Lecture No.8 - Bridge Foundations
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Transcript of 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