Settlement Analysis II

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Settlement Analysis II

• Prof. Jie Han, Ph.D., PE
• The University of Kansas

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Outline of Presentation

• Introduction
• Based on Elastic Theory
• Based on In-Situ Tests
• Settlement of Mat Foundations

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Introduction
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Undrained versus Drained Modulus and Poissons
Ratio
Undrained elastic modulus and Poissons ratio
Clay and silt Ipgt30 or organic
Clay or sandy clay Iplt30 or stiff
Poissons ratio
Drained elastic modulus and Poissons ratio
Sand (OCR1)
Silty or clayey sand (OCR1)
Poissons ratio
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Drained Elastic Modulus from CPT Results
Soil Type
Es/qc
Young, NC clean silica sand (age lt 1000yrs)
2.5 3.5
Aged, NC clean silica sand (age gt 3000yrs)
3.5 6.0
OC clean silica sand
6.0 10.0
1.5
NC silty or clayey sands
3.0
OC silty or clayey sands
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Drained Elastic Modulus vs. SPT N60 Value

• Es ? (5N60)x100kPa (sands with fines)
• Es ? (10N60)x100kPa (clean NC sands)
• Es ? (15N60)x100kPa (clean OC sands)

(Kulhawy and Mayne, 1990)
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Drained Elastic Modulus vs. SPT N60 Value
?0
?1
Soil Type
(psf)
(kPa)
(psf)
(kPa)
Clean sands (SW SP)
100,000
5,000
24,000
1,200
Silty sands clayey sands (SM SC)
50,000
2,500
12,000
600
Use OCR 1 unless clear evidence of
overconsolidation
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Drained Elastic Modulus
For NC clays
For OC clays
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Based on Elastic Theory
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Settlement of a Circular Footing onan Elastic
Semi-Infinite Mass
Settlement of footing
q average vertical stress I 0.785 for rigid
footings or 1.0 for flexible footings E soil
modulus ? Poissons ratio d diameter of
circular footing
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Model and Definitions
Q
qQ/A
B
Layer depth, h
Es, ?
Bedrock or firm soil layer
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Settlement of a Flexible Footing
?
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Settlement of a Flexible Footing with An
Inclined Bedrock
B
p
b
a
c
Soil
H
E, v
?
Bedrock (incompressible)
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Numerical Analysis
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Settlement Influence Factor, I?, at Center of
Foundation
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Settlement Influence Factor, I?, at Shallow Side
Edge of Foundation
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Settlement Influence Factor, I?, at Deep Side
Edge of Foundation
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Based on Insitu Tests
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Strain Influence Factor
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Schmertmanns Method Based on In-situ Tests
- Mainly for cohesionless soil
Strain Influence Factor, Ie
Iep
0.1
0.2
zf/B0.5
zf/B1.0
1
2
L/B1
zf/B
L/Bgt10
3
4
zf depth below bottom of footing
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Schmertmanns Method Based on In-situ Tests
Settlement of footing
qn net applied pressure C1 depth factor, C2
secondary creep factor, C3 shape factor, t
time since application of load (yr) Ici
influence factor at midpoint of soil layer Icp
peak influence factor, ?zp initial vertical
effective stress at depth of peak strain
influence factor
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Settlement of Mat or Raft Foundations
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General Form of Settlement Equation
Settlement of footing
q average vertical stress IG Influence factor
considering Gibson soil IF Influence factor
considering foundation flexibility IE Influence
factor considering foundation embedment E soil
modulus ? Poissons ratio d diameter of
circular footing
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Gibson Soil and Influence Factor
d
kE
1
E0
Influence factor
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Flexibility Factor
Foundation flexibility factor (Brown, 1969)
E soil modulus Emat mat modulus ? Poissons
ratio a radius of circular mat t mat thickness
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Influence Factor
Influence factor can be approximately expressed by
Kf flexibility factor
Perfectly rigid
Kf gt 10
Intermediate flexibility
0.01 lt Kf lt 10
Perfectly flexible
Kf lt 0.01
Mayne and Poulos (1999)
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Influence Factor
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Influence Factor
Approximate influence factor (Mayne and Poulos,
1999)
d footing diameter zE embedment depth ?
Poissons ratio
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Settlement at Edges or Corners
Settlement at the edge of a circular foundation
Settlement at the corner of a square or
rectangular foundation
Mayne and Poulos (1999)