Design of negative skin friction should include checks on the structural and geotechnical capacity of the pile, as well as the downward movement of the pile due to the negative skin friction dragging the pile shaft (CGS, 1992; Fellenius, 1998; Liew, 2002). A pile will settle excessively when geotechnical failure occurs. As the relative displacement between the soil and the pile shaft is reversed, the effect of negative skin friction on pile shaft would be eliminated. Therefore, the geotechnical capacity of the pile could be based on the shaft resistance developed along the entire length of pile. The dragload need not be deducted from the assessed geotechnical capacity when deciding the allowable load carrying capacity of the pile. On the other hand, the structural capacity of the pile should be sufficient to sustain the maximum applied load and the dragload. The dragload should be computed for a depth starting from the ground surface to the neutral plane.
The estimation of downward movement of the pile (i.e. downdrag) requires the prediction of the neutral plane and the soil settlement profile. At the neutral plane, the pile and the ground settle by the same amount. The neutral plane is also where the sustained load on the pile head plus the dragload is in equilibrium with the positive shaft resistance plus the toe resistance of the pile. The total pile settlement can therefore be computed by summing the ground settlement at the neutral plane and the compression of the pile above the neutral plane (Figure 6.21). For piles founded on a relatively rigid base (e.g. on rock) where pile settlement is limited, the problem of negative skin friction is more of the concern on the structural capacity of the pile.
This design approach is also recommended in the Code of Practice for Foundations (BD, 2004a) for estimating the effect of negative skin friction.
For friction piles, various methods of estimating the position of the neutral plane, by determining the point of intersection of pile axial displacement and the settlement profile of the surrounding soil, have been suggested by a number of authors (e.g. Fellenius, 1984). However, the axial displacement at the pile base is generally difficult to predict without pile loading tests in which the base and shaft responses have been measured separately. The neutral plane may be taken to be the pile base for an end-bearing pile that has been installed through a thick layer of soft clay down to rock or to a stratum with high bearing capacity. Liew (2002) presented a methodology using simple analytical closed-form equations to determine the neutral plane and the negative skin friction on a pile shaft. Step-by-step examples are also given by O'Neill & Reese (1999). The method includes the effect of soil- structure interaction in estimating the neutral plane and dragload 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 (Section 6.4.4.3) may be used to estimate the magnitude of negative skin friction on single piles (Bjerrum et al, 1969; Burland & Starke, 1994). For design purposes, the range of β values given in Tables 6.3 may be used for assessing the negative skin friction.
P
Ø
Notes :
(1) The negative skin friction, fn, in granular soils and cohesive soils is determined as for
positive shaft resistance, τs. The effective stress approach can be used to estimate the
negative skin friction as follows : fn = β σv'
where fn = negative skin friction
σv' = vertical effective stress
β = empirical factor obtained from full-scale loading tests or based on the soil mechanics principle (see Section 6.4.4):
(2) Ultimate load-carrying capacity of pile will be mobilised when pile settles more than the surrounding soil. In such case, the geotechnical capacity of the pile can be calculated based on the entire length of pile.
Figure 6.21 – Estimation of Negative Skin Friction by Effective Stress Method
Ultimate resistance of pile (when pile
settles more than surrounding soil) Distribution of
Shaft Resistance
Load Distribution in Pile Settlement Profiles for Surrounding Soil and Pile
v w
w
v w
v w
v
v w
v wv
w
v w
Pile Subject to Negative Skin Friction fn τs Applied load, P Neutral plane Transition zone Axial load distribution atworking stage Ground
settlement profile Pile settlement capacity, Qult settlement, δt Settling soils
v wv
w v w
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, 1969; 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).
The effect of soil-slip at the pile-soil interface has been investigated by many authors (e.g. Chow et al, 1996; Lee et al, 2002 and Jeong et al, 2004). Negative skin friction and dragload tend to be overestimated if the effect of soil-slip is not considered. On the other hand, negative skin friction near the neutral plane is usually partially mobilised, as the relative movement between the soil and pile is smaller than that required for full mobilisation (Lee et al, 2002). As such, negative skin friction estimated by effective stress or β method is conservative.