Cost/benefit analysis

The case for clay

Economic Considerations

General

The Water Services Association of Australia (WSAA) has issued its draft National Sewer Code and included in this publication is the recommendation that Authorities apply WHOLE OF LIFE COST ANALYSIS when comparing tenders of differing materials.

The factors involved in such an analysis are:-

1. Initial cost of supply
2. Construction costs
3. Maintenance costs
4. Material life cycle
5. Replacement costs

With the recent introduction of Hepworth SuperSleve calsined clay pipe into the Australian market, Authorities and Consultants now have access to a high quality, consistent performing pipe and joint which offers a projected life of 150-200 years. This is well in excess of the 30-50 years life that can be expected from the various types of plastic pipes.

The SuperSleve clay pipe is a calsined pipe ie it is produced from blended fine ground clay (ground to a particle size of 0.25mm compared to the Australian Standard allowable of 2.5mm) and fired in automatic continuous roller kilns.

WHOLE OF LIFE COST ANALYSES should be a mandatory exercise for Authorities and Consultants to ensure best asset management practices. Whilst such practices are generally employed in major sewerage schemes, pressure by Developers to achieve low cost subdivisions has resulted in material selection being limited to initial costs only.

Attached (Attachment 1) is an article showing that Auckland City, after consideration of all the above factors, legislated by way of a bylaw to ensure that rubber ringed vitrified clay pipe was the only approved pipe for its public sanitary sewer system. In 1997, Wellington City also made the decision not to allow the use of uPVC in its public drainage systems and circularized Consultants accordingly as per the attached (Attachment 2).

The decision by the Auckland City Council was mainly based on:

• A longer projected life provided an expectation of reduced spending from $12.5M to $5.0M annually to keep its sewerage system operative
• Because clay pipe does not distort its shape (does not become oval) it should provide a framework to line or grout in future rehabilitation

Initial Cost of Supply

In large sewerage schemes all types of pipe are considered by Authorities not only on initial cost of supply but also on the performance of the pipe to provide the most benefits to the Asset Owner.

In subdivisional developments the Developer is mostly concerned with initial costs and it has been on this basis that tender documents have been structured to the extent that in some cases only one pipe material is specified.

Generally clay pipe supply costs are higher than uPVC pipe. Thus it is only when Whole of Life Cost Analysis principles are used, that clay pipe becomes the preferred material.

Construction Costs

Specifications for construction of a sewerage line generally favour rigid clay pipes over flexible uPVC pipes because of the inherent strength of clay pipe. Because of this inherent strength in a clay pipe, the Contractor has the advantage of using Type 1 as the minimum bedding and support requirement specified by Sydney Water with the comfort that the pipe will not deform from construction or backfill settlement loads.

The flexibility of uPVC necessitates the use of Type 3 as the minimum bedding and support requirement specified by Sydney Water to assist the pipe withstand the applied loads without deformation.

The extra costs in construction of a uPVC line for the material and care in placement of the Type 3 support must be taken into account by the Contractor in the preparation of his tender. Unfortunately this is not always the case as the Contractor attempts to meet market prices.

Contractors are obliged to carry out ovality testing on uPVC sewerage pipelines before acceptance and handing over to the Asset Owner. For this ovality testing to be meaningful, the testing should be delayed until all construction and settlement loads have occurred. As deformation is a critical factor in the long term (and short term) performance of uPVC pipes, ovality testing at the end of the maintenance period should be enforced by Asset Owners.

The cost of this testing and the costs determined by risk analysis should again be taken into account by the Contractor.

Notwithstanding the high interface rubber ring pressures design to inhibit root intrusion, the full range of SuperSleve Pipes can be joined easily by the traditional manual crow bar and block method

Maintenance Costs

Maintenance costs of gravity sewerage systems generally relate to clearance of blockages and restriction of infiltration which can result from root intrusion at joints, faulty pipe and joints and loose manhole branch joints.

Hepworth SuperSleve calsined clay pipe and joints offer superior features to past and current Australian manufactured clay pipes - these are explained in detail under “Pipeline System Features�.

The Hepworth SuperSleve Coupling is manufactured from polypropylene which retains its properties better than uPVC at high temperatures with a quoted maximum of 100oC compared with 60oC for uPVC. The copolymer polypropylene materials used have a balance of creep properties and excellent impact properties even at low temperatures.

Hepworth Polypropylene Couplers were first introduced in 1965 and there are no known failures in the many millions of metres of sleeved pipes that have been laid over the period. The SuperSleve Coupler’s performance is enhanced by the inherent strength of the coupler as it is reinforced by internal and external ribbing which is not possible on conventional uPVC pipe. This ribbing together with the support from the rigid clay pipe prevents ovality in the coupling. It is recognised that ovality is the predominant factor to failure of uPVC pipes and especially at joints where differential deformation can occur because of the difference in thickness of the spigot and socket.

Hence the SuperSleve Coupler can be confidently expected to have a projected life at least equal to the projected life of the clay pipe.

Hepworth’s SuperSleve calsined clay pipes also offer advantages over uPVC pipes in the reduction of maintenance costs. In particular long term clearance of roots and blockages utilising the latest technology of powered rodding tools and high pressure jetting is likely to be compromised as the uPVC ages and the pipe becomes brittle.

Unfortunately there is no known published historical financial data on maintenance available to apply to the various pipe materials in a whole of life cost analysis. Hepworth SuperSleve clay pipe and even uPVC pipe have not been used for a sufficient period in Australia for comparative financial data to be collected. However, based on Auckland City Council’s data and assessments in their decision to utilise clay pipe exclusively, for every $1.00 spent on maintenance they expected to be able to reduce this to $0.40 by using clay pipes in their sewerage systems.

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 Attachment 3).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

Attachment 4 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