TRENCHLESS TECHNOLOGY | SEWER REHABILITATION
Determining the remaining life of concrete sewers Traditionally, the large-diameter outfall sewers installed in South Africa from about the 1950s onwards were made of concrete and designed for a 40-year life. In many cases, these systems have lasted beyond their designed lifespan; in others, they have had to be replaced or are in need of rehabilitation or replacement. By Alaster Goyns, Pr Eng*
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he deterioration of these sewers is mainly due to biogenic corrosion and particular problems arise when gradients are inconsistent or sewers are downstream of rising mains. To perform optimally, these pipelines should flow partly full under gravity at reasonable gradients, which ensures effective operation. Where gradients are too flat, there is insufficient oxygen in the slow-flowing effluent, resulting in hydrogen sulfide (H2S) gas being generated. When there are steep gradients, any H2S that has been generated is stripped out of the fast-flowing effluent and then biologically converted to sulfuric acid (H2SO4). This H2SO4 then attacks and corrodes any alkaline pipe material, such as concrete or fibre cement. The structural integrity of the pipes is then compromised; if this is not addressed timeously, the sewer collapses. However, before a sewer collapses, it starts leaking – resulting in cavities forming around the leaks and becoming water paths running adjacent to the sewer. The flow along these paths removes the bedding support around
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the pipes, further reducing their load-carrying capacity and shortening the time to collapse. As these sewers are frequently placed at depth below the surface, such a collapse of the pipe/soil system invariably results in problems for the services above. Although many of the outfalls installed in South Africa since the 1950s and 1960s are still in operation – way beyond their planned operating life – some have deteriorated and collapsed. When this happens, the costs are several times that of replacement, even before the unaccounted-for costs to the public are considered. To prevent this from happening, any sewers where this could happen should be replaced or rehabilitated. In urban areas in particular, the use of trenchless techniques is generally the best approach.
Replace or rehabilitate? Before deciding about replacing or rehabilitating a pipeline, an assessment of its performance and condition should be done. This will establish how well its hydraulic and structural requirements are being met and enable an estimate of its remaining
service life. Recently, the great strides made with CCTV and associated inspection systems mean that a complete picture inside sewers, as well as a quantification of internal dimensions, can be provided. Combining this information with the loading conditions, and an understanding of the corrosion mechanism, provides input for the structural analysis of sewers and an estimate of their remaining life. The extent and severity of the problems can be established and Corrosion mechanism in sewers
H2SO4 FORMATION H2S RELEASE
H2S GENERATION
