Chapter 14: Conduit Strength and Durability

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Section 1: Conduit Durability

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Introduction

When designing a culvert or storm drainage system, you must evaluate aspects of structural design, hydraulic design, and durability design. The first two disciplines are quite familiar to most civil engineers. Durability design, however, is generally beyond the scope of civil engineering and is more closely aligned with the field of chemistry. Experience has shown that culverts most frequently fail as a result of durability problems. This is usually due to improper selection of materials to meet the project design life and site conditions.

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Service Life

For permanent TxDOT hydraulic facilities, an ideal service life expectancy is generally 50 years. However, the scope and intended use of the facility and economic considerations may warrant longer or shorter service life. Many factors affect durability, each independently affecting different aspects of the facility:

With knowledge of these factors, the designer should exercise some control over choice of material, design of the facility, and maintenance practices.

Relative service life of conduit material is a function of the corrosion/abrasion cycle. You can predict the relative service life based on the evaluation of soil and water site characteristics such as the following:

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  • Acidity/alkalinity -- The universal measure for acidity/alkalinity is the pH scale. Acidity can result from either mineral or organic sources. Mineral acidity can be the result of leaching of acidic soil, runoff from mining activities, and acidic rainfall. Organic acidity may result from organic decay such as runoff from a large feedlot. Relative service life of materials used in conduits is a function of the pH value of the soil and water. High acidic values in the soil and water (pH<4) represent a greater threat to the conduit material service life. High alkalinity values in the soil and water (pH>9) also represent a significant threat to the conduit material service life.
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  • Resistivity -- Resistivity is a measure of the electrical current carrying capacity of a material. If the resistivity value (expressed in ohm-cm) is low, the current carrying capacity is high. In such a case, the potential for corrosion is also high. In general, the higher the resistivity, the lower the potential for corrosion due to resistivity.
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  • Abrasion -- Abrasion is a function of flow velocity and bedload. High flow velocity and the presence of an abrasive bedload in the water cause scour or erosion to the conduit material. Abrasive bedloads are typically not transported when flow velocities are less than 5 fps (1.5 m) per second. While this is a damaging mechanism leading to deterioration and further exposure for the mechanism of corrosion, it is not a common problem in most parts of Texas. In very hilly and rocky areas, consider abrasion as a possible threat to the expected service life of the conduit.

The hydrogen ion content (pH) of the soil and water and the resistivity of the soil and water determine the relative effect of a site on the durability of a drainage structure. The geotechnical report of the highway project may include information regarding pH values and resistivity values for soil and water associated with the project. Particularly sensitive cases may justify determining pH and resistivity values at specific facility sites.

Where corrosion is a threat, consider structure material choice and possibilities of material protection. Under no circumstances arbitrarily select the structure material. In some instances due to specific experiences with various materials, local practice or policy may dictate use of certain materials in drainage facilities. Where policy dictates selection of the material, document the basis of the policy.

For alkalinity or acidity and for resistivity consider all soils in contact with the culvert conduit, inside or outside, including:

Acidity in the water may occur in either the runoff water or the ground water in the area of the facility.

The resistivity value correlates directly with the salt content of the soil or water. The presence of salts in the soil or water at a facility site can affect both the pH value and the resistivity. Calcium carbonate inhibits corrosion, and certain chlorides and sulfates increase the potential of corrosion. Generally, the project geotechnical report will address the salt characteristics of soils and water if the resistivity is greater than 7,500 ohm-cm.

Evaluate the abrasion level of the drainage facility. Select conduit material and conduit protection based on the abrasion level. Abrasion is classified by the following levels:

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  • Level 1 - non-abrasive - little or no bedload and very low velocities (less than 5 fps or 1.5 m per second)
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  • Level 2 - low abrasive - minor bedloads of sand and low velocities (less than 5 fps or 1.5 m per second)
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  • Level 3 - moderate abrasive - moderate bedloads of sand and gravel and average velocities (5 to 15 fps or 1.5 to 4.5 m per second)
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  • Level 4 - severe abrasive - heavy bedloads of sand, gravel, and rock, and high velocities (greater than 15 fps or 4.5 m per second)

Countermeasures to level 3 and level 4 abrasion may include one or a combination of the following:

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  • reducing the flow velocities in the conduit.
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  • for metal pipes, selecting a heavier gage metal (sacrificial material).
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  • burying the invert of the conduit.
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  • for metal pipes, installing invert protective linings such as bituminous paved invert, concrete paved invert, bituminous lining, and concrete lining.
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