Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
Bearing piles 875
friction and the ultimate base resistance capacities to be developed. Commonly, load
transfer curves are produced, which are plots of load resistance versus vertical movement
of the pile head for displacements ranging from zero to the ultimate limit or
to a permissible maximum value (e.g. 40 mm). These plots include mobilized
soil–pile shear transfer versus local pile movement and mobilized base resistance
versus tip movement.
Axial load transfer curves for clays and sands have been obtained from full-scale
research static pile load tests. These pile tests monitor both the load and strain at a
number of positions along the pile.Tests that have been performed on clays and fine
granular soils show the general behaviour (see Fig. 29.7).
It is found that, at any position along the pile, full shaft friction resistance is not
developed until the pile has moved axially (relative displacement between pile and
soil) to a magnitude in the range 7–10 mm. Once this movement is reached no
further additional wall friction resistance develops, and apart from any small reduction
in peak value of wall friction (due possibly to strain softening), the curve tends
to a nearly constant resistance. This constant resistance value can be assumed to be
the ultimate unit shaft friction of the soil at any level.
The behaviour of a pile in clay in base resistance is also shown in Fig. 29.7. It is
seen from the base resistance load versus pile tip deflection (equal to pile head
movement) curve that a greater pile tip deflection is required to achieve the ultimate
base resistance capacity than is required to develop the shaft friction capacity,
i.e. in excess of 40 mm.
The mobilization of the total axial resistance with increasing displacement for a
pile is obtained from the summation of the mobilized shaft friction resistance and
the mobilized base resistance (see Fig. 29.7).
R c = R s + R b
R s
Axial
resistance
R b
Fig. 29.7 Axial load transfer curves for soils
Total resistance
Shaft friction resistance
Base resistance
10 20 30 40
Axial displacement (mm)