soil_mechanics ders notlari2

8/3/2019 Soil_mechanics Ders Notlari2

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BEARING CAPACITY OFSOILS

FOUNDATIONS

A foundation is theinterface between thebuilding structure and theground. Foundations aredesigned to distributeloads from buildings to a

suitable soil layer.

B

DfFooting

D: depth of footing

B: width of footing

f

Foundation

Shallow foundation Deep foundation

Shallow Foundation (Df B)

The basic principle for the design of shallowfoundations is to reduce the applied stresses onthe earth by providing a large bearing area directlyunder the load. Shallow foundations are mostlymade of reinforced concrete and are constructed

in many sizes and shapes to accommodatevarious loading conditions.

Types of shallow footings

(1) Individual footing (tekil temel)

(2) Strip-continuous footing (erit-srekli temel)

(3) Radial footing (radye temel)


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Individual footing (tekil temel)

Square or rectangular shape inplan view

1


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A geotehnical engineer must ensure that a

foundation satisfies the following two stabilitycondition

!...

1. Share failure do not occur under footing

2. Settlement of structure must be withintolerable limits

d e

b a

c

B

q

a

0W

b

c

c, makaslama direnci(kohezyon)

450

Df Dfd

Share failure of foundation

Definitions Ultimate bearing capacity (Nihai tama

gc): is the maximum pressure that the soilcan support

Ultimate net bearing capacity (Nihai nettama gc)(qult): is the maximum pressure

that the soil can support above its currentoverburden pressure Allowable (or safe) bearing capacity (izin

verilebirlir tama gc)(qa) : The bearingcapacity which consider factor of safety.

Factor of safety: is the ratio of the ultimate netbearing capacity to the allowable bearingcapacity

Standart penetrasyon deneyi (SPT)

Bu deney; sondaj tijlerine taklm, ortasndan ikiye ayrlabilen (yark) ve iindepirinten yaplm bir i tpn bulunduu bir rnekleyicinin, 63.5 kg arlndabirahmerdann 760 mm ykseklikten tijlerin zerine drlerek zeminesokulmas ilkesini esas alr.


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ahmerdan Darbe blou

SPT uygulamas

Deney sonunda SPT tp ve ar

SPT tp ve ar

SPT rnei


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(1) Overburden correction (CN) on SPT-N

0 0.5 1.0 1.5 2.0CN

0

100

200

300

400

500

Efektifrtgerilimi,

(kPa)

v

'

200.77logC

v

10N =

N1=CNxSPT-N

Allowable bearing

capacity qa isobtained from graphby using N1 for 25mm maximumsettlement

Groundwater

correction mustbe applied ifexist.

BD

D0.50.5C

f

w

w ++= qa=qaxCw

Groundwater correction Example:


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Derinlik(m)

SPT-Nv

(kPa)CN N1

0.75 8 * * *1.55 7 26 2.0 14

2.30 9 39 1.6 14

3.00 13 51 1.4 18

3.70 12 65 1.25 15

4.45 16 70 1.2 19

5.20 20 + + +

Ortalama N1: 16

Derinlik 1.55 m v = h = 17 x 1.55 = 26.3 26 kPa

Derinlik 4.45 m v =h hww = 17 x 4.45 (4.45 3.5) 10 = 66 kPa

B=3 m ve N=16 iin B>1 m kouluna ait grafikten, qa = 165 kPa

89.05.4

5.35.05.0 =

+=

wC qa = 0.89 x 165 = 150 kPa

Bearing Capacity of Cohesive

Soil (Terzaghis method)

qult = c Nc + q Nq+ 0.5 B NBu eitlikte;

c : Kohezyon

: Birim hacim arlk

B : Temel genilii

q : rt yk (=Df)

Nc, Nq, N : Boyutsuz tama gc faktrleri

qult = 1.3 c Nc + q Nq+ 0.4 B NSquare footing:

Strip-continuous footing:

Circular footing: qult = 1.3 c Nc + q Nq+ 0.3 B N

F

qq ulta = F: Factor of safety

50

45

40

35

30

25

20

15

10

5

0

0

0

0

0

0

0

0

0

0

0

0

11 10

100 1000

N, N , N (Tama gc faktrleri) c q

Meyerhof

Hansen

N

N

Nq

Nq

Nc

Nq = exp ( tan) tan2 (45 + /2)

Nc = (Nq 1) cot N = 1.8 (Nq 1) tan (Hansens)

N= (Nq 1) tan (1.4 ) (Meyerhofs)

For pure cohesive soil (=0)

qult=cNc

10

9

8

7

6

5

4

Ta

magc

faktr,N

c

Kare ve dairesel temel, B/L=1

Mtemadi (erit), B/L=0

0 1 2 3 4 5D / Bf

Fqq ulta =

F: Factor of safety


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Stress and Displacement

Point Load

BOUSSINESQ EQUATION

2/5

22zz)

)z/r(1

1(

z2

P3

+

=

r

z

Example

10 m

PA=10000 kNPB=50000 kN

C

2 m

8 m

2/5

22zz)

)z/r(1

1(

z2

P3

+

=

22/5

22zzm/kN1.7095.0x6.74)

)8/10(1

1(

82

10000x3==

+

=

2

22/5

22zz

m/kN8.3277.3201.7

m/kN7.320859.0x2.373))8/2(1

1(

82

50000x3

=+

==

+

=


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Circular area carrying uniform pressure

2/3

2z)

)z/r(1

11(P

+

=

P

D=diameter of circular areaD=2r

cz PxI=

Rectangular area carrying uniform pressure

rz PxI=

Fadum (1948)

xNxP005.0z =

scale

CONSOLIDATON


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Process by which an in causes a deformation of a soilmass by reducing its e (water seeps out of the voids).

How can increase?

1. Water level decreases

2. A load is induced (we will consider fills)

Consolidation


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Field vs. Lab

From Coduto 1998 (text)

Consolidation Curve - LAB

e

(log scale)p

eo yield

Preconsolidation pressure

e

(log scale)p

eo


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(1) ESTIMATION OF SETTLEMENT

Coefficient of volume compressibility (mv)

For any stress increment:

)ee

(

e1

1m 10

0

v

01

+

=

=v

mS


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Design Pressure of Footing Have to be selected by considering both,

Allowable bearing capacity,

Amount of settlement

soil_mechanics ders notlari2

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