Pipe Deterioration

It is important to assess the condition of the conduit early in the process. Further actions all depend on this important factor.

CMP

Corrugated Metal Pipes (CMP) have been used for the outlet works in many small dams. Undetected corrosion or distress in the pipe is common, and may be an initiator of embankment piping. The deterioration of most old CMPs is an area of concern for dam owners, regulators, and engineers. Cases have been documented where CMP has deteriorated in less than 7 years after construction.

Corrosion:
Uncoated CMP is susceptible to corrosion from a number of sources: soil, groundwater, and water in the pipe. When the corrosion advances to a point where holes begin to develop, water flows into or out of the pipe and begins to erode the soil around the pipe. The rate of deterioration often increases with time.

• Deterioration Accelerates with Time
• Extent of Corrosion May Limit Repair Options
• Deterioration at Invert is Hard to Detect
• Corrosion Occurs from Outside In
• Inspect Full Length of Pipe

Joints and Connections:
Connections are often the weakest part of an aging CMP outlet works.

• Connection between Riser and Conduit often First to Fail

Metal Pipe Treatments:
Common treatments for Corrugated Metal Pipe include galvanizing, aluminizing, polymer coating, and bituminous coating. Proper coatings significantly increase the performance of Corrugated Metal Pipe.

• Bituminous Coating Impact on Design Life of Pipe
• Treatment Types

Abrasion of Coatings:
Gravel & Sand in outlet flow can abrade coating and steel over time. Abrasion is one of the common causes of pipe deterioration. For CMPs, it is primarily a concern for pipe coatings. When a coating becomes damaged, the bare steel is in direct contact with water, which significantly increasing the likelihood of corrosion.

Concrete

Reinforced Concrete Pipe (RCP) is often used in small dams. RCP is corrosion resistant, can be placed under large fills, is hydraulically efficient, and has a long design life. Concrete is a high quality product. Understanding common problems is important to effectively assessing your conduit.

Types of Concrete Pipe Damage:
Common issues to look for during inspections of RCP conduits are cracks in the conduit and spalled concrete. Cracks are expected in concrete, knowing what size crack to be concerned about is important. Cracks in the conduit typically occur at the transition immediately downstream of the control structure due to differential settlement. Spalling often occus in precast concrete pipe at the joint locations.

• Exposed Steel
• Longitudinal Cracking

Joints and Connections:
RCP conduits are connected using a bell and spigot type of connection and can be constructed to accomodate some expected settlement along the conduit alignment due to the flexibility provided at each joint location. However, joints and connections are common places for water and soil to flow into your pipe. Inspectors should assess all joints and any connections to the pipe

• Joint Deterioration - Pay me now or pay me later

Quality concrete can provide a very long service life:
Concrete can last a very long time. High quality concrete typically won't burn, rust, tear, buckle, deflect, and it's immune to the attack of most elements. If it looks good, it probably is!

• Service Life

Steel/Ductile Iron/Cast Iron

Smooth steel conduits are typically coated in the factory, but joints are coated in the field. When coatings deteriorate, corrosion begins. Smooth steel pipe is typically much thicker than Corrugated Metal Pipes and will take longer to develop corrosion holes. Corrosion is still a serious concern, regardless of how thick the pipe is.

• Continuous Flows can Reduce Deterioration in Downstream Valves

Note: The content on this page was originally created as part of DamOutletWorks.org (DOWL, 2018). It has subsequently been updated and reformatted as part of the Dam Safety Toolbox.
Revision ID: 3130
Revision Date: 09/15/2022