“Modern Sewer Design” is the result of a - Jensen Bridge & Supply

CHAPTER 8Durability251INTRODUCTIONCorrugated steel pipe (CSP) has been used successfully since 1896 for stormsewers and culverts throughout the United States and other countries. It continuesto provide long service life in installations that cover a wide variety of soil andwater conditions.Since the initial applications before the turn of the century, an estimated 50,000installations have been the subject of critical investigative research to establishdurability guidelines (1,2) . The behavior of both the soil side and the effluent side ofthe pipe have been studied. These studies have shown that CSP provides outstandingdurability with regard to soil side effects, and that virtually anyrequired service life can be attained for the waterside by selecting appropriate coatingsand/or pavings for the invert.Of course, all pipe materials show some deterioration with time, and such effectsvary with site conditions. To aid the engineer in evaluating site conditions andselecting the appropriate CSP system, the main factors affecting durability and theresults of field studies will be reviewed before presenting specific DurabilityGuidelines.FACTORS AFFECTING CSP DURABILITYDurability in SoilThe durability of steel pipe in soil is a function of several interacting parametersincluding soil resistivity, acidity (pH), moisture content, soluble salts, oxygencontent (aeration), and bacterial activity 3,4,5 . However, all of the corrosion processesinvolve the flow of current from one location to another (a corrosion cell).Thus, the higher the resistivity and/or lower the soil moisture content, the greaterthe durability. Table 8.1 lists typical ranges of resistivity values for the primary soiltypes 6 .A study performed by Corrpro Companies in 1986 found that soil-side durabilityis generally not the limiting factor in designing CSP systems. “Survey resultsindicate that 93.2 percent of the plain galvanized installations have a soil-side servicelife in excess of 75 years, while 81.5 percent have a soil-side service life inexcess of 100 years.” In the vast majority of CSP installations, durability is controlledby the invert (water side) of the pipe.The study also found that soil moisture contents below 17.5 percent did notexhibit any accelerated corrosion. “Under most circumstances, corrosion rates aredirectly related to soil moisture content. However, for galvanized steel storm sewerand culvert pipe, the soil moisture content primarily affects the activity of any chlorideions present and the chloride’s acceleration of the corrosion. Where the soilmoisture content was below 17.5 percent, the chloride ion concentration did nothave a significant affect on the corrosion rate of the zinc coating.”Most soils fall in a pH range of 6 to 8, which is favorable to durability. Soils withlower pH values (acid soils), which are usually found in areas of high rainfall, tendto be more corrosive.