M25 Road Widening

Salix were tasked with greening up a 45 degree slope consisting of 6F2 fill. The initial design was a covering of 150mm of topsoil over an area of about 8,000m2 using a geocell product attached to the surface of the slope to contain the soil and prevent it from slipping.

This option is extremely time consuming and expensive to install and then fill with a decent, friable topsoil.

Salix offered a value engineering solution by replacing the need for a geocell and soil layer and applying HydraCX direct onto the 6F2 fill material.

Establishing a grassy vegetation cover on steep embankment with slope angles of more than 40° can be difficult. However there are many steep slopes with a full vegetation cover. The limiting factors for plant life are:

• Moisture
• Air
• Nutrients
• Temperature
• Light

Of the above the most common limiting factor is moisture. Where the vegetation exists on steep slopes it is because the slope allows the ingress of moisture from external sources, eg rain and runoff, and in some cases from weeping lenses of internal origin. If moisture can travel in the slope face there is likely to be sufficient porosity for enough air to be in the soil profile for active root activity. Temperature and light will also affect the growth rate.

If the grass regime can be established and survive its first year, it is often the case that none of the above is a limiting factor. Plants on the slope increase the roughness of the profile allowing water time to ingress into the “soil” of the face, thus improving the growth to sustain the vegetation.

HydraCX is a system that not only hydraulically applies seeds to a slope, but also includes cotton and straw fibres (with polymer and tackifier to prevent wash off in heavy rain) that bridge and arch on impact to form a physical matrix.

• This matrix is stable against the impacting rain drops providing an erosion control solution. This can also be accompanied with soil stabilisation products to bolster the strength of the soil.
• The matrix allows more air passage than the smothering effect of hydraulic paper/wood mulches and so the seeds germinate and establish extremely quickly.
• The cotton/straw breaks down slowly releasing a balanced nutrient resource.

Salix included a grass mix within the HydraCx that could tolerate difficult conditions, plus a nurse crop to aid initial plant establishment and legumes to fix atmospheric nitrogen for long term growth.

Application was in August 2010 in very dry conditions. By the end of September the following month grass cover was exceptional. By May 2011 the UK had experienced the driest Spring in 100 years but by then vegetation had established and continued to survive. The site was visited in August 2013, three years after initial application and the density of growth was surprising. Very lush vegetation with many different species present and 100% coverage over the surface with no bare patches.