Geo-technical Design of Shallow Foundation

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Geotechnical Design ofShallow Foundations

Chapter # 03



GENERAL REQUIREMENTS OFFOUNDATION:- The only requirement is that it should not faili.e. it should work efficiently under allconditions of working.

Failures are of two types; Bearing capacity failure. Excessive settlement. So the foundation must be safe against both

the above failures and also it must be properlylocated. This proper orientation of a foundationcan be well explained with the followingexample;



Case:

Consider a footing ‘A’ say at depth D, and width B. the effective stress envelope isshown in Figure 01. The stresses within

the zone are within permissible limits.Latter on a foundation ‘B’ is constructedsuch that stress envelopes intersect. Nowthe stress at point ‘X’ will be the sum of

the affects of ‘A’ &’B’. This should notincrease than the bearing capacity of thesoil.



Diagram # 01

AB

X



Example

In Badami Bagh area, many tall buildingscollapse. Actually close to these, excavation was carried out and as a result stress pattern

under the old buildings changes and ultimatelycollapses. That is if latter on these changes areto be made, these must be designed alreadyand temporary supports must be given to

existing buildings, called asunderpinning. So for design of a footing, both settlement and bearing capacity are checked.



GENERAL REQUIERMENT FOR ANYDESIGN:-

By design we always mean that what are thestresses acting on a particular member and thecorresponding size of the members that theses

stresses can be carried out efficiently. Thegeneral principle of the design is to design thestructure into different elements. We are goingto discus only one element, i.e. foundation.

Designing is done in two stages; Analysis

Sizing.



STRUCTURAL ANALYSIS:-

In first step of every design, we analyze thestate of stress and see the strain due to thesestresses. In analysis we see the type of

loading, type of strain and the modes offailure. In foundation design these stressesare called as bearing capacity and strains assettlements.

So in foundation design, analysis means the

determination of bearing capacity andsettlement. We have various methods bothfield tests and empirical methods for finding bearing capacity and settlements.



SIZING:-

Step # 01(Material Selection)

Before going to sizing, we decide about thematerial to be used in the construction of thefooting e.g. wood, concrete, steel etc. it dependsupon the availability of the material and

economy. The cost of project mainly depends onit.

Since the foundation system is a very complexsystem, the construction material is not

homogeneous. It consists of soil and othermaterials (wood, concrete etc.). here we will takeconcrete only.



Step # 02 (Dimensioning)

Now using the data from analysis and thematerial selected the dimension are

chosen (i.e. thickness, width, depth ofpad) and the design is completed.

Step # 03 (Documentation)

Now the design is represented in the formof drawings and the constructionspecifications (i.e. procedure, problemsand solutions) are also mentioned.

SIZING:-



SPECIFIC DESIGN OF FOUNDATION:-

Design means the determination of;

Df =?

d =?

B =?

L =?

As=?

P

G.S.L

Df

BXL

B

L



Design Components

Design is divided into two parts; GEOTECHNICAL DESIGN:- The design that takes into account only the properties ofsoil is called as Geotechnical Design.

SCOPE OF DESIGN:- The scope of geotechnical design is; a) Df =? b) B =? c) L =? GOAL:- The goal of geotechnical design is; Bearing capacity. Settlement should be within permissible limits.



STRUCTURAL DESIGN:-

In design that takes into account thetechnical aspects related to concrete is

called as Structural Design.



STEPS OF GEO-TECHNICALDESIGN

1. Selection of the type of foundation system.

2. Fix the vertical location i.e. Df of the

foundation.

3. Bearing Capacity and Settlement Analysis

and from this a suitale value of !"d# i.e. the

design pressure.

$. %he dimensions in plane &B ' ()

*. Construction Specification.



Step No. 1:-Selection of thefoundation type:-

For this the follo+ing steps are ,ept in mind

%ype of structure and its re"uirements

Su soil profile at the site.

-verall impact on the environment. elative cost and construction facilities.

Broadly spea,ing the types of foundation are/

Shallo+ Foundations. Deep Foundations.

Floating Foundations.



Step No. 2:- DEPTH OO!ND"ION#1. For depth of foundation0 the follo+ing t+o

considerations are ,ept in mind02. echanical Consideration.

3. hysical Consideration.

$. n mechanical consideration +e chec, Bearing

Capacity and Settlement.*. For Bearing Capacity %er4aghi5s e"uation &for

general share failure) is applied i.e.

6. qult = ScCNc + γ Df Nq + Sγ0.5γBNγ

6. From this e"uation it is clear that +ith the samesoil0 the properties remain the same and if 75 is,ept constant0 then the Bearing Capacity goes onincreasing y increasing the depth0 ut economyis also given due regards.



For pure clay;

(qult )Net=ScCNc + γDf (Nq-1) + Sγ 0.5γBNγ

C 8 "u92

:c 8 *.6

i.e. for clays0 the depth effect is 4ero. But forsands there is effect of depth.

Similarly in the case of Settlements0 it goes

on decreasing y increasing the depth. S 8 Cc9 &1 ; eo) &<) log &=> ;? =)9&=>)



Phy$ical %e&ui'e(ent$:-

Follo+ing are the different physical re"uirements/

Footing should e elo+

%op organic soil.

Susceptile 4one. Surface erosion 4one.

Frost line.

Scour depth.

%here should e the specified edge distance i.e.

(evel Difference



Step No. 3:- BEARING CAPACITY ANALYSIS:-

FO B!A"NG CAPAC"#$%- qult = CNc + γDfNq + 0.5γBNγ "ult 9 F.-.S 8 safe gross B.C or safe B.C (qult )Net = CNc+ γDf (Nq-1) + 0.5γBNγ

&"ult ):et 9 F.-.S 8 safe net B.C For s"uare footing and circular footing/ qult = 1.&CNc + γDfNq + 0.5γBNγ

For pure clay

:c 8*.60 :"81 ' :@8> B.C calculated y these e"uations is called B.C +.r.t. shear.



)E"%ING *"P"*IT+ ,ITHS.P.T N /"0!ES:- n F..S system/ For s"uare footing0 qult ='N'B + 6(N' + 100)Df For very long footing0 qult =&N'B + 5(N' + 100)Df

here0 "ult 8:et ultimate earing pressure0&SF)

8ressure at ottom of footing in excess of the pressure at thesame level due to the +eight of soil immediately surrounding thefooting.

: 8Standard enetration %est.

B 8idth of footing. Df 8Depth of footing. f the ground levels on oth sides of footing are not e"ual0 D should e

measured from the lo+est ground level. f D B0 use D ; B for computation.



Correction factors for position of water levels

+ ' +5 8correction factors for position of +ater levels.

1. f +ater tale is at a depth 7B5 or greater from the ottom of footing then + 8 +5 8 1 f +ater tale is at ase of footing +5 8 1 + 8 >.* f +ater tale is at top + ' +5 8 >.* And in et+een the linear variation is made. s S. system0

For s"uare footing/ q* = 0.105N'B + 0.&1(N' + 100)Df +ith F.-.S 8 3 <ere B 'Df are in !m# and " 8 a.



:et allo+ale earing pressure in psf for maximum

settlement of 1# is

q, = '0(N-&) (B = 1 'B)' ( /0

here0 d 8 1 ; D 9 B 2

f : value is given and there is no information aout

E ' @ then from ayerhoff5s E 82 ; : 9 1>

And for the minimum density samd unit0

eight @ 8 11* G 12>pcf.



O% SETT0EENT "N"0+SIS:- = 2Cc (1 + l3)4 () l3 (73 +8 7) 73 here0 Cc 8compression index0 for normally consolidated clays. Cc 8>.>>H&(.( I 1>)

? 8initial overurden pressure at that level.&aove midIheightof consolidating layer)

73 8@&Df ; level). 8 7 8additional pressure at that level. 8 9 &B ; J) < 8length of strata &layer). f the soil is drained on top and

ottom as in the consolidation test0 half thic,ness should eused

e> 8natural void ratio of the soil in place.



Settle(ent f'o( S.P.T N alue:- = ' N 2B (B + 0.&)4' C0 C( q,

K 8 Settlement in !mm# Cd 8 Depth factor 81 9 d d 8 1.>33D 9 B 1.33 C+ 8 ater correction factor. 8 1 &al+ays) t is reflected in 7:5 values. "a 8 9 7 So0 = ' N 2B (B + 0.&)4' C(973) <ere0 Cd 8 1 9 d ' d 8 1 ; Df 9 B &.33) 1.33 %he Df used in the layers other than the first one is at the level of the layer under

consolidation. Similarly in L M 8 9 &B ; J)2 here0 J is the distance up to the c9l of layer from the ottom of footing

Geo-technical Design of Shallow Foundation

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