2.6 Piped Culvert
The piped culvert is one of the few cases where synthetic material is introduced into upland pathwork. They are only used where traditional drainage techniques are not possible, mainly when a path crosses soft ground, particularly deep peat, and it is impractical to build cross-drains. Large piped culverts are also required to cope with streams where covered drains are needed, and there is no suitable material for stone culverts. Stone facing, or headwalls, help to hide the plastic pipe.
The piped culvert serves the same function as a crossdrain, or stone culvert - to channel water from one side of the path to the other, generally from drainage ditches installed to protect the path. This situation commonly occurs when the path is traversing wet hillsides or stretches of poorly drained ground. The culvert does not catch water draining down the path; a continuous walking surface is provided over the top.
Bill of Quantities (example)
Construct a piped culvert under the aggregate path, using 300mm diam. twin-walled black pipe bedded on 100mm depth of gravel material. Compact backfill material to 300mm minimum depth. Use local weathered stone to build headwalls, and landscape to hide exposed pipe ends. Allow approximately 10m of ditching.
POSITIONING OF THE PIPED CULVERT
The assessment for positioning piped culverts is largely the same as for stone culverts and cross-drains. When installed with a new path over soft ground, the path alignment can affect location and frequency. To reduce the visual impact of too many piped culverts design the path to reduce the need for water to cross it. Avoid using piped culverts on steeper gradients, where they block quickly and are prone to becoming exposed.
Positions to consider include:
- at the best place for a ditched waterflow to cross under the path, often the lowest point
- where water crosses the path, from a small stream, spring or mossy flush
- at the best place for water to be dispersed away from the path
ComponentsThe piped culvert consists of:
- synthetic pipe laid in excavated trench
- bedding material in base of trench to hold the pipe in position and level
- backfill material over the pipe to protect it from pressure of path use; also provides a compacted base for path construction
- headwalls around exposed pipe ends, comprising base stone, two side walls supporting one top stone, and splash stones at both ends
With careful turfing, headwalls hide the existence of the synthetic pipe, minimising the visual impact in the landscape. Headwalls also serve to retain the backfilled trench and stabilise the path edge. These are weak points of the feature. Without protection they may erode and collapse with path use.
The inflow and outflow to the pipe are also weak points, particularly if there is a steep drop to or from the culvert in soft ground. Without large splash plates they become undermined and eroded by pressure of reduce the depth and fall required to flow into the pipe under the path. The outflow ditch will also need careful alignment to ensure dispersal away from the path edge.
These will depend on the situation in which the piped culvert is being used, and the size of pipe required.
- the culvert will normally be straight across the path, but may need angling to allow for the fall in the pipe, and the alignment and fall of the inflow and outflow ditches
- the drainage fall of the pipe should be at least 5° so that water will flow easily and reduce the amount of silting
- the pipe diameter depends on the volume of water - the size most suited to an upland landscape is 225mm - as this is may quickly block with debris a minimum of 300mm is preferable
- bedding material should be approximately 100mm depth to provide a level base and retain the pipe alignment
- backfill material should be compacted to a depth of at least 300mm, to protect the pipe from pressure of path use, and exposure through path damage; it also provides a firm base for the path surfacing
- the trench and bedded pipe should extend at least 300mm each side of the path width
- headwall dimensions will depend on the pipe size, but the outer edge should extend in front of the pipe by at least 150mm, to hide the pipe; and the top and side stones should retain the trench backfill and path edge, and fully enclose the pipe extending outside the path width
- there should be at least 100mm of backfill or bedding material between the inside faces of headwall stones and the pipe
- the base stone should extend under the pipe and bedding material by at least 150mm; and in front of the pipe by approximately 300mm to provide a splash plate on the outflow side, and splash plate base on the inflow
- the top stone surface should be lower than the finished path surface, and allow turfing over to match adjoining path edge landscaping
The most common pipe used is black polypropylene, which has less visual impact. It can be easily cut to the length required, and for easy transport. Alternatives such as concrete and clay are available but will be heavy to transport to remote sites.
|225, 300 or 450mm||Black||Single walled flexible pipe||30m coils|
|225, 300 or 450mm||Black||Twin walled rigid pipe||6m length|
Local, weathered stone for the headwall should be found within reach of the path. Points to note when selecting stone:
- the base stone should be at least 200mm deep; wide and level enough to support the side stones, if possible; long enough to serve as the splash plate
- the side stones should be large and deep enough to sit on the base stone, or be set in each side of it, and support the top stone above the pipe
- the top stone should be wide enough to span the side stones; and not extend above the path surface level
- top and side stone should be long or deep enough to hide the exposed pipe and retain the trench and path edge
Bedding material should be gravel, or small aggregate, won from borrow pits or stream beds within close reach of the path.
Backfill material must compact well; trench excavated material should be suitable, unless it is peat when material should be won from borrow pits or stream deposits.
Method of Construction
Excavate a trench.
- dig the trench long and wide enough for the required pipe length, diameter and bedding surround; and deep enough for bedding material, pipe diameter and compacted backfill below the path surface
- the angle, and base of the trench must provide the required fall for the pipe
- if the path is on deep peat, and floated on geotextile, the matting and geogrid are taken into the trench to line it; for fitting, and ease of laying, the geotextile should be cut and overlapped across the full trench width
Position the pipe
- cut or trim the pipe to the exact length required
- compact and level the bedding material in the trench base working up from the outflow end to maintain the required draining fall
- position the pipe and align correctly, adjusting the drain fall, if necessary
- pack the sides of the pipe using bedding material topped with compacted backfill
- taking care not to move or damage the pipe with any large or sharp stone, backfill the trench across the path width, compacting in at least two layers, to the required level
- construct the headwalls prior to final compaction
Construct the headwalls
- set in the base stone to extend under the pipe bedding and the required length out from the pipe, making sure it is level if used to support the side stone
- set the side stones firmly on the base stone, or at each side, and set back to retain the trench and enclose the pipe, leaving the required margin around it, and providing level top surfaces for the top stone
- set additional side stones, if needed to achieve the required height, and to provide solid and stable side walls
- position the top stone to span the side stones, set back to retain the trench or path edge and enclose the pipe, leaving the required margin above it, and the top surface to the required level.
- wedge, pin and pack all stonework firmly before completing the backfill, with gaps packed tightly with smaller stone, to prevent any movement
- set splash plate stones, as required, with the surface level below the base stone at the outflow, above the base stone at the inflow
Complete the aggregate path over the backfill base to tie in with the adjoining surface.
Complete the inflow and outflow ditching, connecting to drainage ditches as required. Ensure the required draining fall is maintained to collect and disperse the waterflow.
Restore all areas damaged during construction. The path edges and the area above the headwall should be carefully landscaped using turf and spoil, won from ditching and excavations. Turfing over the top stone must be stable and form a containing edge to the path. Lay turf up to the side stones to minimise the impact of the pipe and stonework, ensuring the feature is as natural and unobtrusive as possible.
Key points to watch out for:
- use the correct diameter of pipe for the volume of water
- maintain the pipe and ditch run - ensure that water will flow through the pipe
- set in headwall base stones and splash plates firmly - prevent undermining by waterflow pressure
- make sure the headwall protects the trench and path edge - prevent path edge collapse over the pipe
- set the pipe at least 300mm below the path surface - if not it will become exposed with use
Larger piped culverts are needed when there is no suitable material for stone culverts, and covered drains are essential for the nature of path use. They are also required to channel streams with high flows. Where large pipes are used, substantially wider and higher revetted headwalls, with several courses of stone, will be necessary.
Another variation for high volumes of water, is to use two pipes side by side with a wider headwall. Two pipes are harder to disguise than one.
Where no block stone is available on site to construct a stone-faced culvert headwall, an alternative is to use geotextile grid and large turves to construct a reinforced turf bank. This is built using alternative layers of strong turf and geotextile grid, build up around the mouth of the culvert pipe. Build the bank so that it shapes back, with a batter of 30°- 45°, and use large turves to prevent movement. This solution will not be as solid as a stone headwall, but is preferable to an unsupported and uncovered culvert end.
Piped culverts are very prone to becoming blocked. Routine clearing after long or high periods of rain or after snow melt is essential:
- carefully clear out debris and silt from the pipe; drain rods or long handled tools will be required for this
- clear out debris and silt from out and inflowing ditches
- check stability of headwalls; re-pack stonework where there is any movement or gaps
- check landscaping around the headwall and path edge, stabilise and re-instate as required
- re-pack surfacing over the culvert, and backfill over pipe if there is any settling, compaction or erosion
- cleared silt or gravel can be re-used in packing and re-surfacing; any spare material should be carefully hidden on site.
- the appropriateness of piped culverts in the landscape must be considered - it is difficult to hide the pipe completely - use an alternative technique, such as cross-drains or stone culverts
- surplus spoil from excavations should be used for landscaping, in-filling borrow pits or discretely hidden on site
- any off-cuts or excess pipe must be removed from the site and disposed of properly
HEALTH AND SAFETY HAZARDS
- be careful when cutting pipe, use sharp tools - blunt ones may lead to excessive force being used and an accident
- take care when working in the trench of a culvert - it may become wet and slippery under foot
- make sure the pipe is well hidden and the path edge and headwall are stable.
- do not under estimate the amount of water to be channelled in the wettest conditions. It is better to choose a larger capacity pipe than one that may be too small.