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 compressibility,
a f. This term is used in the equations:
I c � a f
B 0.7
and
a f � � pi
� q
(5.26)
(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
��i
�q is the increment of foundation pressure in kN/m 2
I c and a f are 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 N is
greater than 15 the Terzaghi and Peck correction factor should be applied, giving
N (corrected) � 15�0.5(N�15) (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 I c values in Figure 5.26 are based on the average N-values over
the depth of influence, z I, of the foundation pressure. The depth of influence is related to the
width of the loaded breadth B in Figure 5.27 for cases where N increases or is constant with
depth. Where N shows consistent decrease with depth, z f is taken as equal to 2B or the base
of the compressive layer, whichever is the lesser. The average N in 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
� i � q��B 0.7 � I c (in mm)
(5.30)
In an over-consolidated sand or for loading at the base of an excavation for which the
maximum previous overburden pressure was � vo and where q� is greater than � vo, the immediate
settlement is given by
� i � � q��2
3 � vo� B0.7 � I c (in mm)
where q� is less than � vo equation 5.31a becomes
� i � q��B 0.7 � I c
3
(in mm)
(5.31a)
(5.31b)