Seismic performance analysis of spun precast concrete pile in reclaimed soil

Abstract

Precast Spun Prestressed Concrete (SPC) piles are a well-established foundation type in many parts of the world, but it is newly introduced in Bangladesh, particularly in soft soil. As a result, assessing the bearing capacity of SPC pile has an added interest for local engineers and researchers. Also, to analyze the seismic performance of pile foundation embedded in liquefiable soil during an earthquake event is imperative. Among different types of precast piles, the spun pile is popular for using in building and bridge structures for attaining proper axial and lateral load considering pile-soil interaction. In a seismic region, a laterally loaded pile needs to be safe against structural and geotechnical failure to sustain large deflection. This study systematically investigates the applicability of SPC pile and also determine the liquefaction potential of the reclaimed soil. At the very beginning, a number of subsoil investigations are conducted and required soil samples are collected for basic laboratory tests. Based on the test result and seismic consideration, the liquefaction assessment is performed following the simplified SPT method and compared with some analytical approaches. The results have showed that the soil of that particular area is liquefiable up to a depth of 4.5 m, and SPC piles may be suitable for this weak soil condition. The ultimate load test of the SPC pile has been performed in the Jolshiri area to determine the in-situ pile capacity. It should be mentioned that the vertical load-bearing capacity of the pile has been reduced in a range of 3.8% to 4.5% after considering liquefaction through analytical approach. In this investigation, the SPC pile is simulated under axial and earthquake loading conditions to observe the static and dynamic pile response through an advanced numerical finite element code PLAXIS 3D. The static numerical model is validated using pile load test data. The PLAXIS 3D soil modeling parameters are determined from field and laboratory tests using established formulas and correlations. For earthquake analysis, Hardening Soil Model is used and liquefaction phenomenon is captured through UBC3D-PLM model. It is found that the pile deflection increases by 40 times for earthquake with large acceleration compared to only static loading. Total stress and shear stress have increased about 7.2% and 8.5% respectively in case of earthquake loading than axial loading. Parametric study has also been conducted in the analysis and noted that pile length and diameter has significant effect to limit pile displacement at considerable level but mesh size has no considerable effect on pile displacement. It has been observed from the liquefaction analysis in PLAXIS 3D that the pore water pressure ratio becomes 1 which means the loose sand layer completely liquefied. In liquefied soil, pile shows larger displacement about 30-60 % than non-liquefied soil condition during earthquake.The liquefaction causes large bending moment at the interface of liquefiable and non-liquefiable soil. A comparison of pile moment generation has been made considering liquefaction, without liquefaction and improved soil under earthquake loading. It has been found that soil improvement up-to almost 15 m can help to reduce large moment generation about 90% to 100% during liquefaction. The soil improvement measures significantly ameliorate the pile response and flexural capacity. SPC pile is a good alternative compared to conventional driven and cast-in-situ pile in reclaimed areas in terms of axial loading but their application under earthquake loading needs to be analyzed to improve the flexural capacity and lateral stiffness.