Section 6a - Suitability by material type

Natural Surfaces

Grading And Reseeding With Grass

Good for routes with average usage where some drainage and surface restoration may be required, but the importation of surfacing material is not required. It relies on the principle of breaking up the old surface and regrading to create a new camber for drainage. It is preferable to harrow the surface in order to break up surface ruts, hardpan, or ironpan. Harrowing can also be carried out where no drainage is needed, but this will require more maintenance if left as a graded surface only.

The key to maintaining a permanent grass surface is securing adequate drainage so that the natural ground does not become cut up by intense equestrian use. If the right balance can be struck between trampling and plant growth virtually no maintenance will be required. Trampling is desirable to maintain the grass sward and prevent scrub invasion. It is however advisable to sow trampling resistant species and lush agricultural mixes. Include species with tough leaves with a growing point below the surface and an ability to reproduce by stem as well as seed. A basic mix of 50% Manhattan Rye, 25% Creeping Red Fescue and 25% Smooth-stalked Meadow Grass is recommended. Seed should be sown in late spring or early autumn when conditions are not too dry or too wet. Seed should be sown at a fairly high density.

Reinforcing fabrics or wire (see below), can be used to bind the surface of the grass.

Woodchips

Providing the surface is well drained woodchips can provide a durable surface that can accommodate both riders and walkers. It should be considered especially where significant scrub clearance is being carried out and the waste can be chipped on site.

Cement Stabilisation

This method involves using cement to strengthen the in situ soil. Between 2-8% by weight of cement is rotovated into the top 0.15m layer, watered then compacted to finished levels.

Granular or cohesive (clay) soils may be treated in this way, although larger proportions of cement are needed for cohesive soils.

Lime Stabilisation

As with cement stabilisation the lime is mixed with the in situ soil in the same way. Lime is only effective on clayey soil. If the process is executed below 15ºC the gain in soil strength will slow. Between 2-5% by weight of lime is required to provide adequate strength (minimum of 5kg of lime per square metre).

Imported Material

Crushed Concrete and Hardcore

In extremely boggy areas where clays or silts are present, layers of free draining material can be used to spread the load on the weak subgrade. The intention is to form a floating platform that will not sink into the weaker soil below.

Crushed concrete is a recycled material in good supply in Hertfordshire, and provides a highly durable surface appropriate for multi-use routes. Hardcore in the form of bricks or rubble can also be used to good effect and is cheap and effective on soft clay.

The irregular size means they both compact well but care should be taken to avoid very large pieces that will penetrate the top surfacing layer. Both should also be checked for undesirable elements such as steel reinforcement, asbestos or other noxious substances, and should be screened at source.

The initial layer should be of a size that when compacted will not sink into the subgrade. Once a firm base has been created the subsequent layers can be formed with smaller and smaller size material until an overall foundation is formed to a depth of 0.3m. This surface provides an excellent floating platform when compacted by heavy machinery.

Care should be taken that the use of heavy machinery in the application does not cause further damage to the surface (2.5m width is usually sufficient for machinery use). The surface may look fairly conspicuous initially and may require covering with a suitable surface material. However, the surface will soon weather and become unobtrusive and is long lasting even with heavy use.

Hoggin

Hoggin is not recommended due to the variable quality of local material.

Granular Sub-Base

This material comprises a well graded granular material which compacts to a dense homogenous mass. It comes in the form of Type 1 and Type 2 sub-base in accordance with the Department of Transport [Manual of Contract Documents for Highway Works (Volume 1: Specification for Highway Works, Series 800, Road Pavements - Unbound Materials)]. Both types are used as a sub-base layer in the construction of new roads.

Type 1 [Clause 803] consists of crushed aggregate and is non plastic. Type 1 material is more expensive but has greater load bearing capabilities.

Type 2 [Clause 804] sub-base material consists of natural sands, gravel, crushed aggregate and a small percentage of plastic material. Type 2 is cheaper and only performs satisfactorily if laid in dry weather.

Geotextiles

Geotextiles are synthetic membranes which are strong and porous fabrics which can be useful in creating a sub-base material. They work on the principle that they act as a barrier and prevent the surfacing materials above mixing with the weak sub-grade below. They can be difficult to apply consistently well.

Because mixing is prevented, effective compaction of the surfacing material can take place. Geotextiles also help to spread the load over the sub-grade so increasing its load bearing capabilities. Recent developments have led to a number of reliable products coming onto the market.

Application Guidelines:

The route to be surfaced must be well prepared being flat, drained and all sharp objects removed. If possible the geotextile should be unrolled in one go away from the contractor's vehicles as these cannot pass over the roll. Avoid driving the machinery across the surfacing as the compaction of this material may stretch and puncture the membrane. At least 0.15m of material should be laid on top to avoid objects from above or in the material puncturing the membrane.

The surfacing should overlap the membrane by 0.15m each side to protect it and prevent any fabric showing. With smaller widths and shorter lengths e.g. footpaths the membrane can be laid by hand. The edge of the path can be cut with a sharp edge and the membrane pushed down below the surface to hold it in place while the surfacing materials are added.

Woven Reinforcing Fabrics

Several products have been developed recently which work on the principle of reinforcing the top soil so grass can take a better hold. Over very soft sub-grade further strengthening from a woven reinforcing fabric may be required to prevent tearing. They can also be used in conjunction with, or in replacement of geotextiles. The application method is similar to geotextiles, however, reinforcing fabrics can be pinned down with steel pins. Reinforcing fabrics can also be used to seed grass in (see above).