Introduction to Group Piles

David M. Marks, E.I.T.Graduate AssistantRowan University

Group Piles

• Piles used in groups to transmit structural load to soil

• Pile caps are used to tie piles together

Pile Cap

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

Close Spaced PilesStress Bulb

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)

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

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.

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%

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

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

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

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

Consolidation Settlement of Group Piles

Step 3:• Calculate the stress increase at the middle

of each soil layer.pi = Qg [(Bg + zi)*(Lg + zi)]

Consolidation Settlement of Group Piles

Step 4:• Calculate the settlement caused by the

increased stress.si = [ei (1 + e0)]Hi

Consolidation Settlement of Group Piles

Step 5:• Total settlement of pilessg = si