Large Diameter Sewer Condition Assessment Using Combined Sonar and CCTV Equiptment

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Mark E. AndrewsAPWA, P.Eng.1 M.E.Andrews and Associates Limited 1203-211 Wurtemburg Street, Ottawa, K1N 8R4, Canada

ABSTRACT

Underwater Sonar equipment combined with enhanced CCTV provides an ideal engineering tool to inspect and assess large diameter sewers with high flow conditions. Using high frequency, rotating Sonar technology the full wetted perimeter of the sewer can be scanned and assessed. Combined with simultaneously retrieved CCTV images a comprehensive structural and hydraulic condition assessment can be prepared. This presentation will focus on the results of the inspection and assessment of over 50 km of large diameter interceptor and trunk sewers in Canada. Examples used to illustrate the technology include a 2900 mm brick-lined interceptor constructed in Toronto in 1910, a 2400 mm, 35 m deep fully surcharged concrete lined tunnel in Ottawa, and a 2100 mm concrete lined tunnel located in a very heavy industrial area in Hamilton, Ontario. An overview of trends and issues flowing from the results of these inspections is provided.

Keywords: trunk sewer, sewer maintenance, Sonar, surcharged sewers

1.0 INTRODUCTION

1.1 Overview and purpose
Large diameter, trunk sanitary sewers are, without question, among the most critical components of municipal infrastructure systems. Increased environmental awareness and a heightened sense of legal responsibility that has been imposed on municipalities reinforce the importance of maintaining the integrity of these critical sewers. The lack of significant preventive maintenance and the advanced age of the sewers are two signals that the time has come to pay more attention to them. Indeed, a 1984 survey of Canadian municipalities regarding the condition of infrastructure indicated, even then, that among other components, sewers “are entering a state where significant rehabilitation needs can be expected”. An analysis of the survey concluded that extensive work needs to be undertaken to assess the Mark Andrews is a principle engineer in a consulting firm which focuses on infrastructure engineering and has particular expertise in sewerage system evaluation. The firm has worked across Canada and in the Middle East. Mark can be contacted at (613) 244-2000 or mandrews@sympatico.ca. This paper was presented at the “APWA International Public Works Congress and Exhibition”, Las Vegas, Nevada, September 1998.condition of infrastructure in a precise manner and to identify options available for rehabilitation, and hence for extending the life of components (Adams, 1987).

But trunk sanitary sewers are difficult to inspect and hence assessment of their condition is a difficult proposition. Remote valley locations, deep manholes, and long reaches between manholes, all combine to frustrate access to the sewer. Ever increasing depths of flow make internal access and inspection difficult, if not impossible. The purpose of this paper, therefore, is to describe one method of inspection that is available to carry out a detailed inspection in spite of these constraints. Sonar equipment, specially adapted for use in sewers, can be used to provide an image of the interior, wetted perimeter. The equipment can be used under fully submerged conditions and is therefore ideally suited to the inspection of surcharged and full to partially full sewers. Alternatively, it can be combined with CCTV equipment, in partially full sewers, to provide details both above and below the water level.

1.2 Purpose
The purpose of this paper is to provide a brief summary of Sonar technology, as applied to sewers, along with an outline of the methodology used to undertake inspections. A series of three case studies which outline how the technology was used in different situations is also presented. The paper concludes with a summary to date of general trends observed in the trunk sewers inspected. These findings are related to three selected criteria and are intended to provide only a sense of trends that have so far been identified.

1.3 Scope
Numerous inspection techniques are available for smaller sewers and the assessment of them is well documented in the literature. In this paper, therefore, attention is focused on the inspection and condition assessment of large diameter sanitary sewers. Large diameter sewers are not nearly so well understood and inspection and assessment techniques are far less available. Large diameter sewers are considered here to be those that typically range from 1.5 or 1.8 meters in diameter up to, at least, 3 meters in diameter; depths of from 6 to 60 meters are common. These sewers are constructed using pre-cast concrete pipe in a cut-andcover type of installation, or more commonly, through a cast-in-place concrete lining in a tunnel type of installation. As will be seen, some older brick lined sewers are also in use.

Service populations for these sewers can range from a few hundred thousand people up to a million people in some larger metropolitan areas. Typically they are located in close proximity to natural watercourses and valley systems. In fact, conventional interceptor systems typically incorporate overflow and diversion facilities to these watercourses. It follows therefore that uninterrupted service is critical. Complete failure, or even partial blockage, could lead to sewer system surcharging, widespread basement flooding, extensive public inconvenience, and significant discharges to the environment. The effects of failure could be experienced for several days up to several weeks or months as repairs could likely be difficult and complex. The ultimate consequences of failure could be catastrophic.