Review of Soil Mechanics III

1
Review of Soil Mechanics III

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

2
Outline of Presentation

• Introduction
• Soil compressibility
• Soil strength

3
Soil Compressibility
4
Definitions of Settlements
L
Structure
S2
S1
Total settlement, S1 or S2
Differential settlement, ?S
Distortion
5
Total Settlement
Total settlement
Se immediate settlement (elastic deformation)
Sc primary consolidation settlement (due
to dissipation of excess pore water pressure)
Ss secondary consolidation settlement (due
to adjustment of soil fabric)
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Consolidation Process
Valve closed
P
Valve closed
A
S0 ??0 ?u0
S0 ??0 ?uP/A
(a) Initial condition
(b) At the moment of load
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Consolidation Process (Continued)
P
Valve opened
P
Valve opened
?(t)
?p
S?(t) ??k?(t) ?uP/A-k?(t)
S?p ??k?pP/A ?u0
(c) At a time, t
(d) At completion of consolidation
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Consolidation Test
Dial gauge
Load
Oedometer
9
Consolidation Curve
Time (log scale)
Stage I Initial compression
Stage II Primary consolidation
Deformation
tp
Stage III Secondary consolidation
10
Over-Consolidation Ratio
Highest ground surface in the past
h
Current ground surface
?
z
A
Preconsolidation stress (pressure) - the maximum
effective stress the soil has experienced in the
past
pc (or ?p) ?(hz)
OCR pc/?z
OCR gt 1 Overconsolidated soil OCR
1 Normally-consolidated soil OCR lt
1 Under-consolidated soil
11
Determination of Preconsolidation Stress from Lab
Results
g
e
a
b
?
Void Ratio, e
d
?
c
f
pc
Pressure, p (log scale)
12
Effect of Soil Disturbance
Lab consolidation curve
e0
Field consolidation curve
Void Ratio, e
Remolded specimen Consolidation curve
Disturbance increases
0.42e0
Pressure, p (log scale)
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e - logp Curve for Normally Consolidated Soil
Lab consolidation curve
e0
Virgin consolidation curve
Cc
Void Ratio, e
Cc Compression index
0.42e0
pc?z
Pressure, p (log scale)
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e - logp Curve for Overconsolidated Soil
e0
Lab consolidation curve
Virgin consolidation curve
Cc
Void Ratio, e
Lab rebound curve
Cr
Cr Recompression index
0.42e0
?z
pc
Pressure, p (log scale)
15
e - logt Curve for Secondary Consolidation
Void Ratio, e
C??e/log(t2/t1)
ep
?e
t2
t1
Time, t (log scale)
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Typical Compression Indices
For soils
Cc 0.1 to 0.8 and Cc 0.009(LL-10)
Cr Cc/5 to Cc/10
C?/Cc 0.01 to 0.07
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Primary Consolidation Settlement of Normally
Consolidated Soil
pc ??z
??
Void Ratio, e
Stress, ? (log scale)
H Thickness of soil layer
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Primary Consolidation Settlement of
Overconsolidated Soil
?z
?
?z
pc
??
pc
??
Cr
Cr
1
Void Ratio, e
1
Void Ratio, e
Cc
1
Stress, ? (log scale)
Stress, ? (log scale)
19
Rate of Consolidation
Hdr
H
Clay
Sand
For Ult60
For Ugt60
20
Soil Strength
21
Direct Shear Test
P
Porous stone
T
Soil
Shear box
Normal stress
Shear stress
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Direct Shear Test Data
Peak shear strength, tf
Shear Stress, t (kPa)
Residual shear strength, tr
Shear Displacement, d (mm)
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Mohr-Coulomb Failure Envelope
Shear stress, tf (kPa)
f
c
Normal stress, sn (kPa)
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Triaxial Shear Test
??
Deviator stress
?1?3??
?3
?1
?3
?3
Rubber membrane
Cell (confining) pressure
Drainage or pore pressure measurement or back
pressure
25
Triaxial Shear Test vs. Direct Shear Test
Triaxial shear test

• Complex but versatile
• Better representation of stress conditions
• Better way to determine soil strength

26
Total Strength Envelope
?1
?
?n
?
?f
?f
?3
?3
?
2?
c
?
?3
?1
?n
?1
27
Effective Strength Envelope
?
Effective strength
?
?
Total strength
?
u
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Undrained Shear Strength
?
?1
?3
?3
?1
cu or Su
?u0
?
Unconsolidated Undrained Test (UU)
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Unconfined Compression Strength
?
?1
cu qu/2
?1
cu or Su
qu unconfined compression strength
?u0
?
?30
?1qu
Unconfined Compression Test