Report No FR0413



MARCH 1994



The development of proposals for the use of planned maintenance, outlined in this report, should lead to savings in operational costs on maintenance of non-critical sewers.


The objective of this study was to identify a more cost-effective approach to the maintenance of non-critical sewers, and to identify future research requirements if appropriate.


There are approximately 235,000km of non-critical sewers in England and Wales, representing approximately 80% of the total length of sewers. Maintenance is currently carried out on a reactive basis. The total expenditure on the maintenance of non-critical sewers in England and Wales is approximately 6.5 million per year to deal with failures and approximately 30 million for clearing blockages. There is concern that the reactive maintenance approach may not prove cost effective in the long term.


  1. The are approximately 4000 sewer failures a year on non-critical sewers in England and Wales. These cost an average of 1600 per failure. The number of failures per year does not appear to have changed since the early 1980's.
  2. Pre-emptive inspection and repair of defects in non-critical sewers to prevent failures is unlikely to be economic except for a very small percentage of non-critical sewers. This is based on current repair costs.
  3. New methods of trenchless repair for isolated defects in sewers may significantly reduce planned repair costs. This may make a pre-emptive approach to maintenance of a much larger proportion of non-critical sewers economic.
  4. Data from a limited study suggests that there are approximately 200,000 blockages each year. Up to 20% of these may be recurring more than once a year. The cost of clearing these blockages is estimated at approximately 30 million per year.
  5. A pre-emptive approach to removing the cause of recurrent blockages is likely to be cost effective in a significant proportion of cases. However further information on the number of blockages and their recurrence is required to confirm this. Information on the extent to which identifiable sewer defects cause recurrent defects is also required.


  1. Planned maintenance of non-critical sewers should only be considered to reduce the number of collapses in a small number of areas where the failure rates are abnormally high.
  2. Further research should be carried out to study the impact that emerging trenchless repair methods will have on the economics of planned maintenance to reduce collapses on non-critical sewers.
  3. Planned maintenance should be considered to reduce expenditure on blockages where they are recurring more than once per year.
  4. Further research should be carried out investigate the extent and causes of recurrent blockages in more detail and to develop detailed guidance on the use of planned maintenance.


This report describe the results of a desk study to determine the feasibility of a new approach to the maintenance of non-critical sewers. The technical and economic appraisals are described and proposals for research to produce definitive methods are described.

Copies of the report are available from FWR, price 15.00, less 20% to FWR Members.