Pile Group

PILE FOUNDATIONS UNIT IV

PILE GROUPS

Some Examples

Multistoried Building Resting on Piles

Some Examples

Piles Used to Resist Uplift Forces

Some Examples

Piles used to Resist lateral Loads

Pressure isobars of single pile

Pressure Isobars of Group of piles with piles placed farther apart

Pressure Isobars of Group of piles closely spaced

Typical Arrangement of Piles in Groups Spacing of piles depends upon the method of installing the piles and the type of soil

Minimum Spacing between Piles  Stipulated in building codes

 For straight uniform diameter piles - 2 to 6 d  For friction piles – 3d  For end bearing piles  passing through relatively compressible strata, the

spacing of piles shall not be less than 2.5d  For end bearing piles passing through compressible strata and resting in stiff clay - 3.5d  For compaction piles - 2d.

Pile Group Efficiency

g 

Qgu

Q

u

CAPACITY OF PILE GROUP  Feld’s Rule  Converse-Labarre Formula  Block failure criteria

FELD'S RULE  Reduces the capacity of each pile by 1/16 for

each adjacent pile

CONVERSE-LABARRE FORMULA

g  1

 n  1m  m  1n 90mn

Qgu  g Qu m = number of columns of piles in a group, n = number of rows, θ = tan-1( d/s) in degrees, d = diameter of pile, s = spacing of piles center to center.

Block Failure

c = cohesive strength of clay beneath the pile group, L = length of pile, Pg = perimeter of pile group, A g= sectional area of group, Nc = bearing capacity factor which may be assumed as 9 for deep foundations.

Settlement of Pile Group

Total Settlement

Elastic Settlement

Consolidation Settlement

Settlement of Pile Group  'Load transfer' method ('t-z' method)  Elastic method based on Mindlin's (1936)

equations for the effects of subsurface loadings within a semi-infinite mass.  Finite Element Method.

Settlement of a group is affected by • the shape and size of the group • length of piles • method of installation of piles and possibly many other factors.

Semi-Empirical Formulas and Curves  Vesic (1977)

S = total settlement, Sp = settlement of the pile tip, Sf = settlement due to the deformation of the pile shaft.

 Qp= point load,  d = diameter of the pile at the

base,  q pu - ultimate point resistance per unit area,  Dr = relative density of the sand,  Cw = settlement coefficient, = 0.04 for driven piles = 0.05 for jacked piles = 0.18 for bored piles,

 Qf = friction load,  L = pile length,  A = cross-sectional area of

the pile,  E = modulus of deformation of the pile shaft,  α = coefficient which depends on the distribution of skin friction along the shaft and can be taken equal to 0.6.

Fg = group settlement factor Sg = settlement of group, S = settlement of a single pile. Curve showing the relationship between group settlement ratio and relative widths of pile groups in sand (Vesic, 1967)

t-z Method

t-z Method 1. Divide the pile into any convenient

segments 2. Assume a point pressure qp less than the

maximum qb. 3. Read the corresponding displacement sp

from the (qp- s) curve. 4. Assume that the load in the pile segment

closest to the point (segment n) is equal to the point load. 5. Compute the compression of the

segment n under that load by

6. Calculate the settlement of the top of

segment n by

Pile subjected to Vertical Load

Load Transfer Mechanis m

t-z Method 7. Use the (τ - s) curves to read the friction in on

segment n, at displacement sn. 8. Calculate the load in pile segment (n – 1)by 9. Do 4 through 8 up to the top segment. The load

and displacement at the top of the pile provide one point on the load-settlement curve. 10. Repeat 1 through 9 for the other assumed values of the point pressure, qp .

Settlement of Pile Groups in Cohesive Soil CASE 1  The soil is homogeneous clay.  The load Qg is assumed to act on a fictitious footing at a depth 2/3L from the surface and distributed over the sectional area of the group.  The load on the pile group acting at this level is assumed to spread out at a 2 Vert : 1 Horiz

Settlement of Pile Groups in Cohesive Soil CASE 2  The pile passes through a very weak layer of depth L1 and the lower portion of length L2 is embedded in a strong layer.  In this case, the load Q is assumed to act at a depth equal to 2/3 L2 below the surface of the strong layer and spreads at a 2 : 1

Settlement of Pile Groups in Cohesive Soil CASE 3  The piles are point bearing piles.  The load in this case is assumed to act at the level of the firm stratum and spreads out at a 2 : 1 slope.

Allowable Load in Groups of Piles

1. Shear failure 2. Settlement

Negative Skin Friction

Negative Skin Friction on Piles

Negative Skin Friction on Piles

Negative Skin Friction on Piles

Occurrence of Negative Skin Friction  If the fill material is loose cohesionless soil.  When fill is placed over peat or a soft clay stratum  By lowering the ground water which increases the

effective stress causing consolidation of the soil with resultant settlement and friction forces being developed on the pile

Magnitude of Negative Skin Friction  Single pile – Cohesionless Soil

 Single pile – Cohesive Soil

 Ln = length of piles in the compressible material  s = shear strength of cohesive soils in the fill  P = perimeter of pile  K = earth pressure coefficient normally lies between the active and the

passive earth pressure coefficients  δ = angle of wall friction

Negative Skin Friction on Pile Group

n = number of piles in the group, γ = unit weight of soil within the pile group to a depth Ln, Pg = perimeter of pile group, A - sectional area of pile group within the perimeter Pg s = shear strength of soil along the perimeter of the group.

Negative Skin Friction on Pile Group

L1 = depth of fill, L2 = depth of compressible natural soil, s1, s2 = shear strengths of the fill and compressible soils respectively, γ1, γ2= unit weights of fill and compressible soils respectively, Fnl = negative friction of a single pile in the fill, Fn2 = negative friction of a single pile in the

Uplift Capacity

Uplift Capacity  Pul = uplift capacity of pile,

 W p= weight of pile,  fr = unit resisting force  As = effective area of the Cohesive Soil

embedded length of pile.  cu = average undrained shear strength of clay along the pile shaft  α = adhesion factor  ca = average adhesion

Uplift Capacity of Pile Group

L = depth of the pile block L & B = overall length and width of the pile group cu = average undrained shear strength of soil around the sides of the group W = combined weight of the block of soil enclosed by the pile group plus the weight of the piles and the pile cap.

Uplift of a group of closelyspaced piles in cohesive soils

Uplift Capacity of Pile Group

Uplift of a group of closelyspaced piles in cohesionless soils

Recap  Capacity of single pile

 Capacity of pile group  Settlement of pile group  Negative Skin Friction  Uplift Capacity