
with
iiiAreasclayedconsolidatoverFor
o
c
es
c
c
avcc
cspc
avco
'
'
'
''
0
0
'
0
'
0
)(
''
0
''
/
log1
log1
:
)()(
conditionoedometerinsettlementSoedc
6
3.15 Three-Dimensional Effect on Primary
Consolidation Settlement
][ )3()1()3( Au7
22.3FigurefromratiosettlementK
conditionoedometerinsettlementS
KSS
oedc
oedcc
B: diameter of a circular
foundation or width of a
continuous foundation.
8
Primary Consolidation Settlement Relationships
Three-Dimensional Effect on Primary Consolidation Settlement
Vertical Stress Increase in a Soil Mass Caused by Foundation Load
(Self review)
Allowable Bearing Pressure in Sand Based on Settlement Consideration
Field Load Test
Tolerable Settlement of Buildings
Primary Consolidation Settlement and Creep Settlement
14
Stress due to a Concentrated Load
Boussinesg (1885) equation is
22
2/52
2 12
3
:
yxrwhere
z
rz
P
increasestressVertical
3.16 Vertical Stress Increase in a Soil Mass Caused by
Foundation Load (for Consolidation Settlement Calculation)
Self review
15
2/3
20
21
11
:
z
B
q
centrebelowincreasestressVertical
Stress due to a Circularly Loaded Area
16
Stress below a Rectangular Area
8.3
,
n)f(m,factorinfluence
)(2
)(3
:
00 0
2/5222
3
0
TableUse
z
Ln
z
Bm
I
Iqzyx
zdxdyq
cornerthebelow
increasestressverticalThe
L
y
B
x
17
Table 3.8 Variation of Influence Value I
18
Table 3.8 Variation of Influence Value I
19
Below any point
say O
20
21
Calculate stress increase
below the centre of a
rectangular area
8.3
28.3.
TableasSame
FigfromI
Iq
Iq
o
o
22
))(( zLzB
LBqo
23
)4(6
1 ''''bmtav
Calculate the average stress
increase of a soil layer
)84.5(
Example:
A flexible rectangular area measures 1.5m3m in plan. It supports a load of 100kN/m2. Determine the vertical stress increase due to the
load at a depth of 3.75 m below the center of the rectangular area.
Solution 1: using Table 3.8
Solution 2: using 2:1 method
kPa12.13
kPa6.12
24
cCoe
3.30
16.5
For a OC soil, c=60kPa, what is Sc ?
What is immediate
settlement for the
clay layer, Se ?
25
26
)84.5(
cavc
czpc
cav
He
CHS
with
iiAreaclayedconsolidatnormallyFor
'
0
''
0
0
)(
'''
0
log1
:
)(
27
28
22.3FigurefromratiosettlementK
conditionoedometerinsettlementS
KSS
oedc
oedcc
29
For each layer Hj, if mv and are constant with depth z, then:
jvjvcj HmHS'
In case of normally consolidated clay, using Cc:
jc
jvcj He
CHS
'
0
'
1
0
log1
For multi-layer Hj (j=1,2,3, n),
summation of settlements in all layers :
called
nj
j
cjc ss1 30
vU
ionconsolidatof
degreeaverageU
SUS
v
cvt
31
Settlement due to Secondary (Creep) Consolidation
Ht
tC
Ht
t
e
CHS
p
ezsc
1
2
1
2)(
log
log1
)/log(loglog 1212 tt
e
tt
eCC e
Ce
C
p
e 1
32
For Hong Kong Marine
Clays:
C = (0.3% to 1%) w (in %)
33
Why a clayey soil creeps?
Creep is due to
viscous adsorbed water (double layers) on clay particles
viscous re-arrangement/sliding/deformation of clay
particles/plates
viscous deformation of clay plates
Adsorbed water is NOT free water
Adsorbed water is NOT free to flow under gravity.
34
Under
effective
stress
Creep movement !
36
Creep always exists under the action of effective stresses
(loading), independent of the excess pore water (or pore
pressure).
Therefore, creep has nothing to do with the primary
consolidation.
Creep exists during and after primary consolidation.
Creep rate depends on stress/strain state:
Creep rate is large in a normally consolidated state.
Creep rate is small in a over-consolidated state. 37
Example 3.11:
a. Determine the primary consolidation settlement of a foundation with 1.5m 3m in plan.
b. Assume the pore water pressure parameter A for the clay is 0.6, estimate the
consolidation settlement considering the 3D effect.
c. Assume that the primary consolidation settlement is completed in 3 years. Also let
C=0.006. Estimate the secondary consolidation settlement at the end of 10 years.
39
3.17 Allowable Bearing Pressure in Sand Based on
Settlement Consideration
Meyerhof (1956) proposed a correlation for the net allowable bearing
pressure for foundations with SPT (N1)60.
Original Meyerhof method, for 25mm estimated maximum settlement:
)22.1)((28.3
128.399.7)/(
)22.1)((98.11)/(
2
60
2
)(
60
2
)(
)(
mmBforB
BNmkNq
mmeterinBforNmkNq
Dqq
allnet
allnet
fallallnet
Researchers observed Meyerhofs results are rather conservative. 40
Example 3.12:
A shallow foundation measuring 1.75m 1.75m is to be constructed over a layer of sand. Given Df = 1m; N60 is generally increasing with depth, the average value of N60 is 10. The
estimated elastic settlement of the foundation is 14.7mm. Use Meyerhofs method
(modified form by Bowles) to calculate the allowable bearing pressure of the sand.
qnet(all) = 115.6 kPa 42
Standard Penetration Test (SPT)
SPT N-Value:
Standard hammer weight is
622.72 N(62.3 kg or 140 lb)
Hammer drop height is
762 mm (or 30 in)
Number of blows for spoon
penetration of three 152.4mm
(6 in) is recorded
The blow number of last 2
penetrations (2 x 152.4=304.8mm)
is the SPT N-Value: (1) 152.4mm 4 blows
(2) 152.4mm 5 blows
(3) 152.4mm 7 blows
SPT N-Value=5+7=12 43
)(8.0 MPainNEs
44
Example 3.13:
A shallow square foundation for a column is to be constructed on sand. The foundation
must carry a net vertical mass of 102,000 kg. The standard penetration numbers (N60)
obtained from exploration are given in the figure. Assume that the depth of the foundation
will be 1.5m and the tolerable settlement is 25mm. Determine the size of the foundation.
Depth (m) N60
2 3
4 7
6 12
8 12
10 16
12 13
14 12
16 14
18 18
45
)(5
)()(
smallerwhicheverBorHz
averagez
zEE
is
s
Consider the non-homogeneous
nature of soil deposits:
3.18 Field Load Test
46
47
Plate load test simulates field loading conditions and predicts settlement on proposed
foundation. Bearing capacity and modulus of subgrade reaction.
48
where
clayintestsFor
PuFu )()(
:
platetestofwidthB
foundationproposedofwidthB
where
B
Bqq
soilssandyintestsFor
P
F
P
FPuFu
)()(
:
(Independent of the size of the plate)
platetestofcapacitybearingultimateq
foundationproposedofcapacitybearingultimateq
Pu
Fu
)(
)(
49
3.20 Tolerable Settlement of Buildings
ratiodeflectionL
EAlinereferencefrom
deflectionrelative
lij
)(
)jandipoints
betweendistanceis(
l
Sdistortionangular
pointssuccessivetwo
betweengradient
''
ij
T(ij)
50
In Hong Kong:
(a)25mm for important structures; (b) 50mm less important
(c) 100 mm for walk road, and (d) 200mm for gardens etc.
Prof. A.W. Skempton
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