BEARING CAPASITY OFSOIL final

8/7/2019 BEARING CAPASITY OFSOIL final

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Bearing Capacity Of Shallow

Foundation


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Bearing Capacity Of Shallow Foundation

* A foundation is required for distributingthe loads of the superstructure on a largearea.* The foundation should be designed

such thata) The soil below does not fail in shear &b) S ettlement is within the safe limits.


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Basic Definitions :

1) Ultimate Bearing Capacity (qu) :The ultimate bearing capacity is thegross pressure at the base of thefoundation at which soil fails in shear.

2) Net ultimate Bearing Capacity (qnu) :It is the net increase in pressure at the

base of foundation that cause shear failureof the soil.

Thus, qnu = qu Df (ovrbruden pressure )


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3 ) Net Safe Bearing Capacity (qns) :It is the net soil pressure which can be

safely applied to the soil considering only shearfailure.

Thus, qns = qnu /FOS

FOS - Factor of safety usually taken as 2.00 - 3 .00

4) Gross Safe Bearing Capacity (qs) :It is the maximum pressure which the soil can

carry safely without shear failure.

qs = qnu / FOS + Df


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5 )Net Safe Settlement Pressure (qnp) :It is the net pressure which the soil can

carry without exceeding allowablesettlement.

6) Net Allowable Bearing Pressure (qna ):It is the net bearing pressure which can beused for design of foundation.Thus,

qna = qns ; if qnp > qnsqna = qnp ; if qns > qnp

It is also known as Allowable Soil Pressure

(ASP).


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M odes of shear Failure :

Vesic (1973) classified shear failure of soil under a foundation base into threecategories depending on the type of soil & location of foundation.

1) General Shear failure.2) Local Shear failure.3 ) Punching Shear failure


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General Shear failure

Strip footing resting on surface Load settlement curve

of dense sand or stiff clay* The load - Settlement curve in case of footing resting on surface of dense sandor stiff clays shows pronounced peak & failure occurs at very small stain.* A loaded base on such soils sinks or tilts suddenly in to the ground showing asurface heave of adjoining soil*

The shearing strength is fully mobilized all along the slip surface & hencefailure planes are well defined.* The failure occurs at very small vertical strains accompanied by large lateralstrains.* I

D> 65 ,N> 3 5, > 3 60, e < 0.55


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2) Local Shear failure -

* When load is equal to a certain value qu(1),* The foundation movement is accompanied by sudden jerks.* The failure surface gradually extend out wards from the foundation.* The failure starts at localized spot beneath the foundation & migrates out

ward part by part gradually leading to ultimate failure.* The shear strength of soil is not fully mobilized along planes & hence

failure planes are not defined clearly .* The failure occurs at large vertical strain & very small lateral strains.

* ID = 1 5 to 6 5 , N=10 to 30 , 0.7 5

Strip footing resting on surface Load settlement curve

Of Medium sand or Medium clay


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3) Punching Share failure -

* The loaded base sinks into soil like a punch.

* The failure surface do not extend up to the ground surface.

* No heave is observed.* Large vertical strains are involved with practically no lateral

deformation.

* Failure planes are difficult to locate 222


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Terzaghis Bearing Capacity Analysis Terzaghi (194 3 ) analysed a shallow continuous footing bymaking some assumptions


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* The failure zones do not extend above thehorizontal plane passing through base of footing

* The failure occurs when the down ward pressureexerted by loads on the soil adjoining the inclinedsurfaces on soil wedge is equal to upwardpressure.

* Downward forces are due to the load (=qu B) &the weight of soil wedge (1/4 B 2 tan)

* Upward forces are the vertical components of resultant passive pressure (Pp) & the cohesion (c)acting along the inclined surfaces.


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For equilibrium:

Fv = 01 B 2tan + quxB = 2Pp +2C Li sin 4

where Li = length of inclined surface CB( = B/2 /cos)

Therefore,qu B = 2Pp + BC tan - B 2tan ------ (1)

The resultant passive pressure (Pp) on the surfaceCB & CA constitutes three components ie. (Pp)r,(Pp) c & (Pp) q ,Thus,

Pp = (Pp) r + (Pp) c + (Pp) q


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qu B= 2[ (Pp) r +(Pp) c +(Pp) q ]+ Bctan - B2 tan

Substituting; 2 (Pp)r - rB 2tan 1 = B BNr2 (Pp)q = B D Nq

& 2 (Pp)c + Bc 1 tan 1 = B C 1 Nc;We get,

qu =C Nc + Df Nq + 0.5 B N

This is Terzaghis Bearing capacity equation fordetermining ultimate bearing capacity of strip footing.Where Nc, Nq & Nr are Terzaghis bearing capacityfactors & depends on angle of shearing resistance ()


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General Shear Failure Local Shear Failure

Nc Nq Nr Nc Nq Nr

0 5 .7 1.0 0.0 5 .7 1.0 0.0

15 12.9 4.4 2. 5 9.7 2.7 0.9

45 172.3 173.3 297. 5 5 1.2 3 5 .1 37.7


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Important points :

* Terzaghis Bearing Capacity equation is applicable

for general shear failure.* Terzaghi has suggested following empirical reduction toactual c & in case of local shear failureMobilised cohesion Cm = 2/ 3 CMobilised angle of m = tan 1 ( tan)

Thus, Nc ,Nq & Nr are B.C. factors for local shear failure

qu = CmNc+ Df Nq + 0.5 B Nr

* Ultimate Bearing Capacity for square & Circular footing - Basedon the experimental results, Terzaghis suggested followingequations for UBC

Square footing qu = 1. 3 c Nc + Df Nq + 0.4 BNr

Circular footing qu = 1 3 c1Nc + Df Nq + 0. 3 BNr


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Thus,

qu = c Nc + [ sub Df +( - sub )Dw] Nq + 0. 5 sub BNr

When, Dw =0

qu =c Nc + sub Nc + 0. 5 sub BNr

& when x = 0

qu = c Nc + Df Nq + 0. 5 sub BNr


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ii) When water table is located at depth y below base :

* Surcharge term is not affected.* Unit weight in term is = sub + y ( sub )

BThus,

qu = c Nc + Df Nq + 0. 5 B Nr When y = B ; W.T. at B below base of footing.

qu = c Nc + Df Nq + 0. 5 B Nr

Hence when ground water table is at b B, the equation is notaffected.


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Methods for improving bearingcapacity

VERTICAL DRAINSTO ACCELERATE THE RATES OF SETTLEMENT

HENCE TO DECREASE THEPRELOADINGTIMES, VERTICAL DRAINS ARE INSTALLED TOSHORTEN THE DRAINAGE PATHS. IT ISESPECIALLY EFFECTIVE IN PRIMARYCONSOLIDATION. PORE WATER PRESSURESDISSIPATE QUICKLY, T a Hdr2 , IN MOSTDEPOSITS kh>kv . IT IS NOT EFFECTIVE INORGANIC SOILS AND PEATS IN WHICHCOMPRESSIONS ARE DOMINATED BY

SECONDARY COMPRESSION


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SAND DRAINSTHEY WERE WIDELY USED BETWEEN 19 3 0 -1980 WITHDIAMETERS CHANGING BETWEEN 20 -60 CM AND WITH 1.5 TO 6M SPACING.


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Vibro-Compaction Densifying granular soil by inserting a vibrating probe

into the ground

Probe spacing ranges from 6 to 14 feet Suitable for sand with less than 15% fines (silt- and clay-size particles)

Vibrator is a torpedo shaped horizontally vibrating probe,10 to 15 feet long, and weighs about 2 tons.

The probe penetrates to the design depth under its ownweight assisted by water jetting


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The action of vibrator and water jetting reduce inter granular forces between soil particles allowing them tobecome denser

The vibrator starts at the bottom of the hole and raised totreat the next interval; the procedure is repeated as backfillsand is added

If backfill is not added, craters with diameters of 10 to 15feet can form around vibrator


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Settlement of foundation :a) Settlement under loadsSettlement of foundation can be classified as-

1.E

lastic settlement (Si):E

lastic or immediatesettlement takes place during or immediately after the construction of the structure. It is also known asthe distortion settlement as it is due to distortionswithin foundation soil.

2. Consolidation settlement (Sc): Consolidationsettlement occurs due to gradual expulsion of water from the voids at the soil. It is determined usingTerzaghi's theory of consolidation.

3. Secondary consolidation settlement (Ss): Thesettlement occurs after completion of the primaryconsolidation. The secondary consolidation is non-

significant for inorganic soils.


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Causes for foundation settlement

Elastic compression of the foundation and the underlying soil, alsocalled immediate settlement,may be one cause.

Plasic or inelastic compression of the underlying soil, called timedependent settlement or settlement due to consolidation.

Groundwater lowering is another major cause for settlement to occur.Repeated raising and lowering of the ground water particularly ingranular soils, tends to reduce the void volume and cause settlement of the ground surface. Prolonged lowering of water table may cause

settlement in fine soils. Vibrations caused by pile driving, machinery, blasting, etc may causesettlement, particulary in granular soils.

Other causes of settlement includes volume change of soils, groundimprovement and excavation for adajacent structure.


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Thus,Total settlement (s) = Si+ Sc + Ss

b) Settlement due to other causes

1. Structural collapse of soil.2. Underground erosion.3. Lowering of water table. .4. Thermal changes.5 . Subsidence etc.


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E lastic settlement of foundation :a) On Cohesive soilsAccording to schleicher, the vertical settlement

under uniformly distributed flexible area is,

Si = q B 1- 2/Es I

whereq -uniformly distributed load.B - characteristic length of loaded area,E s - modulus of elasticity of the soil.

- poisson's ratio.I - influence factor which dependent uponelastic properties of base & shape at base.Alternatively, the value of [1- 2/Es] I can bedetermined from the plate load test.


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b) On Cohesionless SoilsAccording to Stuartmann & Hartman immediate

settlement on Cohesionless soils is given by -

Where, C 1 -Correction factor for depth of foundationembedment

C2 - correction factor for creep is soils.

q - pressure at the level of foundationq -surcharge ( Df)E

s- modulus of elasticity = 766 N (KN/m 2) from SPT= 2q c from SCPT

!

(!ZB

Z S i E

I qqC C S

0

221


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Settlement of foundation on Cohesionless Soils

Settlement of foundations on Cohesionless soils aregenerally determined indirectly using the semi-empiricalmethods.

1. Static Cone Penetration method

In this, the sand layer is divided into small layers suchthat each small layer has approximately constant valueof the cone resistance. The average value of the coneresistance of each small layer is determined.The settlement of each layer is determined using the

following equation-

S = H/C Log ( 0 + ) / 0

Where, c = 1. 5 qc/ 0


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in which q C - static cone resistance0 - mean effective overburden pressure,

- Increase is pressure at center of layer due to net foundation pressure.

H - thickness of layer.The total settlement of the entire layer isequal to the sum of settlements of individual layers.

2. Standard Penetration TestIS 8009 (part I) 1976 gives a chart for the calculation of settlement per unit pressure as a foundation of the widthof footing & the standard penetration number.

3. Plate Load TestThe settlement of the footing can be determined fromthe settlement of the plate in the plate load test.


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Differential Settlement :* The difference between the magnitudes of settlements at any two points is known asdifferential settlement.* If there is large differential settlement

between various part of a structure, distortionmay occur due to additional momentsdeveloped.* The differential settlement may caused due

to tilting of a rigid base, dishing of flexible

base or due to non uniformity of loading.* If S 1 & S 2 are the settlements at two

points,then differential settlement is( = S 1 -S 2

Angular distortion = (S 1- S 2 ) / L = ( /L


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* It is difficult to predict the differentialsettlement.* It is generally observed indirectlyfrom the maximum settlement.* It is observed that the differential

settlement is less than5

0% of themaximum settlement is most of thecases.The differential settlement can bereduced by providing rigid rafts,founding the structures at great depth& avoiding the eccentric loading.


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Allowable Settlement* The allowable maximum settlement

depends upon the type of soil, the type of foundation & the structural framing system.

* The maximum settlement ranging from

20mm to 300mm is generally permitted for various structures.

* IS 1904-1978 gives values of the maximum& differential settlements of different type of building.


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Sand & hardClay

Plastic clay

M ax.Settle. Diff.Settl Angular distortion

M ax.Settle Diff.Settle.

Angular distortion

Isolatedfoundation

i) steel structii) RCC struct

5 0mm5 0mm

0.0033L0.001 5 L

1/3001/666

5 0mm75 mm

0.0033L0.001 5 L

1/3001/666

Raft

foundationi) steel structii) Rcc struct.

75 mm75 mm

0.0033L0.002L

1/3001/5 00

100mm100mm

0.0033L0.002L

1/3001/5 00

Theoretically, no damage is done to the superstructureif the soil settles uniformly.

However, settlements exceeding 150mm may causetrouble to utilities such as water pipe lines, sewers,

telephone lines & also is access from streets.


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BEARING CAPASITY OFSOIL final