Discussion of “Axial Load Tests on Bored Piles and Pile Groups in Cemented Sands” by Nabil F. Ismael

Abstract

The writer reported results of static loading tests in tension and compression on two small-scale, single, bored piles in compact, dry to moist, slightly cemented sand. The tension test was performed on a pile with a diameter of 200 mm and an embedment length of 2.65 m and the compression test was performed on a pile with a diameter of 100 mm and an embedment length of 2.25 m. The load‐movement response in the tension test was that of a gradual rise rather than a plunging failure ~Fig. 7 of the original article!. The writer defined the capacity of the tension pile to the maximum load applied in the tension test—157 kN at a pile head movement of 20 mm. The maximum movement in the compression test was 12 mm and no plunging failure occurred ~Fig. 8 of the original article!. By applying ‘‘the slope tangent method’’ to the shape of this load‐movement curve, the writer determined the capacity of the compression pile to be 90 kN. The compression-pile head movement at this load was 7 mm. For the tension test, the writer calculated the 157 kN capacity to correspond to an average unit shaft resistance of 91.5 kPa. The writer then applied this average value to the 90 kN capacity of the compression tested pile, establishing a 66 kN total shaft resistance for this pile. The balance of the capacity, 24 kN, was assigned as pile toe resistance. The writer based his analysis on a simple total stress analysis, applying constant unit shaft resistance along the piles. The discusser believes that an effective stress analysis is better suited to the analysis of the test results. In contrast to the total stress method, the effective stress analysis accounts for the difference in embedment depth of the piles; the embedded length of the compression pile is 85% of that of the tension pile. Accordingly, for the unit shaft resistance in an effective stress analysis to be equal for the two piles, the average shaft resistance of the compression pile must also be 85% of that of the tension pile. Moreover, the writer’s values of shaft resistance in the two tests were determined at different pile head movements. Considering the load‐movement dependency, in the discusser’s opinion, correlation of the shaft resistance measured for the tension pile to the shaft resistance measured for the compression pile should be made at equal pile movements. In the tension test at a pile head movement equal to the movement at the 90 kN compression test capacity, 7 mm, the tension load was 125 kN, as opposed to 157 kN for the maximum movement. Combining the aspects of