The invention is applicable in the field of retaining structures, known as "reinforced soil", and, more specifically, it refers to a structure for reinforcing slope, embankments and excavation faces, made up of a facing element that is made of a natural and biodegradable material, and a reinforcement element placed in horizontal layers, consisting of a double twisted wire mesh and/or a synthetic geogrid, possibly combined with an anti-erosion component.

"The reinforced soil" is an engineering solution which allows to combine the soil features with those of the reinforcing elements so obtaining a composite structure capable of resisting to tensile stress so expanding the scope of earthworks. Reinforced soil structures and their applications currently represent a key point in the field of the geotechnical engineering because they make it possible to reinforce the soil structure while respecting the natural and various environmental aspects of the sites.

Reinforced soil structure applies to several areas, as for example road and railway infrastructures, soil protection, mitigation of the hydrogeological instability, hydraulic constructions, river works or rockfall protection works, bridge abutments retaining walls, green elements for street furniture and industrial complexes, noise barriers.

Reinforced soil walls or embankments bring wider benefits than the traditional gravity retaining walls or reinforced concrete retaining walls, mainly because they imply a simple technique which does not need any specialised equipment or staff. For doing this works it is in fact only necessary to use an excavator, a compactor and little skilled labour. It is furthermore possible to easily construct very high retaining works able to take significant deformations before reaching their serviceability limit state, so needing no deep foundations and so being particularly suitable for stabilizing landslides. If compared to the rigid concrete structures, they can better bear earthquakes, dynamic traffic loads and impacting loads on them. Using reinforced soil instead of a traditional reinforced concrete wall has a lower environmental impact, especially in consideration of the green grass growing on the front side, and makes it possible to use filling material collected right there as a result of excavation, so considerably reducing the construction costs. As is well known, reinforced soil structures are made of three elements:

- a reinforcing element (usually a geogrid or an hexagonal double twisted wire mesh, having different levels of corrosion protection)

- a facing element (usually a welded wire steel mesh having a variable spacing and cross section diameter usually about 7 to 8 millimetres, mostly having no corrosion protection, combined with a natural or synthetic anti-erosion component

- structural soil (that is the filling material of the structure)

Though widely used, this standard type of reinforced structure has got some drawbacks. One of them is the risk of soil pollution due to the degradation and corrosion of the metallic material making up the facing element. This also applies to the zinc-coated facing elements which nonetheless release harmful particles of rust into the soil, though later than the classic black facing elements (without zinc-coating).

It is worth considering that many countries have laws which forbid (or guidelines suggesting to avoid) the use of steel in certain areas, such as for example some natural parks, rivers and seas, areas in which slowly it will no longer be possible to use the reinforced earth technique, as it is known today, notwithstanding the aforementioned advantages.

Accordingly, in consideration of the growing worldwide trend towards maximising the respect for the environment and reducing the various forms of pollution, the principal aim of the present invention is to provide a reinforced soil structure, embankments and excavation faces comprising a facing element made of a biodegradable and natural origin material in order to avoid the risk of soil and groundwater pollution deriving from the steel degradation and corrosion (the systems currently in use include in fact facing elements, connecting parts and stiffening elements made of iron).

Another aim of the present invention is to provide a structure for reinforced soil and excavation faces comprising a facing element, made of natural and biodegradable material, which can preserve the initial shape of the work even after the facing element progressive biodegradation.

A further aim of the invention is to provide a structure for reinforced soil and excavation faces which can be quickly sited and laid and which can be done by unskilled staff.

Through the present invention the so far described (and further) aims are achieved; they are hereinafter illustrated in a preferred embodiment, but further improvements are still possible, by means of the attached drawings which show:

Fig. 1 , an axonometric view of the facing element provided for in the present invention;

Fig. 2, an axonometric view of a first embodiment of a retaining structure including a facing element provided for in the present invention; Fig. 1 illustrates the facing element referenced in the present invention in a first embodiment. The facing element, indicated as a whole with the reference number 1 , is made of natural origin and biodegradable material. More specifically, said material may be a composite, biological or natural origin material, or, alternatively, synthetic material or a biopolymer.

The facing element 1 is therefore biodegradable and the material which it is made of can also render it suitably rigid, so that the proper shape of the external face of the work is assuredly preserved. If the facing element 1 is made of a natural biological material, the latter is a unique natural origin element naturally occurring in the form used, for example wood.

Alternatively, if the natural origin material is made of a synthetic one deriving from natural elements, it is a biopolymer. A biopolymer shall be understood as a synthetic material, in most cases biodegradable and non-toxic, obtained from renewable natural sources, either produced from biological systems (plants, animals, microorganisms) or chemically synthesized from molecules of biological origin (sugars, starch, oils, fats). In a first embodiment, the biopolymer making the facing element 1 is polylactic acid 100 percent genuine (PLA) or, alternatively, bonded with other natural elements.

Furthermore, if the natural origin material is a composite one, the latter shall be understood as any material variously obtained by combining in different percentages elements of natural origin (synthesized and non-synthesized) with other elements which improve the properties of said materials. In another embodiment, the material of the facing element 1 may be a fibre composite material containing cellulose, as for example WPC wood, or a composite PLA, made of PLA bonded with other elements in different percentages. As to the shape of the facing element 1 , in a first embodiment it is made of a rectangular grid 2, having for example a 2 metres frontal width, so that it can be easily shaped into a curved frame, whereas the commonly known facing elements usually have a 3 to 4 metres width. As an alternative, the grid 2 may have a thickened square shape or an oval shape, like a river stone.

In a different embodiment of the invention, the facing element 1 includes a secondary grid 3 - generally smaller in size than the grid 2 - hinged to the grid 2 in correspondence of its side 21. Grids 2 and 3 are reciprocally hinged by means of binding elements 4 made of a suitably resistant string or natural fibre cord, or other equivalent means, as for example a series of rings, suitable for fastening the grid 2 to the grid 3 in correspondence of the side 21 and for allowing both grids to freely rotate around the axis defined by side 21. A different embodiment includes the use of clatches between grids 2 and 3. Another embodiment involves the use of tightening brackets 5 to be interposed between grid 2 and 3. As a consequence, the tightening brackets 5 make it possible to rapidly set and keep the designed inclination of the work planned for the facing element. In a first embodiment the tightening brackets 5 have a triangular section and are made of the same natural - composite or biological - material of the facing element 1 , namely of the grids 2 and 3. The tightening brackets 5 are fastened to the grid 2 through suitable means 6, for example rings, binding elements or clips, and they are secured to the soil and/or to the secondary grid 3 by means of stakes 7.

In a different embodiment spacer devices (or struts) are included as an alternative to the tightening brackets 5, said spacers having a length suitable for rapidly setting and keeping the designed inclination of the facing element. All that is outlined above clearly shows that the facing element 1 does not include any steel parts, generally used in the known facing elements, because the facing element 1 , any tightening brackets 5 (or any struts), the linking elements 4, the fastening means 6 and the stakes 7 are all made of biodegradable material. Furthermore, considering that the facing element is a formwork to be only used for shaping the work and not having any structural purpose, it is actually not necessary to aim at a higher resistance deriving from the use of a metallic material; lower resistance materials are therefore fittingly used as long as they are suitably rigid in order to keep the soil during the spreading and compaction phases and to help to hold the front anti-erosion element up to when vegetation grows up. From that moment onwards, the facing element no longer has any purpose.

Lastly, when biodegradation of the facing element 1 starts, the reinforcing structure will keep its initial shape, which will be overtime ensured by the structural reinforcements. From the comparison between the facing element provided for in the present invention and removable formwork soil reinforcement techniques, the latter involve the use of the formwork only during the construction operations in order to shape the work and it is then rapidly removed before constructing the upper reinforcing layer. This happens because the same formwork is used more than once and the use of iron is avoided. This technique is much slower than the one using a non-removable welded steel mesh formwork in terms of the extent of work surface done in a day and it also needs at least two types of highly specialised staff: the material positioner and the excavator operator, who carefully removes the formwork trying to avoid deforming or damaging the structure. This technique has been almost completely abandoned in recent years because of its expensive labour requirements and it has been replaced by the technique using a non-removable welded steel mesh formwork, which involves the additional cost of the steel but also a strong labour reduction and which nonetheless has the pollution related disadvantages as above indicated. Therefore, the use of a facing element 1 made of natural, composite, synthetic or biological material firstly does not generate the soil pollution, it has reduced construction times and it does not need any highly specialised staff. The grid 2 and the secondary grid 3, as shown in fig. 1 , have standard size square meshes. In a different embodiment it is anyway possible to use a facing element 1 including grids 2 and 3 having thickened meshes so that it is not necessary to use an anti-erosion bionet or mat combined with the facing element 1. Costs are this way reduced and since it is less necessary to use metallic clips in order to join the various elements, corrosion and the release of pollutants into soil are restricted.

As it can be seen in fig. 2, the facing element 1 is used in the construction of structures 100 for reinforced soil and excavation faces. In a first embodiment a structure 100 for reinforced soil and excavation faces includes a facing element 101 , an anti-erosion net 102 and a reinforcing element 103, the latter being alternatively made of a double twisted wire mesh or a geo-synthetic net.

The facing element 101 , the anti-erosion net 102 (if any) and the reinforcing net can be positioned as it follows: facing element - anti-erosion net - reinforcing net or, rather, reinforcing net - facing element - anti-erosion net, or in any different sequence. All will depend on the landscape visual requirements as chosen by the work designer, but it will change nothing in terms of reducing soil pollution.

The structure 100 for soil reinforcement, made as described above, may be found in a pre-assembled condition so as to facilitate the installation operations by unskilled staff and the folding lines are made in order to help transporting and unfolding said structure in the construction site. By means of the invention here provided for, it is possible to overcome the difficulties deriving from the currently known pre-assembled elements having a fixed height, which means a variation in the height of the reinforced soil layer according to the change of the external facing element inclination, since the facing element is in a standard size welded steel mesh. The facing element in natural biodegradable material as here proposed may be differently sized.

Finally, as for joining distinct structures 100 so as to make a single reinforcing structure, in the present invention all the lateral junctions are eliminated. More specifically, each facing element 100, having a standard width, is assembled to the reinforcing element 103, having a greater width or at least a lateral extension 104, the latter being as wide as the length of 1 or 2 meshes (for a double twisted wire mesh) or at least 10 to 15 centimetres wide (for a geogrid). It is so possible to join distinct structures by simply overlapping two adjacent elements. In this case elements are joined due to the resistance transfer among them, without interfering with the thickness of the facing element 1 and without having therefore to secure each mesh by means of steel clips or steel wire along the entire length of the reinforcing elements. These are often overlooked details because they take a long time to be performed, so elements are left with no junction. If two adjacent elements do not come in direct contact, there will be no resistance progression and no strain transfer. Each single element will separately work and will not co-operate with the adjacent elements. This is something which does not benefit the final result of the whole work. The solution as here proposed much facilitates the production process and it can ensure the resistance continuity and the strain transfer among the various reinforcing elements. It is worth considering that the phases of material filling and compaction are executed by heavy means such as (crawler type) excavators and compact rollers which often cause the reinforcement elements to be moved by few millimetres if they are not properly joined, so rapidly losing the lateral cooperation effect. Overlapping 1 or 2 meshes (for a double twisted wire mesh) or at least 10 to 15 centimetres (for a geogrid) therefore ensures the strain transfer among the elements since, even though they are moved by few millimetres during the installation operations, they lay several centimetres overlapped.