Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
to rock or to a stratum with high bearing capacity. The method includes the effect of soil- structure
interaction in estimating the neutral plane and drag load on a pile shaft. Alternatively, the neutral
plane can be conservatively taken as at the base of the lowest compressible layer (BD, 2004a).
The mobilised negative skin friction, being dependent on the horizontal stresses in the ground, will be
affected by the type of pile. For steel H-piles, it is important to check the potential negative skin
friction with respect to both the total surface area and the circumscribed area relative to the available
resistance (Broms, 1979).
The effective stress or β method may be used to estimate the magnitude of negative skin friction on
single piles (Bjerrum et al, 1919; Burland & Starke, 1994).
In general, it is only necessary to take into account negative skin friction in combination with dead
loads and sustained live load, without consideration of transient live load or superimposed load.
Transient live loads will usually be carried by positive shaft resistance, since a very small
displacement is enough to change the direction of the shaft resistance from negative to positive,
and the elastic compression of the piles alone is normally sufficient. In the event where the
transient live loads are larger than twice the negative skin friction, the critical load condition will be
given by (dead load + sustained live load + transient live load). The above recommendations are
based on consideration of the mechanics of load transfer down a pile (Broms, 1979) and the
research findings (Bjerrum et al, 1919; Fellenius, 1972) that very small relative movement will be
required to build up and relieve negative skin friction, and elastic compression of piles associated
with the transient live load will usually be sufficient to relieve the negative skin friction. Caution
needs to be exercised however in the case of short stubby piles founded on rock where the elastic
compression may be insufficient to fully relieve the negative skin friction. In general, the customary
local assumption of designing for the load combination of (dead load + full live load + negative skin
friction) is on the conservative side.
Poulos (1990b) demonstrated how pile settlement can be determined using elastic theory with
due allowance for yielding condition at the pile/soil interface. If the ground settlement profile is
known with reasonable certainty, due allowance may be made for the portion of the pile shaft over
which the relative movement is insufficient to fully mobilise the negative skin friction (i.e. movement
less than 0.5% to 1% of pile diameter).
9-12 March 2009