Shallow Foundations Settlement (Suite)
Transcript of Shallow Foundations Settlement (Suite)
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Shallow Foundations
– Settlement (Suite)
Reference:
Foundation Design, Principles and Practices,
Donald P. Coduto, Part B, Chapter 7
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Settlement Analysis Based on
Laboratory Tests
Approach used when good quality
“undisturbed” samples can be obtained from
soil
Perform consolidation test
Obtain Cc, Cr , e0, and ´p
Perform settlement analysis
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Two Methods of Analysis
Classical Method
Based on Terzaghi’s Theory
One dimensional compression
Skempton and Bjerrum Method
Considers distortion settlement
Uses an adjustment factor for 3D compression
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Foundation Rigidity Effects• For cylindrical steel water tank, the water inside the tank
weighs much more than the tank itself, and this weight is supported directly on the plate-steel floor. In addition, the floor is relatively thin, and could be considered to be perfectly flexible.
• A square spread footings is much more rigid than plate steel tank floors. Although the center of the footing "wants" to settle more than the edge, the rigidity of the footing forces the settlement to be the same everywhere.
• A mat foundation is more rigid than the tank, but less rigid than the footing. Thus, there will be some differential settlement between the center and the edge, but not as much as with a comparably-loaded steel tank.
• When performing settlement analyses on spread footings, we account for this rigidity effect by computing the settlement values beneath the center of the footing, then multiplying the result by a rigidity factor, r.
• Use of r = 1 is conservative.
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Classical Method; Foundation
Rigidity Factor
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Settlement Predictions
N.C. Clays
00
log1 z
zfcc H
e
Cr
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Settlement Predictions
O.C. Clays…… Case I
00
log1 z
zfrc H
e
Cr
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Settlement Predictions
O.C. Clays…… Case II
c
zfc
z
crc H
e
CH
e
Cr
log
1log
1 000
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Settlement Computation
We compute the consolidation settlement by dividing the soil beneath the foundation into layers, computing the settlement of each layer, and summing.
The top of first layer should be at the bottom of the foundation, and the bottom of the last layer should be at a depth such that:
'
010.0 zz
Unless the soil is exceptionally soft, the strain below this depth is negligible, and thus may be ignored.
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Classical
Method
The classical method
divides the soil beneath
the footing into layers.
The best precision is
obtained when the
uppermost layer is thin
and they become
progressively thicker with
depth.
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Thickness of Soil Sub-layers
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Example 7.3
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Example 7.3
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Example 7.4
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Example 7.4
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Distortion Settlement
The classical method is based on the
assumption that settlement is a one-dimensional
process in which all of the strains are vertical.
This assumption is accurate when evaluating
settlement beneath the center of wide fills, but it
is less accurate when applied to shallow
foundations, especially spread footings, because
their loaded area is much smaller.
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Distortion Settlement
Skempton and Bjerrum (1957) presented another method of
computing the total settlement of shallow foundations.
This method accounts for three-dimensional effects by
dividing the settlement into two components:
Distortion settlement, (also called immediate settlement,
initial settlement, or undrained settlement) is that caused by
the lateral distortion of the soil beneath the foundation.
Consolidation settlement, (also known as primary
consolidation settlement), is that caused by the change in
volume of the soil that results from changes in the effective
stress.
In addition, they accounted for differences in the way excess
pore water pressures are generated when the soil experiences
lateral strain. This is reflected in the parameter ψ.
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Distortion Settlement
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Skempton & Bjerrum Method
Settlement,
Distortion Settlement (d)
Consolidation Settlement (c)
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)(II
E
Bq
u
zDd
cd
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Influence Factors, I1 and I2
I1
D
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I2
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3D Adjustment Factor,
zh
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3D Adjustment Factor,
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Settlement Analysis based on In-Situ
Tests
Techniques for estimating settlements in sands are nearly always based on in-situ test results
In sands, settlement analysis is not performed based on consolidation analysis
Instead, we use Equivalent Modulus of Elasticity, Es
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Es from SPT Data (Table 7.4)
6010 NOCREs
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Es from CPT Data (Table 7.3)
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Schmertmann’s Method
s
zDE
HIqCCC )(321
C1 = depth factor =
C2 = Secondary creep factor =
C3 = Shape factor =
zD
zD
q
5.01
1.0log2.01
t
73.0/03.003.1 BL
30
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Strain Influence Factor
The greatest strains do not occur
immediately below the footing, as one
might expect, but at a depth of 0.5 B to B
below the bottom of the footing, where B
is the footing width.
This distribution is described by the strain
influence factor, I which is a type of
weighting factor.
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Strain Influence Factor
zp
zDp
qI
1.05.0
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Shmertmann method – Analysis procedure
1. Perform appropriate in-situ tests to define the subsurface
conditions.
2. Consider the soil from the base of the foundation to the depth
of influence below the base. This depth ranges from 2B for
square footings or mats to 4B for continuous footings. Divide
this zone into layers and assign a representative E, value to
each layer. The required number of layers and the thickness of
each layer depend on the variations in the E vs. depth profile.
Typically 5 to 10 layers are appropriate.
3. Compute the peak strain influence factor, Iεp using Equation …
4. Compute the strain influence factor, lε at the midpoint of each
layer. This factor varies with depth using Equations …
5. Compute the correction factors C1, C2 and C3 using equations
…
6. Compute the settlement using Equation …
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Example 7.6
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Example 8.1
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Example 8.1
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Simplified Schmertmann Method
When Es is uniform with depth over the
depth of influence
For square/circular footings:
s
pzD
E
BIqCCC )025.0)((321
For continuous footings:
s
pzD
E
BIqCCC )1.02)((321
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Settlements in Stratified Soils
When the soil profile primarily consists of
clays and silts – use procedures described in
Section 7.4 to estimate settlement
For clays/silts use laboratory consolidation tests
to determine Cc/(1+e0) and Cr/(1+e0)
For sands use Cc/(1+e0) or Cr/(1+e0) from Table
3.7
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Settlements in Stratified Soils When the soil profile primarily consists of
sands and silts – estimate settlement based
on Schmertmann Method
Use in-situ test data to determine Es for sand
Use following equation to determine Es for
clays
)1/(
30.2
0eCE
c
zs
)1/(
30.2
0eCE
r
zs
or
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Settlements in Stratified Soils
Alternative Approach
Conduct separate analysis for clays/silts and
sands
Add the computed settlements