Strain measurements during static pile testing: a case study of two piles with diferent diameter and length

Abstract

The paper presents testing of two piles, diameter of Ø1200mm and Ø800mm, length of 26.0m and 18.0m respectively. Tests were conducted on construction project WELLPORT III phase - New Belgrade, Republic of Serbia. The piles were erected using CFA technology by Novkol jsc. - Belgrade. The tests were conducted according to the standard SRPS EN ISO 22477-1: 2019, jointly by the Laboratory for structures of the Faculty of Civil Engineering University of Belgrade and Laboratory of Novkol jsc. - Belgrade. In addition, beside to the common and inevitable measurement of pile settlement with respect to the applied load during test phases, tests include measurements of strain along the pile depth. Measurements of strain were carried using so-called ‘sister-bar’ based on resistance principle (classical foil strain gages), as well as using embedded sensors based on ‘vibrating-wire’ principle of strain measurement, as both principles of strain measurements are state-off-the-art in actual practice. Installation of all instruments were incorporated in pile reinforcement assembly at several levels before casting. During testing, beside to recording force at pile head and settlements by LVDTs which were leaned on the reference beam, there were recorded and realized strain levels on all installed instruments. During data reduction phase, computation of actual force was realized at each section with strain measurement, based on axial stiffness of tested piles. Using such a procedure, forces in tested piles were calculated. Such data gave possibility to explore part of force which was transmitted by pile shaft along the depth to the surrounding soil, as well as the part which goes to the pile base. All results are shown in illustrative diagrams of corresponding stresses between pile shaft and surrounding soil, force levels along the pile depth, and finally level of force which transfer by pile shaft and pile base. According to the facts that pile bases were in different soil level, and with different diameters, the different ratio between force levels which transferred by pile shaft and pile base were established.