Geocomposites on the basis of geomeshes for road and bridge construction

. Geosynthetic structures are widely used to prevent destructive anti-erosion, anti-landslide processes in transportation construction. The purpose of this work is to develop a transport structural protective layer with high strength characteristics and the preservation of drainage and anti-erosion properties due to the formation of a flexible volume membrane structure. This structure can also be utilized as a crushed stone layer of a multilayer structure for construction of a transportation section with variable stiffness. The protective layer is formed by adding a polymer binder into the composition of a structural layer, consisting of grain elements. The produced geocomposite is made of layers with different functional properties that ensure its high homogeneous properties in porosity, strength, density, drainage and anti-erosion capacity, self-cleaning tendency, and long service life.

materials are in rolls and easily rolled out on the surface.During installation, a significantly smaller number of assemble dowels and less soil are used, since they are enough to backfill no more than 3-4 cm of soil over the geomat.Geomats also have their drawbacks, first of all, they have low strength, 1-2 kN/m in average.

Fig.1. Geomats
The way out of this situation may be the reinforcement of geomats with meshes.We know about a geomat for anti-erosion protection of soil surfaces, containing a base plate made of a flat rolled geomesh and three-dimensional chaotic polymer filaments connected to the base plate and forming a coating.The base plate is made in the form of a warp-knitting geomesh, the intersecting systems of warp threads and a filler which are based on glass roving or polyester fibers, knitted with a thread, while the warp-knitting geogmesh has a cell dimension 20-100 mm, and the coating is made of three-dimensional chaotic filaments extruded on a warp-knitting geomesh with a thickness of 0.3-1 mm, producing a geomat height from 8 to 20 mm and a density from 300 to 1500 g/m 2 , while the densities of the geomesh and the filament coating have a ratio of 1:(3-4) [12].
The drawback of this technical solution is the high damageability of the structure caused by the lack of bottom protection, as well as high deformation properties in the vertical direction, due to which the water disposal is heterogeneous in terms of flow speed and consumption per a unit of time, that causes an increased risk of erosion in areas with a higher flow speed.
Geocomposite is also known as an anti-erosion screen for the roadbed of a transportation structure, containing a geogrid and two layers of geotextile fixed on both planes, while a geogrid is made of a three-level structure of its elements, in which the cells have dimensions that prevent the coupling of geotextile layers as they are loaded [13].The disadvantages of this structure are increased deformations during loading in the vertical direction, that have significant variability.This may cause a decrease in the modulus of elasticity and its coefficient of variation.
There are some new materials, for example, 3D A-Mat, that consist of a polyester grid with integrated free fibers, providing optimal adhesion to the topsoil (Figure 2).The material replaces the functions of the root system, that is dissipates water flows while decreasing its speed.After germination, the root system integrates in A-Mat 3D forming the whole unity.
Another advantage of the material is high strength at low cost.The material has a strength of at least 35 kN/m and 50 kN / m, that allows to withstand significant loads and to protect the slope from local distruction.In comparison, plastic volume geogrids have a strength of 15-25 kN/ m.A-Mat 3-D is made of polyester [13].
A study of the creep of various polymers shown in Figure 3 reveals that polyester is the most stable product in terms of strength loss and elongation of plastic geogrids.A geomat is protected by a special impregnation, which makes it stable to acidic and alkaline environments, as well as to ultraviolet light [14].At the same time, polyethylene itself, from which tapes for geogrids are made, is a material that is poorly resistant to the parameters of "creep" (changes in strength and elongation over time).It is obvious that at the initial stage polyethylene receives critical residual creep deformations.Only polyester retains its dimensional and mechanical properties.

Fig. 3. The graph of creep of various materials [14]
Figure 4 shows typical consequences of volume polyethylene geogrids creep.If such violations in the structure are not repaired at once, then this can lead to a loss of slope stability.

Method
In order to improve the operational properties of protective crushed stone structures, reinforced with a volume geogrid, it is proposed to carry out some surface impregnation of the crushed stone layer with a two-component polyurethane binder.This technical solution can be utilized to create anti-erosion protection of highways and railways, bridge cones with the help of flexible and rigid mats on the roadbed surface, as well as to solve other tasks, related to strengthening filled structures made of crushed stone and gravel of various granulometric composition.The composition hardens during its contact with the natural humidity of the air, the hardening time of the system lasts 20 minutes.A binder based on a polyurethane composition has high cohesive bonds with inert fillers (crushed stone, gravel, etc.).Varying the temperature and humidity of the external environment, it is possible to optimize both the viscosity and the polymerization speed of the binder, that allow to obtain a conglomerate with higher rates of strength, elasticity and durability in adhesive compounds [11,13].Polyurethane-based materials also have very low shrinkage characteristics, about 0.001%.
If we put some additional auxiliary components into the polyurethane binder (catalysts, stabilizers, foamers) and others, it is possible to obtain a material of different stiffness (from monolithic to cellular, from hard to soft), which has high resistance to the negative effects of the environment.This property of materials can also be used for stiffness leveling of approach sections in front of the constructional works.
The composites, created with the use of polyurethane binder, have high physical and mechanical properties, tolerate sudden temperature changes very well, without reducing impact resistance, and are characterized by long-term operational resistance.
In comparison with rubbers, polyurethane compositions have the following properties: • strength is 2.5 times higher than rubber has; • high elasticity, characterized by dual elongation at break than rubber has; • low abrasion, characterized by triple conditional wear resistance than rubber has; • Shore hardness -98 units; • high resistance to tearing and multiple deformations than rubber has; • compositions withstand pressure up to 105 MPa; • high resistance to acids and many solvents; • polyurethane binder-based composites operate in a wide temperature range from -50°C to +120°C without losing elasticity at low temperatures; • high biological resistance to various microorganisms; • vibration resistance, oil and gas resistance, ozone resistance; water resistance.A photo report of the polyurethane use for the repair of bridge cones on the highway "Don" is presented in Figures 5-9.The disadvantage of the presented structure is a low modulus of elasticity, which does not allow to use it as a design structural element of a transportation structure in order to ensure its required strength properties and safety.The flat shape of the polymer base itself restricts functional characteristics to the shear deformation resistance (change in the relative position of lying above/below reinforced layers of the transportation structure).

Results and discussion
The task was set to increase the strength indicators of the structural layer of the transportation structure and preserve its draining and anti-erosion properties.The technical result, achieved by solving the task, is the formation of a flexible volume membrane structure by adding into the structure a layer of granular elements bonded by a hardened binder.
This task is achieved by the fact that a geocomposite, consisting of a volume layer of reinforcing geosynthetic material, according to [15], additionally contains a layer of granular elements, covered with a hardened binder, located on a layer of reinforcing geosynthetic material.Polyurethane, bitumen, polyurea, epoxy resins and their analogues and compositions, based on them, are used as the binding layer of granular elements.
The developed geocomposite consists of a volume layer 1 of reinforcing geosynthetic material, layer 2 of granular elements 3 covered with a hardened binder, placed in the volume of layer 1 of reinforcing geosynthetic material (Fig. 1, Figure 9).Fig. 9. Schematic presentation of the geocomposite structure: 1 -volume layer of the reinforcing geosynthetic material; 2 -layer of granular elements; 3 -a granular element of layer 2; 4 -flexible power threads of the base of layer 1; 5 -free 3-D elements of the base of layer 1; 6 -a filler thread of the base of layer 1; 7 -shells of a hardened binder on granular elements 3 of layer 2; 8 -vertical threads of a hardened binder of layer 2; 9 -randomly distributed cavities of layer 2; 10point connections of the contact areas of shells 7 with each other of layer 2.
Layer 1 of reinforcing geosynthetic material is made in the form of a volume two-component structure consisting of a base, including a reinforced volume textile material and organic impregnation, and layer 2 of granular elements 3, covered with a hardened binder, is included in the pore space of volume layer 1, made of a reinforcing geosynthetic material.Layer 1 base of the geosynthetic material presents a reinforced volume textile material from layers of flexible power threads 4 and free 3-D elements 5 knitted together with a filler thread 6 at their intersections, and a liquid polymer based on styrene-butadiene-styrene is used, for example, as an organic impregnation (Fig. 2, Figure 9).Some free space between the reinforcing and free fibers (threads) of the reinforcing geosynthetic material determines its pore space, which includes layer 2 of granular elements 3, covered with a hardened binder.Layer 2 of granular elements 3, covered with a hardened binder, forms a dimentional frame structure in the form of shells 7 of a hardened binder on granular elements 3, vertical threads 8 of a hardened binder, coming from the bottom point of granular elements 3, and randomly distributed cavities 9, generated in the area of point connections 10 of the contact areas of shells 4 with each other (Fig. 3, Figure 9).
The geometry of granular elements 3 determines the spread (scattering field) of the heights of rise heights and depression depths in layer 2.
An average square deviation of height difference of rises and depth difference of depressions in layer 2 of granular elements 3 is 0.5-5.0mm and it is defined empirically depending on the material and dimension parameters of granular element 3 due to the conditions of achieving optimal drainage and strength parameters of layer 2.
When the average square deviation value is less than 0.5 mm, the drainage properties of geocomposite drop dramatically, and when the deviation value is more than 5.0 mm, its strength characteristics decrease.
For example, fractioned crushed stone, sand, gravel, stone, screenings, artificial crushed stone, fractioned secondary breakage (crushing and screening product) of construction materials and their screenings are used as granular element 3.In this case, the size of granular elements 3 of layer 2 are within the range of 3-15 mm.This range corresponds to the sizes of the pore space of layer 1 of reinforcing geosynthetic material and sizes of crushed stone, screenings and gravel that are abundant in the market of construction materials.Smaller sizes do not allow for meeting of the requirements for water-transmitting capacity, and large sizes considerably heavy up the geocomposite.
Polyurethane, polyurea, bitumen, epoxy resins and their analogues as well as compositions on their basis can be used as a binder of layer 2. The usage of the polyurethane composite blend as a binding material allows forming flexible elastic threads 8 in the space between granular elements 3, and strong and durable connections in points 10 of the contact of shells 7 of granular element 3 as it solidifies.
Drainage and anti-erosion properties of reinforced composite according to the first variant are provided by a volume pore structure of a layer, consisting of granular elements covered with a hardened binder, and according to the second variant, including the combined use of a pore structure of a non-woven fabric interlayer and a volume layer of granular elements covered with a hardened binder.This leads to the separation of flows with respect to precipitation and to groundwater above and below soil environments.
The dimensional frame structure of a layer of granular elements due to technological and high operational properties contributes to the formation of its stable geometric and strength uniformity, the required functional properties and stability of the transportation structure.
Figure 10 shows a photo of a produced geocomposite on a geogrid basis.

Conclusions.
Geocomposite is characterized by the increased vertical strength and drainage properties, since it insures horizontal separate distribution of groundwater and precipitation, and the required uniformity of geometric and mechanical parameters of the structural layer of the transportation structure, that leads to a longer inter-repair period and service life of the transportation structure.
In order to strengthen the subgrade and/or to cover a transportation structure, it is more advisable to use a two-component polyurethane system, which has good compatibility with various types of fractional fillers.A binder can be modified in such a way that stiffness of the produced composite can vary from soft to hard monolithic material.

Fig. 8 .
Fig. 8. Repair results of the railway in Sochi (the authors' photo).

Fig. 10 .
Fig. 10.Photo of a produced geocomposite on a geogrid basis