Pipe culvert

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What is it?

A twinwall polypropylene plastic pipe, laid in an excavated trench that has been backfilled, to move water under and away from the path. Stone headwalls hide the pipe ends and retain the pipe bedding and backfill materials. It is the commonest drainage feature used in lowland path construction. Pipe culverts are also used for upland paths where a pipe is a practical way to channel large, fast flowing streams, and where there is no suitable stone available to build stone culverts or cross drains.

small diameter piped culvert oatridge college

How does it work?

The plastic pipe channels water from one side of the path to the other, from a ditch installed close to the path. Stone headwalls are built around the ends of the pipe to hide it, but also to retain the pipe bedding and backfill materials, and to support the path edges, which would otherwise collapse over the pipe ends, stopping water getting into and out of the pipe.

What are the benefits?

Path drainage features such as cross drains create physical barriers, making access for most path users difficult or impossible. Pipe culverts will not create barriers, as they provide a continuous path surface over the top of the pipe. This is essential when a path is expected to be used by people on foot, wheels or horseback.

Is it suitable?

It is important to use the correct size (diameter) of pipe for the expected volume of water, otherwise the flowing water will spill over onto the path surface and damage it. There are various methods to help you choose the right size:

  • Make pipe culverts the same size as, or slightly bigger than, any existing culverts in the area that are working well.

  • Speak to local people and land managers about known stream or ditch water levels.

  • Visit your path site in the wettest weather possible and determine the rate of discharge that the culvert needs to cope with. This can be quite a complex calculation, involving measuring the area and depth of any flooding.

Pipe culverts should never run full – ideally, they should be no more than two thirds full in the worst case water flow.

For deep, slow moving ditches or streams, use a pipe size which is twice that of the watercourse's depth.

Twinwall polypropylene plastic pipe is available in various sizes - 225mm, 300mm, 450mm, 600mm, 900mm, and 1100mm. For ditch systems, use several 225mm or 300mm diameter pipes set at regular intervals – every 10m to 20m depending on the amount of water flowing into the ditches.

How much will it cost?

Depending on size and length of pipe, a pipe culvert may cost between £50 - £300 per culvert (material & labour).

How do you install it?

Good locations for pipe culverts are:

  • The lowest points in the side ditch beside the path.

  • Where flowing water will cross under the path via its own gravity.

  • Where small streams or water springs flow across the path.

  • Where dispersal of water away from the path is easiest.

  • On small streams, place pipe culverts on a straight section, preferably where water speed is likely to be low.

Method of installation:

  • A trench was excavated across the line of the path. The trench must be long, wide and deep enough to take the required length and size of pipe, as well as compacted bedding and backfill materials. Allow for the depth of backfill above the pipe to be 50% - 100% of the pipe diameter, depending on expected loadings, with a minimum cover of 150mm. The bottom of the trench must be angled enough to provide the required fall. Remove sharp stones which could puncture the pipe wall. If the path is to be built on geotextile and/or geogrid, these materials should also line the pipe's trench. For ease of installation the geotextile and/or geogrid are cut and overlapped across the full trench width.

piped culvert cross section

  • Cut the pipe to the exact length required. Starting from the outlet end, lay, level out and compact the pipe bedding material along the bottom of the trench, making sure to maintain the required fall. The top level of the bedding must not be above the streambed level. Position and align the pipe on the bedding material, adjusting the pipe's fall. Lay, level out and compact more pipe bedding material along the sides of the pipe, topped up with compacted backfill. Take care when compacting the backfill to not move or damage the pipe. Continue laying, levelling out and compacting backfill in the trench across the path width in at least two layers to the required level.

  • Construct stone headwalls around both ends of the pipe to retain the bedding and backfill materials, and to hide the pipe ends from view. Headwalls may be dry stone or mortared. The wall should be 150 - 450mm thick, depending on the depth of pipe backfill. All stones must fit tightly together and be firmly set into the stream or ditch sides. If possible, provide a large single mantle stone across top of pipe. Alternatively, build stones to provide an arch around the pipe.

piped culvert end view

  • For multiple pipes, at the inlet end, provide flow 'splitters' – long, thin stones wedged between the pipes to direct water flow into the pipes without washing out backfill materials. Place a bed of flat stones around the inlet to prevent scouring under or around the pipes.

  • At the outlet end of the pipe, provide a 'splash plate' immediately below the pipe outlet to prevent undercutting. Also provide a wider bed of flat stones to prevent scour and erosion.

  • Once the pipe culvert is installed, build the path's sub base and surface over the compacted backfill.

  • If required, dig a ditch to connect the pipe to the side ditch. Also, dig a ditch to disperse the water away from the path and allow it to soak away in the surrounding ground.

  • With the pipe culvert built, it is time to do landscaping and tidying up work. Restore all worn areas around the pipe culvert. Carefully landscape above the headwalls and up to the path edges using turf and suitable excavated spoils, from ditching and formation tray works. Lay turf up to the headwall side stones to make the pipe and stonework as natural and unobtrusive as possible.

What variations are available?

Concrete pipes can be used instead, if a twinwall plastic pipe cannot be installed deep enough below the path surface. If vehicles need regular access on to the path, concrete pipes can take the weight of a heavier vehicle.

Where no stone is available on site for constructing stone headwalls, alternatives such as sand bags or geotextiles with large turfs can be used to construct alternative headwalls. They may not be as solid as stone headwalls, but they are better than having unsupported and uncovered pipe ends.

Detailed drawing - Pipe culvert

You can download a standard detail drawing and specification details here icon Piped Culvert - Standard Detail Drawing & Specification Details

© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh