Reinforced grass paving

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What is it?

Interlocking plastic 'cellular' paving, laid on a level sandy loam soil bedding layer over a Type 1 granular sub base on geotextile in a formation tray. The plastic paving sections, 620mm x 300mm in size, are connected to each other like jigsaw pieces. The individual 'cells' were filled with soil and seeded with an amenity grass seed to provide a grassed surface.

reinforced grass paving with new grass oatridge college

How does it work?

Before looking at how cellular paving works, it is important to understand how a grass path surface can be affected by people walking or riding over it. Then you will have a better understanding of how cellular paving ensures grass can, and does, survive.

Imagine a grass path that is well used by walkers, mountain bikers and horse riders throughout the year, and in all weathers. Many sets of feet, hooves and tyres are causing damage to the grass surface and the soil beneath in four ways:

  1. Feet, hooves and tyres crush the grass blades, which damages their delicate internal structure, disrupts photosynthesis, and causes excessive moisture loss from the damaged external surface.

  2. Feet and hooves tear leaves and stems as they 'kick back' when they are lifted.

  3. Feet, hooves and tyres smear mud over the leaf surface during wet weather, reducing the amount of sunlight which can penetrate into the leaf.

  4. Feet, hooves and tyres compact the soil, damaging the soil structure and greatly reducing the amount of air space between soil particles. This inhibits root respiration, makes physical root growth difficult, and interferes with the movement and availability of water in the soil.

So, how does cellular paving work to reduce grass damage and prevent soil compaction?

Cellular paving is made up of individual cells which contain soil with grass growing in them. The delicate parts of the grass grow below the edges of the cells. Feet, hooves and tyres make contact with the cell edges – they should not make contact with the soil – and contact with the grass is reduced considerably, allowing the grass more chance of survival. The soil in the cells will not compact, making healthier growing conditions for the grass. Water availability in, and movement around, the soil is good because the soil in the cells is not compacted. In wet weather, cellular paving does not become muddy like a poorly drained grass path surface can. This is because the soil is free draining – water can pass easily through the cellular paving and other layers beneath to reach the ground below.

What are the benefits?

Cellular paving, (also known as grass paving, reinforced grass paving or geomatrices), is a relatively recent introduction to path management. Its use is likely to rise, as demand to provide a surface that offers a low cost, environmentally friendly and, most importantly, completely permeable surfacing solution increases. Cellular paving can provide a number of benefits as a path surface:

  • A reinforced grass surface allows path network development where imported aggregate surface materials would not be permitted.

  • A reinforced grass surface is permeable - well established grass paving can work effectively in areas that are prone to flooding.

  • A reinforced grass surface is robust and performs very well with a low quality cell infill material, eliminating the need to import large quantities of soil to the site – topsoil dug from the formation tray can be used once stones and vegetation have been removed.

Is it suitable?

Cellular paving, with a continuous grass covering, provides a natural-looking surface where the path needs to blend in well with the surroundings. Regular maintenance, such as grass cutting is required, otherwise the path can quickly become overgrown. The surface of cellular paving is not perfectly smooth, so some users may find it uneven to walk on or ride over.

A good selection of cellular paving systems is now available, and the choice of the most suitable system for a path will need to take into account anticipated loads, volume and type of users, existing ground conditions and drainage, and cost. The 'open surface void percentage' of cells at the top and bottom surfaces is an equally important consideration when selecting a system. The open surface void area for the top surface indicates how much of the cell is visible once infilled with soil. Most cellular paving systems offer a minimum of 90% open surface void area. Golpla® grass paving, the cellular paving system installed at Oatridge College, offers 94% of open surface void area for infilling. The open surface void percentage for the base affects the load bearing capacity of the system. A fully open cell base offers very good drainage for water to pass through, but the paving section will be forced into the bedding layer when weight is directly applied. A fully closed cell base offers the best load bearing capacity but water cannot pass through. Many of the cellular paving systems have open surface void percentage for the base between 40% to 70%. Golpla® grass paving has 62% of open base, which spreads the load well to give maximum load support and paving stability.

reinforced grass paving with growing grass in cells

How much will it cost?

Reinforced grass paving construction may cost between £25 - £60/ m².

How do you install it?

Most cellular paving systems follow a similar construction method. First, a formation tray is dug along the path line. Often this is lined with a geotextile sheet before filling with a suitable granular sub base, preferably free draining material. The sub base layer needs to be well consolidated before adding a bedding layer that will carry the actual paving. The bedding layer needs to be evenly laid so that paving fits together properly without moving or separating.

The paving comes in sections that are fitted together and often pinned down onto the bedding layer to hold them in place – this is especially important if the paving is to be installed on a gradient – pinned paving is unlikely to move down the slope. The cells are then filled with soil or gravel, depending on the finish required. For a grass surface, the soil is seeded with a hard-wearing grass seed mixture. It is important to water the new grass regularly and to make sure it is well established before the path is used.

Here at Oatridge College, the method of construction was:

  • Along the line of the route the machine operator, in a tracked excavator, removed the existing grass surface and excavated down to form a level formation tray to a width of 1500mm and depth of 240mm. The turfs and excavated topsoil were used for other path edge landscaping work.

  • Path workers lined the excavated formation tray with a geotextile sheet prior to laying the sub base layer to separate the laid recycled Type 1 granular sub base stone from the sub grade formation.

  • At the recycled Type 1 heap, path workers used a tracked excavator to load the tracked power barrows. They then transported their loads to the start of the geotextile lined formation tray and tipped the Type 1 on to the geotextile. Path workers spread the laid material by rake to levels to form a level sub base layer of 100mm thickness, and then the material was compacted to refusal with a vibrating whacker plate, maintaining levels, to finish the compacted sub base layer, 140mm below existing ground levels. This construction method was continued along the total length of the excavated formation tray.

  • Any part of the compacted sub base surface deviating from the required level was raked off or topped up with additional recycled Type 1 to form the correct levels and then re-compacted to refusal again. The compacted sub base layer was now ready for laying the bedding layer.

  • Path workers loaded the tracked power barrows with clean sandy loam soil, and tracked their barrow loads to the far end of the newly laid compacted sub base layer. Working backwards, the path workers tipped the soil onto the surface along the middle. The path workers roughly spread the soil over the surface to a depth of about 100mm, and then screed the soil to levels to form a level bedding layer of 100mm thickness, 40mm below existing ground levels. This construction method was continued along the total length of the compacted sub base layer's surface. The bedding layer works were complete, and ready for laying the Golpla® paving sections.

  • Beginning in the left hand corner of the bedding layer, a path worker aligned the first paving section up against the formation tray's inside edge on the bedding layer. The next paving section, in same orientation as the first, was connected to previous paving section on the bedding layer. The path worker continued to lay paving sections in rows in a forward direction, standing on the already laid paving when the next row of paving sections was laid. The paving sections continued to be placed on the bedding layer in this manner until the area needing to be paved was completed.

  • Because the path was on a gradient, the paving sections were pinned down on the bedding layer with the manufacturer's plastic pins to provide good anchorage and prevent the finished grassed paving moving down the slope.

  • Once all paving sections were laid, the path worker compacted the entire paved area with a medium weight walk-behind vibrating roller (or compaction plate), to bed the open base of each paving section into the bedding layer beneath.

  • The path worker filled the cellular paving sections with a sandy loam soil to the top edges of each cell and scrapped off overfill to expose the top edges of all cells. The soil was allowed to settle in the cells, but was not topped up with any more material, so that path's users will not make contact with it, or the delicate grass stems.

  • The path worker lightly watered the infilled cellular paving surface to make sure the seedbed was damp, and then amenity grass seed mixture was broadcast at a sowing rate of 45grams per square metre (m²) over the entire infilled cellular paving surface. While the grass was growing it was regularly watered, and then stopped once it was green and well established.

Detailed drawing - Reinforced grass paving

If you like the look of the reinforced grass paving at Oatridge College, as a potential path surface for your project, download the detailed drawing here icon Reinforced Grass Paving - Detail Drawing

For a generic standard detail drawing and specification details, download them here icon Reinforced Grass Path - Standard Detail Drawing & Specification Details

What variations are available?

The cells can be filled with a clean angular gravel or grit sand instead of seeding them with grass seed. The cells prevent the gravel or grit sand moving laterally, stabilising the surface, making it much more user friendly and reducing the scatter and drift of materials that are common problems with gravel or sand surfaces. Cellular paving filled with sand has been successfully tried on various coastal cyclepaths by Sustrans.

Detailed drawing - Reinforced gravel path

For a generic standard detail drawing and specification details, download them here icon Reinforced Gravel Path - Standard Detail Drawing & Specification Details

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