LOAD CAPACITY OF SINGLE PILES
We know from Section that accurate estimation of pile load capacity is a rather diffi cult task because it is difficult, if not impossible, to account for (a) the changes in stress and strain states from installation effects, (b) the variability of soil types, and (c) the differences in the quality of construction practice. Therefore, calculations of pile load capacity are approximations and rely heavily on empiricism or semiempiricism (part mechanics, part empirical).
The ultimate load capacity, Qult, of a pile is conventionally taken as consisting of two parts. One part is due to friction (Figure ), called skin friction or shaft friction or side shear, Qf, and the other is due to end bearing at the base or tip of the pile or pile toe, Qb. If the skin friction is greater than about 80% of the end bearing load capacity, the pile is deemed a friction pile and, if the reverse, an end bearing pile. If the end bearing is neglected, the pile is called a fl oating pile. From statics
where Wp is the weight of the pile. In many cases, the weight of the pile is included in the dead load or neglected for piles of small cross-sectional areas (<0.07 m2). Equation , as written, is independent of settlement. Recall from Section that Qf and Qb are generally mobilized at different values of settlement. Therefore, it is desirable to add mobilized skin friction and mobilized end bearing at the same value of settlement. The conventional methods that will be presented in the next section treat the ultimate pile load capacity and settlement separately. Thus, except for pile load tests (see next section) and numerical methods, it is not possible to add mobilized skin friction and mobilized end bearing at the same value of settlement.
The conventional allowable load capacity for ASD is expressed as
where FS is a gross factor of safety usually greater than 2. In practice, the application of a factor of safety greater than 2 is thought of as suffi cient to limit settlement to about 25 mm. Nevertheless, you should
Pile shaft and end bearing resistance
estimate the pile settlement as part of good design practice and use it to control the allowable pile load capacity, if necessary. For LRFD,
where r is load factor; P is load; wf and wb are the performance factors for skin friction and end bearing, respectively; h is a ductility, redundancy, and operational performance factor; and i is the load type