Stability and load-displacement behaviour of axially cyclic loaded displacement piles in sands

Abstract

Uncertainties regarding the axial cyclic behaviour of piles driven in sands led to an extended programme of calibration chamber instrumented pile experiments. Broad trends are identified and interpreted with reference to normalised cyclic loading parameters Qcyclic/QT, Qmean/QT and N. Cyclic damage is shown to be related to changes in the radial effective stress regime close to the shaft. While stable loading leads to little or no change as cycling continues in the sand masses’ effective stress regime, high-level cyclic loading can affect stresses far out into the sand mass. The test systems’ chamber-to-pile diameter ratio has a significant impact on outcomes. Piles installed in loose, fine, sand are far more susceptible to cyclic loading than in denser, coarser sand. Little or no change in pile stiffness was seen in tests that remained within the stable cyclic region, even over 10,000 or more cycles. Unstable tests lost their stiffness rapidly and metastable cases showed intermediate behaviours. The permanent deflections developed under cycling depend on the combined influence of Qcyclic/QT, Qmean/QT and N. While model tests provide many valuable insights into the behaviour of piles driven in sand, they are unable to capture some key features observed in the field.

Cyclic volumetric strain accumulation for sand under drained simple shear condition

“…Herein, Fontainebleau sand NE34, as a geotechnical reference siliceous material, has been selected to develop an analytical model of cyclic strain accumulation accounting for important factors [25,47,48,51]. The simple shear tests for various factors are designed according to the deformation behaviour of soil element around the pile [5,7,20]. the cyclic experimental results…”

“…( 4)), as shown in Eq. (7). Then, the normalized volumetric strain could be used to compare the cases which selecting the normalized volumetric strain (Eq.…”

“…Standing [4,5] demonstrated that a high level of cyclic loading is highly detrimental to shaft capacity, based on their experimental results on an in-situ pile test being subjected to a large amount of axial cyclic loading. More recently, Tsuha et al [6] and Rimoy [7] presented a series of reduced-scale cyclic tests, using the mini-Imperial College pile (ICP), that demonstrated the cyclic degradation of radial stresses adjacent to the pile. All cyclic pile tests show that the shaft capacity depends on the mechanical response of a soil element (strain or excess pore pressure accumulation) adjacent to a pile under cyclic loading [8][9][10][11].…”