Settlement of structure

7/27/2019 Settlement of structure

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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


10/20

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.svg


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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


15/20

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


19/20

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:

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Settlement of structure