performances in access road construction valentin feodorov · performances in access road...

ECOTERRA - Journal of Environmental Research and Protection

www.ecoterra-online.ro 2014, Volume 11, Issue 4

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Performances in access road construction Valentin Feodorov

University of Agricultural Sciences and Veterinary Medicine, Bucharest, Romania. Corresponding author: V. Feodorov, [email protected]

Abstract. The paper deals with access roads to the wind power farms constructed between the years 2008 and 2013 in Romanian county Dobrogea. These unpaved and foundationless roads are devoted to support temporarily heavy trucks with cranes for the establishment, operation and maintenance wind power mills. The main function of access roads is to take over the concentrated forces transmitted by wheels and reduce them through horizontal forces by so called vault effect. It is explained at the beginning of paper how this force conversion is carried out by gravel layers reinforced with triaxial geogrids. Gravel grains of carefully selected sizes and shapes are uniformly confined in the apertures of triaxial geogrids and act together to distribute on large horizontal areas the radial effects of concentrated forces generated by truck wheels. This brilliant application of the ancient vault effect mechanism radically improved the construction technology of the access roads. Further the paper successively presents a physical demonstrative model of vault effect, the registered mechanism of TriAx stabilization, mathematical modeling of trafficked sub-base, the visualization of radial stiffness ratio, comparison of trafficking performance, selection of the appropriate Tensar TriAx geogrids and geogrid foundation solutions in earthquake-susceptible locations. The paper concludes with some Romanian study cases defined by their technologic and economic performances. Key Words: confining, radial stiffness, reciprocal actions, vault effect, wedge effect.

Introduction. The paper deals with access roads to isolate and remote locations like those required for the construction, maintenance and ultimately the dismantling phases of wind farm projects. All types of relief are possible to be encountered for such farm sites, from open, flat plains, to the most rugged forms of terrains. Sometime existing access roads, traditionally used for agriculture or foresting, are met around. If still preserved, such roads that bear the local experience of time are worth to be considered and accordingly reshaped. In most cases new access roads, which always end in crane lifting platforms, should be carefully drawn. The main function of these roads is to provide safe, reliable access for materials, turbine components and the passage of cranes to the turbine locations. However, the most intense traffic is usually the aggregate delivery vehicles which are used in the road construction itself. Generally, such roads and platforms are submitted to enormously heavy loads. Whether local climate conditions are severe, foundation soil is poor, and the area prone to strong earthquakes the traditional solutions of constructions can become costly, time consuming and not environmentally friendly. This is why advanced technologies, based on triaxial geogrids, are adopted for such special access of paved roads. The required thickness of the road is most strongly influenced by the traffic activity that has to be supported during the road construction stage and which is engaged in the act of importing the fill for further construction activity along the route. The in-service-traffic generated by the importation of concrete and steel for the foundation, the delivery of the turbine components and the passage of the turbine erection crane is usually a design check on bearing capacity and edge stability rather than the principal determinant of the designed thickness. In-service-traffic the period is 3-6 months and for maintenance is maxim 20 years depends on the time of operation of the wind farm. Geogrids have been used since the early 1980’s to stabilise aggregate in access roads constructed over compressible peat. Such ‘monolithic’ geogrids, with integrated solid joints, were used to provide safe access over soft ground in public roads in the Shetland Islands and infrastructure works on the Falkland Islands. The first recorded wind farm access road to use geogrids was Ovenden Moor near Halifax, UK in the mid-1980. In Romania the use of triaxial geogrid for wind farms project started in 2008, the first big project was Wind Farm Fantanele. Since that time, numerous wind farms projects have been constructed with triaxial geogrids. The greatest challenge when the wind farm is being constructed can be when the large turbine components are unloaded and lifted into position using a crane. The load spreading capability of a triaxial geogrid



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layer increases the bearing capacity of working platforms for heavy-duty plant, cranes and piling rigs. For the contractor this means that less natural aggregate is required to construct the platform which can result in quicker construction and less cost when compared with traditional unreinforced constructions (Feodorov 2014; Tensar 2011a, b, 2013, 2014a, b). Basic design concept. Only two materials are used for completion the access paved roads, crushed stone aggregates and geogrids. The modes in which the two materials are associated in order to take over and further transmit the analysis forces can be explained with a simple idealised model. Indeed, let’s assume that six identical heavy spherical bodies are freely arranged along a row, with punctual contacts between them, on a horizontal surface almost without friction. Under the gravitational field all spherical bodies remain indefinitely at rest (Figure 1left). Suppose that at one subsequent instant an additional spherical body, identical with the others, is laid somewhere over that original row by two punctual contacts (Figure 1right).

Figure 1. The row of six spherical bodies on a horizontal surface.

Since the weight of the added sphere is transmitted downward, along two directions, equally inclined with 30° to vertical, a vault effect occurs at the level of basic row of spherical bodies. Two identic horizontal forces are developed in both directions. Instantaneously, the former mechanical state of rest ceases. Under the horizontal thrust caused by the upper sphere all six bodies are laterally pushed in both directions on the horizontal surface. It should be noted that the whole experiment is happen in the gravitational field and with the direct involvement of the gravity (Figure 2).

Figure 2. Displacements caused by the additional spherical body.

Eventually, some of the seven spherical bodies, under some frictional forces, stop moving and remain in undefined positions into the so called mechanical state of indifference, while others, further moving, under the gravity action are falling down (Figure 3).

Figure 3. Spherical bodies either in indifferent positions or falling down.

As a rule in Applied Mechanics, bodies at rest mean stability, and the stability is controlled by Newton’s Laws of equilibrium. Moving bodies mean instability, and the instability is controlled by Newton’s Laws of motion. The mechanical state of indifference, that which also often occurs like a limit state between stability and instability, is not governed by any law and therefore cannot be controlled at all. In practice it should be avoided by any means. In the case of the idealised model adopted above, if the row of spherical bodies is fixed at both of its ends it will easily support an additional layer of spherical bodies without laterally moving. By such loading in the basic row of spherical bodies a horizontal state of axial compression is induced (Figure 4).



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Figure 4. Displacement prevention and compression by overloading the basic row.

A row of spherical bodies can be overloaded by successive layers under the two slopes of 60° up to the top of the figure. These upper rows become also axially compressed by the same vault effect. In this way the spherical bodies are kept in permanent contact and the cross section remains compact (Figure 5).

Figure 5. Third and fourth layers of spherical bodies.

The degree of compression of each row is gradually decreasing from the base towards the top. In order to reach a certain level of compression the top can be overloaded (Figure 6).

Figure 6. Fifth layer of spherical bodies and the top piece.

It should be also added that the triangular section is the only stable section in plan, and particularly, the equilateral triangle is a perfect figure. By bringing together two such triangular cross-sections and filling the free space between them with identic spherical bodies one obtains an isosceles trapezoidal shape. Due to the wedging phenomenon, based on the effect of Archimedes’ inclined plan, this cross-section is compact and stable. As concerns the height of this cross-section it is limited to 87% of the distance between two consecutive fixing points of the base (Figure 7).

Figure 7. Lateral extension of bi-dimensional model with triangle units.

By surcharging the above typical cross-section it can be extended geometrically in both directions. The biaxial state of created compression is gravitationally generated, directly by the vertical loads and horizontally by the vault effect. All that process of extension depends on the bearing capacity of the layer that contains the fixing points (Figure 8).



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Figure 8. Further vertical extension of bi-dimensional model with triangle units.

For practical purposes the above bi-dimensional model has to be extended in space. After careful tests regarding the response of the basic layer, which is submitted exclusively to tension, the best results were obtained upon triaxial geogrids with triangular apertures. This is why the tetrahedral model was adopted. Its distinctive quality is the confinement ability, what means developing a triaxial state of compression. Mono and bi axial compression don’t have confining qualities. The bearing capacity of a confined layer can be five times higher than that of a monoaxially compressed layer (Figure 9).

Figure 9. Tri-dimensional model of spherical bodies and the confinement effect.

The confinement produced by the vault effect is essential for the real structure of paved roads. The aggregates of crushed stones are no longer spherical, and therefore the punctual contacts between coarse grains are replaced by little contact surfaces where local forces of Coulombian friction occur. Frictional forces are better conserving the wedging effects. For this reason the crushed stone is preferred to that extracted from rivers and rounded by waters (Figure 10).

Figure 10. Real model of reinforcing the structure of paved roads.



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Newton’s Principle of reciprocal actions: - types of static and dynamic actions; - size of computing actions; - mode of vertical and horizontal actions. Performances of triaxial technology. It was natural that after uniaxial and biaxial geogrids the innovative thinking of the producer would provide an increased capacity to cover the efforts and deformations plan. Maintaining the same topological congruency and auto morphism rules, but also the rigid character of the nodes, by arranging the geogrids according to three directions at a distance of 60°, the so-called triaxial geogrids are obtained (Figure 11). The performance is technological because a few centuries ago, the simultaneous extension on two perpendicular directions was a problem.

Figure 11. The perspective geometry of triaxial geogrids.

The mechanic performance is illustrated suggestively by the comparison between the bearing capacity under concentrated force of the biaxial geogrid in blue and that of the triaxial geogrid in red. It is about the coverage capacity which is greater in triaxial geogrids than in biaxial geogrids (Figure 12). The consequence of this innovation is economical. For road works, where the concentrated forces are dominant, the triaxial geogrids are cost effective than the biaxial ones. For retention works, uniaxial geogrids continue to remain the most efficient.

Figure 12. Load distribution at 360 degrees.

Load distribution is 3-dimensional in nature and acts radially at all levels in the aggregate. For a stabilised layer to be effective it must have the ability to distribute loads through 360 degrees. To ensure optimum performance, the geogrid reinforcement in a mechanically stabilized layer should have a high radial stiffness throughout the full 360 degrees. Biaxial geogrids have tensile stiffness predominantly in two directions. TriAxgeogrids have three principal directions of stiffness, which is further enhanced by



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their rigid triangular geometry. This produces a significant different structure than any other geogrid and provides high stiffness through 360 degrees (Figure 13).

Figure 13. Radial coverage at biaxial geogrid with blue and at triaxial geogrid with red.

Performances of confining mechanism. In a mechanically stabilized layer, aggregate particles interlock within the geogrid and are confined within the apertures, creating an enhanced composite material with improved performance characteristics. The structural properties of the mechanically stabilized layer are influenced by the magnitude and depth of the confined zones. According to Figure 14 is three zones: unconfined zone, transition zone (partial confinement) and fully confined zone. The shape and thickness of the geogrid ribs and the overall structure of triaxial geogrid has a direct influence on the degree of confinement and efficiency of the stabilized layer. Triaxial geogrid increases the magnitude of confinement and increases the depth of the confined zones.

Figure 14. Interlock and confinement of triaxial geogrid.



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Performances in selection of the appropriate triaxial geogrids. Subgrade strength is the principal factor affecting the selection of the most appropriate triaxial geogrid to place on the subgrade. This is expressed as a CBR value, or undrained shear strength (Cu). In some situations, where the subgrade strength is very weak, multi-layer geogrid stabilisation is often appropriate. Guidance, according to subgrade strength and fill having maximum particle size of 75 mm is as in Figures 15 and 16.

Figure 15. Triaxial geogrid selection: subgrade strength with radial secant stiffness at 0.5% strain. The availability of granular fill material used within the construction industry can vary in terms of grading and maximum particle size. To reflect this and allow Engineers to select a triaxial geogrid to optimize the performance of their mechanically stabilized layer, three further grades of triaxial geogrid have been developed to deal with coarser and finer granular fill respectively. As an important part of the selection criteria for the triaxial geogrid component of a mechanical stabilized layer, guidance follows for these grades of triaxial geogrid in terms of appropriate selection based on subgrade strength and maximum particle size of the proposed granular fill.

Figure 16. Triaxial geogrid selection: maximum particle size with radial secant stiffness

at 0.5% strain.

Performances of stabilisation mechanism with triaxial geogrid. From the results of various research projects, interlock and the resulting aggregate confinement are the mechanisms for mechanical stabilization. This contravenes the traditional view of many in the industry that geogrids act primarily as a tensioned membrane and that just one characteristic - the ultimate tensile strength, is of primary importance. With this research we are able to show the actual behavior of the triaxial geogrid, the forces and the actual movements of the geogrid. We are also be able to demonstrate that Radial Stiffness of the hexagonal structure triaxial geogrid at low strain is important and related to



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trafficking performance and that tensile strength is not. Very importantly we will show that it is not just the magnitude of Radial Stiffness that is important – but the uniformity of the radial stiffness in all directions. Something we measure by the ratio of the minimum to maximum Radial Stiffness values and refer to as Radial Stiffness Ratio. The model and displacements of the granular particles the Discreet Element Model (DEM) created (Figure 17) consists of a box, 400 mm x 400 mm and 120 mm high. In the bottom of the box a 40 mm thick layer of the selected soft sub-grade material is modelled, followed by an 80 mm layer of the 5-32 mm size granular soil. The granular layer is then trafficked with a wheel load of 500N travelling up and down over the granular soil.

Figure 17. Trafficking model.

The Figure 18 shows the vertical displacements along the wheel path after the 9th run. The colour indicates the magnitude of vertical displacement. Blue being smallest, starting from zero and red being largest up to 25 mm. It is clearly visible how the triaxial geogrid protects the sub-grade and reduces the permanent settlements.

Figure 18. The vertical displacements non-stabilised (left) and triaxial stabilised (right).

Figure 19 shows the vertical displacements perpendicular to the wheel path after the 9th run. The ruts in the wheel path are as before, but notice the different shape of the displacements. The non-stabilised case shows that the particles move in accordance with a load-spreading angle, whereas the triaxial grid efficiently confines the aggregate reducing the width of the displacement zone. At the soil geogrid interface there is hardly any horizontal movement due to this confinement effect.

Figure 19. The vertical displacements across the wheel path after 9th run on the same scale.



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The advantage of this simulation is that it is possible to look at the movements, stresses and strains of each particle, and also those of the geogrid. The most interesting are the movements of the geogrid in the plane of stabilization. Notice that the geogrid is very stable. The individual ribs flex a little under the action of the wheel loading, but they hardly twist the junctions. The confinement of aggregate hardly causes any movement of the geogrid. The forces in the rib follow the hexagonal pattern (Figure 20). These hexagons are arranged concentrically and are near circular in shape, forming tension rings around the center of the load. Tension rings are well known in civil engineering, they support for example dome structures. Also note that the tensile forces in the rings change to compression forces when the wheel has passed. Again there is no evidence of tension forces aligned in the longitudinal direction or transversal direction.

Figure 20. The forces in the ribs of the geogrid.

This simulation of the triaxial geogrid operating in a granular layer proves that the hexagons of the geogrid confine the granular particles, creating a (near circular) tension in the ribs of the hexagons (Tension-Ring-effect). For this to occur the stiffness of the geogrid must be near uniform in all directions (isotropic Radial Stiffness). A hexagon goes into tension when the wheel arrives and ribs go into compression after the wheel has passed. Consequently, a junction will be subjected to varying forces from the 6 ribs attached to it. The response to the moving load is a Sequential Overlapping Tension Ring effect in the geogrid. These model calculations show for the first time that the ultimate strength of the grids is NOT relevant and is NOT an indicator of the performance of the grid under wheel loading. Neither the ultimate strength nor the aperture stability in a product specification would guarantee that the product with the desired performance would actually be used on site! Radial Stiffness and Radial Stiffness Ratio are key characteristics. Understanding the radial stiffness ratio. As part of the European Technical Approval (ETA) of the stabilization function for triaxial geogrids, both the radial stiffness at low strain and radial stiffness ratio are identified as important components in understanding the stabilization effect for triaxial geogrids. The shape of the polar plot for triaxial geogrids shows a near uniform radial distribution of stiffness - particularly when compared with any biaxial geogrid. This IB explains the Radial Stiffness Ratio, its determination and its importance within a triaxial geogrids specification. The Radial Stiffness Ratio is the quotient of the minimum and maximum Radial Secant Stiffness values. It represents the uniformity (isotropy) of the Radial Secant Stiffness of a product in all directions. A value close to 1.0 would represent perfect isotropy - uniform in all directions. The shape of the polar plot for a typical triaxial geogrid is shown in Figure 21. For comparison, the shape of a typical biaxial geogrid is also shown. The tensile stiffness radial plot in Figure bellow for triaxial geogrids has been updated based on a large volume of data measured in accordance with European Organisation for Technical Approval (EOTA) Technical Report TR41.



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Figure 21. The stiffness polar plot of a biaxial geogrid and triaxial geogrid.

The polar plots of Radial Stiffness for biaxial geogrids and triaxial geogrid have two quite different indicative shapes and they each have their relevance to the performance of a mechanically stabilised layer (MSL). Figure 22 shows the radial stiffness of a laminar engineering material. It shows a variation in radial stiffness where a minimum and a maximum value occurs. The Radial Stiffness Ratio is the ratio of the minimum and maximum radial stiffness values (OB/OA).

Figure 22. Definition of isotropic stiffness ratio.

It can also be seen that, as in any production process, there is some variation and this can be considered statistically. Turning to triaxial geogrids, Figure 23 shows that there are two statistical populations of data; one for the variation in the minimum value and one for the variation in the maximum value. For the purposes of a performance-related specification Radial Stiffness Ratio is derived as follows. Figure 23 indicates the statistical ranges in minimum (OB) and maximum (OA) Radial Secant Stiffness. Radial Stiffness Ratio is the quotient of these minimum and maximum values. It is important to note that the values for Radial Secant



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Stiffness quoted in in the ETA and on triaxial geogrids specification documents are statistically derived from a large population of data and represents the 99.7% confidence limit.

Figure 23. Radial stiffness for triaxial geogrid.

If the shape of the polar plot were the ideal of a circle, the Radial Stiffness Ratio value would be unity. The data on production product that has been amassed for triaxail geogrids allows to declare that the range of characteristic values of Radial Stiffness Ratio for tiraxial geogrids is between 0.75 and 0.8. Typical Radial Stiffness Ratio vales for biaxial geogrids range between 0.2 and 0.4 which suggests that the response to a radially applied load will not be uniform in all directions. A uniform radial stiffness (isotropic) response is believed to be more efficient for stabilisation. Case studies Wind Farm Fantanele, Constanta County, Romania. West Fantanele Wind Farm with 139 wind turbines, each turbine having a power of 2.5 MW. Existing tracks had to be widened and new access roads built over the site which soil (loess) (Figure 24). Using thick stone layers to accommodate site traffic would have involved large numbers of vehicle movements and excessive road settlement. Instead, one triaxial geogrid layers were installed and combined with crushed stone. This solution delivered excellent trafficking performance and achieved a significant carbon emissions saving over an unreinforced solution (Figure 26). The quantity of the triaxial geogrid installed it was over 200.000 sqm.



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Figure 24. Relevant photos during the execution Wind Farm Fantanele.

Road execution phases (Figure 25): natural soil after 20 cm uncovering; geotextile 250 g/sqm; 5 cm repartition layer of split or natural sand; triaxial geogrid; 25 cm crushed stone 25-63 mm graded, compacted; 15 cm crushed granite 16-25 mm graded, compacted and bituminous treated.

Figure 25. Section of the access roads load 11,5 t/axle, Wind Farm Fantanele.

Figure 26. The road of Wind Farm Fantanele after the execution.



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Wind Farm Pestera, Constanta county, Romania. The site for wind farm Pestera was located a long distance away from the nearest source of suitable granular fill material. Access roads were needed for construction traffic, as well as the heavy turbine delivery vehicles and cranes which initially required large thicknesses of granular fill (Figures 27 and 28). Working platforms were also required to support the heavy crane operational loads and once again large amounts of granular fill were going to be needed.

Figure 27. Relevant photos during the execution Wind Farm Pestera.

Triaxial geogrids was designed to form the new access roads and working platforms for the wind farm site, which took into account the low strength soils and anticipated high trafficking loads and produced thinner and therefore more cost effective construction. The quantity of the triaxial geogrid installed it was over 200.000 sqm.

Figure 28. The road Wind Farm Pestera after the execution.

Execution phases (Figure 29): natural soil after 20 cm uncovering; geotextile 300 g/sqm; triaxial geogrid; 30 cm crushed stone 25-63mm graded, compacted; 20 cm crushed stone 0-25 mm graded, compacted.



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Figure 29. Section of the access roads load 11.5 t/axle, Wind Farm Pestera.

Wind Farm Chirnogeni, Constanta County, Romania. The triaxial geogrid it was used in Wind Farm Chirnogeni to stabilise well-graded aggregate over low soils to allow stabilised access onto the areas in question (Figures 30 and 31). The access roads support 100 kN standard axeles. The project involved the erection of 32 wind turbines. There will be access roads to separate working platforms required for lifting operations. The road width at the top of the road construction is 4.0-4.5m. Assessment of these granular layers is based on direct trafficking of the aggregate layers by construction vehicles. The quantity of the triaxial geogrid installed it was over 150.000 sqm.

Figure 30. Section of the access roads, Wind Farm Chirnogeni.

Figure 31. Relevant photos during the execution road Wind Farm Chirnogeni.



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Acknowlegments. The kind permission of British colleagues in Blackburn to use some technical data from their Information Bulletins for educational and research purposes is gratefully acknowledged. Conclusions. Access paved roads to remote locations respond to advanced technology requirements. Achieved with the aid of triaxial geogrids the following performances have been obtained: 1) outstanding bearing capacities; 2) flexibility and versatility; 3) unexpected durability; 4) shortest completion time; and 5) cheapest maintenance. The secrets of these performances were disclosed by one idealised model with spherical bodies under gravity action. They are based on the plan defined by Euclid’s triangle, Newton’s Principle of Reciprocal Actions, horizontally directing gravity by vault effect, using tetrahedral qualities for confining and Archimedes’ wedging effect. From model to practice only Coulombian friction was added. References Feodorov V., 2014 Triaxial geogrids in road technology progress. The International

Conference of the University of Agronomic Sciences and Veterinary Medicine of Bucharest, Agriculture for life, life for agriculture, June 5-7, Bucharest.

Tensar TriAx Brochure, 2011a The properties and performance advantages of Tensar TriAx geogrids.

Tensar, 2011b Wind Farm Access Roads – Two decades of floating roads. Tensar Information Bulletin, 2013 Selection of the appropriate Tensar TriAx® Geogrid. Tensar Information Bulletin, 2014a The mechanism of TriAx® stabilisation – DEM

mathematical modelling of a trafficked sub-base. Tensar Information Bulletin, 2014b Understanding radial stiffness ratio. Received: 05 October 2014. Accepted: 31 October 2014. Published online: 30 December 2014. Author: Valentin Feodorov, University of Agricultural Sciences and Veterinary Medicine, Bucharest, 59 Marasti, District 1, 011464 Bucharest, Romania, e-mail: [email protected] This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. How to cite this article: Feodorov V., 2014 Performances in access road construction. Ecoterra 11(4):49-63.

performances in access road construction valentin feodorov · performances in access road...

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