Deep excavations inevitably induce stress relief and ground movement, which may affect the safety and serviceability of nearby existing pile foundations. Although the effects of excavation on adjacent piles have been studied by some researchers recently, most of the previous studies have focused on the response of end-bearing piles due to a single excavation.

This research aims to investigate behaviour of a floating pile adjacent to single and double deep excavations. Three centrifuge tests were conducted to study influence of a single multi-propped excavation on the response of a floating pile with different restraints. In addition, two centrifuge tests were performed to examine effects of double excavations with different excavation sequences on the pile behaviour. Excavation...[ Read more ]

Deep excavations inevitably induce stress relief and ground movement, which may affect the safety and serviceability of nearby existing pile foundations. Although the effects of excavation on adjacent piles have been studied by some researchers recently, most of the previous studies have focused on the response of end-bearing piles due to a single excavation.

This research aims to investigate behaviour of a floating pile adjacent to single and double deep excavations. Three centrifuge tests were conducted to study influence of a single multi-propped excavation on the response of a floating pile with different restraints. In addition, two centrifuge tests were performed to examine effects of double excavations with different excavation sequences on the pile behaviour. Excavation process was simulated in-flight in the centrifuge tests. Moreover, three-dimensional numerical analyses using an advanced hypoplastic model were carried out to back-analyse the centrifuge tests. Based on the calibrated numerical model parameters, a systematic parametric study was conducted to evaluate effects of excavation depth, pile location with respect to excavation, initial working load, soil density and support system stiffness on the pile-soil interaction.

It is found that a pile-raft settled a similar amount of 0.6% of pile diameter due to excavation as a free-head pile and an elevated pile group during a single multi-propped excavation. During the excavation, a downward load transfer mechanism in the pile can be identified. Shaft resistance in the upper part of a pile decreases due to downward soil movement and a reduction of normal stress acting on the pile. In order to maintain vertical equilibrium, the pile has to settle to mobilise higher shaft resistance in the lower part of the pile and toe resistance. Lateral restraints imposed on the pile head have a significant influence on the induced pile bending moment. Induced bending moment due to excavation can exceed the pile bending capacity. Results from the parametric study show downward and upward load transfer mechanisms at different stages of the excavation, depending on the ratio of the excavation depth to the pile length (H_e/L_p). The pile settlement due to an adjacent excavation could be underestimated by up to 39% if the designed ultimate shaft and base resistance before the excavation are used to calculate the change of shaft and base resistance. The excavation-induced pile settlement is negligible when the pile is located at a distance of more than 2.5 times the excavation depth from the excavation. In addition, the excavation-induced pile settlement can be doubled when the design factor of safety is reduced from 3.0 to 1.5. When the relative density of sand is increased from 30% to 90%, the induced pile settlement is decreased by up to 75% due to an increase of soil stiffness. Moreover, due to the limited stress relief and soil movement, the induced pile settlement is insensitive to the support system stiffness when H_e/L_p < 0.4.

During sequential double excavations, the measured pile settlement during the first multi-propped excavation and the second cantilever excavation are found to be 0.42% and 0.72% of the pile diameter, respectively. The larger pile settlement in the second excavation is due to a decrease of soil modulus and a reduction of normal stress acting on the pile. Horizontal soil movements and pile bending moments are found to reverse during the second excavation. Moreover, the excavation sequences have limited influence on the induced pile head settlement since the soil experiences similar stress paths. However, the maximum induced pile bending moment is more than five times larger if the double excavations are conducted sequentially rather than simultaneously. It is more beneficial to conduct double excavations simultaneously.