Bearing capacity of foundation of different sizes of foundation at different depthAs per IS:6403-1981 Cl. 5.1.2

In case of general shear failure

qnet safe =1/F[ CNcScdcic + q(Nq-1)Sqdqiq + 0.5BgNgSgdgigRw']

In case of local shear failure qnet safe = 1/F[0.67{CN'cScdcic} + q(N'q-1)Sqdqiq + 0.5BgN'gSgdgigRw']

Considering the worst case as fully submersed i.e. water table raises upto the ground surfaceWhere qnet safe = Safe bearing capacity

C = Cohesion B = Width of the foundationg = Bulk unit weight

Nc, Nq, Ng = Bearing capacity factors dc,dq,dg = Depth factors

Sc, Sq, Sg = Shape Factors ic, iq, ig = Inclination factorsSHAPE FACTORS (IS:6403-1981), Clause 5.1.2.1, table-2Shape of foundation

i ) Continuous strip ii) Ractangleiii) Squareiv) CircularDEPTH FACTORS (IS:6403-1981), Clause 5.1.2.2

for F < 100 = 1 for F >100 = 1+ 0.1(Df/B)sqrtNF

for F < 100 = 1 for F >100 = 1+ 0.1(Df/B)sqrtNF

Now , C = 2.2 t/m2g= 1.800 t/m3 SqNF=

F = 0.0 gsub= 0.800 t/m3C'= 1.47

Bearing Capacity factors as per IS:6403-1981 Table 1 Clause 5.1.1

Using the Factor of Safety = 2.5

Sh

ape

of

fou

nd

atio

n

Dep

th o

f fo

un

dat

ion

Rw

qnet safe

from GSFailure

t/m2

qnet safe

from LSFailure

t/m3

SBC

T/ m2

B L Sc Sq Sy dc dq dy ic iq iy

1.50 1.50 2.00 0.50 1.30 1.20 0.80 1.27 1.00 1.00 -- -- -- 7.45 4.97 4.972.00 2.00 2.00 0.50 1.30 1.20 0.80 1.20 1.00 1.00 -- -- -- 7.06 4.70 4.70

2.50 2.50 2.00 0.50 1.30 1.20 0.80 1.16 1.00 1.00 -- -- -- 6.82 4.55 4.55

3.00 3.00 2.00 0.50 1.30 1.20 0.80 1.13 1.00 1.00 -- -- -- 6.66 4.44 4.44

1.50 1.50 2.50 0.50 1.30 1.20 0.80 1.33 1.00 1.00 -- -- -- 7.84 5.23 5.23

2.00 2.00 2.50 0.50 1.30 1.20 0.80 1.25 1.00 1.00 -- -- -- 7.35 4.90 4.90

2.50 2.50 2.50 0.50 1.30 1.20 0.80 1.20 1.00 1.00 -- -- -- 7.06 4.70 4.70

3.00 3.00 2.50 0.50 1.30 1.20 0.80 1.17 1.00 1.00 -- -- -- 6.86 4.57 4.57

1.50 1.50 3.00 0.50 1.30 1.20 0.80 1.40 1.00 1.00 -- -- -- 8.23 5.49 5.49

2.00 2.00 3.00 0.50 1.30 1.20 0.80 1.30 1.00 1.00 -- -- -- 7.64 5.10 5.10

2.50 2.50 3.00 0.50 1.30 1.20 0.80 1.24 1.00 1.00 -- -- -- 7.29 4.86 4.86

3.00 3.00 3.00 0.50 1.30 1.20 0.80 1.20 1.00 1.00 -- -- -- 7.06 4.70 4.70

1.00 1.00

5.14 5.14

1+0.2(B/L) 1+0.2(B/L)

1.000

1.301.20

Siz

e o

f fo

un

dat

ion

(m

)

Shape factors Depth factorsInclination

factors

1.20

Square

BC Factors

1

0

1

0

CALCULATION OF BEARING CAPACITY FOR SHALLOW FOUNDATIONS

Local Shear failuer

dg

General Shear failuer

FROM SHEAR FAILURE CONSIDERATION

sc sq

dc

0.800.60

Nc=

Nq=

Ng=

1+0.2(D/B)sqrt(NF)

dq

sg1.00

1-0.4(B/L)

1.30

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Total settlement S = Si + Sc

Where Si = Immediate settlement

Sc = Consolidation Settlement

As per IS:8009 Part 1-1976 Cl. 9.2.3.2 Equation (11)

Si = (qB (1-m2) x I )/E x df x dr

Where m = poisson ratio = 0.5 for clay q = Applied bearing pressure

I = Influence factor(depands upon length (L ) to breadth (B ) ratio of the footings)

E = Modulas of elasticity

df = depth factor dr = Rigidity factor

Influence factor 1.00

Now let us considered the applied Safe Bearing Pressure =

The safe bearing pressure obtained from the local shear failure

Rigidity factor = 1.0

Fox depth factor has been taken 0.78 (avarage)

from Fig. - No-12 of IS: 8009:(Part I)1976

E = A x Cu = 750 X Cu ( Ref. V.N.S. Murty Eqn. 18.53)

E = 250(N+15) For saturated sand

E = 300(N+6) For silty sand

Immediate Settlements

Depth

of

foundati

on

appli

ed

pre

ssure

Pois

son

rati

o

Modula

s o

f

Ela

sti

cit

y

B L m E

1.50 1.50 2.00 4.97 0.50 1650 1.00 0.78 1.0 2.6412.00 2.00 2.00 4.70 0.50 1650 1.00 0.78 1.0 3.3362.50 2.50 2.00 4.55 0.50 1650 1.00 0.78 1.0 4.0313.00 3.00 2.00 4.44 0.50 1650 1.00 0.78 1.0 4.7261.50 1.50 2.50 5.23 0.50 1650 1.00 0.78 1.0 2.7802.00 2.00 2.50 4.90 0.50 1650 1.00 0.78 1.0 3.4752.50 2.50 2.50 4.70 0.50 1650 1.00 0.78 1.0 4.1703.00 3.00 2.50 4.57 0.50 1650 1.00 0.78 1.0 4.8641.50 1.50 3.00 5.49 0.50 1650 1.00 0.78 1.0 2.9192.00 2.00 3.00 5.10 0.50 1650 1.00 0.78 1.0 3.6142.50 2.50 3.00 4.86 0.50 1650 1.00 0.78 1.0 4.3093.00 3.00 3.00 4.70 0.50 1650 1.00 0.78 1.0 5.003

DETERMINATION OF SETTLEMENTS

Siz

e o

f

foundati

on

(m)

Infl

uence

facto

r

df dr

Sett

lem

ent

in

mm

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Consolidation Settlement

As per Terzaghi's theory of one dimensional consolidation

Sc= (Cc X H)/(1+e0) x (log (p0+ Dp) L oed df dr

Cc = Co efficient of consolidation

H = Thickness of the compressible layer

p0= initial overburden pressure

Dp = Increment in pressure

L oed = Oedometer correction

df = depth factor

dr = Rigidity factor

Rigidity factor = 1.0

L oed = Oedometer correction = 0.7 {IS:8009 (Part-I) -1976}

Consolidation Settlements

Depth

of

foundati

on

appli

ed p

ressure

Thic

kness o

f th

e

com

pre

ssib

le

layer

Cc/(1

+e

0)

B L

1.50 1.50 2.00 4.97 2.25 5.625 1.621 0.70 0.16 0.78 1.0 21.622.00 2.00 2.00 4.70 3.00 6.300 1.536 0.70 0.16 0.78 1.0 24.832.50 2.50 2.00 4.55 3.75 6.975 1.485 0.70 0.16 0.78 1.0 27.463.00 3.00 2.00 4.44 4.50 7.650 1.451 0.70 0.16 0.78 1.0 29.651.50 1.50 2.50 5.23 2.25 6.525 1.707 0.70 0.16 0.78 1.0 19.832.00 2.00 2.50 4.90 3.00 7.200 1.600 0.70 0.16 0.78 1.0 22.842.50 2.50 2.50 4.70 3.75 7.875 1.536 0.70 0.16 0.78 1.0 25.353.00 3.00 2.50 4.57 4.50 8.550 1.493 0.70 0.16 0.78 1.0 27.481.50 1.50 3.00 5.49 2.25 7.425 1.792 0.70 0.16 0.78 1.0 18.462.00 2.00 3.00 5.10 3.00 8.100 1.664 0.70 0.16 0.78 1.0 21.272.50 2.50 3.00 4.86 3.75 8.775 1.587 0.70 0.16 0.78 1.0 23.653.00 3.00 3.00 4.70 4.50 9.450 1.536 0.70 0.16 0.78 1.0 25.71

DETERMINATION OF SETTLEMENTS

Siz

e o

f

foundati

on (m

)

Init

ial overb

urd

an

pr.

t/m

2

Pre

ssure

incre

ment

t/m

2

Oedom

ete

r

corr

ecti

on

df dr

Sett

lem

ent

in m

m