HDPE Conduit Design

Conduit vs. Pipe

In general, plastic conduits and plastic pipes are very similar in structure and composition, but deployment is where they differ.

  • Conduits do not have long-term internal pressure. External forces are unchecked if ovalized during installation, it may not recover during service.
  • Long-term stress rupture is not a factor. (Hydrostatic Design Basis is not required in material selection.)
  • Conduit ID is chosen by cable occupancy, where internal clearances are critical; whereas, for piping applications, ID is based on volumetric flow requirements.
  • Path of installation for conduit is very important - radius of curvature, vertical and horizontal path deviations (undulations) and elevation changes all significantly affect cable placement.

Cable Dimension Considerations

Determination of a conduits dimensions begins with the largest cable, or group of cables or innerducts, intended for occupancy. From a functional viewpoint, selection of diameter can be broken down into the following general considerations:

  • The inside diameter of the conduit is determined by the cable diameter and placement method (pulling or air-assisted pushing).
  • Pulling cables into underground conduits requires sufficient free clearance and is typically further distinguished by classifying the cables into two groups: power and coax (short lengths) and fiber (long lengths). Additionally, electrical cable is controlled by the National Electric Code (Chapter 9), whereas, dielectric, or fiber optic cables, are not.
  • Long pulling lengths require low volume fill, i.e. 36% max.
  • Short pulling lengths may be filled up to 53%, or up to the latest NEC limitations for groups of cables.
  • Push-blow installation methods for long length fiber cables utilize higher volume, i.e. up to 70% max.
  • Innerducts are smaller diameter conduits, intended for placement into larger conduits or casings. Their purpose is to subdivide the larger conduit space into discrete continuous pathways for incorporation of fiber optic cables. Diameters of conduits and innerducts are often specially designed to maximize the conduit fill.
  • Using these guidelines, one can determine the minimum ID of the conduit or innerduct. When over-sizing a conduit for power, coaxial or multi-pair telecom cables, the more room the better. This rule does not necessarily apply for push-blow methods of installation. Here, it is found to be more difficult to push a cable with additional clearance since a cable tends to form a helix, which transfers some of the axial load laterally into the wall causing friction. The air velocity moving over the cable can also be maximized with a minimum volume of air when the free volume is low. Higher air velocities result in improved drag forces on the cable, thus aiding with its placement.

Conduit Wall Determination

Conduit and duct products come in a wide range of sizes, spanning 1/4 -inch (5mm) to 24-inch (610mm) bore casings. The standard dimension ratio, SDR, of a conduit is determined as the ratio of the average conduit diameter divided by the minimum wall thickness. Wall thickness typically ranges between SDR 9 to SDR 17. (Larger SDR numbers indicate a thinner wall thickness.) Conventions exist that work off of either the average outside diameter (SDR) or the average inside diameter (SIDR). Internally sized (SIDR) are usually chosen when the inside diameter clearance must be very carefully controlled. This usually does not apply to most duct installations because, as noted above, the free clearance between the cable and the inner wall of the conduit is not usually that close. Bore casings, on the other hand, offer situations that can benefit from close ID control because many times several innerducts are tightly fit into a casing. In this latter case, the conduit wall can be increased or decreased relative to service conditions without jeopardizing the inside clearance fit. Internally sized dimension tables tend to preserve the minimum ID above the nominal conduit size, whereas, externally sized conduits often fall below the nominal ID as the wall thickness increases. For most conduit installations, SDR sizing is utilized because the OD control lends itself to better joint formation using external couplers. This becomes very important when air-assisted placement methods are used for placing the cable. On the other hand, large diameter conduits (4 and above) typically undergo butt fusion as a means of joining. Determination of the wall thickness becomes a function of either the method by which the conduit is placed, or the nature of environmental stresses that it will be exposed to over the service life. ASTM F 2160, Standards Specfication for Solid Wall High Density Polyethylene (HDPE) Conduit Based on Controlled Outside Diameter (OD), explains the conduit sizing systems fully.

Installation Method vs. Short-Term and Long-Term Stress

The viscoelastic nature of HDPE results in differences in the observed mechanical properties as a function of time (and/or temperature). The apparent stress/strain behavior of the material is time dependent under the influence of a sustained load. This is referred to as creep properties. In this regard, we can distinguish between short-term properties, such as those exhibited during a laboratory tensile test at a strain (stretching) rate of two inches per minute, as compared with long-term properties typical of conduit placement and sustained service loads.

Knowledge of the load-bearing capability of HDPE as a function of loading rate allows one to select appropriate strength values to substitute into design equations. Loads are applied to conduits both by the environment that they are placed into and by the placement means under which they are installed; the chief difference being the duration over which the load is applied. For example, a common means to install multiple conduits is to directly plow them into the ground using either a railroad plow or tractor-drawn plow. During this installation process, a certain amount of bending and tensile stress is encountered over a rather short period of time (only seconds to minutes). Whereas, after the plow cavity collapses about the conduit, the ground continues to settle upon stones that may be pressing directly against the conduit, thus setting up a long-term compressive load. For this application, we see that we would require both long-term and short-term moduli to assess the deflection resistance. Initially the conduit may offer resistance to ovalization, but in time, the resin may yield under the sustained load, resulting in a reduced pathway for the cable.

Numerous approaches to placing conduits have evolved over the years. Each method presents its own unique set of challenges with respect to the potential for conduit damage, or installation related predicaments. Perhaps one way to compare the potential sensitivity to damage of the various methods is the following table. Here the potential for damage is depicted by a numerical scale ranging from 0 to 5, where 5 is the most severe condition, resulting in yielding and permanent deformation of the conduit; 4 is the potential for loads greater than 75% of yield stress; 3 represents loads greater than 50%; 2 representing greater than 25%; 1 less than 25%, and 0 representing no significant load at all. The shaded areas depict the most severe condition.

Relative Damage Sensitivity vs. Installation Method
Installation Method Short-Term Loading Long-Term Loading Recommended SDR Range
Tensile Bending Crushing Impact Crushing Tensile
Conduit* 3-5 3 2 1 1 1-2 9.0-13.5
Horizontal Bore 4-5 2 3-4 0 3-5 1 9.0-11.0
Direct Plow 2 3 4-5 1-2 4-5 1 9.0-11.0
Continuous Trench 2 2 3-4 1-2 3-4 1 9.0-11.0
Open Trench 0 0 1-3 1 1-3 1 11.0-17.0
Aerial 1-2 3-5 12-3 1 1 2 11.0-13.5

The term "conduit" in this chart refers to the placement of HDPE innerducts into a buried 4" and 6" PVC conduit typical of the underground telecom plant. The SDR recommendation range attempts to select safe SDR"s based upon the potential for stressful conditions.

A comprehensive, industry consensus design guide for the proper use of polyethylene pipe is available from the Plastics Pipe Institute (PPI). The engineering handbook is available, free of charge, from the PPI website.

It should be noted that the above table is not intended to be representative of all conduits installed by these methods, but is indicative of what can happen when the wrong diameter, wall or material is used. Check with supplier for specific design recommendations. Perhaps the most serious and least controlled problem for cable placement is that of ovalization or kinking of the conduit. This condition can be brought about through tensile yielding, severe bending, excessive sidewall loading, or probably more frequently, the crushing action of rocks in the underground environment. In direct plow or bore applications, one gets little feedback from the process to indicate that a potential problem is developing. For these applications, the most robust conduit design should be considered.

Below Ground Installations

Open Trench / Continuous Trenching

Conduits intended for buried applications are commonly differentiated into two classes, rigid and flexible, depending on their capacity to deform in service without cracking, or otherwise failing. PE conduit can safely withstand considerable deformation and is, therefore, classified as a flexible conduit. Flexible conduits deform vertically under load and expand laterally into the surrounding soil. The lateral movement mobilizes the soil"s passive resistance forces, which limit deformation of the conduit. The accompanying vertical deflection permits soil-arching action to create a more uniform and reduced soil pressure acting on the conduit. PE stress relaxes over time to decrease the bending moment in the conduit wall and accommodates local deformation (strain) due to imperfections in the embedment material, both in the ring and longitudinal directions. The relationship between pipe stiffness, soil modulus (stiffness), compaction and vertical loading is documented by the work of Spangler and others. The pipe stiffness, as measured in ASTM D2412 and Spangler"s Iowa formula provide a basis for prediction of conduit deflection as related to dimension ratio and resin modulus.

It should be noted, however, that creep affects the pipe stiffness, so the long-term modulus should be used. Additional information pertaining to soil embedment materials, trench construction and installation procedures can be found in the chapter on Underground Installation of Polyethylene Piping in this Handbook. Flexible conduit can occasionally fail due to stress cracking when localized forces (for example, from a large sharp rock) exceed the material"s ability to relax and relieve stress. However, PE resins suitable for conduit applications should have adequate stress relieving properties to avoid these failures. Therefore, the design process should include consideration of the conduit resin"s stress crack resistance, as well as the selection of appropriate embedment material and compaction.

Direct Plow

Flexible conduit materials need adequate compressive strength to safely resist the compressive stresses generated by external loading. However, the usual design constraint is not material failure due to overstraining, but, rather, excessive deflection or buckling under anticipated earth and ground water pressures. Deflection or buckling is more probable when the embedment material does not provide adequate side support. For example, pipe installed by directional drilling and plowing typically does not receive side support equivalent to that provided by the embedment material used in trench installations where bed and back? lt can be engineered to provide a specific level of lateral support.

Plowing installations often encounter rocky soils, which would induce significant crush loads for conduits 2-inch diameter and smaller. In these cases, SDR 11 is the minimum wall thickness that should be used, and if rocky conditions were likely, SDR 9 would be more appropriate.

Pipe stiffness, as calculated per ASTM D2412, gives a measure of flexural stiffness of the pipe. Pipe stiffness equals the ratio of the applied load in units of lbs/lineal inch to the corresponding deflection in units of inches at 5% deflection. It should be understood, however, that although two conduits, 6-inch and 1.25-inch diameter, may possess the same pipe stiffness, the amount of soil load required to induce a 5% deflection in each is considerably different. As a result, the sensitivity of smaller diameter conduits to underground obstructions is that much greater. Another physical parameter for smaller conduits, crush strength, is often employed to establish limits of crush resistance. Unfortunately, there is no universally agreed upon criterion or test method for crush testing. Typically, the conduits are subjected to an increasing load, similarly applied as in ASTM D2412, but to a far greater deflection the order of 25 to 50% of the inside diameter. This deflection-limiting load is then reported on a per-foot basis.

The following table illustrates the difference in the load required to induce a 5% deflection in conduits having different diameters but common pipe stiffness values. These values were generated assuming a flexural modulus of 150,000 psi for the resin. Units for pipe stiffness are in pounds/inch of length/inch of deflection, whereas those for the crush are presented as pounds per foot. It is apparent that a fixed external load more easily deflects smaller diameter conduits. It is also important to remember that, in long-term loading, the resin will maintain only about 22 to 25% of its original modulus; thus, smaller thin-wall conduits can be quite susceptible to localized loads brought about by buried obstructions.

Pipe Stiffness (PS) vs. Crush Strength
SDR 9 SDR 11 SDR 13.5
Conduit Size OD Wall PS Crush Wall PS Crush Wall PS Crush
Inches Inches Lb/1 inch Lb/6 inches Inches Lb/1 inch Lb/6 inches Inches Lb/1 inch Lb/6 inches Inches
1 1.315 .146 1310 804 .120 671 433 .097 344 231
1.25 1.660 .184 1310 1020 .151 671 547 .123 344 292
1.5 1.900 .211 1310 1160 .173 671 626 .141 344 33
2 2.375 .264 1310 1450 .216 671 782 .176 344 417
2.5 2.875 .319 1310 1760 .261 671 947 .213 344 50
3 3.500 .389 1310 2140 .318 671 1150 .259 344 615
4 4.500 .500 1310 2750 .409 671 1480 .333 344 790
6 6.625 .736 1310 4050 .602 671 2180 .491 344 1160
SDR 15.5 SDR 17
Conuit Size OD Wall PS Crush Wall PS Crush
Inches Inches Inch Lb/inch Lb/6 inches Inches Lb/1 inch Lb/6 inches
1 1.315 .085 220 151 .077 164 114
1.25 1.660 .107 220 190 .098 164 144
1.5 1.900 .123 220 218 .112 164 165
2 2.375 .153 220 272 .140 164 206
2.5 2.875 .185 220 .330 .169 .164 .249
3 3.500 .226 220 402 .206 164 304
4 4.500 .290 220 .516 .265 .164 .390
6 6.625 .427 220 760 .390 164 575

The above table is for comparative purposes only. Pipe stiffness values are based on 150,000-psi flexural modulus. Crush values are estimated from empirical data for 6 inch long conduit samples compression tested in accordance with ASTM D2412 to 50% deflection.

Conduit Network Pulling

In the telephone and electrical utility industries, the underground plant is often comprised of a network of 3", 4", and 6" conduit banks. These rigid conduits are composed of clay tile, cement conduit, or more recently, PVC constructions. They are usually separated by manhole vaults or buried pull-boxes. Distances between, and placement of manholes and pull-boxes is largely a function of the following constraints:

  • Location of branch circuit intersections
  • Lengths of cables (or innerducts) available on reels
  • Access to, or limited by physical obstructions
  • Path difficulty for placement of cable or innerducts
  • Surface environment
  • Method of cable placement (mid-assist access)

In addition, Department of Transportation (DOT) regulations often require additional protection and support structure for buried conduits in road bores and traffic areas. Although steel casings have been used in the past, it is becoming more prevalent to horizontally bore under roadways (or waterways) and pull back an HDPE casing into which HDPE innerducts are installed. Pull placement of innerducts has obvious similarity to traditional cable placement methods. Several good references on this subject exist, including Guide For Installation of Extruded Dielectric Insulated Power Cable Systems Rated 69KV Through 138KV, Underground Extruded Power Cable Pulling Guide, AEIC Task Group 28 and IEEE Guide Distribution Cable Installation Methods In Duct Systems.

A comprehensive, industry consensus design guide for the proper use of polyethylene pipe is available from the Plastics Pipe Institute (PPI). The engineering handbook is available, free of charge, from the PPI website.

There are a number of variables that influence loading and selection of innerducts when pulling into conduit structures:

  • Diameter of conduit and innerduct, and number of innerducts to be installed"clearance fit
  • Length and direction changes of conduit run, sweeps
  • Composition of conduit and coefficient of friction
  • Jam combinations
  • Pull speed and temperature
  • Elevation and innerduct weight

Horizontal Directional Bore

For directional drilling the design process should include consideration of tensile forces and bend radii created during these processes. Flexible conduits installed in continuous lengths are susceptible to potential tensile failures when pulled into place, so allowable tensile forces should be determined to avoid neck-down from tensile yield. The engineer should also account for the conduits allowable bend radius, especially on bends with no additional support given to the conduit, to prevent ovalization and kinking from installation. For additional information, please refer to the chapter on horizontal directional drilling in this Handbook.

General Considerations

Mechanical Stress

Regardless of the installation method, mechanical stress is of great concern during conduit placement. Exceeding the maximum allowable pulling tension or the minimum allowable bending radii can damage conduit. Consult the conduit supplier for allowable pulling tensions.

Pulling Tension

During conduit pulling placement, attention should be given to the number of sweeps, bends or offsets and their distribution over the pull. Tail loading is the tension in the cable caused by the mass of the conduit on the reel and reel brakes. Tail loading is controlled by two methods. Using minimal braking during the pay-off of the conduit from the reel at times can minimize tension; no braking is preferred. Rotating the reel in the direction of pay-off can also minimize tail loading. Breakaway swivels should be placed on the conduit to ensure that the maximum allowable tension for that specific conduit type is not exceeded. The swivel is placed between the winch line and pulling grip. A breakaway swivel is required for each conduit.

Bending Radii

Conduit is often routed around corners during placement, and pulling tension must be increased to complete the pull. It is important to determine the minimum radius to which the conduit can be bent without mechanically degrading the performance of the conduit.

Minimum Bend Radius
SDR 9 SDR 11 SDR 13.5
OD Wall Min. Radius Wall Min. Radius Wall Min. Radius
Conduit Size Inches Inches Inches Inches Inches Inches Inches
1 1.315 .146 15.4 .120 20.1 .097 25.9
1.25 1.660 .184 17.1 .151 22.3 .123 28.9
1.5 1.900 .211 18.2 .173 23.8 .141 30.8
2 2.375 .264 20.0 .216 26.3 .176 34.2
2.5 2.875 .319 21.8 .261 28.0 .213 37.3
3 3.500 .389 23.8 .318 31.4 .259 40.9
4 4.500 .500 26.4 .409 35.0 .333 45.8
6 6.625 .736 30.9 .602 41.3 .491 54.4
SDR 15.5 SDR 17
OD Wall Min. Radius Wall Min. Radius
Inches Inches Inches Inches Inches Inches
1 1.315 .085 30.6 .077 34.1
1.25 1.660 .107 34.2 .098 38.1
1.5 1.900 .123 36.4 .112 40.6
2 2.375 .153 40.5 .140 45.2
2.5 2.875 .180 44.3 .169 49.5
3 3.500 .226 48.5 .206 54.2
4 4.500 .290 54.5 .265 61.0
6 6.625 .427 64.9 .390 72.8

A comprehensive, industry consensus design guide for the proper use of polyethylene pipe is available from the Plastics Pipe Institute (PPI). The engineering handbook is available, free of charge, from the PPI website.

Placing the Conduit

An important consideration for open-trench installations of PE conduit is that conduit should be straightened to remove any residual coil memory, which can create a tortuous path for the cable and create significant challenges to cable installation. Conduit pay off can be accomplished by pulling the conduit into the trench from a stationary reel or by laying the conduit into the trench from a moving reel, usually attached to a trailer. Spacers should be used when placing multiple ducts in a trench. Spacers prevent the ducts from twisting over and around each other. By keeping the ducts in straight alignment, cable-pulling tensions are reduced. When water is present in the trench, or when using extremely wet concrete slurry, floating of the conduit can be restricted through the use of the spacers.

Directional Bores

Directional boring allows the installation of conduit under obstacles that do not allow convenient plowing or trenching installations, for example rivers or highways. This unique installation method, which capitalizes on a primary strength of PE conduit its flexibility, can be accomplished over very long distances. Directional boring is accomplished using a steerable drill stem to create a pathway for the conduit. The equipment operator can control the depth and direction of the boring. A detailed discussion of this installation method is presented in the chapter on Polyethylene Pipe for Horizontal Directional Drilling in this Handbook. Also, consult the equipment supplier for detailed operating procedures and safety precautions. It is recommended that DR 11 or DR 9 be used, depending on conditions and conduit diameter.

Installation into Existing Conduit

Conduit (or multiple conduits) is often pulled into existing conduit systems as innerduct.


An important step that should be taken prior to this type of installation is proofing the existing conduit to ensure that all obstructions are cleared and that conduit continuity and alignment is good. It is recommended that a rigid mandrel roughly 90% of the inner diameter of the conduit be used to perform the proof. Proofing conduit is typically performed by pushing a fiberglass fish with a rigid mandrel attached to the end of it through the conduit. Any problem areas should be felt by the person pushing the fiberglass fish and should then be marked on the fish so that the distance to the problem is recorded and if necessary can be located for repair with greater ease. If the fiberglass fish makes its way through the conduit without any difficulties experienced, then the conduit has been proofed out, and no repairs should be necessary.

Before placement of the innerduct inside the conduit can be started, it is important to have all of the necessary equipment to protect the innerduct. The use of sheaves, bending shoes, rolling blocks (45 and 90 degrees) and straight pulleys are required for protection of the innerduct during installation. It is important that they all meet the proper radius for the innerduct size. The use of a pulling lubricant will greatly reduce the tension and stress on the innerduct when pulling innerduct into an existing conduit. Ball bearing swivels are needed for attaching the winch line to the innerduct harness system.

After Pulling

The stress of pulling innerduct through existing conduit will vary with the length of the route and the number of turns it has to make, as well as the condition of the conduit it is being pulled into and the amount of lubrication used. The effects of the stress will cause the innerduct to elongate (or stretch) in proportion to the amount of stress, but should be less than 2% of the total length placed. Due to this effect, it is important to pull past the conduit system slightly to compensate for recovery to the original length. An allowance of at least one hour needs to be given for the innerduct to relax before cutting and trimming it.