Underground drainage pipes are exposed to soil pressure, construction loads and other external forces after installation. Unlike pressure pipes, which are often evaluated mainly in relation to internal pressure, buried drainage pipes must also maintain appropriate structural behavior under external loading.

For thermoplastics pipes used in underground drainage systems, ring stiffness is therefore an important technical parameter.

When selecting perforated corrugated drainage pipes for roads, retaining walls, slopes and other civil engineering projects, buyers may encounter specifications such as SN4, SN8 or references to EN ISO 9969.

What do these terms mean, and how should they be used when selecting a drainage pipe?

1. What Is Ring Stiffness?

Ring stiffness describes the resistance of a thermoplastics pipe to radial deformation under specified test conditions.

In simplified terms, a pipe specimen is placed between two parallel plates and compressed under controlled conditions.

As the load is applied, the pipe deforms. The relationship between the applied force, pipe dimensions and resulting deformation is used to determine ring stiffness according to the applicable test method.

For buried drainage pipes, this parameter is relevant because the pipe is surrounded by soil and may also be affected by loads transferred from the ground surface.

Ring stiffness therefore provides useful information when evaluating the structural characteristics of a thermoplastics drainage pipe.

However, it should not be considered independently from the installation conditions.

2. Why Does Ring Stiffness Matter After a Pipe Is Buried?

A buried flexible pipe does not work independently.

The installed system consists of both:

Pipe + Surrounding Soil

The pipe provides structural resistance, while properly placed bedding and backfill provide support around the pipe.

For this reason, the behavior of an underground drainage system may be affected by several factors:

• Pipe ring stiffness
• Burial depth
• Soil type
• Bedding
• Backfill material
• Compaction
• Groundwater conditions
• Surface loading
• Traffic conditions
• Installation method

This explains why simply asking whether a drainage pipe is “strong enough” does not provide sufficient information for engineering selection.

The pipe specification needs to be considered together with the project conditions.

3. What Is EN ISO 9969?

EN ISO 9969 is associated with the determination of ring stiffness of thermoplastics pipes.

The test uses a pipe specimen subjected to controlled diametric deformation. Force and deformation measurements are then used to determine the ring stiffness according to the test procedure.

For drainage pipe procurement, a reference to EN ISO 9969 means that ring stiffness can be evaluated according to a defined testing method.

However, there is an important distinction:

EN ISO 9969 is primarily a test method.

It should not automatically be treated as the complete product standard for every underground drainage or sewerage system.

A project may also specify other standards covering pipe dimensions, materials, structured-wall construction, fittings and system requirements.

4. What Do SN4 and SN8 Mean?

Buyers of underground drainage pipes frequently encounter classifications such as:

SN4

and

SN8

“SN” refers to a nominal ring stiffness classification.

In procurement and engineering specifications, these classes are used to distinguish pipes according to their nominal ring stiffness.

However, a higher SN class should not automatically be interpreted as:

Higher SN = Better Pipe for Every Project

The appropriate ring stiffness class depends on the actual installation.

For example, selection may need to consider:

• Burial depth
• Soil conditions
• Backfill quality
• Compaction
• Traffic loads
• Pipe diameter
• Pipe wall structure
• Installation environment
• Applicable project standard

The required SN class should therefore be confirmed according to the engineering specification.

5. SN4 or SN8: How Should a Buyer Choose?

There is no universal answer that SN4 or SN8 should always be selected.

Consider two different installations.

Project A

A drainage pipe is installed at a controlled depth in an area with relatively limited surface loading.

Project B

A drainage pipe is installed under different burial and loading conditions, with vehicle traffic above the installation area.

Even if both projects use the same pipe diameter, their structural requirements may be different.

For this reason, buyers should provide more information than simply:

“We need 200 mm drainage pipe.”

A more useful inquiry would include:

Diameter + Ring Stiffness + Burial Conditions + Application + Standard

This allows the technical specification to be matched more closely to the actual project.

6. Is SN8 Always Required for Road Drainage?

Not necessarily.

Road drainage covers many different installation environments.

A drainage pipe installed beneath a traffic lane may be subject to different conditions from a pipe installed beside the road, behind a retaining structure or within a protected drainage trench.

Therefore, the ring stiffness class should be selected according to the actual project design.

Important information may include:

• Installation position
• Burial depth
• Vehicle loading
• Soil conditions
• Backfill
• Pipe diameter
• Required design life
• Project standard

The engineering specification should take priority over a general assumption that one SN class is suitable for all road projects.

7. Why Is Pipe Diameter Not Enough for Procurement?

Pipe diameter determines part of the hydraulic and geometric requirements of a drainage system, but it does not fully describe the structural requirements.

For example, two projects may both require a 200 mm perforated drainage pipe.

One pipe may be installed in a relatively shallow drainage trench.

Another may be installed at greater depth or under different surface loading.

The diameter is identical, but the installation conditions are not.

Therefore, underground drainage pipe procurement should not be based only on:

Diameter + Length + Price

Structural requirements should also be confirmed.

8. Why Does Pipe Wall Structure Matter?

Underground drainage pipes can use different wall structures.

Depending on the product design, buyers may encounter:

• Corrugated pipes
• Structured-wall pipes
• Smooth-wall pipes
• Other thermoplastics pipe constructions

The geometry of the pipe wall contributes to its structural behavior.

For corrugated or structured-wall products, ribs and profile geometry are part of the pipe design.

This means two pipes cannot necessarily be compared only by looking at their nominal wall thickness.

Material, profile design, diameter and manufacturing consistency also need to be considered.

9. Why Is Wall Thickness Alone Not Enough?

A common purchasing question is:

“What is the wall thickness?”

Wall thickness can be a relevant product parameter, but it does not independently describe the structural behavior of every pipe design.

For structured-wall and corrugated pipes, performance can depend on:

• Raw material
• Pipe profile
• Rib geometry
• Wall construction
• Diameter
• Manufacturing process

A pipe with a different wall structure may achieve a particular ring stiffness without having the same wall thickness as another design.

Therefore, where ring stiffness is specified by the project, buyers should compare the applicable SN class and test requirements, rather than using wall thickness alone as the purchasing criterion.

10. Ring Stiffness Is Not the Same as Overall Pipe Performance

Ring stiffness describes a specific response of a pipe under defined testing conditions.

It should not automatically be treated as a complete measurement of all pipe properties.

Other characteristics may include:

• Impact behavior
• Material properties
• Joint performance
• Dimensional consistency
• Perforation design
• Hydraulic characteristics
• Installation behavior

This distinction is important in engineering procurement.

A pipe may meet a specified ring stiffness requirement while other project requirements still need to be checked separately.

11. Why Does Installation Quality Matter?

Even when the appropriate pipe is selected, installation conditions remain important.

Flexible thermoplastics pipes interact with the surrounding soil after burial.

Installed performance may therefore be affected by:

• Trench preparation
• Bedding
• Sidefill
• Backfill
• Compaction
• Burial depth
• Construction equipment
• Installation sequence

Ring stiffness should not be used as a substitute for appropriate installation design.

A pipe with a specified stiffness class still needs to be installed according to the project requirements.

12. Why Are Bedding and Side Support Important?

Bedding helps provide a suitable foundation beneath the drainage pipe.

Sidefill and surrounding material provide support around the pipe.

If the foundation is uneven or the side support is inadequate, the installed pipe may experience different loading conditions from those anticipated in the design.

This is especially relevant for flexible buried pipes because the pipe and surrounding soil work together structurally.

Therefore, when investigating deformation in an underground pipe, it may be necessary to check both:

Pipe Specification

and

Installation Conditions

rather than assuming that the pipe itself is the only variable.

13. What About Perforated Corrugated Drainage Pipes?

Perforated drainage pipes contain openings that allow water from surrounding soil or drainage layers to enter the pipe.

They may be considered for applications such as:

• Road subgrade drainage
• Retaining wall drainage
• Slope drainage
• Foundation drainage
• Landscape drainage
• Other subsurface drainage systems

For these products, ring stiffness is only one part of the selection process.

The system may also need to consider:

• Perforation arrangement
• Water collection requirements
• Pipe diameter
• Hydraulic capacity
• Geotextile filtration
• Surrounding aggregate
• Outlet design
• Fittings

A perforated pipe should therefore be considered as part of a drainage system rather than as an isolated product.

14. Why Is Geotextile Often Used in Subsurface Drainage?

Underground drainage involves both water movement and soil particles.

If fine soil particles migrate continuously into the drainage layer, the drainage system may gradually be affected by sediment accumulation or clogging.

A suitable geotextile may be used as a filtration and separation layer according to the engineering design.

Its function may include allowing water to pass while helping control the migration of surrounding soil particles.

However, geotextile selection should consider the actual soil and drainage conditions.

Simply wrapping a drainage pipe with an arbitrary fabric does not automatically create an appropriate filtration system.

15. Plastic Blind Drain or Perforated Drainage Pipe?

Plastic blind drains and perforated drainage pipes can both be used in subsurface drainage systems, but their structures and installation methods are different.

A plastic blind drain provides a three-dimensional drainage pathway and may be used where distributed water collection is required.

A perforated drainage pipe allows surrounding water to enter through openings and then conveys the collected water toward an outlet.

Selection can depend on:

• Water source
• Drainage direction
• Installation space
• Required flow capacity
• Soil conditions
• Structural requirements
• Project design

In some projects, several drainage materials may be used together rather than choosing only one product.

16. Why Should Ring Stiffness Be Verified Rather Than Judged by Appearance?

Two corrugated pipes may look similar externally while having different structural characteristics.

Visual inspection alone cannot determine the ring stiffness of a thermoplastics pipe.

Where the project specifies an SN class or ring stiffness test, the buyer should review the relevant technical documentation and, where required, test information.

This is especially important in international projects where tender documents or engineering specifications may reference a particular standard.

17. What Is the Relationship Between EN ISO 9969 and EN 13476?

EN ISO 9969 and EN 13476 may both appear in underground drainage pipe specifications, but they are not interchangeable.

EN ISO 9969 concerns the determination of ring stiffness of thermoplastics pipes.

EN 13476 is a standard series for structured-wall piping systems made from unplasticized PVC, polypropylene and polyethylene for underground non-pressure drainage and sewerage applications.

A project may therefore specify that ring stiffness is determined according to EN ISO 9969 while requiring the applicable piping system to meet relevant requirements of EN 13476.

The exact applicable part and requirements should be confirmed according to the pipe material, structure and project specification.

18. What Should Buyers Confirm Before Ordering?

A useful underground drainage pipe inquiry should include more than pipe diameter.

Before requesting a quotation, buyers can provide:

• Pipe material
• Pipe structure
• Nominal or outside diameter
• Required pipe length
• Perforated or non-perforated design
• Required ring stiffness class
• Applicable standard
• Burial depth
• Project application
• Required quantity
• Fitting requirements
• Destination

If project drawings, technical specifications or a bill of quantities are available, these can also help clarify the required drainage system.

19. Do Not Forget Drainage Pipe Fittings

Many underground drainage projects require more than straight pipe sections.

Depending on the system design, fittings may include:

• Couplings
• T-connectors
• Y-connectors
• Reducers or adapters
• End caps
• Bends or elbows
• Outlet connections
• Other required fittings

Pipe dimensions and fitting requirements should be confirmed together.

This is particularly important when a project uses several pipe diameters and requires transitions between different sizes.

20. How Should Different Drainage Pipe Quotations Be Compared?

Price per meter is important in procurement, but it should not be the only comparison.

Before comparing prices, buyers should check whether the quotations are based on the same technical requirements.

Useful comparison points include:

• Material
• Pipe structure
• Diameter
• Ring stiffness class
• Applicable standard
• Perforation design
• Pipe length
• Fitting system
• Packaging
• Required documentation
• Quantity
• Delivery terms

If Supplier A quotes an SN4 pipe and Supplier B quotes an SN8 pipe, or if the two products use different structures, the prices may not represent equivalent technical offers.

The technical specification should therefore be aligned before making the commercial comparison.

Frequently Asked Questions

What Does Ring Stiffness Mean for an Underground Drainage Pipe?

Ring stiffness describes the resistance of a thermoplastics pipe to radial deformation under specified test conditions. It is one of the technical parameters used when evaluating buried flexible pipe systems.

What Is EN ISO 9969 Used For?

EN ISO 9969 provides a test method for determining the ring stiffness of thermoplastics pipes.

What Is the Difference Between SN4 and SN8?

SN4 and SN8 represent different nominal ring stiffness classes. The appropriate class should be selected according to project requirements, installation conditions and the applicable specification.

Is SN8 Always Better Than SN4?

Not necessarily. A higher stiffness class is not automatically the correct choice for every drainage project. The required class depends on burial depth, soil, loading, installation conditions and engineering requirements.

Is Ring Stiffness the Same as Wall Thickness?

No. Wall thickness is a physical dimension, while ring stiffness describes structural response under a specified test. For corrugated and structured-wall pipes, profile geometry and material also influence structural behavior.

Does Ring Stiffness Alone Determine Underground Pipe Performance?

No. Pipe stiffness, bedding, backfill, soil conditions, compaction, burial depth, surface loading and installation quality can all influence the behavior of a buried drainage system.

Can Perforated Drainage Pipe Be Used Together with Geotextile?

Perforated drainage pipes are often incorporated into systems that also use geotextile filtration. The appropriate filtration and drainage design should be selected according to soil and project conditions.

What Information Is Needed for a Drainage Pipe Quotation?

Buyers can provide pipe material, diameter, structure, perforation requirements, SN class, applicable standard, quantity, required fittings, application and destination. Project drawings or technical specifications can also be useful.

Selecting Underground Drainage Materials as a System

Underground drainage performance depends on more than one pipe parameter.

For roads, retaining walls, slopes and other civil engineering projects, perforated corrugated pipes, plastic blind drains, geotextiles and fittings may need to work together as part of the drainage system.

Ring stiffness provides information about one aspect of the structural behavior of thermoplastics pipes. Filtration, hydraulic capacity, installation conditions and outlet design address other parts of the drainage problem.

For this reason, drainage materials should be selected according to the actual project specification, installation environment and required drainage function, rather than according to diameter, wall thickness or unit price alone.