Session 21 – 22 GROUP PILES

Course : S0484/Foundation EngineeringYear : 2007Version : 1/0

GROUP PILES

Topic:• Bearing Capacity of Group Piles• Group Efficiency• Piles in Rock• Consolidation settlement of Group Piles

GROUP PILES

Lg = (n1 – 1)d + 2(D/2)

Bg = (n2 – 1)d + 2(D/2)

Where:

D = pile diameter

d = spacing of pile (center to center)

GROUP PILES

GROUP EFFICIENCY

u

ug

QQ )(

Where:

= group efficiency

Qg(u) = ultimate load bearing capacity of the group pile

Qu = ultimate load bearing capacity of each pile without the group effect

GROUP PILES IN SAND

uug Qnnp

DdnnQ

nnpDdnn

...

422

..422

21

21)(

21

21

If < 1 Qg(u) = .Qu

If 1 Qg(u) = Qu

GROUP PILES IN SAND

GROUP PILES IN SAND

GROUP PILES IN SAND

GROUP PILES IN SAND

Summary:

1.For driven group piles in sand with d 3D, Qu(g) may be taken to be Qu, which includes the frictional and the point bearing capacities of individual piles.

2.For bored group piles in sand at conventional spacings (d 3D), Qg(u) may be taken to be 2/3 to ¾ times Qu (frictional and point bearing capacities of individual piles)

GROUP PILES IN SATURATED CLAY

GROUP PILES IN SATURATED CLAY

Calculation steps:1. Determine Qu = n1.n2 (Qp + Qs)

where:QP = 9 . cu . Ap (ultimate end bearing capacity of single pile)

QS = (.p.cu.L) (skin resistance of single pile)

2. Determine the ultimate capacity by assuming that the piles in the group act as a block with dimensional Lg x Bg x L as follow :- end bearing capacity of the block

QP’ = Ap . qp = Ap . cu . Nc* with Ap = Lg . Bg

- Skin resistance of the blockQS’= (pg.cu.L) = 2.(Lg+Bg).cu.L

- Daya dukung batas tiang grupQu = QP’ + QS’ Qu = (Lg . Bg) . cu . Nc* + 2.(Lg+Bg).cu.L

3. Compare the values obtained in step 1 and 2 the lower of the two values is Qg(u)

GROUP PILES IN SATURATED CLAY

GROUP PILES IN SATURATED CLAY

Problem:

The section of a 3 x 4 group pile layered saturated clay. The piles are square in cross section (14 in. x 14 in.). The center to center spacing, d, of the piles is 35 in. Determine the allowable load bearing capacity of the pile group. USE FS = 4

GROUP PILES IN SATURATED CLAY

PILES IN ROCK

CONSOLIDATION SETTLEMENT OF GROUP PILES

The Terzaghi formula is valid with some rules:

1.The consolidation settlement is occurred from the depth of 2/3 of pile length.

2.The stress increase caused at the middle of each soil layer by using 2:1 method

igig

gi zLzB

Qp

sat = 18 kN/m3

Cc = 0,3eo = 0,82

sat = 18,9 kN/m3

Cc = 0,2eo = 0,7

sat = 19 kN/m3

Cc = 0,25eo = 0,75

Problem:

A group pile with Lg = 3.3 m and Bg = 2.2 m as shown in the figure. Determine the consolidation settlement of the pile groups. All clays are normally consolidated.

CONSOLIDATION SETTLEMENT OF GROUP PILES

ELASTIC SETTLEMENT OF GROUP PILES

• VESIC

ELASTIC SETTLEMENT OF GROUP PILES

• MEYERHOF (Pile groups in sand and gravel)

ELASTIC SETTLEMENT OF GROUP PILES

• PILE GROUP SETTLEMENT RELATED TO THE CONE PENETRATION RESISTANCE

UPLIFT CAPACITY OF GROUP PILES

UPLIFT CAPACITY OF GROUP PILES