"Whole of life" cost benefit
Material Life Cycle
Engineering considerations indicate that clay pipes have a design life of at least 80 years (and in fact is well in excess of this) compared to uPVC pipes of 40 years.
The life of a uPVC pipe however depends entirely on the quality of construction and/or future ground movement and settlement. uPVC pipe and flexible pipes in general rely on backfill side support to maintain circularity. Poor compaction or future settlement which results in this loss of side support would cause ovality in the pipe and a concentration of stresses leading to accelerated aging of the plastics with the ultimate risk of premature failure.
A conservative approach in a whole of life cost analysis would be to use the design life of 80 years and 40 years respectively for Australian produced clay and uPVC pipes. For Hepworth SuperSleve calsined clay pipe because of the production process and materials used a life period of 100 years would be conservative.
Replacement Costs
With the technological advances made in relining equipment and processes, it is more likely that rehabilitation of sewerage pipeline systems will occur rather than replacement. However such rehabilitation will only be possible where the pipe system has maintained its circularity (unless a significant loss of diameter can be tolerated). Thus the cost and feasibility of rehabilitation of uPVC pipelines is highly dependent on the extent that ovality has occurred. If this is a major problem then the more expensive replacement of the pipe would be required. Further, rehabilitation of uPVC lines by grouting is not practical because of poor bonding between the grout and plastic.
The Water Research Centre UK published in 1986 an ‘Economic comparison of rehabilitation options with different lives’ which was agreed nationally throughout the UK for use in project appraisal and current cost accounting as follows: (Refer Table K1).
Assuming an inflation rate of 5%, this table indicates that projects involving pipes of 225mm and less are assessed on a whole cost of life analysis by adding 14.4% (16.5% - 2.1%) to the total supply and construct initial cost of the uPVC system when compared to the clay system costs. Similarly for projects involving 300mm to 1000mm pipes 15.7% (16.5% - 0.8%) is added to the total supply and construct initial cost of the uPVC system to cater for future replacement/rehabilitation costs.
Whole of Life Analysis
Appendix 1 provides a simple approach for whole of life cost analysis of optional pipe systems. Discount rates may vary between 5% to 8% and it is suggested as per Table K.1 that a design life of 40 years for uPVC should be used and 80 to 100 years (and this is conservative) is appropriate for clay pipes. For calsined pipes a minimum of 100 years life would be conservative.
Summary
Cost of supply of pipe and total initial cost of supply and construct should not be the predominant elements in the choice of pipe for a sewerage system/scheme.
Best practice asset management dictates consideration of whole of life cost analyses and this is recommended under the new Australian WSAA Code of Practice.
All factors involved in the long term performance of the system should be considered to determine the whole of life cost analysis.