introduction to group piles
TRANSCRIPT
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Introduction to Group Piles
David M. Marks, E.I.T.Graduate AssistantRowan University
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Group Piles
• Piles used in groups to transmit structural load to soil
• Pile caps are used to tie piles together
Pile Cap
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Load Bearing Capacity
• Extremely complicated due to close pile spacing and overlapping stresses
• Minimum center-to-center spacing– D = 2.5 x diameter of pile
• Efficiency of pile group = Qgu / Qu
– Qgu = Ultimate load bearing capacity of group– Qu = Ultimate load bearing capacity of each pile
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Close Spaced PilesStress Bulb
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Plan View
Lg
d
d
d
d
Bg
Number of Piles in Group = n1 x n2Note: Lg >= BgLg = (n1-1)d + 2(D/2)Bg = (n2 –1)d + 2(D/2)
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Simplified Analysis
• Piles may act in 2 ways– As a block: Lg x Bg x L– As individual piles
• As a block: Qgu = fav*pg*L– Fav = Average unit frictional resistance– Pg = 2(n1+n2-2)d + 4D– No. of piles = nn
– Spacing between piles = d
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Simplified Analysis II• As individual piles: Qu = p*L*fav
– perimeter of cross section of each pile = p = [2(n1+n2-2)d + 4D] [p*n1*n2]
• Qgu = ([2(n1+n2-2)d + 4D] [p*n1*n2])Qu
• If > 1; piles behave individually• If < 1; then,
– Qgu = Qu
• If >= 1– Qgu = Qu
• See Table 9.13 for other Group Efficiency Eqns.
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Feld’s Method: Sand Embeddedd
d
d
d
C
C
C
B
C
B
B
B
A
Pile Type
No. Piles
No. Adjacent
Reduction Factor
Ultimate Capacity
A 1 8 1- (816) 0.5Qu
B 4 5 1- (516) 2.75Qu
C 4 3 1- (316) 3.25Qu
6.5Qu = Qgu
= Qgu Qu
= 6.5Qu 9Qu = 72%
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Ultimate Capacity of Group Piles in Saturated Clay
• Determine Qu = n1n2*(Qp + Qs) (Eqn. 9.130)– Qp = Ap(9cu)– Qs = pcupL, where cup = undrained cohesion at pile tip
• Determine the ultimate capacity by assuming the piles act as a block (Lg x Bg x L)– Skin Resistance
pgcuL = (2Lg + Bg)cuL– Point Bearing Capacity
• Apqp = ApcupNc = (LgBg)cupNc
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Ultimate Capacity of Group Piles in Saturated Clay II
Qu = LgBgcupNc + 2(Lg + Bg)cuL (Eqn. 9.131)– Compare Eqn 9.130 and 9.131– The lower value is Qug
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Consolidation Settlement of Group Piles
Approximated stress distribution 2:1Procedure:Step1:L = Depth of Embedment of pilesQg = Total load acting on group
If pile cap is below surface, Qg is total load minus effective weight of soil removed
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Consolidation Settlement of Group Piles
Step 2:• Assume load (Qg) acts 2L/3 from top of
piles• Load spreads out along 2 vertical: 1
horizontal
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Consolidation Settlement of Group Piles
Step 3:• Calculate the stress increase at the middle
of each soil layer.pi = Qg [(Bg + zi)*(Lg + zi)]
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Consolidation Settlement of Group Piles
Step 4:• Calculate the settlement caused by the
increased stress.si = [ei (1 + e0)]Hi
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Consolidation Settlement of Group Piles
Step 5:• Total settlement of pilessg = si