RELATIVE NDT EVALUATION OF THE SIDE WALLS OF A BRICK CHANNEL

Giovanni Cascante1, Homayoun Najjaran2 and Paola Ronca3
department of Civil Engineering, University of Waterloo, ON, Canada
2Institute for Research in Construction, National Research Council Canada, ON, Canada
3Department of Civil Engineering, Polytechnic of Milan, Milan, Italy

Abstract

The network of channels called Navigli was built in the surrounding country and downtown Milan, Italy. Wave propagation measurements were taken along the channel for a total length of 80 m. Each test consisted of the simultaneous measurement of the response of the wall with 15 transducers to an impulse load. The relative condition of the wall is evaluated by considering three main wave characteristics: group velocity, phase velocity, and attenuation coefficient. A fuzzy logic model is developed to make a relative evaluation of the condition of the sidewall.

Introduction

The evaluation of the condition of existing structures is a key element in the maintenance and safety plans of any municipality. This evaluation should be based on realistic measurements of different variables such as geometry and material strength. These variables are commonly measured with destructive tests. However, these tests are expensive and sometimes impossible to perform at regular spacings. Conversely, nondestructive tests (NDT) are fast and economical, and can be used in-situ without major disruption to normal operations. Wave velocity and attenuation depend on the stiffness and mass of the medium, thus variations in the medium properties should be inverted from the change in wave parameters.

The Navigli channel was mainly built during the XVI century. The current civic administration is facing the difficulty of saving what remains of these complex hydraulic and historic structures. Typical cross-sections of the wall are shown in Figure 1. The restoration of the Navigli should be performed without causing mayor disturbance to this historic structure. Therefore, the use of non-destructive methods for the condition assessment of the channel is an ideal and cost-effective solution. The pulse velocity method is widely used for the assessment of the quality of concrete; this assessment is based on empirical correlations between wave velocity and strength (Popovics and Rose 1994, Ronca 1993). Geophysical methods commonly used for site characterization are seismic reflection or refraction, spectral analysis of surface waves, seismic crosshole and downhole, ground penetrating radar, resistivity, and electromagnetic survey. On the other hand, the spectral analysis of surface waves (SASW) method has been proposed for the evaluation of shear wave profiles in layered media (Nazarian et al. 1988, Rix et al.

2002) .

This paper summarizes the results of the nondestructive testing programme performed on the sidewalls of the Navigli channel. Wave propagation measurements were taken at two different elevations on the sidewall (bottom and top lines). All the results presented in this paper correspond to the bottom line only. The response of the wall was measured at 161 locations; the transducers (accelerometers) were spaced at 0.5 m, the total length of wall tested was 80m, and the total number of tests performed was 86. Each test consisted on the simultaneous measurement of the response of the wall on 15 transducers, thus each test covered a section of 7.5 m in length. The relative condition of the wall is evaluated by considering three main wave characteristics: group velocity, phase velocity, and attenuation coefficient. The sections that showed low values of group and phase velocities and high values of the attenuation coefficient are identified as the weaker sections. In the following sections, the theory of the techniques used for data analysis is presented, and then the experimental setup and the testing programme are described. Finally, the main results and conclusions from the testing programme are outline.

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M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 485-492. © 2006 Springer. Printed in the Netherlands.