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Settlement
Total settlement (S=Se+Sc+Ss) Elastic or Immediate settlement
Important for all cohesionless soils with high permeability (k>10-3m/s)
Important in case of fine-grained soils including silts and clays with
a degree of saturation, S90% Final settlements reach during the construction stage itself due to highpermeability of soil
Consolidation settlement: all saturated for nearly saturated, finegrained soils.
Due to expulsion of pore water from the voids
Completely saturated soils
Time-dependent settlement
Secondary settlement (Creep) Due to distortion of the soil skeleton (internal rearrangement of soil
particles)
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ST(max)DST(max)
W=tilt
b=DSTij/lij
i j
lij
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Total and differential settlements of buildings
(MacDonald and Skempton, 1955)
No. Criterion Isolated
foundation
Raft
foundation
1 Angular distortion (bmax) 1/300 1/300
2 Differential settlement (DSTmax)
Clays
Sands
40 to 50 mm
30 mm
45 mm
32.5 mm
3 Total settlements (STmax)Clays
Sands
75 mm
50 mm
100 mm
62.5 mm
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Elastic theory
Simplified theory to model real soil behaviour
Soil is homogeneous and isotropic
Assumed linear relationship between stress and strain
Used to estimate immediate settlements of cohesive soiland total settlement of cohesionless soil
Uses Youngs modulus (Es) and Poissons ratio (m) of
soil
Produces satisfactory results when stress levels are lowrelative to the failure values
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Elastic settlement (Se)
Beneath the corner of a uniformly loadedflexible area
fS
ne IEBqS
2
1 m=qn=Net applied pressure on the soilB=Width of the footing
m=Poissons ratio of soil
Es=Elastic modulus of soil
If=Influence factor1 3
42 Method of superposition to estimate
the settlement at centre or a point
other than the corner
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Influence factor (If) (Terzaghi,19??)
H=Thickness of
compressible layer
Form=0.5, If=F1
Form=0, If=F
1+F
2
For 0
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Youngs modulus of soil (Es)
Laboratory testing method
Unconfined compression tests
Triaxial compression tests
Field testing method, using
Standard Penetration Test (SPT)
Static cone penetration test (CPT)
Pressuremeter test (PMT), etc.
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Plate load test
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Plate load test
Square footings in granular soils
In clayey soils
p
f
PfB
BSS =
2
)3.0(
)3.0(
=
fp
pfpfBB
BBSS where Bfand Bp arein meters
Bearing capacity:
quf=qup*Bf/Bp (Sands)
quf=qup (clays)
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Correlation of Eswith SPT N and
CPT qc (kPa)
Soil SPT CPT
Sand (Normally
consolidated)
500 (Ncor+15) 2 to 4qc
Sand (saturated) 250(Ncor+15)
Sand
(overconsolidated)
-- 6-30qc
Gravelly sand and
gravel
1200(Ncor+6) --
Clayey sand 320(Ncor+15) 3 to 6qc
Silty sand 300(Ncor+6) 1 to 2qc
Soft clay -- 3 to 8qc
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Range of Poissons ratio (Bowles, 1996)
Type of soil m
Clay (saturated) 0.4-0.5
Clay (unsaturated) 0.1-0.3
Sandy clay 0.2-0.3
Silt 0.3-0.35
Dense sand 0.2-0.4
Coarse sand 0.15
Fine grained sand 0.25
Rock 0.1-0.4
strainLinear
strainLateral=m
http://upload.wikimedia.org/wikipedia/commons/6/67/Poisson_ratio_compression_example.svg7/27/2019 Settlement of structure
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Settlement of footings in sand(Schmertmann et al. 1978)
D
= i
izi
dvbE
zI
qCCs |'
21
C1=Depth correction factor
C2=Creep correction factor
qb=Net applied pressure at footing base level
v|d=Effective overburden pressure at footing base level
Izi=Influence factor of layer i
Dzi=Depth of layer i
Ei
=Youngs modulus of layer i
Based on a simplified distribution ofvertical strain under the
center of a shallow foundation, expressed in the form of a strain
influence factor
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Settlement of footings in Sand
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Depth and time factors
sandsilicaidatedoverconsolforq
sandsilicaedconsolidatnormallyagedforq
sandsilicaedconsolidatnormallyyoungforq
ci
ci
ci
0.6
5.3
5.2
E =
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Design of a shallow foundation
Design a shallow foundation for a column in amultistoreyed framed structure with the following details.
Intended life of the structure=100 years
Vertical load on the column = 4000 kN
Depth of the foundation is restricted to 1.5 m due to siteconditions
Soil properties: Normally consolidated sand with bulkunit weight=17 kN/m3, f=30. GWT well below the
ground surface Cone tip resistance varies from 10 kPa at the ground
surface and increases at a rate of 2000 kPa/m below theground surface
Use a factor of safety of 3 against the shear failure
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Modulus of subgrade reaction
A conceptual relationship
between soil pressure
and deflection
Used in the structural
phase of foundation
design
Used for design of
continuous footings,
mats, and various typesof piling works
Ks=q/d
q
d
Ks(Secant)=q2/d2
Ks(Initial tangent)=q1/d1
d1
d2
q1
q2
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Contact pressure
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Uniformly loaded rigid strip footing resting on perfectly
elastic, homogeneous, and isotropic subgrade
Soil in elastic state
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Smooth rigid footing on a
real, elastic soilSmooth rigid footing on a
cohesionless soilSmooth rigid footing on a
soil with intermediate
characteristics
Cu contact pressure distribution when footing is loaded to ultimate value
Elastic state
Semi plastic to plastic state
plastic equilibrium
Soil stress states: