ACTA GEOTECHNICA SLOVENICA, 2014/2 5. about the authors Mladen Ćosić University of Belgrade, Faculty of Civil Engineering Marka Milanovića 17, 15300 Loznica, Serbia E-mail: mladen.cosic@ymail.com Boris Folić University of Belgrade, Faculty of Mechanical Engineering E-mail: boris.folic@gmail.com Radomir Folić University of Novi Sad, Faculty of Technical Sciences E-mail: folic@uns.ac.rs Abstract Tis paper deals with the development of a discrete numeri- cal 2D and 3D solid pile model with a discontinuity and defects to simulate non-destructive testing using the pile integrity test (PIT). Te pile discontinuity and defects were modelled by reducing the specifc fnite elements and the elastic modulus of concrete. Te wave-propagation response of the pile was analyzed based on a step-by-step numerical integration using the Hilber-Hughes-Taylor (HHT) method in the time domain. Te concept of a system-response analy- sis was originally formulated based on the integration of individual refectograms into a refectogram surface, which is generated in a 3D cylindrical coordinate system. Te use of refectogram surfaces enables an understanding of wave propagation based on their velocity to a higher level than is usually the case with standard, one-dimensional refecto- grams. Changes in the velocity responses on the refectogram, shifing from a positive to a negative value, point to the locations of discontinuities and defects in the discrete 3D pile model, and there is a clear diference in the refectograms, depending on the position of the measuring point. Te study defnes the typological models of the refectogram: without discontinuities and defects, pile-head defect, defect in the middle of the pile length or a reduced modulus of elasticity in the middle of the pile length, pile-base defect or reduced modulus of elasticity in the pile-base zone and reduced modulus of elasticity in the pile-head zone. Keywords refectogram surface, numerical pile model, solid fnite elements 1 INTRODUCTION Dynamic non-destructive testing (NDT) methods are usually accompanied by low stress and strain states in the pile in the linear-elastic behaviour of a material. Te pile integrity test (PIT) is a fast and reliable method, mostly used for evaluating the condition of the pile in the soil. It is based on the refection of the waves emitted from the pile head towards the pile toe. Te quality and integrity of the placed pile is identifed by verifying the actual length and diameter of the pile, its discontinui- ties, defects and damages. However, despite the proven reliability of the PIT test, there are a number of issues regarding this method, such as the efect of the propaga- tion of the emitted waves through the pile at diferent levels and models of the discontinuities and defects. In [1] the authors describe a special method for monitor- ing the dynamic pile-testing sensors, based on strain tensors, installed inside the pile body. Te results of the measured experimental investigation are compared with a numerical study of the pile and soil model in the sofware PLAXIS. Te specifc aspects of measuring the strains of the pile in the interaction with the ground for dynamic testing, where the sensors are fxed to the expansion-reinforcing bars, are shown in the paper too [2]. In addition to testing the piles in real conditions using the PIT test, tests can be conducted based on simulating analytical and numerical methods, such as the fnite-element method (FEM). In [3], the numeri- cal modelling of a tube system pile is discussed using 3D solid fnite elements, while the pile defects were simulated with a series of PIT tests based on changes in the pile-wall thickness and in the Young's elastic modulus. A numerical analysis using the PIT simulation and the wave propagation through the pile and the soil is described in [4] and [5] using the CEFIT sofware. NUMERICAL SIMULATION OF THE PILE INTEG- RITY TEST ON DEFECTED PILES MLADEN COSIC, BORIS FOLIC and RADOMIR FOLIC ´ ´ ´ ´