Surface

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The surface layer is the most important part of the path from the users' point of view. It is the only part of the path with which they will make contact, but it also needs to transfer their weight to the base layer below. The surface layer is therefore critical to both the usability and appearance of the path.

surface in location

Path surface in location...

When choosing a path surface, the following factors should be considered:

  • Durability

  • Maintenance requirements

  • Smoothness

  • Appearance

  • Sustainability

Consider what the users will want in terms of smoothness and all weather usability and then consider this in relation to durability, maintenance implications and appearance when choosing a suitable surface. The sustainability issue of where materials are sourced is increasingly important, to ensure that path networks have minimal ecological and carbon footprints.

The budget usually has the greatest effect on surface choice. Aim to obtain sufficient funds to provide a path specification suitable to the location and needs of users, rather than specifying the path according to how much funding is available. If funds are restricted, consider first reducing the length of the constructed path rather than using a cheaper, less durable and less suitable surface type.

Surface types

The main types of surface are outlined here, and there are pros and cons to each:

Not all path surfaces are suitable for everyone and every location. Unbound surfaces tend to get heavily marked by horses, particularly during or after wet weather. Wheeled users such as cyclists, wheelchair and mobility scooters normally prefer a smoother surface like bound surfaces. Horse riders on the other hand feel safer on softer natural surfaces with more grip and like to avoid loose stones that can damage the horse's hooves. Providing a constructed unbound, semi-bound or bound surface with a parallel natural grass path for horse riders might be a better approach, rather than looking for a surface material suitable for everyone.

Natural surfaces

natural surface

Mown grass path...

In some situations, a path may not require a formal surface. The existing vegetation may be appropriate for the expected users and setting, especially in rural areas. Mowing a strip of grass regularly will help to define the path and improve the strength and wearing quality of the grass. Short grass will be easier for wheeled users. The addition of drainage works may further improve the surface by removing muddy areas and puddles. With high volumes of use, natural surfaces will be susceptible to rapid wear and tear which requires careful maintenance and management to prevent deterioration beyond comfortable use.

Unbound surfaces

unbound surface

Whin dust path...

An unbound surface is generally made of well graded aggregate. It relies purely on the friction between the different sized stone particles to bind them together for strength and durability. Generally, the aggregate is very fine and particle sizes range from 4mm, 5mm or 6mm to dust. Common unbound surface types are whin dust, limestone and granite dust, depending on the local geology. Coarser aggregates can be used to provide more surface texture. Because stone particles are not bound together, these surfaces are very susceptible to being washed out by water, so careful surface drainage is required.

The simplest unbound surface is the path's base layer itself. This can be blinded with whin dust or granite dust in order to fill up any voids and bind together loose stone. This method is very economical as it requires the minimum amount of material. However, use and weather will quickly expose the stone surface below, which will require regular patching and compacting.

Whin dust and granite dust can be laid on top of the base layer (usually about 25mm thick) and compacted. This will provide a reasonably smooth, even surface suitable for a variety of users. However, as whin dust is not free draining, frost heave and heavy rain can turn the surface to mush, making it unusable in winter or wet weather. Granite dust is free draining and less susceptible to frost heave.

Semi-bound surfaces

semi bound surface

Toptrec path...

Semi-bound surfaces are based on locally available sources of natural or recycled aggregates and industrial waste by-products. These surfaces can be composed of many different combined materials such as limestone, shale waste or granite waste and screened road planings. They generally contain significant quantities of fine particles, which when wetted, start to 'set', and then 'harden' when dry, although not to the same extent as a bound surface. Newly laid surfaces can be loose, but will 'harden' to the point of becoming impermeable after a period of continuous use.

Semi-bound surfaces are often thought of as an intermediate between short-life unbound surfaces and long-life bound surfaces. However, the whole life costs of semi-bound surfaces may exceed those of a bound surface equivalent.

Bound surfaces

A bound surface is a combined layer of well graded aggregate glued together by a binder, such as bitumen. This top layer is called the 'surface course', a term used in road and pavement construction. Bound surfaces are much more durable than an unbound or semi-bound surface as the binder will prevent the aggregate being washed or worn away quickly. Bound surfaces will therefore be more suitable for steeper slope gradients and all year round use. It should be noted, however, that choosing a bound surface doesn't mean that drainage provision can be reduced, as flooding may still damage the surface. It is also essential to protect a bound surface from weed growth either by using a suitable geotextile or residual weed killer. If a bound surface is what you require, seek specialist advice from competent surfacing contractors.

Impermeable bound surfaces used for most path applications fall into three categories:

  • Surface dressing

  • Bituminous macadam (Bitmac)

  • Hot rolled asphalt

Surface dressing

surface dressing surface

Tar spray and chip path...

Surface dressing involves spraying a thin layer of bitumen emulsion over an existing bound surface or a newly laid base layer, followed by rolling-in a layer of washed stone chippings. The objective of a surface dressing is to create a stable mosaic of stone chippings that are securely attached to the surface by the bitumen emulsion. The result is a sealed surface that protects against water ingress into the base layer below, and, in the case of resurfacing treatment, slows down the deterioration of the path to prolong its life and restores surface texture. Because the surface dressing is a very thin layer it is liable to crack if the path's base layer surface is uneven or poorly prepared.

Surface dressing provides the appearance of loose stone but creates a non-slip surface for users, even horses. The finished look of these surface treatments will depend on the colour of the aggregate used. Loose stone chippings must be swept from the finished surface as they can cause a slip hazard.

The time period available for surface dressing is very narrow and the summer months are only really suitable. There can be a high risk of surface failure when temperatures are low.

Bituminous macadam (Bitmac)

bitmac surface

Bitmac path...

Bitmac is a combined mixture of well graded aggregate and a bitumen binder. It can be laid to various depths and is commonly used in road and pavement construction. The most common type of bitmac used in path construction is Dense Bituminous Macadam (DBM).

Bitmac is very strong and durable. If laid correctly, the surface can last for 20 years or more without repairs being required. The smooth finish is very attractive to wheeled users, but frost and ice can make these surfaces treacherous in winter, so salt application may be required. Leaf litter can also cause problems, so overhanging trees may need to be cut back and leaf clearance included in the maintenance schedule.

Hot rolled asphalt

Hot rolled asphalt is a combination of a bitumen binder and fine, well graded aggregate. When it cools it sets to a strong flexible 'concrete'. It is very smooth and totally impermeable. Texture is achieved by rolling in chips after laying. Asphalt is more difficult to lay than bitmac and requires a skilled surfacing contractor. However, asphalt is more flexible and tends to deform rather than break apart if the path's base layer settles.

Porous surfaces

porous surface

Reinforced grass path...

The issue of Sustainable Drainage Systems (SUDS) cannot be ignored, particularly in urban situations where a smooth surface is required. If surface water can be dispersed 'at source' it will reduce the need for systems to store and drain storm water.

In order to produce a porous surface the path's base layer also needs special preparation – it must be free draining, uniformly graded aggregate with no fine content, relying on the spaces between particles to allow water to pass through. However, the base layer needs to be constructed above the water table to avoid becoming waterlogged, and therefore ineffective.

Two techniques that give a similar durable finish to bound surfaces are porous asphalt and resin-bound aggregate. Both surface types need to be laid by specialist contractors and use hot bitumen or UV resin respectively to bind the surface layer together.

Reinforced grass or gravel systems can also be used to construct a porous surface, using interlocking plastic cellular pavers to provide structural strength. For further information about these specialist systems read Reinforced grass paving at Oatridge College demonstration path or Reinforced grass path at Battleby demonstration path.

Find out more about path surfaces here.

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© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh