Understanding Conduit Specifications

Last week's newsletter on collapsed conduit generated several emails asking a similar question. If conduit can collapse, deform, stretch, or become damaged during installation, how should it actually be installed?

I thought that was a good question.

Over the years, I have noticed that many conduit problems are treated like mysteries. A pathway that looked fine during construction suddenly becomes difficult to rod. A future cable pull experiences excessive drag. A mandrel stops halfway through a newly installed route. A section of conduit is discovered to be flattened or collapsed. These problems can be traced back to something much simpler: the conduit was installed outside the limits it was designed.

Every reel arrives with specifications attached to it. Most people view those specifications as engineering paperwork. I have always viewed them as installation instructions. The manufacturer is telling us something through those numbers. The wall thickness tells us how much margin exists before the conduit begins to deform. The bend radius tells us how tightly the conduit can be curved before it starts losing its shape. The pull rating tells us how much tension can be applied before damage. Those values are not arbitrary.

That is why understanding conduit specifications matters. The purpose of this article is not to turn anyone into an engineer or overwhelm crews with technical information. My goal is much simpler than that. I want to explain how I think about conduit installation and why the specification sheet is often the first place I look when trying to understand how a product should be installed.

What SDR Means

At its core, SDR is simply a way of describing the relationship between the outside diameter of the conduit and the thickness of its wall. As the wall gets thicker, the SDR number gets smaller. As the wall gets thinner, the SDR number gets larger.

A conduit with a thicker wall generally has more resistance to crushing, deformation, and pulling forces. A conduit with a thinner wall is often lighter and more flexible, but it also provides less margin for error when subjected to the stresses of construction. Neither is automatically better than the other. They are simply designed for different conditions and different applications.

It provides insight into how the conduit will respond when it encounters pulling tension during HDD. It influences how resistant the conduit will be to deformation under load. It helps determine how much margin exists when the conduit is subjected to bends, sidewall pressure, handling, storage, and installation stresses.

This is why two conduits that look almost identical on the outside can behave very differently in the field.

A crew may install one conduit through a challenging bore path and experience no issues. The next project may use a different SDR and suddenly become much less forgiving under similar conditions. The drill is the same. The crew is the same. The installation method is the same. The difference is that the conduit itself was designed to operate within a different set of limits.

That is where installation problems begin.

When crews do not understand the SDR, every conduit gets treated the same. The same pullback practices are used. The same handling practices are used. The same assumptions are made about what the product can tolerate. Sometimes those assumptions work. Sometimes they do not.

Understanding SDR changes the conversation because it forces us to recognize that conduit is not generic plastic pipe. It is an engineered product with defined strengths, defined limitations, and defined installation requirements.

The Three SDR Ratings Crews See

Once you understand what SDR is, the next step is understanding the three ratings most crews encounter in fiber construction: SDR 9, SDR 11, and SDR 13.5.

These numbers are not just labels printed on a reel. They tell you a great deal about how the conduit will behave once construction begins. They provide an indication of how much wall thickness exists, how much margin for error is available during installation, and how resistant the conduit will be to the forces it encounters underground.

SDR 9 is the thickest-walled conduit most crews will encounter in everyday construction. Because it contains more material in the wall, it generally provides greater resistance to crushing, deformation, and installation stresses. That additional wall thickness gives the conduit more ability to tolerate the forces created during directional drilling, pulling operations, and long-term loading after installation. When conditions are challenging, having additional wall thickness often means having additional margin for error.

SDR 11 occupies the middle ground and has become one of the most common choices in outside plant construction. It offers a balance between strength and practicality. There is enough wall thickness to perform well in a wide variety of installation methods while still remaining manageable from a handling and installation standpoint. For many crews, SDR 11 becomes the conduit they encounter most often because it performs well across a broad range of construction conditions.

SDR 13.5 moves further toward flexibility and reduced wall thickness. That thinner wall can make the conduit lighter and easier to work with in certain situations, but it also means there is less material available to resist deformation when forces begin increasing. Pulling tension, sidewall pressure, tight bends, improper handling, and external loading all become more significant as wall thickness decreases. The conduit may still perform exceptionally well when installed properly, but there is less room for mistakes during construction.

A useful way to think about these three SDRs is to think in terms of margin. As wall thickness increases, the conduit generally becomes more forgiving. As wall thickness decreases, installation practices become increasingly important. Small mistakes that may not create immediate issues with a thicker-walled conduit can become much more significant when the wall thickness is reduced.

That does not mean thicker conduit eliminates the need for good construction practices. Every conduit can be damaged if it is subjected to enough force. What changes is how much force it takes to reach that point.

How SDR Affects Directional Drilling

If there is one place where conduit specifications stop being numbers on a specification sheet and start becoming reality, it is during directional drilling.

Directional drilling places the conduit under multiple forces at the same time. The conduit is being pulled through the ground, forced to follow the shape of the bore path, pushed against the outside of curves, and subjected to friction throughout the installation. Every change in direction creates additional stress that the conduit.

That is why so many conduit problems can be traced back to the installation process itself.

The conduit may have been overstressed during pullback. It may have been forced through aggressive bore shape. It may have experienced excessive sidewall pressure through a series of corrections. The installation may have been completed successfully while the conduit was already carrying hidden damage.

This is where SDR begins influencing the outcome.

A thicker-walled conduit generally provides more resistance to deformation when installation forces begin increasing. A thinner-walled conduit generally provides less margin before those same forces begin changing its shape. The exact limits depend on the conduit size, wall thickness, manufacturer, material, and installation conditions, but the principle remains the same. As wall thickness decreases, the installation becomes less forgiving.

One area where crews often get into trouble is focusing too much on entry and exit angles and not enough on bend radius.

Many HDD conversations revolve around angles.

• "What entry angle should I use?"

• "What exit angle should I use?"

The reality is that the angle itself is usually not the specification. The bend radius is.

Two bores may have the exact same entry angle while creating completely different stresses on the conduit. One may transition gradually over a long distance. The other may force the conduit through a much tighter curve. This is why experienced drillers focus on smooth shape.

• Every steering correction creates a curve.

• Every curve creates pressure.

• Every pressure point creates stress.

A smooth bore path distributes those forces gradually across the installation. An aggressive bore path concentrates those forces into smaller areas. When multiple corrections occur close together, the conduit may be forced through a series of bends that increase friction, increase sidewall pressure, and increase stress throughout the pullback.

As conduit diameter increases, the required bend radius generally increases as well. Larger conduit typically requires longer, smoother transitions than smaller conduit. Likewise, as wall thickness decreases and SDR increases, the margin for error generally becomes smaller.

The goal is not simply getting from Point A to Point B. The goal is creating a pathway that allows the conduit to maintain its shape while getting there.

How SDR Affects Pulling Forces

Once the conversation turns to pullback, most crews start asking the same question.

How hard can we pull on the conduit before we start damaging it?

The answer begins with a specification called Safe Working Pull Strength, often abbreviated as SWPS.

SWPS is the maximum recommended pulling load the conduit is designed to experience during installation. It is one of the most important numbers on the specification sheet because it helps define the point where the risk of stretching, deforming, or damaging the conduit begins to increase.

The important word here is "working."

SWPS is not a failure number. It is not the point where the conduit instantly breaks. It is the manufacturer's recommended operating limit for installation.

That distinction matters.

Many products can survive forces above their SWPS. The problem is that surviving a force and surviving without damage are two different things.

Crews look at the pullback gauge on the drill and assume that number represents the force being applied directly to the conduit. The gauge is measuring the force being generated by the machine. Those are not necessarily the same thing.

Everything between the drill and the conduit affects that reading.

The drill rods create friction inside the bore. The reamer creates resistance. The drilling fluid affects how easily the tooling moves through the hole. Ground conditions create drag. Bore shape creates drag. The conduit itself creates drag.

All of those forces become part of the number displayed on the machine.

A pullback reading of 1,500 pounds does not automatically mean the conduit is experiencing 1,500 pounds of tension. Some of that force is being consumed by the drill string. Some is being consumed by the reamer. Some is being consumed by friction throughout the bore path.

That is one reason experienced drillers rarely focus on a single pullback number. They pay attention to trends.

A pullback that gradually increases as more conduit enters the bore may be completely normal. More conduit in the hole often means more friction. More friction often means more force.

What gets attention is when the trend changes unexpectedly.

A pullback that has been steady suddenly spikes. A bore that was pulling smoothly suddenly begins demanding significantly more force. Those changes often indicate that something underground has changed.

This is also where bore path shape becomes extremely important.

Many people think of pulling force as a straight-line problem. In reality, every bend in the bore path affects how much force the conduit experiences.

• Every entry angle creates a curve.

• Every exit angle creates a curve.

• Every steering correction creates a curve.

• Every sweep creates a curve.

As conduit moves through those curves, friction increases. Sidewall pressure increases. The amount of force required to continue the pull increases.

Think about pulling a rope across an empty parking lot. Now imagine wrapping that same rope around a telephone pole before pulling. The force required changes dramatically because the curve creates additional friction and pressure.

The same thing happens underground.

A long, smooth sweep distributes force over a larger area. A tight sweep concentrates force into a smaller area. That concentrated force increases drag and increases the stress being placed on the conduit.

This is one reason conduit manufacturers publish pull ratings and bend radius requirements together. A conduit may remain below its SWPS and still experience excessive sidewall pressure if it is forced through aggressive bends, sharp corrections, or poor bore shape.

That is why smooth bore paths matter.

• Smooth entry angles reduce stress.

• Smooth exits reduce stress.

• Smooth corrections reduce stress.

• Long-radius sweeps reduce stress.

The goal is not simply to keep the pullback number low. The goal is to create a bore path that allows the conduit to move through the ground without accumulating unnecessary force in the first place.

Because once pulling forces, sidewall pressure, and bend radius begin working together, the conduit starts responding to the total load being placed on it, not just the number shown on the drill screen.

How SDR Affects Bend Radius

Every conduit has a minimum bend radius. That specification exists because the conduit was designed to remain round only within certain limits. Once those limits are exceeded, the shape of the conduit begins to change.

The easiest way to understand this is to think about a drinking straw.

A drinking straw can bend quite a bit without a problem. As the bend becomes tighter, however, the round shape begins to flatten. Keep bending it and eventually the straw kinks.

Conduit behaves much the same way.

The difference is that conduit damage is often hidden underground. The conduit may never fully kink. It may never completely collapse. Instead, it may ovalize slightly. The inside diameter may become smaller. A restriction may develop. Future cable pulls may become more difficult.

• That is why bend radius matters.

• Every entry angle creates a bend.

• Every exit angle creates a bend.

• Every steering correction creates a bend.

• Every elevation change creates a bend.

The conduit is forced to follow every one of those transitions. The goal is to get conduit through the hole without changing its shape.

Because once the conduit begins losing its shape, every future use of that pathway becomes more difficult. A cable pull that should have been routine becomes a struggle. A mandrel that should have passed easily suddenly finds a restriction.

That is exactly why manufacturers publish bend radius requirements in the first place. They are not trying to make installation harder. They are defining the point where the risk of deformation begins to increase.

Before It Goes In The Ground

One of the easiest mistakes to make is assuming that because conduit is designed to live underground for decades, it can be handled any way we want before installation. I hear this all the time when discussing storage or weather exposure.

"It's outside anyway."

That sounds reasonable until you think about the difference between a conduit installed underground and a conduit sitting on a reel in a yard.

Once conduit is buried, the surrounding soil helps support it. Temperature changes occur gradually. The conduit is protected from direct sunlight. The environment becomes relatively stable.

A reel sitting in a laydown yard experiences something entirely different.

The conduit is supporting its own weight. It may be exposed to direct sunlight for extended periods. Temperatures can swing dramatically between hot afternoons and cool nights. Forklifts move it. Straps hold it in place. Equipment gets parked near it. Materials get stacked around it. Every one of those things introduces forces that the conduit wasn’t intended to experience during its normal service life.

Most crews have seen examples of this without necessarily connecting it back to future performance.

• A reel gets dropped during unloading.

• A forklift tine bumps the side of a coil.

• Storage straps are tightened excessively.

• A reel sits on uneven ground for months.

The problem is that conduit damage is often cumulative. Those small stresses begin stacking on top of one another. That is why manufacturers publish handling and storage recommendations alongside installation specifications. They understand something many people overlook. The conduit begins accumulating its history long before it enters the ground.

This does not mean crews need to treat conduit like fine china. Construction materials are meant to be handled in the real world. What it does mean is that understanding the product's limitations should influence how it is stored, transported, unloaded, and staged for installation.

By the time the conduit reaches the bore path, the goal should be simple. The conduit should arrive in the same condition it left the manufacturer. Because once installation begins, the conduit will experience enough stress on its own. There is no reason to start the process already carrying damage from the yard.

The Bigger Lesson

As I was putting this article together, I realized the conversation was never really about SDR, pull ratings, bend radius, or conduit specifications.

The conversation is about understanding what is happening to the conduit while we are installing it.

One of the things I have noticed over the years is that most crews are taught how to install conduit, but very few are taught how conduit actually behaves during installation. They learn how to run a drill, pull conduit, place handholes, and complete a project. They learn the process. They learn the equipment. They learn the sequence of work.

What often gets overlooked is the product itself.

It is responding to pulling forces, sidewall pressure, bend radius, ground conditions, handling practices, and bore shape. When you understand what creates sidewall pressure, you begin looking at bore paths differently. When you understand what pull ratings actually represent, you begin looking at pullback gauges differently. When you understand bend radius, you begin paying closer attention to steering corrections and transitions. The equipment has not changed. The process has not changed. The thinking has changed.

Many of the problems we encounter in the field are not the result of crews who do not care. Most crews want to build good networks. Most crews take pride in their work. The challenge is that people cannot account for factors they have never been taught to consider.

A locator may not immediately recognize how several small corrections can create additional stress on a conduit. A drill operator may not realize how bore shape influences sidewall pressure throughout a pullback. Someone unloading conduit in the yard may not appreciate how a small deformation can follow that conduit for the rest of its service life.

Once people understand the reasons behind the specifications, many of the decisions become easier. The specifications stop looking like engineering paperwork and start looking like practical guidance. They become a way of understanding where risk begins and how small installation decisions can influence long-term performance.

That is really the purpose of this article. Every specification on a reel of conduit exists because someone discovered where a problem begins.