RST Environmental Solutions Ltd

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RST has invested in technology, design and systems which enable it to build a range of retaining walls. The common theme across all our structures is that they all have a "TOUCH OF GREEN" and they are living. The walls range from bio-engineered structures through to structural design systems with a living front face. This means that a wall can be constructed with a low maintenance vegetative front face that is aesthetically appealing.


These walls are constructed using ancient technology. We commonly use a shrub willow to create the layers but it is possible to use natives or a combination of both. The shrub willow is used as it is a dwarf species that grows as a bush not a tree and is male and sterile so will not seed or have roots the sucker. Once established it is easily mown with a reach mower if height becomes an issue. In general terms this species does not grow higher than 6-8 metres.


The root mass from the brush grows back into the wall adding cohesion to the soil profile and dewatering the site. Willows can take up large amounts of water per day if it is available. The brush cover on the front face also stops a lot of surface erosion during rain events.

The fill material used is generally the in-situ material or one from a local site. It can range in quality and the level of wetness is not an issue as once the willows start to grow the water up take is rapid and the site will dry quickly.

Some sinking of the site can occur during this process usually in the first year and the top of the site may need additional fill. It is not uncommon for sites to shrink by 150-200mm but this does not affect the structural integrity of the wall.

If natives are used then the system must be modified to include a geo-grid to offer additional stability. The natives are then layered into the front face area and will grow out of the wall.

Geo-grid is also used if the internal depth of the wall is greater than the length of the brush available. It is then tied into the brush so that the root mass of the brush will grow through the grid giving structural support.

Brush-layered wall can be used on both slopes and in waterways to secure slips. They can also be successfully used where the base of the slip cannot be founded on hard material as the root mass creates the structural support required to bind the site together.

In areas where additional weight at the top of a slope is a problem they are ideal as they do not increase the slope loading significantly compared to more traditional gabion structures or where rock is used.

Modified Brush-Layer
The modified brush-layer is where the system is either modified with the use of a geo-grid or a Filtrexx front facing system added. Geo-grid is commonly used when the internal depth of the wall is greater than the length of the brush or natives used as outlined on the Brush-layer section.

A Filtrexx front facing system can be assessed to modify the brush-layer if it is used in a waterway situation or when the fill material that is being used is liable to erode before the willow brush has established. This does not apply to all waterway situations but if flooding is likely in the first 3-4 months of installation it is recommended.

The Filtrexx fron facing system uses a synthetically woven sock that is filled with a composted and aggregate mix. The socks are laid on site and filled with a specialised machine. They are seeded generally with grass which once it germinates grows through the socks and bonds them together. Natives can also be used in this system.

The brush-layer acts like a geo-grid in the system giving support to the socks and the site. In a waterway the willow brush creates hydraulic friction when water levels are high which protects the face. It can also create habitat for aquatic species and deposition areas if required.

Fish structures can be added to these walls to enhance habitat creation.

If required aggregate can be used as the back fill and compacted as this does not affect the performance of the brush system.

Living Walls
These walls are mechanically stabilised structures that are engineered for the site. They use a geo-grid to tie back into the failure zone and generally have a coarse aggregate fill. The Filtrexx front facing system is used to create the living system as anything can be grown in this. Anchors can also be used in the structure if the wall needs to be tied back for support.

These walls are designed for both internal and global stability.

A geo-grid is used in this system to fully wrap the front face. The Filtrexx soxx allows vegetation to grow out of it making the wall blend into the natural terrain as many plant species can be grown in it.

Drainage is installed behind the wall then the grid is laid and a Filtrexx soxx is installed on the front face line. Fill is then placed and compacted on top of the grid and the grid is then wrapped back into the wall. The process is repeated until the wall height is reached.

The Filtrexx soxx system also allows you to follow the natural terrain as it is a continuous system and can follow the ground contour rather than having to work in square sections that most other walls need to conform to.

The soxx also allows the wall to be constructed very quickly as it gives good front face support so that the fill material can be compacted right to the front face without any fear of the face moving. This means that walls can be constructed in significantly less time than using traditional systems and have the added advantage of a "green" look to them.

Anchors can also be installed into the side to allow for the wall to be tied back into the slop. The anchors used to date are either a Shot-Rod system fired pneumatically from a soil nail launching gun or a helical screw anchor that is screwed at low revolutions into the slope.

Once installed the anchors are tied together with netting mesh which is lapped back into the wall. This then ties the anchor and the wall together.

Using this style of anchor means that they can be quickly installed under the road without having to close the road or be too invasive. This is a significant advantage over other systems.

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