Axial Load Capacity of Steel Pipe Piles in Sand
Authors: Olson R.E. 
Published: October 2008

ABSTRACT
The pile load test database, used previously1 in recommending properties to use in API RP 2A2 for analysis of the axial load capacity of steel pipe piles in sand, is reviewed and updated, and suggestions are made for properties that lead to improved agreement between measured and predicted axial pile capacity.

INTRODUCTION
The API recommended practice for analysis of the axial load capacity of steel pipe piles in cohesionless soils (RP 2A) has undergone a continued evolution during the last decade. Major changes have involved taking into account the effect of relative density and changing the numerical values of the earth pressure coefficient (K). Some of the changes have been controversial. The function of this paper is to review relevant data and to make suggestions for further changes that would bring the API standard more into accord with the existing database. A restricted amount of data on piles other than steel pipe will be included where they are relevant.

ANALYTICAL METHOD
The calculated axial load capacity of piles (Qc) is taken as: (mathematical equation) (available in full paper) where Qs is the capacity in side shear, Qp is the tip capacity (taken as zero for tensile loads), and Wp is the weight of the pile submerged in soil (usually small enough that it is ignored) and is positive for tensile loading and negative for compressive loading. In the absence of more definitive data, side shear is calculated using: (mathematical equation) (available in full paper)where fs is the local side shear between the pile and the surrounding cohesionless soil (limited to a value flim), C is pile circumference, ?L is an increment of pile length in the Lth layer, K is the earth pressure coefficient (1.0 for full displacement piles, 0.8 for non-displacement piles), −? is the free-field vertical effective stress in the middle of the Lth layer, and ? is the friction angle between the pile and the surrounding soil. Similarly, the tip capacity is calculated using: (mathematical equation) (available in full paper) where qp is the net pressure between the pile tip and the subsoil (limited to qlim), Ap is the area of the pile tip, −? is the free field vertical effective stress at the pile tip, and NQ is a dimensionless bearing capacity factor. Numerical values used for the various parameters, in the 1989 API RP2A, are shown in Table 1, for comparison with alternative values recommended later. It is widely recognized that Eqs. 2 and 3 are used mainly as a means of generalizing pile load test data. The actual numerical values, taken separately, have a significant possible error. For example, the effective stress (−?) near a loaded pile cannot be the free field value. The value of ? is not likely to be the friction angle of the original cohesion less soil on the pile material, e.g., as obtained from direct shear tests, because cohesion less soils are likely to be ground up due to the high stresses and large shearing strains near a pile tip. The earth pressure coefficient, K, is empirical because the state of stress right next to- a pile may involve a substantial amount of arching, thus making it difficult to apply cavity expansion, and other, models.