Culverts

These take water under the path, and are built where the flow is too great for a cross drain, or where vehicular access is needed. They do need regular maintenance to keep them clear, and depending on the situation and the materials available, it may be more effective to construct a simple bridge.

Size

Culverts block easily, and should always be built as big as is practical. Before designing the culvert, walk the path in wet weather, compare with any similar culverted flows near the site, and get advice from landowners and the local authority as necessary. It is possible to estimate the diameter required for given areas, using the table below. This only gives a guide, and it is impossible to predict exact flows.

Headwall

A headwall should be constructed at either end of the pipe to protect it, and to stop any flow through the fill around the pipe which will cause erosion. If the pipe is over-topped, a strong headwall will act as a dam and cause increased flow through the pipe due to greater water pressure.

Usually the height of the headwall should equal the diameter of the pipe plus 300mm, to take it level with the surface of the path. Headwalls can be constructed of stone or brick. Stone headwalls can be dry or mortared, the latter being the more durable, but less visually acceptable in remote areas. A compromise may be to only mortar the upper headwall, and build the lower headwall of dry stone, as it should not be subject to any water pressure.

Talbot's Formula

If the headwall is being mortared, it is worth constructing aprons. Aprons are usually specified for pipes over 440mm diameter.

If no material is available to build the headwall, the bank should be sloped as shown below. A headwall is preferable however, as a sloping bank will require a greater amount of fill and a longer and more expensive pipe.

In mountainous country, where the culvert is taking large flows of water from a steep mountainside, it may be necessary to construct stone-built sumps, which collect the water from stone-lined drains and direct it through the culvert. Many culverts of this type have been constructed on the Pyg track on Snowdon.

Vehicular access

If there is any chance of a vehicle crossing a culvert, even if it is not legally entitled to do so, then build the culvert strong enough to withstand it. A piped culvert can be broken by the passage of a single vehicle, and is awkward to repair. If it is possible to get a vehicle up the path, it is likely to be useful to do so for future path management, so put in a pipe long enough to give a path width of 2.5 metres. Check with the supplier that you obtain a strong enough pipe, and protect it with at least 300mm of backfill.

Piped culverts

Except for the seepage culverts described below, pipes should be at least 225mm diameter. Use either plastic, clay, concrete or steel pipes. The choice will depend on local availability, and on the means of transport to the site.

1. Excavate trench if necessary. Existing stony stream beds and stony subsoils should provide sufficient support. In peaty soil the pipe should be supported by a 50mm layer of 10-15mm diameter aggregate.
2. Lay concrete foundation/apron if specified. Lay pipe and build up headwalls around and above the pipe.
3. Compact stone-free filling at the sides and top of the pipe. It should be put down in layers not more than 150mm thick and thoroughly consolidated, to a total depth over the pipe of at least 300mm.
4. Surface with sub-base, base and surfacing, crowning each layer so that water runs off.

Stone culverts

These are traditional in many upland areas of Britain where suitable stone can be found. The edges of the culvert should be constructed in the same way as a cross drain, using the largest boulders which can be found and moved into position. Make the inside edge as smooth as possible, so that debris will wash through and not wedge inside. The bottom must be pitched with stone to prevent scour.

Box culverts

These can be made of slate, stone or treated softwood. Stone and slate culverts rely for their strength on being carefully constructed and backfilled. Align all the stones so there are no protruding edges which will catch debris. Pack angular stones along the sides and up to the top of the culvert, and then cover with at least 300mm of stone-free material, well tamped down.

Wooden culverts can be nailed together, in a similar way to the wooden cross drains described here. They are not recommended for culverts crossed by vehicles.

Heavy vehicle crossings

The following is the Ministry of Agriculture’s design for culverts crossed by heavy farm machinery. Pipes must be covered by at least 900mm of backfill, and should be of the British Standard strength class shown below.