Centrifuge Model Tests of Liquefaction-Induced Downdrag on Piles in Uniform Liquefiable Deposits

Abstract

Earthquake-induced soil liquefaction can cause settlement around piles, which can translate to negative skin friction and the development of drag load and settlement of the piles. A series of centrifuge model tests were performed to assess liquefaction-induced downdrag and understand the interplay and effects of (1) pile embedment and pile-head load, (2) excess pore pressure generation and dissipation, and (3) reconsolidation and ground settlement on pile response during and postshaking. The model included a layered soil profile (clay, liquefiable sand, and dense sand) with two 635-mm-diameter instrumented piles. One pile was placed with its tip at the bottom of the liquefiable deposit; the other pile was embedded five diameters into the dense sand layer. The model was shaken with multiple earthquake motions with their peak horizontal accelerations ranging from 0.025 to 0.4 g. For each shaking event, the drag load on the piles first decreased during shaking and then increased during reconsolidation, exceeding its preshaking value. With multiple shaking events, the net drag load on the piles increased. The maximum observed drag load was found equal to the drained interface shear strength calculated from the interface friction angle of δ ¼ 30°and a lateral stress coefficient of K ¼ 1. Larger drag loads and smaller settlements were observed for the pile embedded deep in the dense sand layer. Most of the pile settlements occurred during shaking; postshaking pile settlement was less than 2% of the pile's diameter. The mechanisms behind the development of liquefaction-induced drag load on piles and settlements are described. Select ramifications concerning the design of piles in liquefiable soils are also described.

Numerical Modeling of Liquefaction-Induced Downdrag: Validation against Centrifuge Model Tests

“…Fleming et al (2008) showed that displacements of 0.5% to 2% of pile diameter are required to mobilize the shaft capacity. Sinha et al (2022b) performed a series of centrifuge model tests and found that soil displacements of 1%-3% of the pile's diameter were sufficient to mobilize the total skin friction capacity. The nonlinear backbone curve for t-z material (Boulanger et al 1999;Mosher 1984;Reese and O'Niel 1987) is a hyperbolic curve that takes displacement equal to about four times z 50 to mobilize >90% of the ultimate capacity (t o ult ).…”

“…The piles were instrumented with strain gauges in full-bridge configuration inside the pipe to measure the axial load distribution. The interface was made significantly rough to achieve full interface friction angle (δ ¼ 30°) (Sinha et al 2022b). The 0D, 3D, and 5D naming annotations indicate the embedment depth of the pile tip; that is, the 0DPile had its tip placed at the bottom of the loose sand layer, whereas the 3D and 5D piles had their tips embedded 3 and 5 times their diameter into the dense sand.…”

“…Video recordings showed water coming to the surface from the sides of the container. The continuous dissipation of excess pore pressures with no surface settlement suggested the formation of a water film at the sand-clay interface (Sinha et al 2022b). The continuous Fig.…”

“…The ultimate capacity of TzLiq and QzLiq materials can be obtained from the empirical correlations developed on data from soil investigation methods such as cone penetration tests, static penetration tests, and others. The stiffness of TzLiq material can be defined considering that a small relative displacement of 10-30 mm or 1%-3% of the pile's diameters is enough to mobilize the full shaft resistance in piles (Sinha et al 2022b). Determining site-specific QzLiq material stiffness is essential for accurately modeling pile settlement and drag load.…”

“…Fellenius and Siegel (2008) used the unified pile design method to describe the effect of liquefaction and reconsolidation settlement relative to the static neutral plane (i.e., the neutral plane before liquefaction) on the developed drag load and settlement of the pile. Recently, Sinha et al (2022a, b;Ziotopoulou et al 2022) performed a series of centrifuge model tests on end-bearing piles and described the evolution of their axial load distribution and settlement during and postshaking ]. Results showed that the negative skin friction was fully mobilized in the liquefiable layers and became equal to the interface shear strength.…”