Pile Design and Construction Practice, Fifth edition

330 Piles to resist uplift and lateral loading
Table 6.5 Relationship of modulus of subgrade reaction (k 1) to undrained
shearing strength of stiff over-consolidated clay
Consistency Firm to stiff Stiff to very stiff Hard
Undrained shear strength (c u) kN/m 2 50–100 100–200 �200
Range of k 1 MN/m 3 15–30 30–60 �60
For most normally consolidated clays and for granular soils the soil modulus is assumed
to increase linearly with depth, for which
5
stiffness factor T ��
EI
n (in units of length)
h
(6.12)
where soil modulus K � n h � x/B (6.13)
Values of the coefficient of modulus variation n h were obtained directly from lateral loading
tests on instrumented piles in submerged sand at Mustang Island, Texas. The tests were made
for both static and cyclic loading conditions and the values obtained, as quoted by Reese
et al. (6.10) , were considerably higher than those of Terzaghi (6.9) . The investigators recommend
that the Mustang Island values should be used for pile design and these are shown together
with the Terzaghi values in Figure 6.20.
Other observed values of n h are as follows:
Soft normally-consolidated clays: 350 to 700 kN/m 3
Soft organic silts: 150 kN/m 3
Having calculated the stiffness factors R or T, the criteria for behaviour as a short rigid
pile or as a long elastic pile are related to the embedded length L as follows:
Pile type Soil modulus
Linearly increasing Constant
Rigid (free head) L � 2T L � 2R
Elastic (free head) L � 4T L � 3.5R
Brinch Hansen’s method (6.12) can be used to calculate the ultimate lateral resistance of
short rigid piles. The method is a simple one which can be applied both to uniform and
layered soils. It can also be applied to longer semi-rigid piles to obtain a first approximation
of the required stiffness and embedment length to meet the design requirements before
undertaking the more rigorous methods of analysis for long slender piles described in
Sections 6.3.4 and 6.3.5. The resistance of the rigid unit to rotation about point X in
Figure 6.21a is given by the sum of the moments of the soil resistance above and below this
point. The passive resistance diagram is divided into a convenient number n of horizontal
elements of depth L/n. The unit passive resistance of an element at a depth z below the