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Steel Designers' Manual - 6th Edition (2003)
and ground beams of a piled foundation, and on the frictional sliding resistance
beneath the ground floor slab and the foundations. Therefore, the lateral loading on
the bearing piles would be insignificant.
Lateral load resistance from vertical bearing piles is particularly dependent on
soil type; in the extreme case of very soft soil, raking piles to an underlying more
competent soil or rock would be required to provide any significant resistance.
Lateral load resistance from vertical bearing piles also requires significant pile
displacement, of the order of many centimetres, so if this is unacceptable then the
designer should think of alternative foundation elements to provide it, such as
raking piles or embedded sheet pile perimeter walls.
Where the contribution to lateral loading resistance of vertical bearing piles is
vital and is an acceptable solution, the designer is recommended to use CIRIA
Report 103 Design of laterally loaded piles 14 and the textbooks by Poulos and
Davis, 15 or Tomlinson. 16
Methods of analysis
The two most extensively validated methods are the P–Y curve method and elastic
continuum analysis FE programs. Both are explained in CIRIA Report 103. 14
P–Y curves originate from instrumented lateral load tests carried out on 762 mm
OD tubular piles in the USA in the 1960s for offshore design. The models of load
resistance were derived from soil resistance distributions required to match the
bending stresses measured by the pile shaft strain gauge instrumentation, i.e. curvefitting
to match bending moment diagrams. The P–Y curve method is the only one
in which it is possible to allow for significant cyclic loading of piles. This is useful
for the structural design of the pile section, but does not give accurate displacements
because the single piles had no head restraint. It is explained in detail in the
American Petroleum Institute Code RP2A 4 and in computer programs like ‘ALP’
in the OASYS suite. 13
Assessment of soil properties
Bearing piles 879
There are no clear guidelines for the assessment of soil property values to be
adopted for the design of laterally-loaded piles. Many factors influence the actual
mobilized values. In particular, the disturbing effect of pile installation is difficult to
quantify. Most soils exhibit considerable loss of stiffness under the action of cyclic
loading, which is virtually impossible to relate to laboratory test data. Generally, it
is desirable to select upper and lower limits to the critical soil properties, and to
check the sensitivity of the design to variation within the chosen range.
Structural designers would be wise to specify their design requirements for soils
data carefully within a site investigation contract, so that the work includes soil tests
appropriate to the type of loading to be applied to the pile. For example, if granular
soils are found on site then in situ soil testing, such as static cone penetration