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Report on the market on Ribbed Culvert Pipe in NZ Conditions

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Traditionally the material used for culvert pipe from sizes 110mm – 400mm has been PVC (polyvinyl chloride). In larger sizes concrete or metal pipes or rotationally moulded HDPE (High Density Polyethylene) have been used. As pipe making technology improved, extruded ribbed PE (polyethylene) became widespread in Europe for stormwater and sewer. It has been manufactured in NZ in smaller sizes up to 500mm in the last 15 years, but in recent years NZ has taken on the concept of using larger diameter extruded ribbed PE culvert pipe. There are two companies in NZ that can make very large sizes to order (normally used for civil projects); they are Frank PKS and Solo Plastics.


Why use culvert pipe?

Small streams often meander across farm land, and even those that become dry for some of the year can have steep banks that prevent vehicles from crossing. The simplest way to provide access and protect the stream is to install a culvert. Other reasons for piping a stream are to prevent stock access and to eliminate the need for regular cleaning out and the expense of getting an excavator in. Culverts are also used to convey water for irrigation canals.


Why is ribbed culvert pipe more commonly used?

The three main reasons:

  1. Its lightness and ease of handling.

  2. Its high strength to weight ratio

  3. Larger diameters measure up well with concrete which is heavy to install and can break down over time in some soils. Metal culverts tend to rust out.


What materials are used in ribbed culvert pipe manufacture?

Generally ribbed pipe is made from PE or PP. Other materials used are galvanised steel, and aluminium. PE is the material of choice for most applications as it has the following characteristics.

  • Very resilient with high impact resistance.

  • Can stand low temperatures – Remains tough in winter conditions.

  • Higher resistance against UV rays compared to PVC

  • Polyethylene has excellent resistance to chemicals in soils. This is especially valuable in some areas  where there is an acidic peat content in the soils which can decrease the life of alternative products

  • HDPE is very resistant against abrasive substances. See table below as an indicative comparison with other options.



What are the features of ribbed culvert pipe?

Generally culvert pipe is round in shape and comes in 5-6m lengths. Culvert pipe often comes with a socket one end; otherwise a separate joiner is needed.  Normally double wall ribbed culvert pipe has another smooth inner wall to improve the flow and increase stiffness ratings.


What are the downsides of ribbed culvert pipe?

Ribbed culvert pipe generally have a smaller inside diameter compared to alternative products, as the ribs take up some of the space. Normally ribbed culvert pipe starts at 250mm as below this size it is more cost effective and you get more capacity using a smooth bore PVC culvert. Above this and up to 400mm size, a farmer would move towards the ribbed option as the price advantage tends to outweigh the loss in decreased sizing. Above that ribbed wins in every way.

In situations where there may be water in the trench it can be better to use a heavy culvert like concrete to prevent floating. Contractors have devised methods of laying ribbed culvert in these conditions and some still prefer it even in a high water table.






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