lecture no.8 - bridge foundations

8/9/2019 Lecture No.8 - Bridge Foundations

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esign of Bridges

ovember 2011

SD 470

DESIGN OF BRIDGES

FOUNDATIONS FOR BRIDGES

Lecture No. 8


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Introduction

Functions of Foundations

The foundation system for a structure is the critical link in thetransmission of loads down to the ground. Bearing directly

soil, the foundation system must;

Distribute the loads so that the settlement of a structure is

negligible or uniform under all parts of the structure.

Anchor the superstructure to preent uplifting due to wind a

earth!uake forces.


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Aspects to be considered during the foundation design

The task is to be able to select and proide the right type of foundation which in

a choicefrom different types of foundations. #t is therefore imperatie to know

behaiourof foundations, constructionof foundations and costof foundation

also important to hae a correct choice of the ana!ysis"ethod#The right cho

results into considerable financia! saingand of course a safe design. The ma

bridges on a highway are small and "ediu"si%es.


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Aspects to be considered during the foundation

'asy, si"p!eand !uick to construct types of bridge designs should be faoured

need for specia!ists. (ormally speed results in saing and this can be achieed +epetitie and simple procedures+eadily aailable construction "ateria!s+-imple foundation shapes+A leel bottom foundation+-imple detai!swhich can be ad$usted on site+$ori%onta! and ertica! surfacesfor foundations as much

as possible+For"&or'which could be reused seeral times and

epensie fa!se &or'should be aoided

"ediu" si%ed barsas ery large bars are difficult to bend


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The construction se(uencehas to be considered along with the "aintenanc

aspects such as de*silting.The possibility of f!oodsand flood debrisduring construction should be looke

because they are a problem at that stage. Apart from that scouris a ma$or ca

bridge failures during its serice time. 0ence foundations hae to be below the

scour !ee! or bed protection &or's ) gabions, rip rap, etc* hae to be used

are ine+pensie# The bed protection works slow the flow and silting occurs fi

oids in the process stopping the scouring action of water oreoer, strea"!i

edgeson pier aboe the foundation also reduce scour#


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,rocedure

-ite reconnaissance for access of the machines obstruction, traffic, etc, to the s

this case also soi!inestigation is conducted and the e+isting foundation

structuresin the neighbourhood and their perfor"anceis obsered. #n addition

en!uiries are made to the !oca! contractors# The ana!ysis of the resu!ts of the

reconnaissanceare conducted by )*

+-tudying the "aps#i.e. geological, topographical etc

+'amining aeria!photographs

+Discussing with geo!ogistsand hydrologists

+Analy%ing the hydrau!ic surey results


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4rocedure

#t is cheaper to diert the rierand construct in the dry. But when the conditionallow then foundations are constructed in cofferda"s#

The sheet pi!!ing used for cofferda"s can be !eft for scour protection# A!sosubsidence of a foundation is aoided by grouting the &ea' soi! areas )oby bridging the oids# In addition- spread foundation is better in resistingdifferentia! sett!e"ent# Soi! description shou!d be in accordance to standa&hich are !oca!!y used# Soi! data shou!d be obtained for depth of up to the!oading depth )pressure bu!bs*# The actua! soi! conditions shou!d be co"pto the !aboratory resu!ts# So"eti"es pi!e tests are done on the site to get

rea!istic data for design# On !ocations re(uiring pi!es- p!ate bearing tests "done to see the possibi!ity of e!i"inating use of pi!es#


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#t is always adisable that the designer should be ino!ed in the site inestig

whereby the designer gets the feedbac'-proposeslocations for inestigations

superises the inestigation.

Ground &atermay present a problem as it leads to)

. Reduced bearing capacity of substrata

* #ncreased !atera! pressure* educesstabi!ity ofs!opes and ban's

Therefore ground &ater !ee!should be known. But most ground &atercan

remoed and controlled by pu"ps#7here it is i"possib!e to de*water then

construction is done under &ater by mass concrete#

The other data is obtained from the bridgedec' designwhereby the following

determined)*

+ Nu"berof foundations+ /oadingon foundations+ Si%eof foundations+ /ocationof foundations

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Types of Foundations

There are two basic types of foundations)

S$A//O0 FOUNDATIONS1transfer the load to soil at the

base of the substructure

DEE, FOUNDATIONS1transfer loads far below the

substructure.

These foundations penetrate through 9poor5 soil until a

satisfactory bearing stratum is reached.

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bed rock

firm ground

Sha!!o& Foundations

or transferring building loads to underlying ground

ostly for firm soils or light loads

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Sha!!o& Foundations


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-hallow oundations

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g g

bed rock

weak soil

Deep oundations

*Deep foundations transfer loads far below the substructure*mostly used in weak soils or for heay loads

*These foundations penetrate

weak soil until a satisfactory

bearing stratum is reached.

*>sed to reach deeper layers

with greater bearing

stratum

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g g

Bearing ,i!es

Transfers load through the unstable surface soils to the denser soils below suc

bedrock

Friction ,i!es

* Depends on friction resistance between the soil it passes through and

surface of the pile.

* >sed in clay soils

* Drien 4iles ?


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Bearing ,i!es

Qu

V e r t i c a ll o a d

R o c k

F u n c t i o n :

T o t r a n s f e r

l o a d s t o a

s u i t a b l e

b e a r i n g

s t r a t u m b ym e a n s o f t i p

r e s i s t a n c e o f

t h e p i l e

E n d B e a r i n g P i l e

opu pAQ

Ap= bearing area of the pile at tipp

o= ultimate bearing capacity of the

rock

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Friction ,i!es

F u n c t i o n :

T o t r a n s f e r

l o a d s t o t h e

s o i l g r a d u a l l y

b y m e a n s o f

s i d e r e s i s t a n c e

a l o n g t h e

l e n g t h o f

t h e p i l e

F r i c

Pile capacity in granular soil

where

Qu= ultimate resisting capacity

Ap= area of cross section at tip

qdi= effective over burden pressure at the ith layer

D = size of the pile at tip

= unit weight of soil at toe

NrandNq= bearing capacity factors of the soil at toe

Ki= coefficient of lateral earth pressure at ith layer (it

varies from 1 to 3 in loose to medium sands)Si= surface area of the pile in the ith layer

i= angle of wall friction between pile and soil in the ith

layer, it may be taken equal = angle of internal friction of the soiln = number of different layers through which the pile rests

qndrpi

n

1idu NqDN2A)tanSKq(Q

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Re"ar's on ,i!e Foundations

,i!e foundations are suited for adoption in the fo!!o&ingsituations3

aailability of good founding strata below large depths of need to hae ery deep foundation beyond the limit of

pneumatic operations @usually depths beyond "/.: m or m founding strata underlying deep standing water, the strata

ery hard not permitting ease sinking of wells

economic factors deciding the use of piles as compared twells.

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4ile oundations

2!assification of ,i!es

4recast drien piles * soil displacement type

Drien cast in*situ piles * soil displacement typeBored cast in*situ piles * soil replacement type

Bored pre*cast in*situ piles * soil replacement type

Drien steel piles * soil displacement type

Spacing of ,i!es

* riction piles) spacing center to center, not less than

perimeter of the pile* 'nd bearing piles) spacing center to center, not less than

twice the least width of the pile

* enerally, 2./ the bigger dimension of the pile section

plan

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,i!e Foundation

Si%e of 2oncrete ,i!es(ot less than :.3/m diameter or e!uialent section area for bridge foundation i

riers, and not less than :.&:m diameter or e!uialent section for other location

Grade of 2oncrete

The grade of concrete shall not be less than 245 or mi ratio by olume of 63 6

Ra'e in ,i!es

The maimum rake normally should not be more flat than the following

* 1 in 6 for pile diameter :.3/m and more

* 1 in / for smaller diameter bored piles

* 1 in & for smaller diameter drien piles

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Safe /oad 2arrying 2apacity

The load of resistance shall be the lesser of the following two

alues;

>ltimate load carrying capacity based on the soil paramete

surrounding the pile diided by a suitable factor of safety

-tructural strength of the pile

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Deep oundations

Drien timber piles

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Tunneling

Mechanically stabilised Earth wall

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Sheet ,i!es

* sheets of interlocking steel or timber drien into the ground,

continuous sheet

ship

warehouse

sheet pile

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Sheet ,i!es

* resist lateral earth pressures

* used in ecaations, waterfront structures, etc

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Sheet ,i!es

used in temporary works

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Sheet ,i!es

interlocking sections

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Sheet pi!e

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2offerda"

sheet pile walls enclosing an area, to preent water seeping in

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/ands!ides

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G


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Ground I"proe"ent

I"pact Ro!!erto


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Ground I"proe"ent

Sheepsfoot Ro!!erto


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Ground I"proe"ent

S"ooth.&hee!ed Ro!!er

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Typica! Safety Factors

Type of Design Safety Factor ,robabi!ity ofFai!ure

'arthworks 1."*1./ 1=/::

etainingstructures

1./*2.: 1=1/::

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-oil (ailing

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4il d i i


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4ile driing

A pile group

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C i T f 4i


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Ceaning Tower of 4isa

ur blunders become monumentsE

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einforced 'arth 7allsF using geofabrics to strengthen the soil

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round #mproement

Big weights dropped from 2/ m,

compacting the ground.

Craters formed in compaction

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-oil Testing


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-oil Testing

Variety of Field Testing e!ices

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Dynamic


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Dynamic

lecture no.8 - bridge foundations

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