Non-destructive inspection of the surface of underground structures based on the propagation of surface waves

S.P. Tremblay, M. Karray, M. Chekired, C. Bessette, L. Jinga

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Soil Mechanics

ABSTRACT: The use of non destructive techniques (NDT) for the inspection of different types of structures is becoming increasingly popular due to the advantages they may offer over destructive techniques. When it comes to the inspection of an underground structure located in an urban setting where direct access to the structure is limited, the use of an NDT that can be performed rapidly and with minimal disturbance to the population is advantageous. Although several NDT™s are available to estimate the location of such structures, none of them have so far shown the potential to evaluate their state of degradation without requiring a direct access to them. This article presents a new NDT based on the study of the propagation of elastic waves that is intended to evaluate the state of degradation of an underground structure directly from the surface of the soil. With this new technique, the variation of the vertical acceleration of the soil following a mechanical impact is recorded along a given profile. The variation of the acceleration of the soil in time, space and frequency is then studied in order to determine the surface condition of an underground structure. Through the use of numerical modelling, this article demonstrates how the propagation of elastic waves is affected by the presence and the state of degradation of an underground structure. The proposed inspection technique is carried in 3 steps. First, the variation in energy content of each recorded signals is presented in the form of a contour map. The second and third steps are based on the evaluation of the variation of the group and phase velocities of the elastic waves along the investigated profile.

RÉSUMÉ: L'utilisation de techniques non destructives (NDT) pour l'inspection de différents types de structures est de plus en plus populaire en raison des avantages qu'elles peuvent offrir par rapport aux techniques invasives. En effet, les techniques non destructrices peuvent s™avérer particulièrement efficaces lorsqu™il s™agit de réaliser l™inspection d'une structure souterraine située dans un milieu urbain, là où l'accès direct à la structure est limité et la densité de population importante. Bien que plusieurs NDT sont disponibles afin de localiser ce type de structures, aucunes d'elles n'a jusqu'à présent montrée le potentiel d'évaluer l™état de dégradation d™une structure souterraine sans nécessiter un accès direct à cette dernière. Cet article présente une nouvelle NDT basée sur l'étude de la propagation des ondes élastiques à travers le sol. Cette méthode est destinée à l™évaluation de l'état de dégradation d'une structure souterraine directement à partir de la surface du sol. Avec cette nouvelle technique, la variation de l'accélération verticale du sol causée par l™application d™un impact mécanique est enregistrée le long d'un profil donné. L™étude de la variation de l'accélération du sol dans le temps, l'espace et la fréquence permet l™évaluation de l'état du toit de la structure souterraine. Grâce à l'utilisation de modèles numériques, cet article montre comment la propagation des ondes élastiques est affectée par la présence et l'état de dégradation d™une structure souterraine. La technique d™inspection présentée dans cet article se déroule en trois étapes. Premièrement, la variation de l™énergie de chacun des signaux enregistrés à la surface du sol est présentée sous la forme d'une carte de contour. Les deuxième et troisième étapes sont basées sur l'évaluation de la variation des vitesses de groupes et de phases des ondes élastiques enregistrées le long du profil investigué. 1.0 INTRODUCTION Modern cities have an important part of their infrastructures buried underground. Although these structures are protected from many environmental hazards, they may however be subjected to other problems. One of these problems arises due to their inaccessibility which may seriously impair their inspection and maintenance. Just as it is the case for any other aging structures, the maintenance of underground structures must be carefully planned in order to insure safety of the public. However, the planning of the maintenance of these structures is often based on the results obtained from inspections requiring the use of different destructive techniques such as excavation or drilling. Although these techniques provide a good estimate of the state of degradation of an underground structure, they are often not an alternative due to their associated costs and the inconveniences that they caused to the population. Moreover, they require a direct access to the structure as well as the use of heavy machinery which can be problematic in many cases. An approach involving the use of a non-destructive technique to evaluate the state of degradation of an underground structure directly from the surface of the soil would therefore offer several advantages over the techniques

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