Impact compression behaviors of high-capacity long piles

Abstract

The Hiley formula underestimates driving resistance of long piles. Methods using affected pile length have been suggested, but have been found to be inapplicable for high-capacity piles. The impact compression behaviors of about 4700 high-capacity H-piles that were 14–80 m long at final set were studied. Measured data revealed that maximum impact forces are very scattered, but their means are independent of the hammer type, ram weight, ram drop, impact velocity, and pile length. Maximum impact compression of pile and affected pile length exist in both long and short piles. The affected pile length in turn is significant to the blow efficiency, hammer constant, and energy transfer ratio. This length is governed by the impact momentum and impact energy, and can be estimated by an energy-based equation. If the affected pile length determined by this equation is substituted into the Hiley formula to back-analyze the driving resistance, predictability of the driving formula can be improved by about 8%. This improvement is significant enough to reduce the number of hammer blows required at very hard driving conditions and reduce pile damage. Furthermore, this proposed equation is simple to use in the field and is more economical compared with stress-wave monitoring techniques.