Statistical Observations on Reliablity of Pile Driving Formula

Yorihiko OHSAKI

建築研究報告  No.23,  1957  建設省建築研究所

＜概要＞

  The allowable loads on piles driven by drop hammers shall not exceed the value R determined by the following formula;

  Where

R : allowable pile load (ton),

W : weight of hammer (ton),

H : height of fall (m),

s : final penetration of pile per blow (m).

  The pile driving formula (1) specified in this provision is of the type of Engineering News formula.

  In 1952, the Architectural Institute of Japan issued Standard Design Specifications for Building Foundations, in which Kreuter's formula was proposed for determining the allowable loads on piles. In 1954, the present Building Code has been enforced by the Government and the same formula of Kreuter's is adopted therein. No information, however, is available for the writer on the reason why the old Engineering News had to be withdrawn.

  According to the present provision, the allowable loads on piles (driven by drop hammers) should not exceed the value R obtained by the following formula except when determined by pile loading tests:

  Where

R : allowable pile load (ton),

W : weight of hammer (ton),

P : weight of pile (ton),

H : height of fall (m),

H0: height of fall for zero penetration (m),

s : penetration of pile for the height of fall, H (m).

  The height of fall for zero penetration, H0, should be estimated from a diagram as shown in Fig. 1, in which pile driving should be repeated three times with different heights of fall of the drop hammer. It will be readily known that this height for zero penetration, H0, corresponds to the energy loss during pile driving.

  However, experienced engineers accustomed to the use of Engineering News formula in the past make complaint quite often that the old formula is far more preferable from the view-point of practice than the new formula of Kreuter's type. The reasons of their complaints may be summarized as follows;

1. it is troublesome and time-wasting to repeat trial drivings to find out whether the proposed allowable loads are reached or not,
2. the accurate determination of H0 is often difficult since the measured points on the diagram shown in Fig. 1 largely scatter,
3. sometimes, even a negative value of H0 is obtained, which can never be understood theoretically,
4. if the arbitrariness of drawing a straight line on the diagram of Fig. 1 would be taken advantage of, almost any desired value of R might be computed by the formula (2).

  In addition, it may also be undeniable that Engineering News formula was favored because of its simplicity in expression.

  It is the writer's opinion, however, that the superiority of a pile driving formula over other ones must be judged deliberately on the basis of a large number of accumulated informations on the comparison of computed values and true carrying capacities of the piles under various soil conditions; and, furthermore, the comparison should be made by dealing with accumulated data in a statistical manner.

  In this paper, experiences of pile driving in two building sites are presented; and deviations in allowable pile loads computed by means of above two pile driving formulas are compared and discussed; furthermore, in one of two sites, the correlation between allowable pile loads determined by tests and computations is discussed to supply a basis for comparing the reliability of pile driving formulas.

  However, it is not intended in this paper to draw a conclusion immediately, but the scope is limited within just presenting a few examples in a methodological point of view.