Abstract: Adopting the field piling test, laboratory test of model pile and engineering examples of field piling, the principle of distribution of tensile stress in pile body is discussed by the introduction and analysis of dynamic measurement of sixteen piles. The results show that the maximum tensile stress always occurs at the top of pile, especially when pile end is embedded into the soft soil; the maximum tensile stress is located at 1/4l (l is pile length) to the top during the beginning of piling, which leads to occur transverse cracks which are perpendicular to axis of pile and the rupture of pile body. The results also show that the disk-spring pile cap can decrease hammer tensile stress, diminish the phenomenon of over-tensile stress when piling aided by conventional pile cap and make measured stress below allowable one (below 5 MPa which is considering effective method to prevent pile body from rupture).

Key words: dynamic piling, tensile stress in pile, prestressed concrete pile, quality of pile body, disc-spring pile cap