Mastering the Design of CMP Detention and Infiltration Systems

Introduction

As land values rise and site complexity increases, civil engineers are under pressure to deliver efficient and cost-effective stormwater management systems. These systems must satisfy increasingly stringent regulatory requirements while maximizing usable land and minimizing lifecycle costs. Corrugated metal pipe (CMP) detention and infiltration systems have emerged as a versatile and adaptable solution that addresses these needs, particularly in urban and constrained project sites.

This article outlines key considerations for selecting and designing CMP systems, providing practical guidance informed by field experience, engineering principles and best practices in constructability. Whether you’re designing for shallow footprints or deep storage, infiltration or detention, the right approach can result in more durable and cost-effective outcomes for engineers and owners.

Why Go Underground?

While traditional surface basins are often less expensive upfront, underground detention systems offer broader value, particularly in developed or high-value areas where space is at a premium.

Maximized Real Estate Utilization

Underground systems free up valuable surface space for parking lots, roadways, structures or landscaped amenities. This is especially critical in urban or commercial developments where every square foot has a direct financial value. By storing stormwater underground, engineers help landowners maximize usable space, allowing for more building, selling, or leasing, ultimately increasing return on investment.

Regulatory Compliance in Tight Spaces

Many jurisdictions mandate stormwater detention, retention or infiltration, but allow flexible configurations to meet those requirements. Underground systems make it easier to meet volume and flow control standards on small or irregularly shaped sites where above-ground basins aren’t feasible.

Lower Long-Term Maintenance Costs

Properly designed and accessible underground systems require less maintenance than vegetated basins or surface features. Strategically placed risers and baffles minimize the need for full-system cleaning, reducing labor and equipment costs.

Enhanced Site Aesthetics and Safety

Removing open water bodies from a site eliminates safety risks and allows for cleaner, greener community spaces. This can improve public perception, reduce liability and eliminate the need for fencing or signage.

Material Selection: Balancing Performance, Cost and Site Conditions

Choosing the right material for an underground system isn’t just about structural integrity; it’s about matching the product to the project’s unique needs.

Storage Volume and Footprint Constraints

The size and shape of the site will largely dictate the layout of your system. When horizontal space is limited, maximizing vertical storage with larger-diameter pipes is often more efficient. Deeper systems reduce the linear footage of the pipe and the overall footprint, cutting excavation and stone backfill costs while preserving surface area.

Site Conditions and Structural Loads

Key site parameters such as live load (e.g., vehicular traffic), construction loading (e.g., crane or equipment weight) and water-table elevation all influence the structural design of the CMP such as pipe gauge, cover requirements and installation techniques. CMP can be customized to handle a wide range of load conditions, from light pedestrian areas to HS-25 truck loading and railroad loading.

Cost and Layout Flexibility

CMP can be fabricated in nearly any shape or configuration, accommodating odd footprints, tight corners or utility conflicts. Custom tees, elbows and manifolds enable engineers to design systems around obstacles rather than redesigning the site to accommodate the stormwater solution, saving time and money.

Durability and Service Life

Service life depends on both structural performance and corrosion resistance. Aluminized Type 2 CMP provides excellent durability in most native soil conditions, with service lives exceeding 75 years. For aggressive soil or water conditions, polymer-coated CMP offers a lifespan of more than 100 years, ensuring that systems remain structurally sound for generations to come.

Soil Chemistry and Water Quality

The native soil’s pH and resistivity influence corrosion potential. Aluminized Type 2 CMP is suitable for soils with a pH between 5 and 9 and resistivity above 1,500 ohm-cm, which applies to approximately 75 to 80 percent of sites across the United States. Knowing this data before selecting materials can prevent premature failure and unplanned maintenance.

Durability and Service Life of Corrugated Metal Pipe

One of the most critical, and often misunderstood, aspects of underground stormwater system design is the long-term durability of the materials used. CMP has a long-standing track record of performance in various soil and water conditions; however, its service life depends heavily on selecting the appropriate coating for the site’s environmental conditions and ensuring proper installation.

Aluminized Type 2: Proven Performance for Most Sites

Aluminized Type 2 CMP features a steel base with a Type 2 aluminum coating, offering excellent resistance to corrosion in soils with a pH between 5 and 9 and resistivity above 1,500 ohm-centimeters. This coating forms a protective layer of aluminum oxide that shields the underlying steel from chemical attack. In most native soils across the United States, Aluminized Type 2 provides a service life of 75 years or more with minimal maintenance.

Polymer-Coated CMP: Enhanced Protection in Aggressive Environments

For sites with more corrosive soils or where extended service life is a priority, polymer-coated CMP provides additional protection. This coating provides superior chemical and abrasion resistance, extending service life to more than 100 years when installed correctly. It’s especially well-suited for stormwater detention systems in industrial areas, landfills or sites with contaminated soil.

Galvanized Steel: Limited Use in Modern Practice

While galvanized CMP was once the industry standard, its use has declined due to its reduced corrosion resistance compared to newer coatings. Galvanized CMP may still be acceptable in select applications with highly neutral soil; however, designers should approach its use cautiously and conduct site-specific testing.

Modern Manufacturing Advances Improve Longevity

Today’s CMP is not the same as the product from decades past. Advances in coating technology, quality control and fabrication have significantly improved the uniformity and performance of CMP materials. As shown in Table 1, field data and long-term studies by organizations such as the NCSPA and the U.S. Department of Transportation (DOT) confirm that modern CMP offers durable and predictable performance.

Installation Matters

The service life of CMP isn’t solely determined by material selection. Proper backfill, compaction and jointing during installation are essential. Infiltration systems require washed, angular stone and separation fabrics to protect the pipe and stone from sediment-laden runoff. Additionally, systems should be checked for any damage done to the pipe during installation, as undiagnosed damage to the pipe may cause issues years later.

Designing for the Life of the Site

Stormwater infrastructure should last as long as the development it serves. With the right combination of coating selection, structural design and installation quality, CMP detention and infiltration systems can meet or exceed the design-life expectations of most commercial, municipal and industrial projects.

Flexible, Custom Layouts

The adaptability of CMP systems provides engineers with unmatched flexibility when designing around site constraints:

Custom Tees, Elbows and Manifolds

As shown in Figure 1, CMP can be fabricated to fit almost any layout, allowing engineers to efficiently navigate existing underground utilities, easements and irregular site geometry. Unlike precast systems that require specific alignments, CMP can be adjusted to site-specific conditions, making it ideal for retrofit and infill projects.

Variable Gauges and Corrugation Options

Engineers can tailor wall thickness and corrugation type to meet structural requirements based on load and depth. This customization ensures that systems are neither under- nor over-designed, achieving optimal performance with minimal material cost.

Strategic Placement of Access Risers

Access points can be positioned at inlets, outlets and high-maintenance areas to facilitate inspection and cleaning. These risers eliminate the need for surface structures, thereby streamlining long-term maintenance planning.

In-System Flow Controls

Orifices, weirs and baffles can be integrated directly into the CMP system, reducing the need for external outlet structures. This not only saves space and material but also simplifies construction, inspection and maintenance process.

Optimizing System Design

Effective stormwater system design is as much about construction efficiency as it is about hydraulic performance:

Go Deep, Not Wide

Increasing system depth allows for the same volume of storage in a smaller horizontal footprint. This reduces excavation volume, minimizes site disruption and conserves valuable surface area for other uses.

Double the Diameter = Quadruple the Volume

As shown in Figure 2, increasing the diameter of the pipe from 48 inches to 96 inches increases the volume per linear foot by a factor of four. This not only increases storage but also reduces the number of pipes, joints and fittings, lowering installation and material costs.

Use Longer Pipe Runs

Long, continuous runs of pipe reduce the need for fittings, such as tees and elbows, which are more expensive and require additional excavation and installation labor. Fewer joints also means fewer potential maintenance points.

Reduce Barrel Spacing

Bringing barrels closer together can reduce system width. However, the spacing must be optimized based on the structural characteristics of the backfill material. Consult with a manufacturer or geotechnical expert to confirm safe spacing for your site.

Simplify Manifold Design

A single manifold with bulkheads or end caps is usually more cost-effective and easier to install than systems with multiple distribution lines. Using smaller-diameter manifolds for larger systems can also reduce material cost and speed up installation.

Infiltration System Design

When designing infiltration systems, attention to detail is critical for performance and compliance:

Verify Infiltration Rate and Drawdown Time

Infiltration systems must empty within a specific timeframe—often 24 to 72 hours—to meet regulatory standards. Accurate testing and conservative assumptions help ensure your design performs as expected.

Use Free-Draining, Washed Angular Stone

Angular stone with high void space (typically 37 to 40 percent) allows for both structural support and temporary water storage. Clean stone ensures rapid infiltration and resists clogging. Be aware that some agencies may apply more conservative void assumptions, which may require a larger stone volume.

Include Geotextile Fabric

Wrapping the stone envelope in non-woven geotextile prevents fines from migrating into the voids, which could clog the system through time. This barrier is essential for maintaining infiltration capacity and long-term function.

Account for Groundwater Levels

Systems must be installed above the seasonal high-water table to avoid flotation and loss of infiltration capacity. If shallow groundwater is present, designers may need to raise the system invert or switch to a watertight detention design.

Maximize Storage with Stone Layer Adjustments

Increasing the thickness of the stone layer beneath or above the pipe provides additional storage without enlarging the system footprint. Adjusting stone layer depth in 1-inch increments enables fine-tuning of volume and cover, especially in shallow cover conditions.

Maintenance Considerations

Reliable long-term performance depends on accessible and maintainable system design:

Strategically Locate Risers

Positioning access points near inlets and outlets simplifies inspection and cleaning. Use risers as surface inlets to reduce the need for separate catch basins and improve hydraulic efficiency.

Incorporate Sediment Isolation

Internal baffles can isolate sediment near inlets, keeping the rest of the system clean. This reduces the frequency and cost of full-system cleanouts.

Use Pretreatment Units

As shown in Figure 3, installing hydrodynamic separators or other pretreatment devices upstream of the CMP system captures debris and fine sediment before it enters the storage area. This protects the stone voids in infiltration systems, thereby extending the system’s lifespan.

Plan for Long-Term Ownership

Maintenance should be discussed with the owner early in the design process. Systems that are easy to access and clean are more likely to be properly maintained, protecting your reputation and the client’s investment.

Common Design Pitfalls to Avoid

Even the best designs can be undermined by avoidable mistakes:

Ignoring Construction Loading Requirements

Temporary cover may be needed during construction to support equipment. Failing to plan for this can lead to system damage before the site is complete.

Inadequate Setback from Structures

Underground systems must not undermine foundations or be placed within their zone of influence. Coordinate closely with structural and geotechnical engineers when working near buildings.

Overlooking Tree Root Intrusion

Trees can damage all underground systems through time. Use root barriers and choose non-invasive species when planting above these systems.

Incomplete Soils Data

Design decisions depend on accurate pH, resistivity and infiltration rate data. Conduct geotechnical investigations early in the design process.

Incorrect Pavement Cover Assumptions

Concrete and asphalt behave differently in load transfer. Include rigid pavement in your cover-depth calculation but exclude flexible pavement layers when determining the required cover thickness.

Conclusion: Tools and Support for Smarter Design

CMP systems offer unmatched versatility and efficiency when designed thoughtfully. From material selection to layout optimization and long-term maintenance, engineers who understand how to use CMP effectively can deliver high-value, low-risk stormwater solutions.