shallow foundation 01

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SHALLOW FOUNDATIONSHALLOW FOUNDATION&&

RETAINING WALLRETAINING WALL

Part 01

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. .

SHALLOW FOUNDATION & RETAINING WALLContents

Part one :

Shear strength of soilsShear strength of soils

Bearing capacity of soilsBearing capacity of soils

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a ow oun a ona ow oun a on



SOIL MECHANICS & FOUNDATION ENGINEERING30 % UTS

Penilaian 40 % UAS

15 % kehadiran ≥ 9 kali

15 % keaktifan kelas/tugas

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Target

pencapaian

Mampu mendesain fondasi dangkal

Mampu mendesain dinding penahan tanah

Main References

Das, B.M. (2002). Principles of, ,

Brooks/Cole Thomson Learning

Das, B.M. (2004). Principles of FoundationEn ineerin 5th edition Brooks Cole

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Thomson Learning



Typical Geotechnical Project

Geo-Laboratory Design Officesoil ro ertiessoil ro erties

~ for testing ~ or es gn ana ys s

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Shallow Foundations

~ for transferring building loads to underlying ground

~ mostly for firm soils or light loads

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bed rock

firm

ground



Deep Foundations

~ for transferring building loads to underlying ground

~

P

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weak soil

I

L

E

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Shear failure

Soils generally fail in shear

strip footing

embankment

failure surface mobilised shear

resistance

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At failure, shear stress along the failure surfacereaches the shear strength.

Shear failure

The soil grains slide overeach other along the

failure surface

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a ure sur ace.

No crushing ofindividual grains.



Shear failure

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At failure, shear stress along the failure surface(τ) reaches the shear strength (τf ).

Mohr-Coulomb Failure Criterion

τ

f

φ

cohesion friction angle

τ

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τf is the maximum shear stress the soil can take without

failure, under normal stress of σ.

σc

σ



Mohr-Coulomb Failure Criterion

Shear strength consists of two components:

cohesive and frictional.τ

φ σ τ tan f f c +=τf

σ tan

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σc σf

c

component

c and φ are measures of shear strength.

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Higher the values, higher the shear strength.



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Transcosna Grain Elevator Canada

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West side of foundation sank 24-ft



Bearing capacity failure

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Bearing Capacity of Soils

Shallow foundation must have two main

characteristics :

have to be safe against overall shear failure

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Failure mechanism

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Failure zone

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Physical model

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Physical model

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General shear failure

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Local shear failure

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Punching shear failure

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Vesic, 1973

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General guidelines

Footings in clays - general shear

Footings in Dense sands (Dr > 67%)- general shear

Footings in Loose to Medium dense

(30%< Dr < 67%) - Local Shear

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Footings in Very Loose Sand (Dr< 30%)- punching shear



Bearing capacity formulas

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Soil bearing capacity

loadquq’ uQ (T, kN, Lb, Kips)

s e t t l e m e n t

A (m2, ft2)

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Ultimate bearing capacity (qu)

Allowable bearing capacity (qall)

Local shearfailure

General shearfailure

Q/A ≤ qall



Karl Terzaghi at Harvard, 1940

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Terzaghi Bearing Capacity Formulas

q = .D

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Terzaghi’s bearing capacity equation

General shear failure

NNNq B5,0qc qcu γγ++=

NNNq B4,0qc3,1 qcu γγ++=

NNNq B3,0qc3,1 qcu γγ++=

Continuous/strip footing

square footing

Df

γ,c,φ

γ1

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circular footing

Terzaghi : BDf ≤Bearing capacity factor=

==

γ

∑γ

NNN

D

,,

.qcohesionc

qc

f 1

Terzaghi Bearing Capacity Factors

2

θ = a

N

07.5 =′= φ when Nc

)2/45(cos2 φ ′+[ ]φ φ π θ

′′−= tan)360/75.0(expa

−

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⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −′

′= 1

cos2

tan2 φ

φ γ

γ

pK N

0tan >′′= φ

φ when N q

c



Bearing Capacity Factors

40

Nγ Nq

10

20

30

( d e g r e e s )

φ

Nc

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BEARINGCAPACITYFACTORS[AfterTerzaghiandPeck(1948)]

60 50 40 30 20 10 0 20 40 60 80

N andN

0

q c Nγ

φu = 0 Nq = 1, Nγ = 0 and Nc = 5.14

Terzaghi’s bearing capacity equation

For local shear failure : 2' 2' c

3c = φ=φ tan

3tan

NNNcq ''q

'c

'u

B5,0q γγ++=

NNNcq ''q

'c

'u

B4,0q3,1 γγ++=

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NNNcq ''q

'c

'u

B3,0q3,1 γγ++=



qallowable

(net)

=

B

Df

γ,c,φ

γ1

,,)gross(ult

qqq )gross(ult)net(ult −=

γγ+−+= NB4,0)1Nq(qcNc3,1

∑γ= Df 1.q

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FSqq )net(ult

)net(all =

General bearing capacity equationMeyerhof, 1963

FFF qiqdqs ,,FFFNFFFNFFFNq idsqiqdqsqcicdcscu

B5,0qc γγγγγ++=

: shape factor

: depth factorFFF

FFF

dqdcd

sqscs

,,

,,

γ

γ

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FFF iqici ,, γ : inclination factor



Nc Nq N

0 5.14 1.0 0.0

5 6.5 1.6 0.5

10 8.3 2.5 1.2

15 11.0 3.9 2.6

20 14.6 6.4 5.4. . .

25 20.7 10.7 10.8

30 30.1 18.4 22.4

32 35.5 23.2 30.2

34 42.2 29.4 41.1

36 50.6 37.7 56.3

38 61.4 48.9 78.0

40 75.3 64.2 109.4

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42 93.7 85.4 155.6

44 116.4 115.3 224.6

46 152.1 156.5 330.4

48 199.3 222.3 496.0

50 266.9 319.1 762.9

Shape, depth & inclination factors

Shape

Depth

or =

Fcs 1+0.2(B/L)

Fqs=Fγs 1

For ≥10o

Fcs 1+0.2(B/L)tan2(45+φ /2)

F =F 1+0.1 B L tan2 45+ 2

or =

Fcd 1+0.2(Df /B)

Fqd=Fγd 1

For ≥10o

Fcd 1+0.2(Df /B)tan(45+φ /2)

F =F 1+0.1 D B tan 45+ 2

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q γ

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −==

β

90FF o

o

qici 1

2

i 1F ⎟ ⎠

⎞⎜⎝

⎛ φβ

−=γ

InclinationInclination



Bearing capacity-water table

γ γ N BqNqcNcqult

4,03,1 ++=D1

Case-1

Case-1

γγ −+γ=water sat2wet1 DDq

γγγ =−=γ '

water sat

Df

d

∇⊇

D2

B

∇⊇Case-2

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⊇Case-3

Case-3 : 0≤

d≤

B

Df .q γ=

( )γγγ −γ+= '' _

B

d

Case-2

γγγ =−=γ '

water sat

wetf Dq γ=

Ultimate Load for Shallow Foundationunder Eccentric Load

The eccentricity is in the X-direction (ex)

B

ex

e2BB' −=

The effective area of plate is B’ times L

Effective width

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B/2 B/2

B eff

If the eccentricity is in the Y-direction (ey)

e2LL' −=Effective length



Foundation

Foundation is the part of structure to transmit the load

into the soil Adequate depth

Selection o f

foundation type

Soil Condition

Applied load

Bearing

capacity failure

Settlement

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Installation/cost

Quality/

adequate

strength

Cost efficiency

Shallow or Deep

Foundation

Flow chart

For designing

shallow foundationCPT, boring

start

Field investigation

Depth of Found.

Shallow foundation must have two main

characteristics :

have to be safe against overall shear failure

cannot undergo excessive settlement

Allowable bearingcapacity (qall)

.

q≤qall?

yes

no

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end

Settlement

safe?yesno

ConcreteStructure budget?

overOK



0

1

2

3

4

qc (kg/cm2)

0

1

2

3

4

0 50 100 150 200 250 300 350 400 450 500 550 600

total friction

0

1

2

3

4

0 0,01 0,02

friction ratio (%)

Menentukankedalamanfondasi

5

6

7

8

9

10

11

D e p t h ( m )

5

6

7

8

9

10

11

qc5

6

7

8

9

10

11

D e p t h ( m )

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12

13

14

15

16

0 20 40 60 80 100 1 20 140 160 1 80 200

12

13

14

15

16

tf (kg/cm)

12

13

14

15

16

⎟⎟ ⎞

⎜⎜⎛

⎟⎟ ⎞

⎜⎜⎛

+=φ −'c1' q

tan log38.01.0

Korelasi antara qc dengan φ dan c

N

q c

k

0cu

σ−=

σ0 = tegangan total

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σ’0 = tegangan efektif

Nk = 15 for electric cone

= 20 for mechanical cone



Footing Footing

Definition

Footings are structural members used to support

columns and walls and to transmit and distribute

their loads to the soil in such a way that the load

bearing capacity of the soil is not exceeded,

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, ,

rotation are prevented and adequate safety

against overturning or sliding is maintained.

Types of Footing Types of Footing

Wall footings are used to

support structural walls that

carry loads for other floors

or to support nonstructural

walls.

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Isolated or single footings

are used to support single

columns. This is one of the

most economical types of

footings and is used when

columns are s aced at

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relatively long distances.


om ne oot ngs usua y

support two columns, or

three columns not in a row.

Combined footings are used

when tow columns are so

close that single footings

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column is located at or near

a property line.




ant ever or strap oot ngs

consist of two single

footings connected with a

beam or a strap and support

two single columns. This

type replaces a combined

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economical.


ont nuous oot ngs

support a row of three or

more columns. They have

limited width and continue

under all columns.

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Ra ted or mat oundation

consists of one footing

usually placed under the

entire building area. They

are used, when soil bearing

capacity is low, column

loads are heavy single

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footings cannot be used,

piles are not used and

differential settlement must

be reduced.


e caps are t c s a s

used to tie a group of piles

together to support and

transmit column loads to the

piles.

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Shallow foundations

S uare footin

Strip/combine footing

Rectangular footing

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Mat foundation

Distribution of Soil PressureDistribution of Soil Pressure

When the column load P is

app e on t e centr c o t e

footing, a uniform pressure is

assumed to develop on the soil

surface below the footing area.

However the actual distribution of the soil is not uniform,

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the soil and degree of flexibility of the footing.



Distribution of Soil PressureDistribution of Soil Pressure

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Soil pressure distribution in

cohesionless soil.

Soil pressure distribution in

cohesive soil.

Mat footing

Jogja International hospital

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M

Q

B

L

M6

L.B

Q

Bq

2max +=

qmax

qmin M6Qq

2min −=

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.

shallow foundation 01

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