The Relationship between Pile Formulas and Load Tests

Pile-driving formulas are often used as the sole criterion of pile bearing capacity, without load tests. When load tests have been made, the results often have been at variance with the computed values. Consideration of the physical properties of soils and piles, and of the interpretations of terms in the formulas, indicates the reasons for such discrepancies, and also reliable means of avoiding them. Driving resistances bear definite relationship to static load resistances only in soils in which the predominating resistance is supplied by cohesionless materials. A study of forty-five cases in soils of this type, in which test load failure values and resistances computed from various formulas in use are compared, indicates the unsatisfactory spread of values obtained from the most common abbreviated formulas and the generally satisfactory results from the more comprehensive formulas. Use of a static formula for preliminary determination of pile lengths and as a check in all other cases appears advisable.