Design of single piles - Piling Engineering - page 155

Figure 4.42 Idealized variation of limiting resistance with depth.
of the pile.) This profile of P u is shown in Figure 4.42. The limit of 9 c u d at depth
is likely to be conservative, except for the case of piles in stiff overconsoldiated
clay, particularly if there is free water readily available to flow into any gap behind
the pile.
Adopting the profile of limiting force per unit length shown in Figure 4.42, charts
may be prepared giving the lateral capacity of a pile in terms of its geometry and limiting
plastic moment. The charts, shown in Figure 4.43, are similar to those presented by
Broms (1964), giving marginally higher failure loads as a result of taking P u equal to
2 c u d at the ground surface. Stewart (1999) has shown that the resulting pile capacities
are also very close to those obtained using the approach of Murff andHamilton (1993),
with the conservative assumption of weightless soil.
For other calculated variations of P u with depth, the expressions derived in
section 4.3 may be used to produce similar sets of charts to those shown in Figure 4.43.
For layered profiles, the procedure may involve some iteration choosing a value for the
depth of plastic hinge, following the principles described in section 4.3 to calculate a
value of H f , and then checking that the position of the hinge corresponds to the point
of zero shear force in the pile.
4.4 Deformation of single piles under lateral loading
The deformation of a pile under lateral load is generally confined to the upper part of
the pile, seldom extending beyond about 10 pile diameters below the ground surface.
This has led to a common - though imprecise - idealization of laterally loaded piles in
terms of equivalent cantilevers. The pile is replaced by a cantilever, fixed at some depth
determined by folklore and ignoring the soil support above that depth. The limitations
of this idealization are discussed further in section 4.4.3. More accurate modelling of
the response of a pile to lateral load is obtained by allowing for the soil support over
the upper part of the pile. Two alternative approaches are possible, one where the soil
is represented by discrete springs down the length of the pile, and one where a more
accurate, continuum model of the soil is adopted. The two approaches are discussed
in the following two sections.