Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 265

Burland and Burbidge(5.19)have developed an empirical relationship between standard
penetration test N-values and a term they have called the foundation subgrade compress-
ibility, af. This term is used in the equations:

(5.26)

and

(in mm per kN/m^2 ) (5.27)

where Ic is a compressibility index

B is the foundation width

is the immediate settlement in mm

is the increment of foundation pressure in kN/m^2

Icand afare related to the standard penetration test results shown in Figure 5.26 for normally
consolidated granular soils. In very fine and silty sands below the water table where Nis
greater than 15 the Terzaghi and Peck correction factor should be applied, giving

N (corrected) 15 0.5(N15) (5.28)

Where the material is gravel or sandy gravel, Burland and Burbidge recommend a
correction:

N (corrected) 1.25 N (5.29)

It should be noted that the Icvalues in Figure 5.26 are based on the average N-values over
the depth of influence, zI, of the foundation pressure. The depth of influence is related to the
width of the loaded breadth Bin Figure 5.27 for cases where Nincreases or is constant with
depth. Where Nshows consistent decrease with depth, zfis taken as equal to 2Bor the base
of the compressive layer, whichever is the lesser. The average Nin Figure 5.26 is the
arithmetic mean of the N-values over the depth of influence.
In a normally consolidated sand the immediate average settlement, i,corresponding to
the average net applied pressure, q, is given by

(5.30)

In an over-consolidated sand or for loading at the base of an excavation for which the

maximum previous overburden pressure was (^) voand where qis greater than (^) vo, the imme-
diate settlement is given by
(5.31a)
where qis less than (^) voequation 5.31a becomes
i qB0.7 (5.31b)
Ic
3
(in mm)
i (^) q^2
3
(^) voB0.7Ic (in mm)
i qB0.7Ic (in mm)
q
i
af
pi
q
Ic
af
B0.7