3.3 Anchor Bars: Aggregate Paths on Slopes
Where an aggregate path is constructed on a slope greater than 8°, there is the risk of the material migrating down the slope, particularly if the binding properties are not good, or there is a high level of path use. To prevent this occurring stone anchor bars are incorporated into the path structure.
Anchor bars form solid, immovable structures within the path construction and, depending on their spacing, hold the aggregate on the slope above. The anchor bar may be used with water bar construction, as the stabilising stone below the shedding bar stones. Anchor bars can be built into existing paths that are showing signs of movement.
Bill of Quantities (example)
Re-construct existing path with aggregate to a variable width, between 600-1000mm. Use large block stone to construct anchor bars every 5m, across the full path width, and flush with the path surface.
POSITIONING OF ANCHOR BARS
Anchor bars will generally be used on paths with a gradient between 8° to 16°. If the surface material has no binding properties anchor bars are useful on slopes as low as 5°. Depending on the gradient they should be positioned at intervals of between 3 and 20 metres.
The following table gives a general guide to spacing.
|Gradient||low 8-10°||medium 10-12°||high 12-16°|
Not all paths on gradients require anchor bars. They may not be necessary if the surface and base material binds well, or if the path is well protected by drainage features, and the level of use is low.
The anchor bar is an informal structure, comprising one or two large block stones, set across the path line. The block stone is sunk into the path with the top face just visible as a part of the path surface. Depending on the gradient and the size of stone available it may be necessary to have a double row, or two courses, of stone.
- the bar should span the full width of the path line; this may require the use of more than one stone
- the bar should be positioned at approximately 90° to the path line
- stone should be set in approximately 200mm deeper than the path construction depth, so that the bar is an immovable, "independent" structure, which will withstand the weight of aggregate and the pressure of use
- the top surface, or tread, of the stone should be flush with the path surface; the lower edge should not normally form a step up from the surface below
- on steeper paths it may be necessary to have a slight step, to avoid the tread being at an uncomfortable angle to walk on
- a double course of stone may be used to provide the height gain required without creating too high and unnatural a step
The local stone selected should be in its natural form, preferably weathered.
- the stone should be large enough to hold the compacted aggregate above and the pressure of path use - if it can be moved and lifted easily it will be too small
- the stone should be at least the width of the constructed path, if two stones are used each should be at least half the path width; it is better for stone to extend outside the path edges than be too narrow
- the stone should be deep enough to bury into the ground by approximately 200mm below the path base
- it should have a level, but rough top face for the tread; it should have no large protrusions, but not be so smooth that walkers will slip with gravel on the surface
Method of Construction
Anchor bars are built into the excavated path tray before the aggregate is laid.
Excavate a trench
- dig a trench approximately 200mm deep across the full width of the path tray
- the trench should be wide enough to allow for the width of the bar stone and the depth required for bar stone tread to be flush with the path surface
Position the anchor bar stone or stones
- set the anchor bar stone so that the surface will be flush with the compacted path surface, and not create a step, unless the path is steep
- if a second stone is necessary they should be tightly butted together to form a solid bar across the path and provide an even tread surface
- wedge and pack any gaps with smaller stone, and backfill the trench firmly, to form an immovable structure
Construct the aggregate path as detailed.
- take care not to dislodge the anchor bar when compacting the path material above and below the bar
- make sure that the surface layer is compacted to be flush with the top and bottom edges of the bar stone or stones
Key points to watch out for:
- use large stone, if possible one to span the full path width - too small a stone will become loose with the weight and pressure of the path
- always keep the bar flush with the uphill path surface - avoid steps up from the downhill surface
- avoid using anchor bars on too steep and mobile a gradient - pitching may be the better solution
If large block stone is not available the anchor bar may be formed by constructing short sections of pitching. This will also be suitable on the steeper gradients where double rows of large block stone, or longer sections of pitching, may be required to "take up" the gradient without creating high and formal steps.
An anchor bar can be built 2 or 3m down a path from a water feature, such as a waterbar. The anchor bar will hold the surfacing on the ramp below the waterbar, creating a more durable walking surface and preventing erosion behind the face stones.
Anchor bars require maintenance on an occasional basis:
- check the stability of the stonework - re-pack where there is movement or any visible gaps
- re-pack aggregate surfacing above and below the bar where compaction or erosion may have taken place
- if anchor bars are not preventing downhill movement of aggregate a pitched path may be required
Often anchor bars are added to an existing aggregate path on a slope, at time of maintenance, to solve problems of surface movement.
- use natural looking weathered stone, that will blend in with the surrounding landscape
- turf over the edges of the anchor bar where they extend outside the path edge
HEALTH AND SAFETY HAZARDS
- use safe lifting techniques when moving or positioning stone for the anchor bar
- the paths dynamics must be carefully considered before deciding to use anchor bars to stabilise the path, in particular the gradient of the path, the mobility of path material, and the levels of use