Double-Wall Corrugated Pipe: SN Class, Sizes and Installation

Double-wall corrugated pipe is polyethylene pipe with two walls: a smooth inner wall and a ribbed outer wall. It is used for gravity sewer collection networks in cities and industry, for carrying surface water and run-off, for highway drainage and for water crossings. "Gravity" means water flows only by slope and there is no pressure inside the pipe; so this pipe is never used for a pressure line.

The main advantage of the double-wall structure is that it gets high ring stiffness with less material: the outer ribs stiffen the pipe against soil pressure and traffic load, while the smooth inner wall keeps the flow moving and collects no sediment. The result is a pipe lighter than single-wall pipe of the same diameter, easier to carry and fit.

In our catalogue this product is P004, "double-wall corrugated sewer pipe", listed to EN 13476-3 and ISO 21138. In this guide you will find the structure, the SN stiffness classes, the size table, how it differs from single-wall pipe, jointing and trench notes.

For housing-estate, road and stormwater collection projects this pipe is one of the most common choices, because it installs quickly in long lengths at gentle slopes and needs no welding. It should be noted, however, that its performance depends on correct trench work, and the quality of installation cannot be made up for by choosing a higher class.

• Double-wall corrugated sewer pipe
• What is corrugated pipe?

The two layers of the pipe are extruded together and fuse into one piece. The smooth inner layer has low hydraulic roughness and lets sewage and run-off pass without catching. The ribbed outer layer raises the second moment of area of the section, like corrugated sheet that is stiffer than flat sheet, and gives the pipe resistance to soil load and traffic.

This geometry lets the pipe have high ring stiffness at low weight. Low weight means the bars can be moved with lighter equipment, and the smooth inner surface means the capacity of the network is kept over the years.

A further practical consequence of this structure is that cutting and jointing must be done at the right place on the corrugation so the ring seats properly. Cut the pipe end through the valley of the corrugation, not the crest, and smooth the cut edge so the ring is not damaged as it goes into the socket.

Ring stiffness, shown as SN, is the pipe's resistance to being crushed and deformed under soil and surface loads, expressed in kilonewtons per square metre. SN8 means a ring stiffness of 8 kN/m²; the higher the SN, the more load the pipe resists. Our corrugated pipe comes in three classes: SN4, SN8 and SN16.

In general SN4 is chosen for shallow burial and routes without traffic, SN8 for most city streets and SN16 for deep burial or heavy traffic load. But the final choice depends on burial depth, soil type, groundwater level, traffic load and the quality of compaction around the pipe, and must be made on the designer's calculations, not just by habit or price.

A practical point: corrugated pipe is a flexible pipe and its performance depends on the surrounding soil. Correct backfill and compaction around the pipe is often as important as the SN class; an SN16 pipe in a poor bed can also run into trouble.

When comparing quotations from several suppliers, make sure the SN class and inside diameter are identical. Two pipes of the same diameter but different classes have different weight and cost, and comparing them without regard to SN gives a misleading result.

Corrugated pipe is known by its inside diameter (ID), not outside diameter. In gravity lines the flow depends on the clear inside section, and because rib height differs between classes, holding the inside diameter constant keeps the design hydraulic capacity the same in every class. So, unlike pressure pipe which is ordered by outside diameter, here always state the ID.

The table below shows every published inside diameter of P004. Each diameter is supplied in all three classes SN4, SN8 and SN16 and in 6 m bars. If the diameter you need falls between two rows, check the larger diameter.

• PE sewer and drainage pipes

Our single-wall sewer pipe (P005) has a smooth solid wall, with an outside diameter of 110 to 1200 mm in SDR 26, 33 and 41 and a brown stripe, and is sealed by butt fusion. Double-wall corrugated pipe is identified by inside diameter and SN ring stiffness and is joined with a socket and rubber ring, without a welding machine.

Single-wall is chosen when a continuous fused joint is needed, and double-wall when speed of installation, low weight and high ring stiffness matter. Neither is for a pressure line, and both are for gravity routes or low pressure; for a pressure line, go to PE100 or PE80 water pipe.

There is another difference in hydraulic behaviour: the inner surface of both types is smooth, but the inside diameter of single-wall pipe depends on its wall thickness, whereas with corrugated pipe the inside diameter is the very number printed on the pipe and used in the order. This makes calculating the network capacity simpler.

• Solid-wall PE sewer pipe

Double-wall corrugated pipes are joined with a socket joint and a rubber ring. The ring sits in the proper groove on the pipe end, its surface is wetted with the special lubricant, and the next pipe is pushed in aligned to the bottom of the socket. This joint needs no welding machine or power, and its seal stops tree roots getting in and sewage leaking out.

To join two bars without a socket or to repair a line, a corrugated coupler is used, which in our catalogue is listed in inside diameters of 160, 200, 250, 315, 400 and 500 mm; the matching sealing ring is available in the same diameters. Clean the pipe groove of mud before fitting and seat the ring evenly in the groove.

On long routes, settle with the designer in advance the spacing of manholes, the points where direction changes and where branches join, and put the fittings you need, such as corrugated elbows and tees, in the same order so work is not held up during construction.

• Corrugated pipe coupler
• Corrugated pipe sealing ring
• Corrugated elbow and tee

Prepare the trench bed with even granular material and check the design longitudinal slope along the route, because gravity flow depends on slope. Compact the soil around the pipe in thin layers evenly on both sides; because the pipe is flexible, the compaction of the surrounding soil is part of the system's load capacity.

The bars are 6 m long and light, and easy to handle, but when lowering pipe into the trench keep it from dropping and from a rock point striking the outer wall. The position of manholes, burial depth and the SN class of each section are set by the network designer.

Before filling the trench, check that the inside of the pipe is clear of soil and debris and that the slope as built matches the drawings. Correcting slope after backfill is almost impossible, and the cost of the fault is far greater than a few hours of inspection.

In your enquiry put the inside diameter (ID), the SN class, the number of bars or total length and the fittings needed (couplers, sealing rings, elbows, tees). If the designer has set the class, copy it word for word, and if you do not know, tell us the burial depth and traffic load so we can advise on choosing the class.

We also deliver to other cities and issue an official invoice. Send your list from the contact page and a pro-forma invoice will be prepared; if you are unsure about a combination of diameter and class, say so there.

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