Pile Design and Construction Practice, Fifth edition

248 Pile groups under compressive loading

Values of the shape factors scand sfor centrally applied vertical loading are obtained
from Figure 5.7, and sqfrom the equation:

(5.2)

Inclined loading is considered in relation to the effective breadth Band the effective
lengthLof the equivalent block foundation. The plan dimensions of the block, as derived
by Meyerhof(5.5), are shown in Figure 5.8. Thus for loading in the direction of the breadth:

BB– 2 ex (5.3a)

where exis the eccentricity of loading in relation to the centroid of the base.

Similarly,

LL– 2 ey (5.3b)

The shape factors sare modified for inclined loading by the equations:

sCB 1 0.2iCBB/L (5.4)

sCL 1 0.2iCLL/B (5.5)

sqB 1 sin iqBB/L (5.6)

sqL 1 sin iqLL/B (5.7)

sB 1 – 0.4iBB/L (5.8)

sL 1 – 0.4iLL/B (5.9)

Where Bis less than L, approximate values of the shape factors for centrally applied
vertical loading which are sufficiently accurate for most practical purposes are

sq

sc(sc1)

Nq

Shape of base sc sq s

Continuous strip 1.0 1.0 1.0

Rectangle 1 0.2 B/L 1 0.2 B/L 1 – 0.4 B/L

Square 1.3 1.2 0.8

Circle (diameter B) 1.3 1.2 0.6

Values of the depth factor dcare obtained from Figure 5.9. The values on the right-hand
side of the figure are for D infinity. dqis obtained from

(5.10)

The depth factor dcan be taken as unity in all cases, also when  0 º, dq 1.0. Where
 is greater than 25ºdqcan be taken as equal to dc. A simplified value of dcand dqwhere
is less than 25ºis 10.35 D/B.The use of the depth factors assumes that the soil above
foundation level is not significantly weaker in shear strength than that of the soil below this
level. However, in the case of pile groups, the piles are usually taken down through weak
soils into stronger material, when either the depth factors should not be used or the depth

dq

dc 1

Nq