Prudent Risk Analysis
When an engineer analyzes a stormwater drain site there are numerous and complex factors to consider before selecting a pipe material or materials from the many pipe materials available today. Phase I of the selection-elimination process would be to determine the size of pipe or pipes needed to carry the 20-50-100 year projected storm flows of a particular site. Once a project has been pipe sized, the Phase II step would be to perform a Life-Cycle-Economic Cost Analysis. The engineer would consider the chemical characteristics of the liquid flow, sediment load in the flow and the soil properties along the drain feed and main lines. The best choice of a pipe material will be one that yields the best performance over the expected project life. Some materials have longer life prospect in a chemically aggressive environment than others. The engineer is looking for Durability. In culverts and storm drains Durability means comparing the projected life of pipe materials to the desired service life of the project, or the capability of the material to withstand wear and decay. While a single material may be so superior that no alternative materials are further considered, the reality is that the pre-programmed mindset of the modern day analyzing engineer is likely to produce several pipe materials. The allowable use of two or more materials may in fact be necessary to meet both commercial product availability and the lifecycle Durability requirements; the thought being that no single pipe material is likely to provide the optimum capability for all design conditions in every feeder line or main.
Once two or more pipe materials have been selected, Risk Analysis considerations are made of the material or materials proposed. This is Phase III in the selection-elimination process. Risk Analysis is the quantification of a pipes exposure, vulnerability and probability of site-specific premature failure. It involves the evaluation of alternative means to reduce risk, and the determination of acceptable levels of risk. Risk Analysis considers numerous factors in evaluating what are the Risks to a pipe system’s Durability lifecycle. The suitability and limitations of each material to survive the selection process to this point is considered, and the probability of an unwanted consequence occurring weighed with a tempered judgement.
Most culvert storm sewer pipes are buried and never seen, as they are apart of streets with curb inlets, trash racks, catch basins and the like. All these interlocking components reduce the probability of some unforeseen harm to a storm pipe from the outside.
The main concern however, is not exterior harm but interior harm to pipes located in chemically aggressive environments where the process of corrosion, abrasion and erosion have a history of affecting the Durability of a pipe’s expected service life. The interior of corroded, abrasion worn pipes is not easily repaired, and is a risky unknowable to municipal, state and federal accountants, who do not like budgetary surprises. Budgetary managers consider storm sewer pipe line maintenance to be apart of the life of a pipe systems life from installation to full depreciation and replacement 50 or 100 years later. Engineers and government cost accountants recognize in their Life-Cycle-Cost studies that pipe materials which employ corrosion inhibitor coatings never stop, halt or prevent the abrasion-corrosion process, but only slow the process. The degree of retardation is always unpredictable at chemically aggressive sites because the pH level is never stable or easily quantified. This in turn produces unpleasant budgetary surprises.
The uncertain Durability track record of storm drain pipes in chemically active systems and resulting uncertain cost have caused engineers to look at High Density Polyethylene (HDPE) Corrugated Pipes since the mid 1980’s. Corrugated HDPE pipes have been approved for culvert storm drains by many state DOT’s because they have been shown to be structurally H-20 liveload capable in depths from 12 inches to 104 feet (see June 1989 CPPA PIPELINE ISSUES). They have been the product of choice because they offer better long term Durability than protective coated pipes whose coats can be easily stripped of by abrasive sediment flows and corrosive waters. The pipes interior wall is protected by the chemical inertness of the HDPE plastic raw material. Consequently long term, unknown maintenance cost surprises have more predictable future.In the instances where HDPE plastic pipes are likely to be exposed to the exterior environment, prudent Risk Analysis management will note that plastic pipes, like Bituminous coated metal or concrete pipes must be protected with fire proof headwalls or other management practices that reduce the probability of fire, damage by mowers, snow plows, errant automobile traffic, etc. as noted by Hurd (Oct 87 public works) and others. These same protective measures also protect plastic culvert ends from sunlight and Bituminous coated culverts from both sunlight or freezing weather that weakens the tar bond. Firm headwalls also prevent erosion around culvert ends made of any material, and assures hydraulic performance satisfaction with less maintenance.
In the few remaining installation possibilities where headwalls might not be used, it is normally the DOT practice to mow from three times per year in a semi arid climate to six cuttings in less arid areas. HDPE plastic does not burn at temperatures below 700 ˚ F. Ample cuttings as described above will prevent the buildup of weed-grass fuel potential for plastics. Bituminous covered pipes however have a lower flash point of approximately 400 ˚ F and would still require some type of headwall.
In summary, there are numerous risk reduction installation management practice techniques already in use that can reduce or eliminate external damage to CPP culvert pipe systems. However, there are no known installation techniques that significantly improve the Durability (Life-Cycle Cost Ratios) of non-plastic pipe products in corrosive-abrasive situations.