Description and validation of structural condition

All wastewater networks deteriorate over time. This deterioration increases the probability of failures such as obstruction and collapse, which may lead to flooding, street and building damage, traffic disturbances and inconveniences for users connected to this sewer. Partially collapsed sewers have reduced flow capacity, which may lead to flooding. In the worst case of collapse, large holes may result in loss of lives.

The structural condition of a pipe depends on material, construction practices, external load and wastewater characteristics. Previous research has shown that pipes laid within certain time periods have been structurally under-designed, and that construction practises within some time periods have not been appropriate, thus leading to particularly frequent failures.

The formation of hydrogen sulphide that occurs in some wastewater networks will lead to a microbiological deterioration and loss of strength for concrete sewers, and is a very important reason for rehabilitation actions. Other reasons are root intrusion and substantial in- and ex-filtration through fissures and leaky joints.

Structural analysis of sewers is normally based on results from CCTV inspection. In Europe there are several systems currently being used for the classification of individual data from such investigations. A standard European code for the description of sewer damages has been developed and recommended to the member States. In general, the condition of sewers is classified according to its most severe damage or overall condition to determine the urgency of rehabilitation and calculate the cost of rehabilitation.

In this project, different models for the classification and assessment of the sewer condition were analysed and tested with CCTV data provided by end-users. Models describing the process of deterioration for specific failure modes rely on this type of data. Models were developed or improved and calibrated with such data from the end-user sewer networks. These models allow a forecast of the year in which a sewer will enter a critical class of condition, thus determining the next inspection date or rehabilitation measure.

Published January 14, 2008

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