Pile Design and Construction Practice, Fifth edition

218 Resistance of piles to compressive loads
progressive increase in negative skin friction on two precast concrete piles driven through
40 m of soft compressible clay and 15 m of less-compressible silt and sand. Reconsolidation
of the soft clay disturbed by pile driving contributed 300 kN to the drag-down load over a
period of 5 months. Thereafter, regional settlement caused a slow increase in negative skin
friction at a rate of 150 kN per year. Seventeen months after pile driving a load of 440 kN
was added to each pile, followed by an additional load of 360 kN a year later. Both these
loads caused yielding of the pile at the toe to such an extent that all negative skin friction
was eliminated, but when the settlement of the pile ceased under the applied load the
continuing regional settlement caused negative skin friction to develop again on the pile
shaft. Thus with a yielding pile toe the amount of negative skin friction which can be developed
depends entirely on the downward movement of the pile toe relative to the settlement
of the soil or fill causing the drag-down force. If the drag-down force is caused only by the
reconsolidation of the heaved soil, and if the pile can be permitted to yield by an amount
greater than the settlement of the ground surface due to this reconsolidation, then negative
friction need not be provided for. If, however, the negative skin friction is due to the
consolidation of recent fill under its own weight or to the weight of additional fill, then
the movement of the ground surface will be greater than the permissible yielding of the pile
toe. Negative skin friction must then be taken into account, the distribution being as shown
in Figure 4.40c or Figure 4.41.
Much greater drag-down loads occur with piles driven onto a relatively unyielding
stratum. Johannessen and Bjerrum (4.62) measured the development of negative skin friction
on a steel pile driven through 53 m of soft clay to rock. Sand fill was placed to a thickness
of 10 m on the sea bed around the pile. The resulting consolidation of the clay produced a
settlement of 1.2 m at the original sea-bed level and a drag-down force of about 1500 kN at
the pile toe. It was estimated that the stress in the steel near the toe could have been about
190 N/mm 2 , which probably caused the pile to punch into the rock, so relieving some of the
drag-down load. The average unit negative skin friction within the soft clay was equal to
100% of the undrained shearing strength of the clay.
4.8.2 Safety factors for negative skin friction
Safety factors for piles subjected to negative skin friction require careful consideration. The
concept of partial safety factors can be applied to the two components of permanent working
load and negative skin friction. Thus if the negative skin friction Pn has been conservatively
estimated before deciding on a value of Qp to give a safety factor of 2.5 or more on the
combined loading, it is over-conservative to add this to the working load W on the pile in
order to arrive at the total allowable pile load. It is more realistic to obtain the required
ultimate pile load Qp by multiplying the working load only by the normal safety factor, and
then to check that the safety factor given by the ultimate load divided by the working load
plus the negative skin friction is still a reasonable value.
When applying the EC7 recommendations to the design of piles subjected to drag-down,
the resulting axial load is treated as a permanent unfavourable action in Table 4.1 (Section
4.1.4). This is classed as a geotechnical action in Clause 7.3.2.1(3)P which can be calculated
either by a pile–soil interaction analysis (Method (a)), or as an upper-bound force exerted
on the pile shaft (Method (b)). As noted above, Method (a) is the more effective of the two,
particularly in determining the depth to the neutral point. It is evident that if Method (b) is