Shielding Piles from Downdrag in Consolidating Ground

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 con- solidating ground. The effects of various shielded lengths have also been investigated. Comparisons between centrifuge test results and finite- element(FE)analysesaremadeanddiscussed.Basedoncentrifugetests,itisclearthattheshieldingeffectonthedragloaddecreasesonlygently 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 sheetpilessleevehangsupthesoil,leadingtoasignificantreductionintheverticalandhorizontaleffectivestressesinthesoilandintheNSFon 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. DOI: 10.1061/(ASCE)GT.1943-5606.0000764. © 2013 American Society of Civil Engineers. CE Database subject headings: Skin friction; Piles; Centrifuge; Soil consolidation; Compressive strength. Author keywords: FEA; Shielding; NSF; Pile; Sleeve; Centrifuge.