Every day, we face problems related to hydrogeologic collapse, such as erosion and landslides.
These problems originate from man’s need to develop urban areas, transportation routes, and the need to use soil and land for farming, whilst also trying to maintain the delicate balance of the surrounding habitat.
The construction of new roads will always impact negatively on the environment since the design priority for a road structure is to be functional.
Environmental restoration
However, it is possible to reduce the negative aesthetic and environmental effects of existing or new roads with innovative design solutions, which will minimise the need for further environmental restoration. Any solutions adopted must naturally blend in with the surroundings.
For any restoration works carried out in urban areas or in water courses, it is fundamental to study and implement naturalistic engineering which will allow the fluvial ecosystem to blend into the urban environment. This will allow better use of the riverbanks, the berm’s banks, green areas, and natural trails.
Our article will discuss:
- Mass gravity retaining walls using Gabions, including important design considerations
- Protection bridge foundations and culvert outlets
Mass gravity retaining walls using Gabions
Gabion walls utilise their purpose-built weight to contain and resist lateral pressures arising from soils, water, or imposed loading. These walls are durable, flexible, permeable and can easily be constructed using mechanical or labour-intensive methods.
Gabions also provide a versatile method of constructing a retaining wall as they are both time and cost effective, and they can be placed where conventional techniques would be impractical or financially prohibitive.
Typical Locations for Gabion retaining walls:
- Mountainous regions
- Marshy ground, where the wall serves as a drain as well as a retaining structure
- Areas liable to subsidence, where its ability to deform without fracture gives it a marked advantage over concrete
- In emergency works – e.g., slips on railways and roads – gabions are used to make repairs which are speedy, effective, and permanent, obviating the need for a second repair job.
Construction of Gabion retaining walls
Construction of a gabion wall is essentially simple. The gabions are built up row on row like toy bricks, preferably with the joints being staggered to improve the structure’s appearance.
- They can be built to any height, size, or shape, subject to a minimum width of 1m, as recommended for toe walls.
- Widths are usually in multiples of 1m, but walls can be built to the nearest 0.5m by incorporating 0.5m gabions placed on their side.
- To give the wall an even more attractive appearance, the filling can be arranged with selected large stones on the outside face. This gives the impression of natural stonework, and the wall in due course merges with its natural surroundings, with soil and vegetation collecting in the voids.
- The top of the wall can be turfed to match the adjacent ground.
Important design considerations
- The design calculations for a permanent gabion will follow the same principles as for a conventional gravity wall of masonry or unreinforced concrete.
- For temporary structures, some tension can be allowed on the inside face, and the resultant thrust can pass slightly outside the middle third.
How to calculate the width of a Gabion wall
- A rule-of-thumb method of calculating the required width of walls up to 6m high is to take it as being two-thirds of the wall height at the base, reducing the steps to 1m wide at the top if there is no surcharge or to 2m if there is a surcharge.
- When successive courses of the wall are only 1m wide, steps of at least 0.15m must be incorporated for each rise of 1m.
- The width should be increased if a light material like broken concrete is used to fill the gabions but may be reduced if indicated by data from soil investigations on site.
- An economical way of strengthening a wall without using excessive materials is to incorporate buttresses or counterforts.
- The steps may be on the outer or inner face of the wall or on both. On the outer face they give more stability for less material; on the inner face they tend to allow more space behind the top of the wall. Whether stepped or not, the outer face can be erected at a batter, simply by tilting the gabions to the required angle. The wall can be curved in plan by shaping the gabions to the required radius.
- The design must allow for risk sliding. If necessary, the base of the wall should be let into the ground to obtain a sufficient key; or, if a slip plane exists, the wall must be taken down to a firm foundation below it.
- Drainage to remove water from the base must always be provided. Where seepage into the ground below the wall must be prevented, the wall should be built over a waterproof membrane such as a thin concrete slab or butyl rubber sheeting.
Reinforced soil structures help prevent slope collapse
The term “collapse of a slope” means rapid sliding of, or rotation of a large or small soil mass or wedge. This movement may be either deep or superficial. The main causes of collapse may be due to erosion or inadequate filtration along with the geomorphology area.
Soils subjected to dynamic or static loading must be stabilised to ensure equilibrium of the surrounding environment. When soil is confined or loaded, disturbing forces are set up that may give rise to sliding, overturning, and bearing failures.
To counteract these effects, a series of interventions may be applied:
- mass gravity walls,
- segmental block or mechanically stabilised walls,
- slope reinforcements, or even
- basal platform reinforcement
Mechanically stabilised earth (MSE) walls or slopes can have flexible or rigid facades, and the reinforcement is characterised by high strength, low elongation properties.
These walls are incorporated into most environments simply by choosing the most appropriate facing system and reinforcement.
They are uncomplicated to construct, adapt easily to changing field conditions and offer significant cost savings over more traditional reinforced concrete structures.
Where the gabion rock is not easy to source, when it is required for mass retaining walls or when an embankment protection is required, a Terramesh System, combined with or without geogrids represents the ideal solution.
The external face can be built using stones, plants, or prefabricated blocks, allowing the best choice to be made in any solution, from a technical, environmental, economic, and landscaping point of view.
Culvert Outlets and Bridge Foundations
Meteorological forces, such as rainfall and temperatures acting on an area will, through soil erosion, modify the physical and structural characteristics, as well as its morphologic aspects of the area.
Landslides or surface erosion due to water runoff, are the main causes of structural instability. To solve these problems, it is necessary to control the undesired effects, without upsetting the environmental equilibrium.
Human settlements (urban or industrial), and the development of communication routes can be the cause of land alterations.
In the case of human settlements, it is necessary to take action in order to limit the damage trying to reach an equilibrium between safety and environmental solutions.
In other cases, the protection work carried out, may alter the environmental balance, carefully selected to, in time, restore the original conditions.
How do we protect bridge foundations and culvert outlets?
Erosion at the foot of the bridge foundations is a serious problem in road engineering. Bridge abutments and bridge piers often collapse as consequence of scour around and under the foundation from the erosive power of flowing water.
Gabions and mattresses provide the ideal protection to the foundation of the piers and abutments and the terrain around the bridge structure. In remote places, gabions themselves are used to support the bridge deck structure.
Gabion mattresses are used widely, for protecting road and railway bridges – e.g. in reveting banks upstream and downstream from a bridge and slopes in front of spill-through abutments; and in protecting solid abutments and piers against scour, neither individually nor by a mattress covering the whole width of the invert.
A sloping head wall and invert of Gabion mattresses, or a mattress apron combined with a head wall and wing walls of gabions or other constructions, can form the downstream protection to culverts under a road or railway embarkment.
Protection of entry and exist points to culverts provided by a combination of gabions head and wing walls with gabion mattresses scour protection.
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