Shielding Piles from Downdrag in Consolidating Ground: Fid-Bau Portal

Shielding Piles from Downdrag in Consolidating Ground

Lam, S. Y. / Ng, C. W. W. / Poulos, H. G.

Abstract

Negative skin friction (NSF) can induce an increased compressive force on piles, called dragload, and additional pile settlement caused by the downward pull of the soil, called downdrag. To investigate the efficiency of shielding effects on dragload and downdrag of piles, centrifuge tests have been carried out to study the shielding mechanisms created by installing sheet piles sleeves around an existing pile in consolidating ground. The effects of various shielded lengths have also been investigated. Comparisons between centrifuge test results and finite-element (FE) analyses are made and discussed. Based on centrifuge tests, it is clear that the shielding effect on the dragload decreases only gently with a decrease in the shielded length, whereas the shielding effect on the downdrag decreases exponentially with a decrease in the shielded length. Numerical simulations of the centrifuge model tests on the sleeved center piles reveal that the observed shielding effects on the center pile are attributed to the stress transfer from the consolidating soft soil to the sheet pile sleeve. As consolidation proceeds, the relatively stiff sheet piles sleeve hangs up the soil, leading to a significant reduction in the vertical and horizontal effective stresses in the soil and in the NSF on the center piles. The deeper the depth, the greater the hang-up effects. Thus, the shielding effect increases with the shielded length of the center pile. The reduction in the NSF on the center piles protected by the sheet pile sleeve is more significant than the reduction in the NSF from the sacrificial piles with the same shielded length.