Upland Pathwork

Construction Standards for Scotland

Contents

• Home
• Preface
• Section 1. Introduction to Upland Pathwork
• 1.1. Principles of Upland Pathwork
• 1.2. Environmental Impact
• 1.3. Path Assessment
• 1.4. Path Survey
• 1.5. Maintenance
• 1.6. Working Practice
• 1.7. Tools and Equipment
• 1.7a. Mechanised Equipment
• 1.8. Health and Safety
• Section 2. Upland Path Drainage
• 2.0. Introduction to Drainage Techniques
• 2.1. Ditching
• 2.2. Stone Lett
• 2.3. Stone Waterbar
• 2.4. Stone Cross-drain
• 2.5. Stone Culvert
• 2.6. Piped Culvert
• 2.7. Stone Ford
• Section 3. Upland Path Surfaces
• 3.0. Introduction to Surface Techniques
• 3.1. Aggregate Path
• 3.2. Geotextiles: Aggregate Path on Peat
• 3.3. Anchor Bars: Aggregate Path on Slopes
• 3.4. Stone Pitched Path
• Section 4. Upland Path Site Restoration
• 4.0. Introduction to Restoration Techniques
• 4.1. Materials and Use
• 4.2. Defining Path Alignment
• 4.3. Containing Path Use
• 4.4. Bank and Slope Stabilisation
• 4.5. Restoring Vegetation
• Glossary of Symbols
• Glossary of Terms
• Further Reference

3.1 Aggregate Path

The traditional hill path uses natural aggregate material found at, or near to, the path site. The resulting path is one that is not out of place in an upland environment, blending with the surrounding landscape without appearing too formal.

FUNCTION

The aggregate path provides a hard wearing, durable surface to withstand the expected pressure of use. It should be comfortable to use so that walkers will keep to it and not walk on surrounding vegetation or take alternative routes. Path edge definition with turfs and boulders, and site restoration, will help to control this. The path should be free draining, with drainage features incorporated, to withstand the expected weather and waterflow.

CONSTRUCTION

Components

The aggregate path comprises layers, or grades, of angular, interlocking stone laid in a path tray.

• sub-base - the load bearing path foundation, required for deep construction over wet or rough ground
• base - provides strength to the construction and a solid base for the path walking surface
• surfacing - forms a durable, and firm surface over the path base
• binding - protects and prevents movement of loose surface material; provides a good walking surface

Each construction layer should ideally contain a range of stone sizes graded on site. This ensures that the aggregate interlocks when compacted, to form a strong and solid layer without any gaps which may weaken the construction. Building a path in several layers of differing grades of aggregate will significantly increase the path durability, compared to using ungraded ‘as dug’ material in one, single layer.

The construction layers are compacted to form a free draining camber, or cross-fall, for surface water to run off either one or both path sides, depending on the site. Generally a traversing path built into the hillside will have a cross-fall, and a path on more open ground will have a camber.

Drainage features are incorporated in the path, as identified during the path survey, and the path edges defined, or contained, with turf and boulders.

Dimension Guidelines

• the width should be naturally varied along the length of construction; the average width will be determined by the path assessment - this may be as little as 600mm, or up to 2000mm
• the average tray depth should be no less than 250mm; the path tray base should be a solid, natural mineral soil foundation; where path tray excavation reaches 300mm and the ground is still soft, or wet, geotextile will be required
• the depth of construction, or path tray, will depend on the nature of the ground and depth of erosion; softer ground, and heavier use will require a deeper tray and a sub base
• the depth of surface, base and sub-base will depend on the tray depth, and material available; minimum depths should be:
  • 50mm of compacted surface material
  • 100mm of compacted base material
  • 150mm of sub-base material
• the surface layer should always be at least 50mm to prevent exposure of the rougher base course through pressure of use; combined base and sub-base depths can be varied, depending on material source and stone size available (see below)
• the surface camber or crossfall should be between 2° to 5°, to effectively shed surface water
• the finished path surface should be no lower than the ground at the path edge to avoid water collecting here

Materials

On-site aggregate is won from the surrounding area and should be sourced during the path assessment (see also Path survey). Material should not be used ‘as dug’ but graded for each path layer. Where feasible this may be done using purpose built screens with different size wire mesh.

The source available may dictate the grading but, as a rough guide, the largest size stone for each layer should be at least 50% of the layer depth. For minimum depths:

• sub-base stone would be between 75 - 150mm, graded down to approximately 10mm
• base stone would be between 50 - 100mm, graded down to 5mm, with some fine particles
• surface stone would be 25 - 50mm, graded down to fine particles
• binding stone should always be no more than 5mm graded down to very fine particles

Stone should be angular for good interlocking. Binding material should have a high mineral content and be free draining, i.e. with not too much peat or soil. In some places the binding layer may be clay.

Material sources include:

• borrow pits
• streams or rivers
• scree slopes or embankments
• stone from around the path, broken up to make smaller aggregate

Borrow Pits

Borrow pits are small quarries or excavations dug in the vicinity of the path to win materials. They are normally a good source for all grades of stone, but a trial dig will be needed to ascertain the suitability of the material.

Careful assessment of the surrounding landscape will give clues to borrow pit locations:

• mounds of glacial deposits, or moraines; normally vegetation covered but there could be suitable material in the mound
• evidence of exposed material, alongside streams, on steep banking or ridges

Borrow pits can be dug into the top or side of a mound. Avoid areas of deep peat, particularly in the bottom of dips or gullies. Peat layers are thinner on the side of mounds or embankments. Once the borrow pit is finished with, it should be filled in with excess stone and spoil, and carefully landscaped with turves (see Introduction to Restoration Techniques).

Borrow pits are a potential health and safety hazard and excavation should be subject to risk assessment and control measures. For deep borrow pits, precautions should be taken to shore up the sides to avoid the danger of collapse.

Rivers and streams are also a good source of material, particularly for surfacing. Turbulent water grades the stone naturally, depositing it as gravel under banks and on the outside of bends. Larger, angular stone from stream beds is suitable for base and sub-base material, but water smoothed and rounded stone will not interlock.

Method of Construction

Step 1

Form the path tray.

• excavate the tray to the variable width required, and the depth required or down to a solid base; infill eroded sections to form a level base
• if the path is badly eroded into a wide, deep gully, use excavated material to infill the gullied width and form the tray base and sides; the tray edges may need to be formed with good size turfs
• form the tray sides so they are capable of containing the aggregate layers; if the aggregate spreads out, with the pressure of use, the thinner surface will erode and expose the path base
• allow for incorporation of drainage features within the path construction
• set aside excavated material for path edges or site restoration

Step 2

Incorporate drainage features

• construct the selected drainage features at the positions required
• depending on the feature these may be constructed at any stage of laying the subbase and base, but to achieve the required draining levels they should be completed before final compaction

Step 3

Lay the sub-base and base

• place the graded stone into the tray to the depth required for each layer; using larger graded stone for the bottom layer, and smaller graded stone for the top layer, below the surfacing
• ideally, for good compaction, the graded material should be in layers that are no more than twice the depth of the maximum stone size
• incorporate shaping for the camber or crossfall required in each layer; with the material slightly thicker in the centre of the path to form the camber, or on the uphill side to form the cross-fall
• compact each layer with a tamper or, if site access allows, a vibrating plate, maintaining the camber or crossfall
• if barrows are used along the path during the construction process a camber may be difficult to achieve; compacting the camber level throughout the sub-base and base construction will help. The surface and camber may need to be topped up after barrow use along the path
• the level of the top layer should allow for the surfacing and binding layer to be flush with path edge vegetation after some settling of the sub-base. If there is any doubt about the compaction, add more surfacing, as it will settle with use

Step 4

Lay the surface and finish with binding material

• lay and compact the surface material to the depth required over the full path width; this should be at least 50mm, but more may be required depending how well compacted, and interlocking, the base is
• allow for some settling and loss of material into the base during compaction, and over time
• allow adequate depth at the path edges. If surfacing is too thin the material will protrude and lead to breaking up of the path, with use
• spread and carefully compact the binding material over the top of the surface, with the path edge surface flush with the ground level

Step 5

Edge finishing

• turf and landscape the path edges to ensure the path construction is contained, the line defined and the appearance "softened", using turf, boulders and spoil from path tray excavation
• use excess turf and spoil to re-instate any eroded or damaged ground, and for infilling any borrow pits (see Restoration techniques)

TROUBLE-SHOOTING

Key points to watch out for:

• long, straight, even lengths - break up with curves, undulations, and variations in width
• surface layer too thin - quickly erodes exposing the path base, which is uncomfortable to walk on
• single size large stone in the base - unstable path structure; surface layer disappears into gaps
• small stone used for the sub-base and base - needs more stone and more handling to reach the level required for surfacing
• use drainage features within the path construction to protect it from water erosion

VARIATIONS

Over wet, but firm ground the path construction can be raised above the water table as a causeway. The aggregate is laid directly onto the ground, using large boulders and turfs to contain it and reinforce the path edges (see Bank and Slope Stabilisation). Over wet peat the causeway path can be floated on geotextile.

Long stretches of newly constructed path require a large amount of aggregate and imported aggregate may be the only option if on-site material is not available or suitable. Approximately one tonne of aggregate will be required to cover three metres of 1000mm wide path, at 200mm depth. Wherever possible a local quarry of the same, or similar geology as the path site should be used. The aggregate should be graded, as required for the base and surface material, and have suitable binding properties.

Aggregate paths can be built using tracked excavators, where access allows. Path drains are constructed as part of this process. The technique requires sensitive machine work and hand finishing to produce a path that fits in with the upland environment. Machine work should only be carried out by a trained and competent contractor.

MAINTENANCE TASKS

With drainage features protecting it from water erosion, a well constructed path should withstand the pressure of use. Depending on the path dynamics it will need some maintenance, but generally not as frequently as the drainage features.

• top up surfacing where it has settled, compacted or eroded, incorporating a camber or cross-fall as required
• repair path edges with turf where they have collapsed and aggregate has spread out of the path tray

next » 3.2. Geotextiles: Aggregate Path on Peat