阅读说明：本技术 铺设于软弱路基之上的路面系统及其铺设方法 (Pavement system laid on weak roadbed and laying method thereof ) 是由 伊扎尔·哈拉米 奥德艾德·埃雷兹 奥弗·亚伯拉罕姆·兹维·基夫 于 2014-09-30 设计创作，主要内容包括：本公开文件涉及铺设于软弱路基之上的路面系统及其铺设方法,所述路面系统包括直接置于路基上的第一土工格栅层；置于第一土工格栅层之上的第一颗粒层；置于第一颗粒层之上的第一土工格室层,其包括土工格室和填充材料；以及任选地置于所述土工格室层之上的覆盖层。所述路面系统适合铺设于包括加州承载比(CBR)为4以下的常规软弱路基的位置。(The present disclosure relates to a pavement system and method of laying on a weak subgrade, the pavement system comprising a first geogrid layer disposed directly on the subgrade; a first granular layer disposed over the first geogrid layer; a first geocell layer disposed above the first granular layer, the first geocell layer comprising geocells and a filler material; and optionally a cover layer disposed over the geocell layer. The pavement system is suitable for being laid in a position including a conventional weak roadbed with a California load bearing ratio (CBR) of 4 or less.)

1. A pavement system for laying on a weak subgrade, comprising:

a first geogrid layer placed over said weak subgrade and comprised of at least one geogrid, each said geogrid being comprised of intersecting rib members to form geogrid apertures;

a first granular layer comprising a first granular material disposed over the first geogrid layer;

a first geocell layer disposed above the first granular layer, including at least one geocell filled with a filler material; and

optionally a cover layer disposed over the first geocell layer, the cover layer being made of a compacted second particulate material.

2. The pavement system of claim 1, further comprising a facing disposed over the first geocell layer, the facing comprising asphalt, or concrete, or ballast, or particulate material.

3. The pavement system of claim 1, wherein the average thickness of the first particle layer is from 0.5 to 20 times the mesh distance of the first geogrid layer.

4. The pavement system of claim 3, wherein the mesh distance is between 10 millimeters and 500 millimeters.

5. The pavement system of claim 1, wherein the first particulate material is sand, gravel, or crushed stone.

6. The pavement system of claim 1, wherein the first particulate material also enters the geogrid mesh of the first geogrid layer.

7. The pavement system of claim 1, wherein the filler material comprises sand, crushed stone, gravel, recycled asphalt pavement, quarrying screen, or mixtures thereof.

8. The pavement system of claim 1, wherein the second particulate material in the overlay comprises sand, gravel, or crushed stone.

9. The pavement system of claim 1, wherein the first geocell layer has a cell height of between 50 mm and 300 mm.

10. The pavement system of claim 1, wherein the first geocell layer has a cell size between 200 mm and 600 mm.

11. The pavement system of claim 1, wherein the at least one geogrid is made of polypropylene, polyethylene, polyester, polyamide, aramid, carbon fiber, fabric, wire or mesh, fiberglass, fiber reinforced plastic, multi-layer plastic laminates, or polycarbonate.

12. The pavement system of claim 1 wherein the first particulate material has an average particle size greater than an average particle size of the filler material.

13. The roadway system of claim 1, further comprising:

a second geocell layer or a second geogrid layer disposed over the first geocell layer;

wherein the cover layer is disposed over the second geocell layer or the second geogrid layer.

14. The pavement system of claim 13, further comprising a layer of second particles having a thickness between 1 and 300 millimeters, the layer of second particles being located between (i) the first geocell layer and (ii) the second geocell layer or the second geogrid layer.

15. A method of laying a pavement system over a weak subgrade, comprising:

laying at least one geogrid over the weak subgrade to form a first geogrid layer, each geogrid being made up of intersecting rib members to form geogrid apertures;

laying a sufficient amount of a first particulate material on the first geogrid layer and then compacting the first particulate material to form a first particulate layer;

providing at least one geocell on the first layer of particles;

filling the at least one geocell with a filler material, thereby forming a first geocell layer;

optionally laying a second particulate material over the first geocell layer and compacting the second particulate material to form an overlayer over the first geocell layer, the overlayer having a thickness between 0 and 500 millimeters.

16. The method of claim 15, further comprising the step of laying a facing over the overlay, the facing comprising asphalt, or concrete, or ballast, or particulate material.

17. The method of claim 15, wherein the average thickness of the first particle layer is from 0.5 to 20 times the mesh distance of the geogrid layer.

18. The method of claim 15, further comprising the step of removing soil to expose the weak subgrade.

19. The method of claim 15, wherein the first and second particulate materials are each independently sand, gravel, or crushed stone.

20. The method of claim 15, wherein the first particulate material also enters the geogrid mesh of the first geogrid layer.

21. The method of claim 10, wherein the packing material comprises sand, crushed stone, gravel, recycled asphalt pavement, quarrying screen, or mixtures thereof.

22. The method of claim 15, further comprising:

laying other geocells or geogrids above the first geocell layer to form a second geocell layer or geogrid layer below the cover layer.

23. The method of claim 22, wherein the second geocell layer or the second geogrid layer is spaced from the first geocell layer by between 0 and 500 millimeters.

Technical Field

The present disclosure relates to pavement systems suitable for application on weak roadbeds, natural soils, expansive soils, or soils prone to frost heaving in cold seasons. These pavement systems are located on foundations and are used in a variety of applications such as roads, park roads, sidewalks and railways. These pavement systems are particularly suitable for weak roadbeds.

In traffic engineering, a plurality of layers are laid during road construction. These layers include road base layers, underlayers, base layers, and topcoats. The roadbed is a natural material and serves as a foundation for the pavement. An underlayment may optionally be laid over the subgrade. The underlayment and base layers are used to carry and distribute the load to a level acceptable for the top layer. Depending on the desired roadway application, additional layers may be placed over the base layer, and this layer may be referred to as a paver base layer. A facing layer is then disposed over this layer, the facing layer being an exposed layer on the surface of the pavement. The surface layer may be, for example, asphalt (such as a road or parking lot), or concrete (such as a sidewalk), or ballast (such as located below rails), or compacted particulate material (dirt road).

A weak subgrade refers to a subgrade with a California Bearing Ratio (CBR) of 4 or less, or more typically, 3 or less, as measured when saturated with water. The weak roadbed has low rigidity and low load resistance. Specific weak subgrades include subgrades with swelled soil or soil that is susceptible to frost heaving in cold seasons. Frost heaving is the upward expansion of the soil caused by ice formation beneath the surface of the earth. The presence of water can cause some conditions to occur that are extremely disruptive to the road surface. First, water molecules can swell soil particles and reduce cohesion between soil particles. Secondly, the expansion of the water causes the soil to expand, thereby increasing the upward pressure on the road surface above. Again, water expands when frozen and hardens due to ice formation, which can damage the road surface. These upward stresses generated during expansion (e.g., expansion of clay or soil) are much greater than the stresses generated on the subgrade during transportation. The pavement laid on such a weak roadbed may be prematurely broken.

In many cases where the subgrade is weak and shallow, it is removed and replaced with a stronger and more dimensionally stable particulate material. However, in other cases this is not possible for the following reasons: (a) the soft soil of the roadbed is too deep; or (b) there is no stronger and more dimensionally stable particulate material locally, or the cost of transporting such material is prohibitive. Examples of these situations can be found in peat ponds in northern russia, expanded soil beds in texas, and moss beds in canada and siberia.

An example of a road surface is shown in figure 1. The pavement comprises a weak roadbed 2, a gravel base 4 and a surface layer 6. Similarly, weak roadbeds may be attributed to soft soils, expansive soils, or soils prone to frost heaving. Typical failures include rutting (grooves or ruts formed in the pavement), cracking of the asphalt or concrete overlay of the pavement, deformation or misalignment of the rails on the ballast, and pumping of the base layer beneath the overlay. These failure conditions are caused by irreversible deformation of the base layer and/or underlayment due to (1) lack of tensile strength, (2) stiffness (modulus), (3) interfacial strength between the layer and the subgrade, and/or (4) bending moment (resistance to bending).

One method commonly used to prevent these failure conditions involves chemically modifying the subgrade. The roadbed is mixed with an inorganic binder (such as lime, cement or fly ash) or an organic binder (such as a polymer emulsion). However, this approach has undesirable characteristics such as: slow curing, poor performance when applied in humid and cold climates, leaching of inorganic binders in humid climates, high cost of polymeric binders, brittleness, difficulty in mixing on site resulting in reduced quality, poor resistance during freeze-thaw cycles, and difficulty in obtaining a uniform subgrade (e.g., texture or composition) over a large area.

It would be desirable to provide a pavement system having improved performance when laid on a weak subgrade, natural soil, expansive soil, or frost-prone soil. It is also desirable to build such pavement systems in an economical and easy to lay method.

Disclosure of Invention

In various embodiments, a pavement system and method of laying the pavement system over a weak subgrade (e.g., expansive or frost-prone soil) having a CBR of 4 or less is disclosed. The pavement system generally includes a geogrid layer, a first particle layer, and a geocell layer over a subgrade. The first layer of particles has a particular thickness or height. The facing may be laid directly over the geocell layer or an additional geocell or geogrid reinforcement layer may be laid over the geocell layer prior to laying the facing.

Disclosed in some embodiments is a pavement system for placement on a weak subgrade having a California Bearing Ratio (CBR) of 4 or less, particularly expansive or frost-heaving soils, the pavement system comprising: a first geogrid layer placed over the subgrade and comprised of at least one geogrid, each geogrid being comprised of intersecting rib members to form geogrid apertures; a first granular layer comprising a first granular material disposed over the first geogrid layer, the first granular layer having an average thickness of 0.5 to 20 times the mesh distance of the geogrid layer; a first geocell layer disposed above the first granular layer, including at least one geocell filled with a filler material; and optionally a capping layer disposed over the first geocell layer, the capping layer being made of a compacted second particulate material.

The pavement system can further include a facing disposed over the optional overlay layer, or over the first geocell layer, the facing comprising granular material, asphalt, or concrete, or ballast. In some embodiments, the pavement system will have railroad tracks and ties laid thereon.

The first particulate material may be sand, gravel or crushed stone. Typically, the first particulate material also enters the geogrid mesh of the first geogrid layer.

The packing material may be sand, crushed stone, gravel or a mixture thereof.

The second particulate material in the optional cover layer may be sand, gravel or crushed stone.

The mesh distance of the geogrid can be between about 10 millimeters and about 500 millimeters, including between about 25 millimeters and about 100 millimeters.

The first geocell layer has a cell height of between about 50 millimeters and about 300 millimeters. The first geocell layer can have a cell size of between about 200 millimeters and about 600 millimeters.

At least one geogrid can be made of polypropylene, polyethylene, polyester, polyamide, aramid, carbon fiber, fabric, wire or mesh, glass fiber, fiber reinforced plastic, multi-layer plastic laminates or polycarbonate.

In some embodiments, the first particulate material has an average particle size greater than an average particle size of the filler material in the first geocell layer.

In some other embodiments, the pavement system further comprises: optionally a second particulate layer disposed over the first geocell layer; and a second geocell layer or a second geogrid layer disposed over the second granular layer or over the first geocell layer; wherein the cover layer is disposed over the second geocell layer or the second geogrid layer. The thickness of the second particle layer may be between about 1 millimeter to about 300 millimeters.

In some other embodiments, the pavement system further comprises a second geocell layer or a second geogrid layer disposed directly above the first geocell layer; wherein the cover layer is disposed over the second geocell layer or the second geogrid layer.

In other contemplated embodiments, a geotextile layer may be laid anywhere between the subgrade and the overburden. Such layers may be particularly useful if the pavement is used where the water table is high, or is subject to heavy rain or flooding, or where fine particles penetrate up and down between layers.

The invention also discloses a method for paving a pavement system on a weak roadbed (such as expansive soil or soil easy to frost heave) with the California Bearing Ratio (CBR) of less than 4, which comprises the following steps: laying at least one geogrid over the subgrade to form a geogrid layer, each geogrid being made up of intersecting rib members, thereby forming geogrid mesh; laying a sufficient amount of a first particulate material on the geogrid layer and then compacting the first particulate material to form a layer of first particles having an average thickness between 0.5 and 20 times the mesh distance of the geogrid layer; providing at least one geocell on the first layer of particles; filling the at least one geocell with a filler material, thereby forming a first geocell layer; optionally laying a second particulate material over the first geocell layer, and compacting the second particulate material to form an overlayer over the geocell layer, the overlayer having a thickness of between 0 and about 500 millimeters. Optionally, a second geogrid or geocell layer is laid directly over the first geocell layer, or is separated from the first geocell layer by a second layer of particles comprised of a granular material.

The method further comprises the step of laying a face layer over the overlay layer, the face layer comprising asphalt, or concrete, or ballast. The method further includes the step of removing soil to expose the weak subgrade.

In some particular embodiments, the method further comprises: forming a second granular layer over the geocell layer; and disposing additional geocells or geogrids above the second granular layer/above the first geocell layer to form a second geocell layer or geogrid layer below the cover layer. The second geocell layer or second geogrid layer can be spaced from the first geocell layer by a distance of 0 to about 500 millimeters.

The invention also discloses an improved pavement system suitable for long-term performance maintenance on a relatively weak subgrade, comprising, in order from bottom to top: subgrades with CBR below 4; geogrids placed directly on the subgrade or incorporated within a layer of granular material; a layer of granular material disposed over said geogrid, said layer of granular material having a thickness between 0.5 and 20 times the geogrid mesh distance; a geocell filled with sand, crushed stone, gravel, ash, Recycled Asphalt Pavement (RAP), quarrying screen, or a mixture thereof; optionally another layer of particulate material having a second geocell or a second geogrid laid thereon; a cover layer of compacted gravel, gravel or sand; and optionally a facing based on bitumen, or concrete, or ballast.

These and other non-limiting aspects of the disclosure are described in detail below.

Drawings

The following is a brief description of the drawings that illustrate exemplary embodiments of the disclosure and not to limit the invention.

Fig. 1 is a cross-sectional view of a conventional pavement system that does not include a geocell layer or a geogrid layer.

Fig. 2 is a perspective view of the geocell in an expanded state.

Fig. 3 is an enlarged perspective view of the polymeric strip in the geocell of fig. 2.

Fig. 4 is a top view of a portion of a geogrid.

Fig. 5 illustrates a pavement system having a geogrid layer and a geocell layer of the present invention.

Fig. 6 illustrates another pavement system having a geogrid layer, a first geocell layer, and a second geocell layer located above the first geocell layer.

Fig. 7 illustrates another pavement system having a first geo-grid layer, a geo-cell layer, and a second geo-grid layer positioned over the geo-cell layer.

FIG. 8 shows the base layer thickness in a conventional unreinforced design calculated as a function of the CBR of the subgradeDegree (H)_SUB-A) To thereby obtain a desired elastic modulus (E) of the base layer_V2-T) The figure (a).

Detailed Description

The disclosed components, methods, and apparatus may be more fully understood by reference to the accompanying drawings. For purposes of facilitating an understanding of the present disclosure, the drawings are merely schematic representations, not intended to illustrate the relative sizes and dimensions of the devices or components, and/or to define or limit the scope of the exemplary embodiments.

Although specific terms are used in the following description for the sake of clarity, these terms are intended to represent only the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the invention. In the following drawings and description, it is to be understood that like reference numerals refer to components having like functions.

As used herein, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

Numerical values in the specification and claims of this application should be understood to include values that differ from the stated value by less than the experimental error that would result from conventional measurement techniques described in this application.

All ranges of the invention are inclusive of the recited endpoints and independently combinable (e.g., ranges of "from 2 mm to 10 mm" are inclusive of the endpoints 2 mm and 10 mm and all intermediate values).

A value modified by the terms "about" and "substantially" may not be limited to the precise value specified. It is to be understood that the use of the modifier "about" also discloses the range defined by the absolute values of the two endpoints. For example, the expression "from about 2 to about 4" also discloses a range of "from 2 to 4".

When referring to the California Bearing Ratio (CBR) herein, the values provided are measured when the layer is saturated with water.

The present application relates to a pavement system for a surface. The present application also relates to different layers that are "on" or "over" or "above" each other. When these terms are used to describe the position of the second layer relative to the first layer, the first layer is located deeper in the ground than the second layer, or in other words, the second layer is closer to the surface than the first layer. It is not required that the first and second layers be in direct contact with each other; there may also be other layers present between the two. Further, each layer has a length, width, height/depth/thickness. Length and width refer to the dimensions of the layer in the horizontal dimension. The terms height, depth and thickness are used interchangeably and refer to the dimension of the layer in the vertical dimension.

Geogrids have been used to repair the failure conditions described above. Geogrids can be made of polymers (such as polyester yarns or extruded polymers) arranged in a network of ribs and meshes to impart uniaxial or biaxial tensile reinforcement to the soil. Geogrids can include coatings that can provide further chemical or mechanical benefits. Alternatively, the geogrid can be formed by stamping the sheet material and then stretching, as is used by Tensar corporation. Geogrids can also be formed by bonding bars or strips of polyester or polypropylene together in a grid pattern by means of laser heating or ultrasonic bonding. Geogrids are typically mechanically and chemically durable so that they can be laid in corrosive soils or water environments. Geogrids are two-dimensional structures without an effective height and are flat structures.

Geocells have also been introduced into pavement systems to prevent failure conditions. A geocell, also known as a cell reinforcement system (CCS), is an array of closed cells with an internal fill in a "honeycomb" like structure. CCS is a three-dimensional structure with internal force vectors in all the walls of each cell, whereas geogrids are only two-dimensional. However, when the geocell is used to reinforce a base or sub-base on a weak subgrade, the pavement may still be damaged due to the internal filler "running" out of the bottom of the geocell and settling down to the weak subgrade, as well as due to its insufficient tensile strength. This may result in undesirable differences in modulus and tensile strength between the base layer/underlayment and the subgrade, and may result in poor tensile properties along the interface direction.

The combination of geocells and geogrids has been investigated for use in conventional pavement systems. For example, one system has been to place geogrids in the subgrade layer and then place the geocells directly on the geogrid-reinforced subgrade layer (i.e., the sub-base) and fill the geocells with excavation material. The layers were then compacted and a clean stone layer (0.75 inch in height) was applied. However, systems using geogrid roadbed with geocell cover layers only partially solve the previously identified problem of pavement system failure. Because the rigidity of the geocell layer is high, the tension applied to the geogrid reinforcement layer is low. The geogrid must be significantly deformed to help achieve significant tensile reinforcement and thus the geogrid does not provide significant reinforcement to the system as a whole.

The present application is therefore directed to an improved pavement system suitable for long term use on weak roadbeds with a California Bearing Ratio (CBR) of 4 or less, or on expansive soils, or on frost heaving soils (i.e., freezable soils). These soils may include organoclay, peat, moss, montmorillonite soils and bentonite soils. The pavement system of the present invention includes a geogrid reinforcement layer separated from a geocell reinforcement layer by a layer of granular material. Additional geogrid layers or geocell layers can be placed above the original geocell reinforcement layer. Such a system is well suited for use in locations where stresses are also generated below the road surface (i.e., upward stresses).

Geocell, also known as a Cell Consolidation System (CCS), is a three-dimensional geosynthetic product that is suitable for many applications of geotechnical engineering techniques such as prevention of soil erosion, canal lining, reinforced retaining wall construction, and supporting pavements. CCS is an array of closed cells resembling a "honeycomb" structure, filled with a filler, which may be cohesionless soil, sand, gravel, ballast, or any other type of aggregate. CCS is used in civil engineering applications to prevent erosion or to provide lateral support for roads, pavements and railroad beds, such as soil retaining walls, sandbag walls or alternatives to gravity walls. Geogrids are generally flat (i.e., two-dimensional) and are used for planar reinforcement, while CCS is a three-dimensional structure with internal force vectors to all sidewalls within each cell. CCS can also provide effective reinforcement for relatively fine fillers such as sand, loam and quarry waste.

Fig. 2 is a perspective view of the geocell in an expanded state. The geocell includes a plurality of polymeric strips 14. Adjacent strips are bonded together along discrete physical first seams 16. The bonding may be by gluing, sewing or welding, but is typically by welding. The portion of each polymer strip between the two first seams 16 forms a cell wall 18 of an individual cell 20. Each cell 20 has cell walls made of two different polymeric strips. The polymeric tapes 14 are bonded together so that when unfolded a honeycomb pattern is formed from a plurality of tapes. For example, the outer and inner side straps 22, 24 are bonded together at a first seam 16, the first seam 16 being regularly spaced along the length of the outer and inner side straps 22, 24. A pair of inner side straps 24 are bonded together along a second seam 32. Each second seam 32 is located between two first seams 16. Thus, as the plurality of polymeric strips 14 are stretched or unfolded in a direction perpendicular to the strip faces, the strips bend in a sinusoidal pattern to form a geocell. At the edge of the geocell where the ends of the two polymer strips meet, a short distance is left between the end-weld 26 (also referred to as a joint seam) and the end 28 to form a short tail 30 that serves to stabilize the two polymer strips. Such a geocell can also be referred to as a section, and can be used in conjunction with other geocells to cover a larger area, especially when it is not possible to actually cover a single section.

Fig. 3 is a close-up perspective view of polymeric strip 14, schematically illustrating length 40, height 42, and width 44 of polymeric strip 14, and first seam 16. The length 40, height 42 and width 44 are measured along the indicated directions. The length of the geocell is measured when it is in a collapsed or compressed state. In the compressed state, each cell 20 can be considered to have no volume, while the expanded state refers to the state when the geocell has been expanded to its maximum possible volume. In some embodiments, the height 42 of the geocell is in the range of about 50 millimeters (mm) to about 300 mm. The size of the geocell (measured as the distance between the seams in the unfolded state) can be in the range of about 200 mm to about 600 mm.

Geocells can be made of linear low density Polyethylene (PE), Medium Density Polyethylene (MDPE), and/or High Density Polyethylene (HDPE). Hereinafter the term "HDPE" is meant to be characterized by a density greater than 0.940g/cm³The polyethylene of (1). The term Medium Density Polyethylene (MDPE) refers to a polyethylene characterized by a density greater than 0.925g/cm³To 0.940g/cm³The polyethylene of (1). The term Linear Low Density Polyethylene (LLDPE) is meant to be characterized by a density of 0.91g/cm³To 0.925g/cm³The polyethylene of (1). Geocells can also be made from polypropylene, polyamide, polyester, polystyrene, natural fibers, woven fabrics, blends of polyolefins with other polymers, polycarbonate, fiber reinforced plastic, fabric, or multi-layer plastic laminates. The strips used to construct the geocell are welded together in an offset (offset) manner, with the distance between the welds being in the range of about 200 mm to about 600 mm.

A typical width of the walls of a geocell is 1.27 millimeters (mm), and can also vary from 0.9 mm to 1.7 mm. The cell walls may be perforated and/or embossed.

Fig. 4 is a partially enlarged top view of the geogrid. The geogrid is made up of rib members 62, the rib members 62 intersecting each other to define geogrid mesh 64. Geogrids can be made of polypropylene, polyethylene, polyester, polyamide, aromatic polyamide (e.g., KEVLAR), carbon fiber, fabric, wire or mesh, fiberglass, fiber reinforced plastic (e.g., blends or alloys), multi-layer plastic laminates, or polycarbonate. As shown, the geogrid mesh is rectangular, but in general the geogrid mesh can be any shape, including square, triangular, circular, etc. Any geometric shape may be used. The rib members are less than 50% of the area of the geogrid, in other words, the open area of the geogrid is greater than 50%.

The mesh distance of each geogrid mesh is the average length of the rib members surrounding the mesh. As shown, for example, in a rectangular mesh, the mesh distance is the average length of the shorter rib members 66 and the longer rib members 68. In some embodiments, the mesh distance of the geogrid is in the range of about 10 millimeters to about 500 millimeters, or in the range of about 25 millimeters to about 100 millimeters.

The geocell and geogrid can be distinguished by the vertical thickness of their respective strip and rib members. The height of the geocell is at least 20 millimeters, and the height of the geogrid is between about 0.5 millimeters and 2 millimeters.

FIG. 5 is a cross-sectional view of an exemplary pavement system of the present disclosure. Typically, the geogrid reinforcement is separated from the geocell reinforcement by a layer of granular material.

First, a geo-grid layer 60 is formed on a road base layer 50. The geogrid layer is formed by at least one geogrid. It is noted that the subgrade may be natural subgrade, may be chemically modified (e.g., with lime, cement, polymer, or fly ash), or may be physically modified (e.g., replaced with a more stable particulate material). The thickness of the roadbed-modified portion may vary from about 50 mm to about 1000 mm.

Next, a first granular layer 70 is placed over the geogrid layer 60. The first particulate layer comprises a first particulate material, which may be sand, gravel or crushed stone. The thickness 75 of the first particle layer is between 0.5 and 20 times the mesh distance of the geogrid layer. It is noted that the first particulate material may fall/enter the geogrid mesh of geogrid layer 60. The first particulate layer may be densified if desired.

Assuming that all geogrids are the same, the mesh distance of the geogrid layer is typically the same as the mesh distance of the geogrids that make up the geogrid layer. When different geogrids having different mesh distances are used in the geogrid layer, the average mesh distance should be calculated as the mesh distance of the geogrid layer by weighting the surface area covered by each geogrid.

Next, the geocell layer 80 is placed over the first granular layer 70. The geocell layer is comprised of at least one geocell 82, the geocell 82 being filled with a filler material 84. The filler material is compacted to harden the filler. Exemplary packing materials include sand, crushed stone, gravel, and mixtures thereof. The filler material may also include other finer grades of particulate material if desired. In this regard, in some embodiments, the average particle size of the first particulate material of the first particle layer is greater than the average particle size of the filler material.

In order to improve the tensile and shear forces and the performance of the geocell layer 80, it is necessary to combine the geogrid layer 60 with the first particle layer 70. The combination of the geogrid layer and the first particle layer provides: (1) a hard, impermeable "floor" which ensures high hardness of the geocell layer during compaction of the fill material; (2) a barrier to prevent fine particles from backfilling up the subgrade to the geocell layer; (3) an interface having high shear; (4) mechanical isolation between the subgrade and the geocell layer ensures that the geocell layer acts as a rigid and elastic beam, while limiting its stresses to within the elastic range.

Optionally, a cover layer 90 is then placed over the geocell layer 80. The layer is formed of compacted material such as crushed stone, gravel or sand. This layer can be considered to consist of the second particulate material.

A facing layer 100 may optionally be disposed over the cover layer 90, with the cover layer 90 being distributed over the geocell layer 80. The facing may comprise asphalt, or concrete, or ballast.

This design can allow the geogrid layer 60 to deform so that the geogrid layer can harden and reinforce the first particle layer 70 below the geocell layer 80. This configuration significantly reduces the stresses and deformations that are transferred to the subgrade 50 and to the interface between the subgrade and the sub-base. The geogrid layer 60 and first particle layer 70 also provide a rigid base for the geocell layer 80 by improving the tensile and shear strength properties of the uppermost region of the subgrade 50. Geogrid layer 60 improves the fatigue resistance of the subgrade and helps reduce the "leakage" of the filler material from the geocell layer down the pavement system over its useful life. Notably, the first granular layer 70 separates the geogrid layer 60 from the geocell layer 80; the geogrid and the geocell are not in contact with each other during assembly.

The geocell layer 80 acts as a rigid and very hard cushion that distributes stress over large area pavement systems and helps to avoid localized overstressing. These local overstresses are the main cause of failure of the pavement system laid on the weak foundations. The inner filling material may be sand, gravel, or crushed stone or a mixture thereof.

When the geogrid layer and the geocell layer are separated by the first granular layer, a synergistic relationship is created between the geogrid layer and the geocell layer. The geogrid layer 60 is located below the geogrid layer 80 a distance that ensures sufficient deformation room along the geogrid layer to provide tensile stiffness to the subgrade to resist the stresses created by the subgrade expansion. The design scheme of the invention can elastically absorb a large amount of mechanical stress and has higher fatigue resistance. In particular, the pavement system of the present invention has improved resistance to a variety of mechanical cycling loads, a variety of expansion-contraction conditions of the subgrade, and long-term freeze-thaw cycles.

Without being bound by theory, it is believed that disposing one or more geogrids layers over the subgrade is not sufficient to successfully reinforce the subgrade because (1) the bending moment is not sufficient; and (2) the hardness of the geogrid layer is not enough. Similarly, the use of only one geocell layer above the subgrade is not successful because (1) the tensile strength is insufficient; (2) the inner filler overcomes the tendency of upward/downward stress due to traffic or expansion and contraction of the soil.

The invention also includes a method of laying a pavement system over a weak subgrade. Generally, the soil is removed to expose the weak subgrade. Next, at least one geogrid is laid over the subgrade to form a geogrid layer. A sufficient amount of the first particulate material is then laid on the geogrid layer to form a first particulate layer having an average thickness between 0.5 and 20 times the pore size distance of the geogrid layer. At least one geocell is laid over the first granular layer and then filled with an inner filler material to form a geocell layer. The second particulate material is laid over the geocell layer and then compacted to form a cover over the geocell layer. If desired, a top layer may be laid over the overlay.

Fig. 6 and 7 are cross-sectional views of two other embodiments of pavement systems that include additional layers.

As described above, in fig. 6, the pavement system includes a geogrid layer 60 formed over a base course layer 50, a first granular layer 70 disposed over the geogrid layer 60, and a geocell layer 80 disposed over the first granular layer 70. The first particle layer 70 has a thickness 75. The second granular layer 110 is then placed on the geocell layer 80. The second particle layer may be made of the same material as the first particle layer 70 or the same material as the filler of the geocell layer. The second particulate layer may be considered to be made of a third particulate material (as described above, the cover layer is formed of the second particulate material). The thickness 115 of the second particle layer may be in a range of about 10 millimeters to about 500 millimeters. A second geocell layer 120 is then placed on the second granular layer 110. This second geocell layer, like the geocell layer 80 described above, is also comprised of at least one geocell and is filled with a filler material. The cover layer 90 is then placed over the second geocell layer 120, and a cover layer 100 may optionally be laid over the cover layer 90. The composition of the cover layer and the facing layer may be as shown in fig. 5. The second geocell layer 120 provides additional tensile strength to the system and resists buckling of the subgrade that may occur during clay swelling or freeze-thaw cycles.

As described above, in fig. 7, the pavement system includes a geogrid layer 60 formed over a foundation layer 50, a first granular layer 70 disposed over the geogrid layer 60, and a geocell layer 80 disposed over the first granular layer 70. The first particle layer 70 has a thickness 75. A second granular layer 110 is also placed over the geocell layer 80, the composition of which is described above. The thickness 115 of the second particle layer may range from about 1 millimeter to about 300 millimeters. A second geogrid layer 130 is then placed over second particle layer 110. The second geogrid layer is made up of at least one geogrid. Cover 90 is then placed over second geo-grid layer 130, and cover 100 may optionally be laid over cover 90. The composition of the cover and face layers is as described in fig. 5. The material used to form the cover layer may fall into the mesh of the second geo-grid layer 130. Second geo-grid layer 130 also provides additional tensile strength to the system and resists buckling of the subgrade that may occur during clay swelling or freeze-thaw cycles.

In other contemplated embodiments, the second geogrid layer or the second geogrid layer can be placed directly on top of the first geogrid layer after the filler in the first geogrid layer is compacted. No second particle layer is required. The distance between the first geocell layer and the second geogrid layer or the second geocell layer can thus be adjusted as needed within a range from approximately 0 to about 500 millimeters to achieve the desired overall road modulus and fatigue resistance.

Furthermore, if desired, a geotextile layer may be laid in the pavement system at any location between the subgrade and the top layer of the system (i.e., the geotextile layer must not be the uppermost layer of the system). Geotextiles are two-dimensional permeable fabrics that can be woven or nonwoven and are used to prevent fine particles from leaching or penetrating into the surface layer of the pavement. Geotextiles are clearly distinguished from geogrids because the mesh of the geogrid is large enough to allow soil to penetrate from one side of the geogrid to the other, while geotextiles do not allow soil to penetrate. Geotextile layers are particularly useful in areas subject to flooding, heavy rain, or high ground water levels. The geotextile layer may be formed from a material having a specific gravity of 50 grams per square meter (g/m)²) To 3000g/m²In between.

The invention will be further illustrated by the following non-limiting working examples, which are to be understood as being illustrative only and not limiting with respect to the materials, conditions, process parameters and the like mentioned.

Examples

The rails are laid on an expansive soil roadbed with the CBR of 3 when the rails are saturated with water. The rails require regular maintenance and the trains traveling on the foundations need to be speed limited. The conventional design is compared to the alternative design described in this disclosure.

FIG. 8 shows the calculation as a function of the CBR of the subgradeThickness of base layer (H)_SUB-A) To obtain the desired modulus of elasticity of the substrate. For example, if the modulus of elasticity of 100kPa is to be obtained with a CBR of 3 for a roadbed, the thickness of the base layer needs to be 750 mm. This modulus is sufficient to meet the design requirements of israel conventional railway pavement.

The conventional design is achieved by using sand or lime to consolidate the first 600 mm of the subgrade. Next, 920 mm gravel was laid and compacted, followed by 300 mm gravel. Ballast and railroad ties are then laid over the pavement system.

Alternative designs are described below. In a model road surface in which a geogrid reinforcement layer and a geocell reinforcement layer are laid on a roadbed having a known CBR, the modulus after the geogrid reinforcement layer and the geocell reinforcement layer are combined is additionally measured. The pressure cell is located under the geogrid layer. Above the geocell layer, increasing pressure is applied with flat plates or wheels until plastic (irreversible) deformation occurs. The modulus of the layer was back-calculated from the pressure drop curve. The degree of "immunity" to long-term stress was evaluated from plastic deformation after a series of repeated loads.

An alternative design in the art is first to flatten the subgrade. A first geogrid layer was laid, covered with a 200 mm thick layer of crushed stone. Thereafter a first geocell layer is laid over the gravel layer. The height of the first geocell layer was 150 mm, and the distance between the joints of the geocells was 330 mm. The filling material is broken stone. Thereafter a second layer of particles 50 mm thick was laid over the first geocell layer and a second geocell layer of the same construction as the first geocell layer was laid. Thereafter, the ballast and the railroad ties are laid on the second geocell layer.

The materials required in the two designs are significantly different. The conventional design requires treatment with 600 mm sand or lime followed by 1220 mm of particulate material. In contrast, the alternative design requires only 750 mm of particulate material, resulting in significant cost savings.

The performance of both designs was studied in israel for one year. The conventional design suffers from plastic deformation, and the plastic deformation increases continuously over time. The result is a slower train speed and shorter maintenance intervals. An alternative design, employing one geogrid layer and two geocell layers, exhibits purely elastic properties without irreversible deformation.

It will be appreciated that variations of the above-disclosed and other features, functions, or alternatives may be combined into many other different systems or uses. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.