Development of an impact sound diagnosis system for tunnel concrete lining

Abstract

In Japan, various non-destructive test techniques are in use for evaluating the soundness of tunnel concrete lining. Among these evaluation methods, impact sound diagnosis by human inspectors is most widely used because it is the easiest to conduct and requires only a minimum of equipment. Impact sound diagnosis by human inspectors, however, has several problems. First, since two or more persons conduct impact sound diagnosis, inspection results tend to vary among inspectors. Second, quantitative judgement required for impact sound diagnosis is difficult to make and the evaluation process is not recorded. Finally, impact sound diagnosis forces inspectors to work in a tunnel, which has an unfavorable working environment for long time. In order to solve these problems, the authors developed ''Sonic Meister'', the impact sound diagnosis system for tunnel lining concrete. This system has a number of characteristics: (1) The equipments are mounted on an eight-ton truck because mobility is considered to be an important requirement; (2) a general-purpose arm robot has been adopted to speed up the inspection process; (3) the impact sound unit has been developed to apply stable impact forces to concrete; and (4) manpower requirements have been reduced dramatically by mechanizing the inspection process. The newly developed impact sound diagnosis system has made it possible to evaluate the soundness of concrete speedily, safely and accurately. To evaluate the soundness of a concrete lining, the impact sound diagnosis system pushes the five-hammers unit, which is equipped with a parallel array of five hammers spaced 30 cm apart, against the concrete wall and activates the five hammers successively at 0.2 s intervals. After collecting the sound generated during the 0.1-s period after each impact, the system analyzes the collected sound signals by using a digital signal processor in real time and displays the results. These results are stored in real time together with raw sound data. Then, the system moves the impact sound unit by 30 cm in the transverse direction and repeats the same procedure. By using this method, the system is capable of inspecting concrete surfaces at a rate of 250-350 m2/h, covering 30 cm grid spaces one after another. The system controls the movement of the multi-hammer unit by using data received from cross-section measurement system and an electronic distance measuring (EDM) instruments system. On the basis of the information thus obtained, the system automatically avoids obstacles such as lighting equipment, piping and fans while conducting impact sound diagnosis. Since the robot mounted on the truck sways back and forth violent, the system is equipped with outriggers to reduce this swaying. Since the outriggers have non-motorized wheels, the vehicle can be moved forward without raising the outriggers. This design has helped to speed up the inspection process. To investigate the nature of sound generated by cavities and exfoliation areas, a series of impact sound diagnosis was conducted using specimens prepared for the examination. The concrete blocks used as test specimens were provided with artificial cavities and the exfoliation. In the tests, three parameters were measured: vibration of concrete surface (measured with an accelerometer), hammer impact force and impact sound. Comparison of impact sound and surface vibration showed that they show similar waveform tendencies although there are differences in degree. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.