2.5 Stone Culvert
Stone culverts, also known as box culverts, are traditionally used on stalkers paths, military roads, and estate roads, as well as upland paths. They allow all path users to pass over the drainage feature without difficulty, and are ideal for ponies, bikes and wheeled traffic.
The purpose of the culvert is to channel water from one side of the path to the other. It can be wider and deeper than a cross-drain to cope with larger volumes of water. Normally used for water courses crossed while traversing a glen or hillside, it can also be used with ditched waterflow. A continuous walking surface is provided over the top of the drain - it does not catch water draining down the path.
Bill of Quantities (example)
Use local stone to construct a stone culvert with a depth and width of approximately 500mm. Extend by 300mm on each path side. Side walls and lid stones must be large and stable enough to support the path surface and use. Stone line the culvert base, with a minimum gradient of 5°. Allow for splash plates at the inflow and outflow, and approximately 10 metres of ditch to or from the culvert. Construct the path surface to 300mm depth over the culvert and at least 3 metres on each side.
POSITIONING OF THE STONE CULVERT
The assessment for positioning culverts is largely the same as for cross-drains. The key situations where they will be required are:
- where a watercourse flows across the path from small streams, springs or mossy flushes
- where uphill surface water is ditched, or side ditches protect the path
The most suitable positions will be:
- at the best place for water to flow under the path, preferably at a low point, close to where the flow is intercepted
- connected directly, and frequently with protective side ditching
- where there is scope for dispersing water away from the path, and not back on to it lower down
The culvert has three main components: side walls, a lined channel base, and a lid. These create a boxed and solid hard-wearing channel under the path which should not become damaged from path use, does not collect path surface silt, and is relatively self cleansing.
- the side walls provide the channel width and depth, and support the lid; they are comprised of two lines of large block stone across the path, placed with faces to channel the water flow and with additional courses as required to achieve the channel depth and support the lid stones
- the lined channel base, between the side walls, is comprised of a row of liner stones to prevent undermining of side walls by water
- the lid encloses the channel and supports the path surface on large flat stone slabs, capable of spanning the culvert width
Section across box culvert width
Splash plate stones extend the liner stones to prevent erosion, especially at the outflow where there may be a steep drop from the culvert channel. A splash plate extension may also be required if the inflow ditch drops steeply to enter the culvert below the path surface.
Where the culvert is connected to drainage ditches, the in and out flowing ditches must be carefully aligned, allowing extra depth to connect with the culvert channel under the path.
An aggregate path surface will usually be required over the top of the lid stones, tying in with the adjoining surface levels. These may need regrading to accommodate the depth of the culvert. If the lid stones are large and stable enough they may serve as the path surface.
Dimensions will vary according to the volume of water to be channelled. The channel must be large enough to take the greatest expected volume of water. Culverts are prone to blocking with large debris. A larger channel will reduce this risk, will be relatively selfcleansing, and easier to maintain as a result.
- the culvert normally passes straight across the path, but may need slight angling to provide a fall in the channel.
- the draining fall of the channel should be no less than 5°, to ensure a clear run.
- the channel size will be variable, the larger the better - on average 500mm deep and wide, but an absolute minimum of 300mm; the availability of lid stone may dictate the channel width
- the level of the lid stone surface should allow for a layer of backfill material compacted to approximately 50mm, and a minimum of 150mm path surface over the top, increased to 300mm if heavy, wheeled traffic use is expected.
- the full culvert structure should extend at least 300mm either side of the path, to prevent path edge collapse into the ditch or watercourse.
A large amount of good size stone is required to build culverts. Selecting the right stone is fundamental, especially for the lid, which must span the full culvert width and support the weight of the path. Large block stone must support this lid. It should also be large enough to withstand the pressure of the greatest waterflow, and frost heave. Some sites may not have suitable stone available and stone culverts will not be an option (see variations).
If possible local stone should be used, found within reach of the path. All visible stone in the structure should remain in its natural form, preferably weathered on visible surfaces. Permanently hidden stone may be shaped as required.
Points to note when selecting stone.
- if possible, large side stones should make up the full channel depth required; if not additional courses of block stone should be used to gain the required depth
- all side stones should be of a size to provide a stable wall, which supports the lid
- side stone faces forming the channel side should be as even as possible, with no protrusions to hamper water flow or collect debris
- channel liner stones can be smaller, but must be wide enough for the required channel width, and at least 200mm deep, to prevent undermining by fast, high volumes of water
- upper surfaces should provide an even surface with adjoining liners, and have no protrusions which hamper water flow and collect debris
- lid stones must be wide enough to span the full width of the culvert and side walls, and deep enough to support the path and its expected use
- there must be enough lid stone available to cover the full length of the culvert
- if lid stone is to provide the path surface it should be as even as possible, with no protrusions for walkers to trip on
- the shape of all construction stones should match evenly and tightly with adjoining stones
Method of Construction
Excavate a large trench, or clear the bed of the watercourse, across the path.
- dig the trench wide enough for both side walls and the required channel width; and deep enough for the liner stones, the required channel depth, the lid stones, backfill and the depth of the aggregate surface over lid stones
- the angle and depth of the trench must complement the watercourse or drainage ditch, and provide an adequate fall
Construct the bottom course of the side walls.
- set the side stones vertically, or angled slightly back from the channel - stones leaning into the channel will be unstable
- the base of the side stone should be at the same level as the base of liner stone
- the top surfaces should be as level as possible to provide a base for additional courses or the lid stone
- butt stones tightly to form even front faces to the water channel, with no protrusions
- wedge and pack gaps firmly with smaller stones so that each side wall is solid and immovable once the liner is in position
Position the liner stones
- set the liner stones to fit tightly between the side stones to give the required channel depth
- maintain the drainage fall by working from the lower to the higher end
- join liner stones tightly, with joins off-set from the side stones to provide a stable construction
- adjoining stones should have top faces even, with no protruding edges
- add splash plates at the outflow, set slightly lower than the channel, and inflow, set slightly higher than the channel
- wedge and pack all gaps firmly with smaller stones so that whole drain construction is solid and immovable, will not allow water to sink in
Construct additional courses for side walls to achieve the channel depth and support the lid.
- tightly match adjoining stones with all joins over-lapped and front faces even, with no protrusions
- use larger stone for side wall ends, or corners, for a strong structure
- the top course must be even and horizontal to provide a solid platform for lid stones
- pin from behind, wedge and pack gaps firmly with smaller stones so that each side wall is solid and immovable
Position the lid stones over the culvert side walls.
- place lid stones so that they extend over the channel walls, and will not collapse into the channel
- butt lid stones tightly together, with any gaps filled to prevent the over-laid path falling through
- pin and wedge lid stones securely to ensure that there is no movement
- use material excavated from the trench to backfill over the lid stone, to provide a compacted base for the path surface
Construct an aggregate path over the backfill. Soundly compact the base and surfacing material, to prevent the path eroding and exposing the lid from pressure of path use. If wheeled traffic or heavy use is expected the depth should be increased to 300mm. Re-construct the path either side of the culvert as far as required for regrading levels to accommodate the culvert.
Restore all areas damaged during construction, particularly broken path edges and the ground above and below the culvert. Continue path edge turfing over the top of the culvert, along the end of the lid stones, to stabilise the edge and ensure the feature is as natural and unobtrusive as possible.
Key points to watch out for:
- make sure the angle of the culvert compliments that of the waterflow
- make sure all joints over-lap for a strong, stable structure
- make sure side walls and channel have no protrusions - debris caught in culverts is difficult to clear
- don’t use thin liner stone - it will quickly be under-mined and wash out
- make sure adjusted path levels don’t cause drainage problems further along the path
Most variations to this design reflect the type of stone and its availability on the site.
Good quality lid stones can be used as the path surface, particularly if large and stable with a level and flat top surface. They will need to tie in carefully with the adjoining path surface which may need regrading to provide the height required over the culvert.
A simple variation often used is a standard cross-drain with lid stones over the top. This avoids building up the sides with additional courses, but relies on large enough side stones to make up the required height. The capacity of the culvert will be reduced by the reduction in the channel size.
Where a continuous surface is required over a larger volume of water, and there is no large stone available there are two other options. If there is plenty of small stone a ford may be the answer, if not, a piped culvert may be the only option.
Stone culverts can be subjected to very fast flowing, high volumes of water, particularly after snow melt or periods of heavy rain. This can result in undermining of stonework, and blocking with large quantities of debris. They are difficult to maintain due to their enclosed structure, however it is essential that the following tasks are carried out regularly.
- clear out debris and silt that is blocking the culvert; use long handled tools or drain rods
- clear debris and silt from in and out-flow ditching
- check stability of stonework - re-pack where there is any visible movement or gaps
- re-pack surfacing over the culvert if there has been compaction or erosion
- check the landscaping around culvert and path edge, re-instate as required
- cleared silt or gravel can be used in packing and re-surfacing; spare material should be carefully hidden
- select stone away from the path edge and out of sight; if this is unavoidable ensure that holes are reinstated
- use surplus turf and spoil from excavations for site restoration, or hide discretely
- restore damage from culvert construction to look as natural as possible
HEALTH AND SAFETY HAZARDS
- take particular care when moving and manoeuvring heavy wall and lid stones; use safe manual handling techniques; beware of crushing fingers or dropping stone onto feet
- when wedging side wall and lid stones watch out for movement that may cause crush injuries
- use eye protection when shaping stone
- the trench will become very wet and slippery; divert or block water flow during construction, if feasible
- do not under estimate the amount of water that culverts will channel in the wettest conditions. An inadequate capacity can lead to extensive damage to the path surface.