UNIT IV
Some Examples
Some Examples
Piles Used to Resist Uplift Forces
Some Examples
Piles used to Resist lateral Loads
Pressure Isobars of Group of piles with
Typical Arrangement of Piles in Groups
Spacing of piles depends upon the method of installing the piles and the
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
Pile Group Efficiency
u
gu
g
Q
Q
CAPACITY OF PILE GROUP
Feld’s Rule
Converse-Labarre Formula
FELD'S RULE
Reduces the capacity of each pile by 1/16 for each adjacent pile
CONVERSE-LABARRE
FORMULA
mn
n
m
m
n
g90
1
1
1
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.
g
u
gu
Q
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.
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
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 Load Transfer Mechanis m Pile subjected to Vertical Load
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
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 slope.
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
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
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
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
Pul = uplift capacity of pile,
W p= weight of pile,
fr = unit resisting force
As = effective area of the 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 closely-spaced piles in cohesive soils
Uplift Capacity of Pile Group
Uplift of a group of closely-spaced piles in cohesionless
Recap
Capacity of single pile
Capacity of pile group
Settlement of pile group
Negative Skin Friction
