Effect of shaft area on ball resistances in soft clays

Abstract

This paper presents the effects of shaft−ball area ratio on penetration resistance and cyclic behaviour of Busan clay. Five balls of different sizes were used at two sites in the Nakdong River delta, South Korea. The cyclic resistance decreased gradually with increasing cycle number. Interestingly, the penetration resistance obtained from relatively large balls in a leached clay layer suddenly dropped to nearly zero after the fifth loading cycle. Test results were also compared with those from two Japanese clays and an existing numerical simulation. Net (or total) ball resistance decreases with decreasing area ratio until the resistance reaches a minimum value, and then increases with further decreasing area ratio. The possibility of soil flow around balls, which have an area ratio smaller or equal to that at the minimum net resistance, is proved through the offset resistance. The increase in net penetration resistance after the minimum point increases approximately linearly with the normalised overburden stress. The area ratio at minimum resistance, ranging from 0·15 to 0·4, tends to decrease with the increase in the normalised shear strength. Thus, balls with an area ratio ≤ 0·15 rather than the generally used 0·10 value are recommended for the three clays.